astronomy150the planets The Solar System 9 Planets Earth Mars, Mercury, Venus, Uranus, Neptune, Jupiter, Saturn, Pluto. The Abundance of Elements in the Sun Hydrogen Helium Carbon Nitrogen Oxygen Neon Magnesium Silicon Sulfur Iron 4. Look at Figure I-6. How can you tell that Mercury follows an elliptical orbit? Can you detect the elliptical Our goal has been to preview the scale of astronomical ob- jects. To do so, we journeyed outward from a familiar cam- pus scene by expanding our field of view by factors of 100 meters, or astronomical units for measuring certain Only 12 such steps took us to the largest structures in the 5. Which is the outermost planet in our solar system? Why .does that change? Pluto. Pluto is smaller than the 4 jovian planets. shape of any other orbits in this figure or the next? 6, Why are light-years more convenient than miles, kilo- universe. 7. Why is it difficult to detect planets orbiting other stars? metric system to simplify distances? our calculations and scientific no- 8. What does the size of the star image in a photograph 9. What is the difference between the Milky Way and the Milky Way Galaxy? galaxy is only one of many billions of galaxies in the universe. 10. What are the largest known structures in the universe? to the sun. Of the eight other planets in our solar system, ter in inches? In yards? The numbers in astronomy are so large it is not conve- nient to express them in the usual way. Instead, we use the tation to more easily write big numbers. The metric system and scientific notation are discussed in Appendix A. tell us? We live on the rotating planet Earth, which orbits a rather typical star we call the sun. We defined a unit of distance, the astronomical unit, to be the average distance from Earth Mercury is closest to the sun, and Pluto is the most distant. The sun, like most stars, is very far from its neighbor- p ing stars, and this leads us to define another unit of distance, the light-year, the distance light travels in 1 year. The near- 2.If a mile equals 1.609 km and the moon is 2160 miles in diameter, what is its diameter in kilometers? 3. One astronomical unit is about 1.5 x 108 km. Explain why this is the same as 150 x 106 km. 4. Venus orbits 0.7 AU from the sun. What is that distance in kilometers? 5. Light from the sun takes 8 minutes to reach Earth. How long does it take to reach Mars? 6. The sun is almost 400 times further from Earth than is the moon. How long does light from the moon take to Chapter 2 Review Questions 1, The diameter of Earth is 7928 miles. What is its diame- est star to the sun is Proxima Centauri at a distance of 4.2 ly. As we enlarged our field of view, we discovered that the sun is only one of 100 billion stars in our galaxy and that our verse. Galaxies appear to be grouped together in clusters, superclusters, and filaments, the largest structures known. As we explored, we noted that the universe is evolving. Earth's surface is evolving, and so are stars. Stars form from the gas in space, grow old, and eventually die. We do not yet understand how galaxies form or evolve. Among the billions of stars in each of the billions of galaxies, many have planets. Although astronomers can now detect planets orbiting other stars, we know very little about the nature of these planets. Yet we suppose that there must be many planets in the universe and that some are like Earth. We wonder if a few are inhabited by intelligent beings like ourselves. solar system Milky Way scientific notation Milky Way Galaxy astronomical unit (AU) spiral arm light-year (ly) Local Group galaxy 1. What is the largest dimension you have personal knowl-edge of? Have you run a mile? Hiked 10 miles? Run a marathon? 2. In Figure I-4, the division between daylight and dark-ness is at the right on the globe of Earth. How do we know this is the sunset line and not the sunrise line? 3. What is the difference between our solar system, our galaxy, and the universe? reach Earth? Part 1 The AstronomeYs Sky 7. If the speed of light is 3 X 105 km/s, how many kilome-ters are in a light-year? How many meters? 8. How long does it take light to cross the diameter of our Milky Way Galaxy? 9. The nearest galaxy to our own is about 2 million light-years away. How many meters is that? 10. How many galaxies like our own would it take laid edge to edge to reach the nearest galaxy? (Hint: See Problem 9.) C 1. Locate photographs of Earth taken from space. What do cities look like? Can you see highways? Is the presence of our civilization detectable from space? 2. Locate photographs of nearby galaxies and compare them with photos of very distant galaxies. What kind of detail is invisible for distant galaxies? 3. One of the biggest clusters of galaxies is the Virgo clus-ter. Find out how many and what kind of galaxies are in the cluster. Is it nearby or far away? Exploring TheSky 1. Locate and center one example of each of three differ-ent types of objects: a. A planet, such as Saturn. Find its rising and setting time. Such objects have distances measured in astro-nomical units (AU). ob How to proceed: Decide on the object you want to lo-cate. Then find and center the object by clicking the Find button on the Object Toolbar. The second method is to press the F key. The third is to click Edit, then Find. Once you have the Object Information win-dow, click the center button. b. A star. All stars in TheSkybelong to our Milky Way Galaxy. Give the star's name, its magnitude, and its distance in light-years. How to proceed: Click on any star, which brings up an Object Information window. c. A galaxy. Give its name and/or its designation. How to proceed: Click on the Galaxies button in the Object Toolbar, then click on any galaxy. Distances to galaxies are millions and billions of light years. 2. Look at the solar system from beyond Pluto by clicking on View and then on 3D Solar System Mode. Tip the solar system edge-on and then face-on. Zoom in to see the inner planets. Under Tools, set the Time Skip Incre-ment to 1 day and then go forward in time to watch the planets move. 3. Identify some of the brightest constellations located along the Milky Way. (Hint: See View, Reference Lines.) r 6o to tbo tiroaRs/COIO ~strosorll Resource Center Inww.Rrookscole. comlasirouors) lor criticsl tRInYtug erercisos, articlss, and addl-tianal resdings Iror IutoTrse Collegs EWUog, Rrsob:ICole's sgugo student Ilbrary. Introduction: The Scale of the Cosmos 9 corporated into their monument the cycles of the sun away from the moon. As the rotating Earth carries the conti- and moon, nents through these bulges of deeper water, the tides ebb The cycles in the sky are a rich part of our culture, and flow. Friction with the seabeds slows Earth's rotation, but those same motions reveal an astonishing fact- and the gravitational force the bulges exert on the moon Earth is a planet. In the next chapter, we will see how force its orbit to grow larger. humanity made that discovery. When the moon passes through Earth's shadow, sun-light is cut off, and the moon darkens in a lunar eclipse. If the moon only grazes the shadow, the eclipse is partial, or - penumbral, and not total. - If the moon passes directly between the sun and Earth, Astronomers divide the sky into 88 areas called constella- it produces a total solar eclipse. During such an eclipse, the tions. Although the constellations originated in Greek and bright photosphere of the sun is covered, and the fainter Middle Eastern mythology, the names are Latin. Even the corona, chromosphere, and prominences become visible. An modern constellations, added to fill in the spaces between observer outside the path of totality sees a partial eclipse. If the ancient figures, have Latin names. The names of stars the moon is in the farther part of its orbit, it does not cover usually come from ancient Arabic, though modern astrono- mers often refer to a star bv constellation and Greek letters ~e Photosphere completely, resulting in an annular eclipse. - assigned according to brightness within each constellation. scale. First-magnitude stars are brighter than 2nd-magnitude constellation on. The magnitude we see when we look at a star in the sky is its apparent visual magnitude. magnitude scale The celestial sphere is a model of the sky, carrying the celestial objects around Earth. Because Earth rotates east- axis. The northern and southern celestial poles are the pivots on which the sky appears to rotate. The celestial equator, an imaginary line around the sky above Earth's equator, divides the sky in half. horizon Astronomers often refer to angles "on" the sky as if the stars, sun, moon, and planets were equivalent to spots painted on a plaster ceiling. Then the angle on the sky between two objects is independent of the true distance between the ob- jects in light-years. south celestial Pole The gravitational forces of the moon and sun act on the spinning Earth and cause it to precess like a gyroscope. Earth's axis of rotation swee s around in a conical motion with a points P New Terms The magnitude system is the astronomer's brightness autumnal equinox stars, which are brighter than 3rd-magnitude stars, and so asterism summer solstice winter solstice apparent visual magnitude perihelion ~m~) aphelion ward, the celestial sphere appears to rotate westward on its scientific model spring tide precession neap tide celestial sphere lunar eclipse zenith penumbra umbra nadir sidereal period north celestial pole synodic period solar eclipse north, south, east, and west photosphere chromosphere period of 26,000 years, and consequently the celestial poles celestial equator and celestial e uator move slowl angular distance of the stars. prominence angular diameter diamond ring effect node a ainst the back round corona minute and second of arc annular eclipse Milankovitch hypothesis Solar cycle zodiac circumpolar constellations perigee ecliptic apogee The motion of Earth changes in ways that can affect the vernal equinox climate. Changes in orbital shape, in precession, and in axial tilt can alter the planet's heat balance and may be responsible for the ice ages and glacial periods. Because we see the moon by reflected sunlight, its shape appears to change as it orbits Earth. The lunar phases wax from new moon to first quarter to full moon and wane from constellation? Give some examples. full moon to third quarter to new moon. A complete cycle of 1. What is the difference between an asterism and a con- 2, Do people in other parts of the world see the same con- stellations, asterisms, lunar phases, lunar eclipses, and solar eclipses that you see? Explain. lunar phases takes 29.53 days. The moon's gravitational field exerts tidal forces on 3` Earth that pull the ocean waters up into two bulges, one on the side of Earth facing the moon and the other on the side 3~ What does the word apparent mean in apparent visual magnitude? 36 Part 1 The Astronomer's Sky 4. In what ways is the celestial sphere a scientific model? 8. he average distance of Mars from the sun is 1.52 AU. 5. If Earth did not rotate, could we define the celestial poles Use the small-angle formula to calculate the angular di- and celestial equator? the ecliptic? ameter of the sun as seen from Mars. 9. Draw a diagram showing Earth, the moon, and shadows during (a) a total solar eclipse, (b) a total lunar eclipse, (c) a partial lunar eclipse, (d) an annular eclipse. 6. Where would you go on Earth so you could see both north and south celestial poles at the same time? Equator Where would you go to see a celestial pole at your zenith? 7. What is the difference between the daily and annual motion of the sun? 8. What would our seasons be like if Earth were tipped 35° instead of 23.5°? What would they be like if Earth's axis were perpendicular to its orbit? criticat Inquiries 9. Why are the seasons reversed in the Southern Hemi- sphere Because Earth orbits the sun, the sun appears to move eastward along the ecliptic through the constellations. Be- cause the ecliptic is tipped 23.5° to the celestial equator, the sun spends half the year in the northern celestial hemisphere and half the year in the southern celestial hemisphere, pro- ducing the seasons. The seasons are reversed south of Earth's equator. 1. Nearly all cultures have populated the sky with gods, heroes, animals, and objects. What can you learn on the Web about non-Western constellations? 2. Who was Orion? How is he related to the scorpion in 10. Where would you look in the sunset or dawn skies to find the full moon? the first quarter moon? the waning crescent moon? the waxing gibbous moon? sphere? 10. A total solar eclipse was visible from eastern Canada on July 10, 1972. When did this eclipse next occur? When will it next be visible from eastern Canada? the sky? 11. What phase would Earth be in if you were on the moon when the moon was full? first quarter? waning crescent? 3. What holidays, rituals, special foods, and beliefs are as- 12. How does the moon slow Earth's rotation? How does sociated with the winter solstice? Earth slow the moon's revolution? 4. What can you find out about Milutin Milankovitch? What is the latest news about the Milankovitch hypothesis? 13. Why isn't there an eclipse at every new and at every full moon? 14. Why is the moon red during a total lunar eclipse? Exploring TheSky 15. Why have most people seen a total lunar eclipse while fewer people have seen a total solar eclipse? 1. As discussed in this chapter, Earth's rotation about its own axis gives us the impression that the whole sky ro-tates around the north celestial pole in a period of one day. This apparent motion of the celestial sphere is dif-ficult to notice because it happens so slowly. However, 1. All cultures around the world named constellations. TheSky makes it possible to simulate this motion at a Why do you suppose this was such a common practice? pace that is easy to observe by using a feature called 2. Do planets orbiting other stars have ecliptics? Could Time Skip. Observe and describe the apparent motion of they have seasons? Must they have seasons? the sky as you see it looking north> east, south, and west. How to proceed: Set the Time Skip Increment (a drop-down menu on the Time Skip Toolbar) to 1 minute, and click on the Go Forward button to begin the simulation. View the sky from the four cardinal directions, due north, south, east, and west. (You'll find Time Skip under the Tools menu as well.) 3. Why would it be difficult to see prominences if you were on the moon during a total lunar eclipse? Prominences are eruptions of ionized gas flowing along magnetic field lines near sunspots. Hot gas boils out of the Sun's surface. Cools, and falls back down. A prominence may last several weeks. 1. If light from one star is 40 times more intense than light from another star, what is their difference in magnitudes? 40.0 magnitude 80.0 magnitude 40.0 magnitude 2. If two stars differ by 8.6 magnitudes, what is their intensity ratio? 3. Star A has a magnitude of 2.5; Star B, 5.5; and Star C, 9.5. Which is brightest? Which are visible to the unaided eye? Which pair of stars has an intensity ratio of 16? Star C, 9.5 is brightest. Star B, 5.5 and Star C, 9.5 are visible. Star A, 2.5 and Star B, 5.5. 4. By what factor is sunlight more intense than moonlight? (Hint: See Figure 1-4.) 5. If you are at a latitude of 35 degrees north of Earth's equator, what is the angular distance from the northern horizon up to the north celestial pole? from the south-ern horizon down to the south celestial pole? 6. If Earth is about 5 billion (5 x 10 9) years old, how many precessional cycles have occurred? 7. Identify the phases of the moon if on March 21 the moon were located at (a) the vernal equinox, (b) the autumnal equinox, (c) the summer solstice, (d) the winter solstice. 2. In which constellation was the sun located on the date of your birth? (Hint: Click Data and then Site Informa-tion to set the location, date, and time of your birth. Turn on constellation figures, constellation boundaries, and labels. Then find the sun.) 3. Set your location to Earth's North Pole and the date to the summer solstice. Turn on the ecliptic and then step forward in time through a day to see what happens to the sun. Repeat for the autumnal equinox and the win-ter solstice. 4. Repeat Activity 3 above for a location on Earth's equator. • Bo to tbe BrookslCole Astronomy Resobrce Ce~tsr (www.brookscols. comlastronomYl Chapter 1 The View from Earth 8) In their search for evidence, modern astronomers have extended their senses with powerful instruments, the subject of the next chapter. uniform circular motion hypothesis geocentric universe theory parallax (p) natural law retrograde motion mass 2. Why did classical astronomers conclude that Earth had to be motionless? Born in 1642, the same year that Galileo died, Isaac Newton used the work of Kepler and Galileo to discover three laws of motion and the law of gravity. These laws made it possible to understand the orbital motion of the planets as a consequence of the sun's gravity. In addition, Newton's work made it possible to analyze the motion of any celestial body The 99 years from the death of Copernicus to the birth of Newton marked the birth of modern science. From that time lied on the analytic methods first demonstrated by Newton. 4. In what ways were the models of Ptolemy and Coperni- Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets. Copernican model, that fit Tycho’s complex mass of detailed observations. In the end, he had to abandon Copernicus’s original simple notion of circular planetary orbits the moon with the statement, "The moon is falling." on, science depended on evidence to support theories and re- teachings of Aristotle had become part of Church teachings. epicycle inverse square relation deferent circular velocity geosynchronous satellite were carried around Earth by great crystalline spheres. This center of mass paradigm closed orbit ellipse escape velocity placed the concentric, Earth-centered heavenly spheres equant semimajor axis (a) open orbit with eccentricity (e) dict the positions of the sun, moon, and planets. Review Questions or sun centered. One advantage of a heliocentric universe is 1. Why did Greek astronomers conclude that the heavens were made up of perfect crystalline spheres moving at constant speeds? Classical astronomy was based on the writings of the Greek philosopher Aristotle. He taught that Earth was the immo- bile center of the universe and that the stars and planets model of the universe was given mathematical form about heliocentric universe AD 140 in the Almagest, the great work of Ptolemy. Ptolemy preserved the classical belief in geocentrism, but he re- a system of epicycles, deferents, and equants and tried to create a mathematical model that could accurately pre- In contrast to the geocentric universe of classical astron- omy, the universe devised by Copernicus was heliocentric, that retrograde motion, the occasional westward motion of the planets, is easily explained. Copernicus did not publish his book De Revolutionibus until 1543, the year he died. The As a critic of the classical view that Earth is at the center of the universe, Copernicus was exploring controversial ideas, The Danish astronomer Tycho Brahe did not accept the Ptolemaic or the Copernican model but rather developed his own, in which the sun and moon circled Earth, and the plan- ets circled the sun. Although his hypothesis was not correct, Tycho made precise observations of planetary positions that later led to a true understanding of planetary motion. Johannes Kepler, Tycho Brahe's assistant, inherited the Danish astronomer's records in 1601 and used his observa- tions to uncover three laws of planetary motion. Kepler dis- focus, that they move faster when near the sun, and that a planet's period squared is proportional to its orbital radius cubed. 3, How did the Ptolemaic model explain retrograde motion? ideas that some would claim were heretical. Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets. Copernican model, that fit Tycho’s complex mass of detailed observations. In the end, he had to abandon Copernicus’s original simple notion of circular planetary orbits, but even greater simplicity emerged as a result. Kepler determined the shapes and relative sizes of each planet’s orbit by triangulation not from different points on Earth but from different points on Earth’s orbit, using observations made at many different times of the year. Noting where the planets were on successive nights, he was able to infer the speeds at which they moved. cus similar? 5. Why did the Copernican hypothesis win gradual accep- tance? A hypothesis is a reasonable supposition made in describing the results of experiments and observations. It attempts to make predictions of future behavior. Once the evidence for its validity is strong, it becomes a theory. the heliocentric model to the observed behavior of the sky. The heliocentric universe explained many observations more simply than the geocentric model. The planets could now be put in order by distance from the Sun, and that information both explained planetary regression and provided a constant increase in sidereal period with distance. The problem with the Copernican theory was inability to abandon the idea that planets could move in anything other than perfect circles. This idea had persisted since Ptolemy, and by keeping it, Copernicus had to add epicycles and deferents to his model to explain existing planetary observations. The new model needed even more epicycles than the Ptolemaic model. The general principle in science (called Occam's razor) is that the simplest explanation that fits the facts is the correct one. More epicycles made this theory more complicated. 6. Why is it difficult for scientists to replace an old para- digm with a new paradigm? 7, Why did Tycho Brahe expect the new star of 1572 to show parallax? Why was the lack of parallax evidence against the Ptolemaic model? covered that the planets follow ellipses with the sun at one 8. How was Tycho's model of the universe similar to the Ptolemaic model? How did it resemble the Copernican model? Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets. Copernican model, that fit Tycho’s complex mass of detailed observations. In the end, he had to abandon Copernicus’s original simple notion of circular planetary orbits, even greater simplicity emerged as a result. Kepler determined the shapes and relative sizes of each planet’s orbit by triangulation not from different points on Earth but from different points on Earth’s orbit, using observations made at many different times of the year. Noting where the planets were on successive nights, he was able to infer the speeds at which they moved. 9. Explain how Kepler's laws contradict uniform circular motion. Kepler's first and second laws of planetary motion. The first law: The orbits of the planets are ellipses with the sun at one focus. The second law: A line extending from the sun to a planet sweeps out equal areas in equal times. These laws give a precise description of planetary motion, accounting for the details of non-uniform speed in orbit, the changing size of the moon, the unequal length of the seasons-everything that was observable at that time. This made possible accurate predictions of planetary positions without the cumbersome, ad hoc patchwork of epicycles. The simplification was more than impressive it was, of course, revolutionary. Kepler determined the shapes and relative sizes of each planet’s orbit by triangulation not from different points on Earth but from different points on Earth’s orbit, using observations made at many different times of the year. Noting where the planets were on successive nights, he was able to infer the speeds at which they moved. 10. What is the difference between a hypothesis, a theory, and a law? A hypothesis is a reasonable supposition made in describing the results of experiments and observations. It attempts to make predictions of future behavior. Once the evidence for its validity is strong, it becomes a theory. a theory: such validity means that many scientists have repeated and verified confirmed the experi- ments observations. No theory can be proven to be true. However, data can prove a theory to be false. 11, How did the Alfonsine Tab]es, the Prutenic Tables, and the Rudolphine Tables differ? 1633, Galileo was finally condemned before the Inquisition 12. Review Galileo's telescope discoveries, and explain why they supported the Copernican model and contra-dicted the Ptolemaic model. Galileo Galilei was a great defender of the Copernican hypothesis. Galileo was the first person to use a telescope to observe the heavens and to recognize the significance of what he saw. His discoveries of the phases of Venus, the satellites of Jupiter, the mountains of the moon, and other phenomena helped undermine the Ptolemaic universe. In for refusing to halt his defense of Copernicanism. In 1610 Galileo made the first telescope survey of the Milky Way and discovered that it is composed of a multitude of individual stars. 13. Galileo was condemned by the Inquisition, but Kepler, also a Copernican, was not. Why not? 14. Why did Newton conclude that gravitation had to be universal? and predict its path in the future. 15. Explain why we might describe the orbital motion of 6Y Part 1 The Astronomer's Sky 1. Historian of science Thomas Kuhn has said that De Revo- lutionibus was a revolution-making book, but not a rev- olutionary book. How was it classical? 2. Why might Tycho Brahe have hesitated to hire Kepler? Why do you suppose he appointed Kepler his scientific heir? 3. How does the modern controversy over creationism and evolution reflect two ways of knowing about the physi- 1. The trial of Galileo is an important event in the history of science. We now know, and the Church now recog- nizes, that Galileo's view was correct, but what were the arguments on both sides of the issue as it was un- folding? Research the Internet for documents chroni- cling the trial, Galileo's observations and publications, and the position of the Church. Use this information to outline cases for and against Galileo in the context of the times in which the trial occurred. cal world? 2. It's hard to imagine that an observatory could exist be-fore the invention of the telescope, but Tycho Brahe's observatory at Hveen was a great astronomical center of its day. Search the Web sites on Tycho and his instru- 1. If you lived on Mars, which planets would describe ret- rograde loops? Which would never be visible as cres- cent phases? 2. angular diameter (61 seconds of arc) when it is a cres- cent and a small angular diameter (10 seconds of arc) when it is nearly full. Use the small-angle formula to distance. Is this ratio compatible with the Ptolemaic universe shown on page 41? 3. Communications satellites are obvious uses of the geo- synchronous oribt, but can you think of other uses for Galileo's telescope showed him that Venus has a large such orbits? Find an Internet site that uses or displays information gleaned from a geosynchronous orbit that provides a useful service. find the ratio of its maximum distance to its minimum ments and describe what an observing session at Hveen might have involved. Exploring TheSky 3. Galileo's telescopes were not of high quality by modern standards. He was able to see the moons of Jupiter, but he never reported seeing features on Mars. Use the small angle formula to find the angular diameter of Mars when it is closest to Earth. How does that compare with the maximum diameter of Jupiter? 4. If a planet had an average distance from the sun of 10 AU, what would its orbital period be? 5. If a space probe were sent into an orbit around the sun that brought it as close as 0.5 AU to the sun and as far away as 5.5 AU, what would its orbital period be? 6. Pluto orbits the sun with a period of 247.7 years. What is its average distance from the sun? 7. Calculate the circular velocity of Venus and Saturn around the sun. (Hint: The mass of the sun is 2 x 103° kg.) 8. What is the orbital velocity of an Earth satellite 42,200 km from Earth? How long does it take to circle its orbit once? 1. Observe Mars going through its retrograde motion. (Hint: Use Reference Lines under the View menu to turn on the ecliptic. Be sure you are in Free Rotation under the Orientation menu. Locate Mars and use the time skip arrows to watch it move.) 2. Compare the size of the retrograde loops made by Mars, Jupiter, and Saturn. 3. Can you recognize the effects of Kepler's second law in the orbital motion of any of the planets? (Hint: Use 3D Solar System Mode under the View menu.) 4. Can you recognize the effects of Kepler's third law in the orbital motion of the planets? r 6o to the Brooks/Cole Astronomy Resource Center (www.krook:eole. comlastronomy) Chapter 2 The Origin of Modern Astronomy 63 magnifying power alt-azimuth mounting refracting telescopes cannot bring all colors to the same focus. The light gathered by an astronomical telescope can be telescope. For many decades, astronomers have used photo- graphic plates to record images at the telescope, but modern 2. electronic systems such as CCDs have now replaced photo- graphic plates in most applications. Spectrographs spread Why? starlight out according to wavelength to form a spectrum. P°lar axis mers refer to as seeing. Atop a mountain, the air is steady, and sidereal drive 1. Why would you not plot sound waves in the electro- magnetic spectrum? thin? What problems does this cause? gey~ew questions dry, and it is more transparent, especially in the infrared. recorded and analyzed by special instruments attached to the If you had unlimited funds to build a large telescope, which type would you choose, a refractor or a reflector? Reflecting telescope reflecting telescope A telescope which uses a mirror to gather and focus light from a distant object. refracting telescope A telescope which uses a lens to gather and focus light from a distant object. astronomical telescopes often have large diameters. Astronomical telescopes are of two types, refracting and reflecting. A refracting telescope uses a lens to bend the light and focus it into an image. Because of chromatic aberration, lens partially corrects for this, but such lenses are expen- sive and cannot be made larger than about 1 m in diameter. Reflecting telescopes use a mirror to focus the light Cassegrain focus and are less expensive than refracting telescopes of the same diameter. In addition, reflecting telescopes do not suffer from chromatic aberration. most recently built large telescopes are reflectors. 3. Why do nocturnal animals usually have large pupils in their eyes? How is that related to astronomical tele- scopes? an antenna, an amplifier, and a recorder. Such an instrument and construct radio maps. The poor resolution of the radio tele-scope can be improved by combining it with another radio telescope to make a radio interferometer. Radio telescopes have three important features-they can detect cool hydrogen, they can see through dust clouds in space, and they can detect certain objects invisible at other wavelengths. 4. Why do optical astronomers sometimes put their telescopes at the tops of mountains, while radio astronomers sometimes put their telescopes in deep valleys? Astronomers build observatories atop high mountains for two reasons. Turbulence in Earth's atmosphere blurs the image in an astronomical telescope, a phenomenon that astronomers the seeing is better. The air at a mountaintop is also thin and 5. Optical and radio astronomers both try to build large telescopes but for different reasons. How do these goals differ? To observe radio signals from celestial objects, we need a radio telescope, which usually consists of a dish reflector, can measure the intensity of radio signals over the sky Earth's atmosphere admits radiation primarily through two wavelength intervals, or windows-the visual window and the radio window. At other wavelengths, our atmosphere absorbs radiation. To observe in the far infrared, astrono- high mountaintops. To observe in the ultraviolet, X-ray, or gamma-ray range and some parts of the infrared, they must send their telescopes into space to get above our atmosphere. 6. What are the advantages of making a telescope mirror A mirror can redirect incoming sunlight to another area inside the telescope. 7. Small telescopes are often advertised as "200 power" or "magnifies 200 times." As someone knowledgeable about astronomical telescopes, how would you improve mers must fly telescopes high in balloons or aircraft, though such advertisements? they can work at some wavelengths in the near infrared from 8. An astronomer recently said, "Some people think I should give up photographic plates." Why might she change to something else? The first advantage is CCD's are much more efficient than photographic plates, recording as many as 75 percent of the photons striking them, compared with less than five percent for photographic methods. This means that a CCD instrument can image objects 10 to 20 times fainter-or the same object 10 to 20 times faster-than can a photographic plate. The second advantage is CCDs produce a faithful representation of an image in a digital format that can be placed directly on magnetic tape or disk, or even sent across a computer network to an observer's home institution for analysis. 9. What purpose do the colors in a false-color image or false-color radio map serve? 10. How is chromatic aberration related to a prism spectro-graph? electromagnetic radiation eyepiece Newton separated sunlight into its component colors with a prism. 11, Why would radio astronomers build identical radio telescopes in many different places around the world? radio astronomers can combine two or more radio telescopes to improve the resolving power. 12. Why do radio telescopes have poor resolving power? Radio astronomers can sometimes overcome the problem of poor angular resolution by using a technique known as interferometry. radio telescope can be improved by combining it with another radio telescope. Radio telescopes are built large in part because cosmic radio sources are extremely faint. 13. Why must telescopes observing in the far infrared be cooled to low temperature? To observe in the far infrared, astronomers place telescopes on high mountaintops. 14. What might we detect with an X-ray telescope that we could not detect with an infrared telescope? For X-ray images showing the ejection of matter from the Sun's corona. Since the Earth's atmosphere absorbs X-rays, solar X-rays can only be studied from spacecraft above our atmosphere. some infrared observations be made from mountain observatories while all x-ray observations must be made from space Earth's infrared absorbing atmosphere. Among the black hole candidates found by x-ray astronomy are stellar black holes in binary star systems. When using a infrared telescope to see in the infrared observations we learn about planetary systems information. Star formation occurs in regions that contain a large amount of dust which both obscures visible and shorter wavelength light and produces large amounts of infrared light. Additionally, protostars are not generally very hot and produce most of their energy at infrared wavelengths. 15. If the Hubble Space Telescope observes at visual wave-lengths, why must it observe from space? Optical Telescopes Astronomers build optical telescopes to gather light and focus it into sharp images. This requires sophisti- cated optical and mechanical designs, and it leads astronomers to build gigantic telescopes on the tops of high mountains. 86 Part 1 The AstronomeYs Sky 9. Astronauts observing from a space station need a tele-scope with a light-gathering power 15,000 times that of 1. Why does the wavelength response of the human eye the human eye, capable of resolving detail as small as match so well the visual window of Earth's atmosphere? 0.1 second of arc, and having a magnifying power of 250. Design a telescope to meet their needs. Could you test your design by observing stars from Earth? 2. Basic research in chemistry, physics, biology, and similar sciences is supported in part by industry. How is as-tronomy different? Who funds the major observatories? 3. Most people like beautiful sunsets with brightly glow-ing clouds, bright moonlit nights, and twinkling stars. Most astronomers don't. Why? 10. A spy satellite orbiting 400 km above Earth is suppos-edly capable of counting individual people in a crowd. What minimum-diameter telescope must the satellite carry? (Hint: Use the small-angle formula.) fro:. .: :,.ads 1. The thickness of the plastic in plastic bags is about 0.001 mm. How many wavelengths of red light is this? 2. Measure the actual wavelength of the wave in Fig-ure 3-1. In what portion of the electromagnetic spec-trum would it belong? 3. Compare the light-gathering powers of a 5-m telescope and a 0.5-m telescope. 4. How does the light-gathering power of one of the Keck telescopes compare with that of the human eye? (Hint: Assume that the pupil of your eye can open to about 0.8 cm.) 1. How do professional astronomers go about making ob-servations at major astronomical facilities? Visit sev-eral observatory Web sites to determine the process an astronomer would go through to secure observing time and make observations at the facility. 2. NASA is in the process of completing a fleet of four space-based "Great Observatories." (The Hubble Space Telescope is one; what are the others?) Examine the cur rent state of these missions by visiting their home pages on the Internet. What advantages would these facilities have over ground-based observatories? 5. What is the resolving power of a 25-cm telescope? What do two stars 1.5 seconds of arc apart look like through this telescope? 6. Most of Galileo's telescopes were only about 2 cm in di-ameter. Should he have been able to resolve the two stars mentioned in Problem 5? 7. How does the resolving power of the 5-m telescope compare with that of the Hubble Space Telescope? Why does the Hubble Space Telescope outperform the 5-m telescope? 8. If we build a telescope with a focal length of 1.3 m, what focal length should the eyepiece have to give a magnification of 100 times? Exploring TWO 1. Astronomical telescopes using equatorial mountings must be aligned precisely with the north celestial pole. Locate Polaris and determine how far it is from the north celestial pole. (Hint: Use Reference Lines under the View menu and check Grid under Equitorial. Be sure the spac-ing is set to auto/fine. Then locate the Little Dipper and zoom in on Polaris.) 6o to the Brooks/Cole Astronony Resource Center Iwww.broohscole. com/astronowy) for critical thinking exercises, articles, sod addi-tional readings from Infoirac College Edition, Brooks/Cole's online atallat ubrary. Chapter 3 Astronomical Tools 81 m- ,alled a (J) transition ,ontinuous spectrum Lyman series absorption spectrum (dark- Balmer series line spectrum) Paschen series absorption line Doppler effect emission spectrum (bright- blue shift line spectrum) red shift emission line radial velocity (V,) Kirchhoff's laws ., or dense The hotter ,roduces a condense at the wave-.ch depends on the ostly short-wavelength mostly long-wavelength ues to the temperatures of jol stars are red. electrons surrounding an atomic ous permitted orbits. An electron ,ner orbit during a collision between re from one orbit to another by absorb-' of the proper energy. If the energy photon is too large, the atom may lose an jecome ionized. spectra? ,e only certain orbits are permitted, only photons n wavelengths can be absorbed or emitted. Each ,f atom has its own characteristic set of spectral lines. 8. Why do hot stars look bluer than cool stars? If light passes through a low-density gas on its way to ar telescope, the gas can absorb photons of certain wave- g. What kind of spectrum does a neon sign produce? lengths, and we will see dark lines in the spectrum at those 10. Why does the Doppler effect detect only radial velocity? positions. Such a spectrum is called an absorption spec- 11, How can the Doppler effect explain shifts in both light trum. If we look at a low-density gas that is excited to emit and sound? photons, we see bright lines in the spectrum at those posi- tions. Such a spectrum is called an emission spectrum. When a source of radiation is approaching us, we ob-serve shorter wavelengths, and when it is receding, we ob- serve longer wavelengths. This Doppler effect makes it pos- 1 In what ways is our model of an atom a scientific model? sible for the astronomer to measure a star's radial velocity, In what ways is it incorrect? that part of its velocity directed toward or away from Earth. 1. Why might we say that atoms are mostly empty space? 2. What is the difference between an isotope and an ion? 3. Why is the binding energy of an electron related to the size of its orbit? 4. Explain why ionized calcium can form absorption lines but ionized hydrogen cannot. 5. Describe two ways an atom can become excited. 6. Why do different atoms have different lines in their 7. Why does the amount of black body radiation emitted depend on the temperature of the object? Discussion QUIU: New Terms nucleus permitted orbit proton energy level neutron excited atom electron ground state isotope heat ionization temperature Problems 1. Human body temperature is about 310 K (98.6°F). At what wavelength do humans radiate the most energy? What kind of radiation do we emit? 2. If a star has a surface temperature of 20,000 K, at what wavelength will it radiate the most energy? 3. Infrared observations of a star show that it is most in-tense at a wavelength of 2000 nm. What is the tempera-ture of the star's surface? ion Kelvin temperature scale molecule absolute zero Coulomb force binding energy wavelength of maximum quantum mechanics intensity (;.m.) 4. Dust orbiting distant stars may be evidence that planets have formed there. If the dust is brightest at the far- black body radiation infrared wavelength of 100,000 run, what is the temper- ature of the dust? 5. If astronomers observe that a liquid flowing out onto the surface of a planet is brightest at a wavelength of 100 Part I The Astronomer's Sky ; 7 f 8 1700 nm, what is its temperature? Do you think it is liq-uid water or liquid rock? 6. If we triple the temperature of a black body, by what fac-tor will the total energy radiated per second per square meter increase? 7. If one star has a temperature of 6000 K and another star has a temperature of 7000 K, how much more energy per second will the hotter star radiate from each square meter of its surface? 8. In the laboratory, the Balmer beta line of hydrogen has a wavelength of 486.10 rim. If the line appears in a comet's spectrum at 486.15 nm, what is the comet's ra-dial velocity? 9. The highest velocity stars an astronomer might observe have velocities of about 400 km/s. What change in wave-length would this cause in the spectral line described in Problem 8? 1. The name for the element helium has astronomical roots. Search the Internet for information on the discovery of helium. How and when was it discovered, and how did it get its name? Why do you suppose it took so long for helium to be recognized? 2. How was the model of the atom presented in the text you read developed? Search the Web for information on historical models of the atom and compile a time line of important developments leading to our current under-standing. What evidence exists that supports our model?  6o to the Brooks/Cole Astronomy Resource Center Iwww.broowscole. com/astronomyl for critical thinking exercises, articles, and addi-tional readings from Igfoirac Collage Edition. Brooks/Cole's online student library. Chapter 4 Atoms and Starlight 101 in that chain is planet building. As we explore the solar system in detail in the following chapters, we must stay alert for further clues to the birth of the planets. Summary and allowed it to differentiate into a dense metallic core and a lower-density silicate crust. In fact, it is possible that the solar nebula cooled as the protoplanets grew so that the first planetesimals were metallic and later additions were sili-cate. It is also likely that the planets grew rapidly enough that the heat of formation released by the in-falling material melted the planets and allowed them to differentiate as they formed. The Jovian planets probably grew rapidly from icy ma-terials and became massive enough to attract and hold vast amounts of nebular gas. The heat of formation raised their temperatures very high when they were young, and Jupiter and Saturn still radiate more heat than they absorb from the sun. Once the sun became a luminous object, it cleared the nebula as its light and solar wind pushed material out of the system. The planets helped by absorbing some planetesi mals and ejecting others from the system. Once the solar sys-tem was clear of debris, planet building ended. The solar nebula theory proposes that the solar system be-gan as a contracting cloud of gas and dust that flattened into a rotating disk. The center of this cloud eventually became the sun, and the planets formed in the disk of the nebula. Observational evidence gives astronomers confidence in this theory. Disks of gas and dust have been found around many young stars, so astronomers suspect that planetary systems are common. Planets orbiting other stars are too faint and too close to their star to image directly, but astron-omers have found many of these planets by observing the motion of the star as the star and planet revolve around their center of mass. The solar nebula theory explains many of the charac-teristic properties of the solar system. For example, the solar system has a disk shape. The orbits of the planets lie in nearly the same plane, and they all revolve around the sun in the same direction, counterclockwise as seen from the north. With only three exceptions, the planets rotate counterclock-wise around axes roughly perpendicular to the plane of the solar system. This disk shape and the motion of the planets appear to have originated in the disk-shaped solar nebula. Another striking feature of the solar system is the divi-sion of the planets into two families. The terrestrial planets, which are small and dense, lie in the inner part of the system. The Jovian planets are large, low-density worlds in the outer part of the system. In general, the closer a planet lies to the sun, the higher its uncompressed density. The solar system is now filled with smaller bodies such as asteroids, comets, and meteors. The asteroids are small, rocky worlds, most of which orbit the sun between Jupiter and Mars. They appear to be material left over from the for-mation of the solar system. Another important characteristic of the solar system bodies is their similar ages. Radioactive dating tells us that Earth, the moon, Mars, and meteorites are no older than 4.6 billion years. it seems our solar system took shape about 4.6 billion years ago. According to the condensation sequence, the inner part of the nebula was so hot that only high-density minerals could form solid grains. The outer regions, being cooler, con densed to form icy material of lower density. The planets grew from these solid materials, with the denser planets forming in the inner part of the nebula and the lower-density Jovian planets forming farther from the sun. Planet building began as dust grains grew by condensa-tion and accretion into planetesimals ranging from a kilome-ter to hundreds of kilometers in diameter. These planetesimals settled into a thin plane around the sun and accumulated lion years ago? into larger bodies, the largest of which grew the fastest and eventually became protoplanets. Once a planet had formed from a large number of plan-etesimals, heat from radioactive decay could have melted it 8. Why is almost every solid surface in our solar system solar nebula theory gravitational collapse extrasolar planet uncompressed density asteroid condensation sequence comet planetesimal terrestrial planet condensation Jovian planet accretion Galilean satellites protoplanet solar wind differentiation meteor outgassing meteoroid heat of formation meteorite radiation pressure half-life heavy bombardment Critical Inquiries 1. How does our solar system compare with the others that have been found? Some of the other solar systems contain planets, and a star oribiting each other. Search the Internet for sites that give information about planetary systems around other stars. What kinds of planets have been detected by these searches so far? Discuss the selection effects (see Win-dow on Science 16-2) that must be considered when in-terpreting these data. did they come from and what processes brought them together? 2. The process of protoplanetary accretion is still not well understood.1. Discuss the history of the atoms in your thumb. Where steps in the formation of a protoplanet through accre- Search the Web for current research in this field. From the results of your search, outline the basic tion. What specific factors are important in these mod- els of planet building? Do these models produce plane-tary systems similar to the ones we know to exist? 2. If the solar nebula theory is correct, then there are probably more planets in the universe than stars. Do you agree? Why or why not? 1. The nearest star is about 4.2 ly away. If you looked back at the solar system from that distance, what would the maximum angular separation be between Jupiter and the sun? (Hint: 1 ly equals 63,000 AU.) 3. How is radioactive dating carried out on meteorites and rocks from surfaces of various bodies in the solar sys-tem? Look for Web sites on the details of radioactive dating and summarize the methods used to uncover the abundances of radioactive elements in a particular sam-ple. (Hint: Try looking for information on how a par-ticular meteorite-for example, the Martian meteorite ALH84001-was studied, what age range was deter-mined, and what radioactive elements were used to ar-rive at the age.) 2. The brightest planet in our sky is Venus, which is some-times as bright as apparent magnitude -4 when it is at a distance of about 1 AU. How many times fainter would Exploring TheSky it look from a distance of 1 parsec (206,265 AU)? What would its apparent magnitude be? (Hint: Remember the inverse square law, Chapter 2.) 1. Look at the solar system from space. Notice how thin the disk of the solar system is and how inclined the or-bits of Pluto and Mercury are. (Hint: Under the View menu, choose 3D Solar System Mode, and then zoom in or out. Tip the solar system up and down to see it edge-on.) 3. What is the smallest-diameter crater you can identify in the photograph of Mercury on page 112? (Hint: See Ap-pendix A to find the diameter of Mercury in kilometers.) 4. A sample of a meteorite has been analyzed, and the re-sult shows that out of every 1000 nuclei of 4°K origi-nally in the meteorite, only 125 have not decayed. How old is the meteorite? (Hint: See Figure 5-9.) 5. In Table 5-2, which object's density differs least from its uncompressed density? Why? 6. What composition might we expect for a planet that formed in a region of the solar nebula where the tem-perature was about 100 K? 7. Suppose that Earth grew to its present size in 10 million ® Bo to the Brooks/Cole Astronomy Resource Center Iwww.broehscole. years through the accretion of particles averaging 100 g comtastronomYl for critical Making exercises, articles, and addl- each. On the average, how many particles did Earth cap- ~ tlonal readings from Infoirac CoINpe EdiUOt1. BrooksiCole's online ture per second? (Hint: See Appendix A to find Earth's student library. mass.) 2. Look at the solar system from space and notice how small the orbits of the inner planets are compared to the orbits of the outer planets. They make two distinct groups. (Hint: Use 3D Solar System Mode.) The innerplanets are the terrrestrial planets. The outerplanets are the jovian planets. 3. Watch the comets orbiting around the sun. Can you lo-cate the comet C/198 M5 (Linear)? What is its orbit like? (Hint: Use 3D Solar System Mode and set the time step to 30 days (30d).) 5. Which is the outermost planet in our solar system? Why does that change? Pluto. 6, Why are light-years more convenient than miles, kilo- meters, or astronomical units for measuring certain distances? Light years are farther distances than miles. Chapter 5 The Origin of the Solar System 125 is part of a large supercluster-a cluster of clusters. Other galaxies are not scattered at random throughout the universe but lie in clusters within larger super-clusters. To represent the universe at this scale, we use a diagram in which each dot represents the location of a single galaxy. At this scale, we see superclusters linked to form long filaments outlining voids that seem nearly empty of ga)axies. These appear to be the largest structures in the universe. Were we to expand our field of view yet another time, we would proba-bly see a uniform sea of filaments and voids. When we puzzle over the origin of these structures, we are at the frontier of human knowledge. Our problem in studying astronomy is to keep a proper sense of scale. Remember that each of the bil-lions of galaxies contains billions of stars. Most of those stars probably have families of planets like our solar system, and on some of those billions of planets liquid-water oceans and a protective atmosphere may have spawned life. It is possible that some other plan-ets in the universe are inhabited by intelligent crea-tures who share our curiosity and our wonder at the scale of the cosmos. shape of any other orbits in this figure or the next? 4. Look at Figure I-6. How can you tell that Mercury fol-lows an elliptical orbit? Can you detect the elliptical Our goal has been to preview the scale of astronomical ob- jects. To do so, we journeyed outward from a familiar cam- pus scene by expanding our field of view by factors of 100. Only 12 such steps took us to the largest structures in the universe. 7. Why is it difficult to detect planets orbiting other stars? Lots of interstellar dust, and scattering of light. metric system to simplify our calculations and scientific no- 8. What does the size of the star image in a photograph 9. What is the difference between the Milky Way and the Milky Way Galaxy? The Milky Way and many other spiral galaxies have spurs and branches sticking off of the spirals. the Milky Way is a spiral galaxy with white stars and interstellar dust in red. the Milky Way galaxy in visible light is due to the obscuration caused by the dust contained in the disk of our Galaxy. the Milky Way Galaxy is that when Galileo pointed his telescope at the Milky Way, he discovered that it was actually composed of a huge number of very dim stars. the Milky Way is divided into two portions for much of its length. the center of the Milky Way galaxy is invisible in ordinary light because the interstellar dust in that direction is so thick. 10. What are the largest known structures in the universe? to the sun. Of the eight other planets in our solar system, The numbers in astronomy are so large it is not conve- nient to express them in the usual way. Instead, we use the tation to more easily write big numbers. The metric system and scientific notation are discussed in Appendix A. tell us? We live on the rotating planet Earth, which orbits a rather typical star we call the sun. We defined a unit of distance, the astronomical unit, to be the average distance from Earth Mercury is closest to the sun, and Pluto is the most distant. The sun, like most stars, is very far from its neighbor- p ing stars, and this leads us to define another unit of distance, the light-year, the distance light travels in 1 year. The near- est star to the sun is Proxima Centauri at a distance of 4.2 ly. 1, The diameter of Earth is 7928 miles. What is its diame- ter in inches? In yards? 2. If a mile equals 1.609 km and the moon is 2160 miles in diameter, what is its diameter in kilometers? 3. One astronomical unit is about 1.5 x 108 km. Explain why this is the same as 150 x 106 km. 4. Venus orbits 0.7 AU from the sun. What is that distance in kilometers? 5, Light from the sun takes 8 minutes to reach Earth. How long does it take to reach Mars? 24 minutes. Among the billions of stars in each of the billions of 6. The sun is almost 400 times further from Earth than is the moon. How long does light from the moon take to solar system Milky Way scientific notation Milky Way Galaxy astronomical unit (AU) spiral arm light-year (ly) Local Group galaxy 1. What is the largest dimension you have personal knowledge of? Have you run a mile? Hiked 10 miles? Run a marathon? 2. In Figure I-4, the division between daylight and dark-ness is at the right on the globe of Earth. How do we know this is the sunset line and not the sunrise line? 3. What is the difference between our solar system, our galaxy, and the universe? our Solar System contains one star and nine planets. sun is only one of 100 billion stars in our galaxy and that our galaxy is only one of many billions of galaxies in the uni- verse. Galaxies appear to be grouped together in clusters, superclusters, and filaments, the largest structures known. Earth's surface is evolving, and so are stars. Stars form from the gas in space, grow old, and eventually die. We do not yet understand how galaxies form or evolve. galaxies, many have planets. Although astronomers can now detect planets orbiting other stars, we know very little about the nature of these planets. Yet we suppose that there must be many planets in the universe and that some are like Earth. We wonder if a few are inhabited by intelligent beings like ourselves. The Universe is evolving. reach Earth? Part 1 The AstronomeYs Sky 7. If the speed of light is 3 X 105 km/s, how many kilome-ters are in a light-year? How many meters? 8. How long does it take light to cross the diameter of our Milky Way Galaxy? 9. The nearest galaxy to our own is about 2 million light-years away. How many meters is that? 10. How many galaxies like our own would it take laid edge to edge to reach the nearest galaxy? (Hint: See Problem 9.) 1. Locate photographs of Earth taken from space. What do cities look like? Can you see highways? Is the presence of our civilization detectable from space? 2. Locate photographs of nearby galaxies and compare them with photos of very distant galaxies. What kind of detail is invisible for distant galaxies? 3. One of the biggest clusters of galaxies is the Virgo clus-ter. Find out how many and what kind of galaxies are in the cluster. Is it nearby or far away? Exploring TheSky 1. Locate and center one example of each of three differ-ent types of objects: a. A planet, such as Saturn. Find its rising and setting time. Such objects have distances measured in astro-nomical units (AU). ob How to proceed: Decide on the object you want to lo-cate. Then find and center the object by clicking the Find button on the Object Toolbar. The second method is to press the F key. The third is to click Edit, then Find. Once you have the Object Information win-dow, click the center button. b. A star. All stars in TheSkybelong to our Milky Way Galaxy. Give the star's name, its magnitude, and its distance in light-years. How to proceed: Click on any star, which brings up an Object Information window. c. A galaxy. Give its name and/or its designation. How to proceed: Click on the Galaxies button in the Object Toolbar, then click on any galaxy. Distances to galaxies are millions and billions of light years. 2. Look at the solar system from beyond Pluto by clicking on View and then on 3D Solar System Mode. Tip the solar system edge-on and then face-on. Zoom in to see the inner planets. Under Tools, set the Time Skip Incre-ment to 1 day and then go forward in time to watch the planets move. 3. Identify some of the brightest constellations located along the Milky Way. (Hint: See View, Reference Lines.) r 6o to tbo tiroaRs/COIO ~strosorll Resource Center Inww.Rrookscole. comlasirouors) lor criticsl tRInYtug erercisos, articlss, and addl-tianal resdings Iror IutoTrse Collegs EWUog, Rrsob:ICole's sgugo student Ilbrary. Introduction: The Scale of the Cosmos 9 corporated into their monument the cycles of the sun away from the moon. and moon, The cycles in the sky are a rich part of our culture, but those same motions reveal an astonishing fact- Earth is a planet. In the next chapter, we will see how humanity made that discovery. the tions. Although the constellations originated in Greek and penumbral, and not total. Astronomers divide the sky into 88 areas called constella- New Terms The magnitude system is the astronomer's brightness asterism summer solstice is its apparent visual magnitude. magnitude scale winter solstice aphelion ward, the celestial sphere appears to rotate westward on its apparent visual magnitude perihelion precession neap tide scientific model spring tide celestial sphere lunar eclipse south celestial pole solar eclipse umbra zenith penumbra nadir sidereal period north celestial pole synodic period north, south, east, and west photosphere chromosphere angular diameter diamond ring effect prominence Milankovitch hypothesis S~.os cycle zodiac node apogee circumpolar constellations perigee ecliptic minute and second of arc annular eclipse angular distance a ainst the back round corona Chapter 1. What is the difference between an asterism and a con- stellation? Give some examples. asterism is a group of stars that people informally associate with each other to make a simple pattern, such as the Big Dipper and Square of Pegasus. The stars in an asterism can come from one or more official constellations. The Winter Circle is an asterism-or pattern of stars-made up of six bright stars in five constellations: Sirius, Rigel, Betelgeuse, Aldebaran, Capella, and Procyon. The Chinese divided the ecliptic into 28 lunar mansions, somewhat analogous to our zodiac (which is solar), and the stars into almost 300 groupings that are smaller than our constellations and that we would call asterisms. 2, Do people in other parts of the world see the same con- stellations, asterisms, lunar phases, lunar eclipses, and solar eclipses that you see? Explain. lunar phases takes 29.53 days. The moon's gravitational field exerts tidal forces on When the moon passes through Earth's shadow, sun-light is cut off, and the moon darkens in a lunar eclipse. If the moon only grazes the shadow, the eclipse is partial, or - - If the moon passes directly between the sun and Earth, it produces a total solar eclipse. During such an eclipse, bright photosphere of the sun is covered, and the fainter corona, chromosphere, and prominences become visible. An observer outside the path of totality sees a partial eclipse. If the moon is in the farther part of its orbit, it does not cover Photosphere completely, resulting in an annular eclipse. 3` Earth that pull the ocean waters up into two bulges, one on the side of Earth facing the moon and the other on the side As the rotating Earth carries the conti- nents through these bulges of deeper water, the tides ebb and flow. Friction with the seabeds slows Earth's rotation, and the gravitational force the bulges exert on the moon force its orbit to grow larger. 3~ What does the word apparent mean in apparent visual magnitude? 36 Part 1 The Astronomer's Sky 4. In what ways is the celestial sphere a scientific model? The celestial sphere is a model of the sky, carrying the celestial objects around Earth. Because Earth rotates east- axis. The northern and southern celestial poles are the pivots on which the sky appears to rotate. The celestial equator, an imaginary line around the sky above Earth's equator, divides the sky in half. horizon Astronomers often refer to angles "on" the sky as if the stars, sun, moon, and planets were equivalent to spots painted on a plaster ceiling. Then the angle on the sky between two objects is independent of the true distance between the ob- jects in light-years. The gravitational forces of the moon and sun act on the spinning Earth and cause it to precess like a gyroscope. Earth's axis of rotation swee s around in a conical motion with a point period of 26,000 years, and consequently the celestial poles celestial equator and celestial e uator move slowly of the stars. 5. If Earth did not rotate, could we define the celestial poles and celestial equator? the ecliptic? Use the small-angle formula to calculate the angular di- ameter of the sun as seen from Mars. 6. Where would you go on Earth so you could see both north and south celestial poles at the same time? Where would you go to see a celestial pole at your zenith? Equator 7. What is the difference between the daily and annual motion of the sun? Sun's annual path on the celestial sphere is the ecliptic, a line that runs through the center of the zodiac, the 18°-wide strip of sky within which we always find the Moon. Because of its motion on the ecliptic, the Sun rises about 4 minutes later each day with respect to the stars. The Earth must make just a bit more than one complete rotation with respect to the stars to bring the Sun up again. As the Sun's annual path in the sky is not lined up with the Earth's equator. This is because our planet's axis of rotation is tilted by about 23° from the plane of the ecliptic (Figure 1.6). Being tilted in this way is not at all unusual among planets; Uranus and Pluto are actually tilted so much that they or- bit the Sun "on their side." The inclination of the ecliptic is the reason the Sun move north dsout hi nth esk ya sth e m seasons change. In Chapter 3 we will discuss the progres-that it is the Earth that is going around the Sun, but the ef- sphere each year is called the ecliptic (Figure 1.5). to the unaided eye-Mercury, Venus, Mars, Jupiter, and Saturn-also slowly change their positions from day to day. During a single day, the Moon and planets all rise and set as the Earth turns, just as the Sun and stars do. But like the Sun, they have independent motions among the stars, 8. The average distance of Mars from the sun is 1.52 AU. 9. Draw a diagram showing Earth, the moon, and shadows during (a) a total solar eclipse, (b) a total lunar eclipse, (c) a partial lunar eclipse, (d) an annular eclipse. 98 CHAPTER 4 RADIATION AND SPECTRA months go by and we look at the Sun from different places We have described the movement of stars in the night sky. The stars continue to circle during the day, but the bril- liance of the Sun makes them difficult to see. in our orbit, we see it projected against different stars in the background, or we would, at least, if we could see the stars in the daytime. In practice, we must deduce what stars lie behind and beyond the Sun by observing the stars visible in the opposite direction at night. After a year, when the Earth has completed one trip around the Sun, the Sun will appear to have completed one circuit of the sky along the ecliptic. The ecliptic does not lie along the celestial equator but is inclined to it at an angle of about 23°. Fixed and Wandering Stars a campfire at night; we see the flames appear in front of i;each person seated about the fire in turn. The Sun is not the only object that moves among the fixed -The path the Sun appears to take around the celestial stars. The Moon and each of the five planets that are visible For thousands of years, astronomers have been aware that the Sun does more than just rise and set. It gradually changes position on the celestial sphere, moving each day about 1° to the east relative to the stars. Very reasonably, the ancients thought this meant that the Sun was slowly moving around the Earth, takin g aperiod of time we call one year to make a full circle. Today, of course, we know fect is the same: The Sun's position in our sky changes day to day. We have a similar experience when we walk around 8. What would our seasons be like if Earth were tipped 35° instead of 23.5°? What would they be like if Earth's axis were perpendicular to its orbit? criticat Inquiries 9. Why are the seasons reversed in the Southern Hemi- sphere? Because Earth orbits the sun, the sun appears to move eastward along the ecliptic through the constellations. Be- cause the ecliptic is tipped 23.5° to the celestial equator, the sun spends half the year in the northern celestial hemisphere and half the year in the southern celestial hemisphere, pro- ducing the seasons. The seasons are reversed south of Earth's equator. 1. Nearly all cultures have populated the sky with gods, heroes, animals, and objects. What can you learn on the Web about non-Western constellations? Middle Eastern mythology, the names are Latin. Even the modern constellations, added to fill in the spaces between the ancient figures, have Latin names. The names of stars usually come from ancient Arabic, though modern astrono-- mers often refer to a star bv constellation and Greek letters assigned according to brightness within each constellation. scale. First-magnitude stars are brighter than 2nd-magnitude constellation autumnal equinox stars, which are brighter than 3rd-magnitude stars, and so on. The magnitude we see when we look at a star in the sky 2. Who was Orion? How is he related to the scorpion in 4. What can you find out about Milutin Milankovitch? What is the latest news about the Milankovitch hypothesis? 10. Where would you look in the sunset or dawn skies to find the full moon? the first quarter moon? the waning crescent moon? the waxing gibbous moon? 10. A total solar eclipse was visible from eastern Canada on July 10, 1972. When did this eclipse next occur? When will it next be visible from eastern Canada? the sky? 11. What phase would Earth be in if you were on the moon when the moon was full? first quarter? waning crescent? 3. What holidays, rituals, special foods, and beliefs are as- Earth slow the moon's revolution? 12. How does the moon slow Earth's rotation? How does sociated with the winter solstice? The moons gravitational influence with the rising of the tides. The gravitational forces exerted by the Moon at several your shoulders in the exact same time that you revolve Early in history it was clear that tides must be related to the Moon, because the daily delay in high tide is the same as the daily delay in the Moon's rising. A satisfactory explanation of the tides, how-ever, awaited the theory of gravity supplied by Newton. The Pull of the Moon on the Earth points on the Earth. These gravity with more than two bodies. Anyone living near the sea is familiar with the twice-daily rising and falling of the tides. Until now we have considered the Sun and a planet (or a planet and one of its satellites) as nothing more than a pair of bodies revolving around each other. In fact, all the planets exert gravitational forces upon one another as well. These interplanetary attractions cause slight variations from the orbits that would be expected if the gravitational forces between planets were neglected. Unfortunately, the problem of treating the motion of a body that is under the gravitational influence of two or more other bodies is very complicated and can be handled properly only with large computers. The Interactions of Many Bodies Here's an example of what we mean: Suppose you have a cluster of a thousand stars all orbiting a common center (such clusters are quite common, as we shall see). If we know the exact position of each star at any given instant, we can calculate the combined gravitational force of the entire group on any one member of the cluster. Knowing the force on the star in question, we can therefore find how it will accelerate. If we know how it was moving to begin with, we can then calculate how it will move in the next in-stant of time, thus tracking its motion. Such complex calculations have been carried out with modern computers to track the evolution of hypothetical clusters of stars with up to a million mem-bers. Within the solar system, the problem of computing the orbits of planets and spacecraft is somewhat simpler. We have seen that Kepler's laws, which do not take into ac count the gravitational effects of the other planets on an or-bit, really work quite well. This is because these additional influences are very small in comparison to the dominant gravitational attraction of the Sun. Under such circum-stances, it is possible to treat the effects of other bodies as small perturbations (or disturbances) on the force exerted by the Sun. During the 18th and 19th centuries, mathe-maticians developed many elegant techniques for calculating perturbations, permitting them to predict very precisely the positions of the planets. Such calculations eventually led to the discovery of a new planet in 1846. 13. Why isn't there an eclipse at every new and at every full moon? 14. Why is the moon red during a total lunar eclipse? The Phases of the Moon The appearance of the Moon during the course of a complete monthly cycle. The upper part shows a perspective from space, with the Sun off to the right in a fixed position. Imagine yourself standing on the Earth, facing the Moon in each part of its orbit around the Earth. In position A, for example, you would face the Moon from the right side of the Earth, in the middle of the day The strip below shows the appearance of the Moon from Earth as you would see it from each lettered position. (Please note that the distance of the Moon from the Earth is not to scale in this diagram: The Moon is roughly 30 Earth diameters away from us, but we would have needed a big expensive foldout to show you the diagram to scale!) on nights with a full moon, but rather that we are more likely to notice such behavior with the aid of a bright celes-tial light that is up all night long. During the two weeks following the full moon, the Moon goes through the same phases again in reverse order (points F, G, and H in Figure 3.13), returning to new phase after about 29.5 days. About a week after full moon, for example, the Moon is now at third quarter-meaning that it is three quarters of the way around, not that it is three-fourths illuminated. In fact, half of the visible side of the Moon is again dark. At this phase, the Moon is now rising around midnight and setting around noon. Note that there is one thing quite misleading about Figure 3.13. The Moon is actually 30 Earth diameters away from us; in the Prologue you will find a diagram that shows the two bodies to scale. In reality, the Earth's shadow misses the Moon most months, and we regularly get treated to a full moon. The times when the Earth's shadow does fall on the Moon are called lunar eclipses and will be discussed in Section 3.7. 3.5 PHASES AND MOTIONS OF THE MOON 69 around him or her, you will continue to face your room- mate during the whole "orbit." only at night. At other times of the month, it may be visible all morning (third quarter) or all afternoon (first quarter) in the daytime sky The differences in the Moon's appearance from one night to the next are due to changing illumination by the Sun, not to its own rotation. You sometimes hear the back The Moon's Revolution and Rotation side of the Moon (the side we never see) called the "dark side." This is a misunderstanding of the real situation: Which side is light and which is dark changes as the Moon moves around the Earth. The back side is dark no more frequently than the front side. Since the Moon rotates, the Sun rises and sets on all sides of the Moon. The Moon's sidereal period-that is, the period of its rev-olution about the Earth measured with respect to the stars-is a little over 27 days: 27.3217 days to be exact. The time interval in which the phases repeat-say from full to full-is 29.5306 days. The difference is again the fault of the Earth's motion around the Sun. The Moon must make more than a complete turn around the moving Earth to get back to the same phase with respect to the Sun. As we saw, the Moon changes its position on the celestial sphere rather rapidly: even during a single evening, the Moon creeps visibly eastvvard among the stars, traveling its own width in a little less than 1 hour. The delay in moonrise from one day to the next, caused by this eastward motion, averages about 50 minutes. The Moon rotates on its axis in exactly the same time that it takes to revolve about the Earth. As a consequence (as shown in Figure 3.14), the Moon always keeps the same face turned toward the Earth. Exploring The Sky 15. Why have most people seen a total lunar eclipse while fewer people have seen a total solar eclipse? 1. As discussed in this chapter, Earth's rotation about its own axis gives us the impression that the whole sky ro-tates around the north celestial pole in a period of one day. This apparent motion of the celestial sphere is dif-ficult to notice because it happens so slowly. However, 1. All cultures around the world named constellations. Why do you suppose this was such a common practice? Know where the stars are in the universe. Be able to identify where the stars are. 2. Do planets orbiting other stars have ecliptics? Could TheSky makes it possible to simulate this motion at a pace that is easy to observe by using a feature called Time Skip. Observe and describe the apparent motion of they have seasons? Must they have seasons? the sky as you see it looking north> east, south, and west. How to proceed: Set the Time Skip Increment (a drop-down menu on the Time Skip Toolbar) to 1 minute, and click on the Go Forward button to begin the simulation. View the sky from the four cardinal directions, due north, south, east, and west. (You'll find Time Skip under the Tools menu as well.) 3. Why would it be difficult to see prominences if you were on the moon during a total lunar eclipse? The earth maybe blocking partial views of the prominences. FIGURE 1.4 Star Circles at Different Latitudes The turning of the sky looks different depending on your latitude on Earth. (a) At the north pole, the stars circle the zenith and do not rise and set. (b) At the equator, the celestial poles are on the horizon, and the stars rise straight up and set straight down. (c) At intermediate latitudes, the north celestial pole is at some position between overhead and the horizon. Its angle turns out to be equal to the observer's latitude. Stars rise and set at an angle to the horizon. 1. If light from one star is 40 times more intense than light from another star, what is their difference in magnitudes? 2. If two stars differ by 8.6 magnitudes, what is their in-tensity ratio? 3. Star A has a magnitude of 2.5; Star B, 5.5; and Star C, 9.5. Which is brightest? Which are visible to the unaided eye? Which pair of stars has an intensity ratio of 16? 4. By what factor is sunlight more intense than moonlight? (Hint: See Figure 1-4.) 5. If you are at a latitude of 35 degrees north of Earth's equator, what is the angular distance from the northern horizon up to the north celestial pole? from the south-ern horizon down to the south celestial pole? 6. If Earth is about 5 billion (5 x 10 9) years old, how many precessional cycles have occurred? 9 + 1=10 x 5=50 7. Identify the phases of the moon if on March 21 the moon were located at (a) the vernal equinox, (b) the autumnal equinox, (c) the summer solstice, (d) the winter solstice. 2. In which constellation was the sun located on the date of your birth? ( Hint: Click Data and then Site Informa-tion to set the location, date, and time of your birth. Turn on constellation figures, constellation boundaries, and labels. Then find the sun.) 3. Set your location to Earth's North Pole and the date to the summer solstice. Turn on the ecliptic and then step forward in time through a day to see what happens to the sun. Repeat for the autumnal equinox and the win-ter solstice. 4. Repeat Activity 3 above for a location on Earth's equator. • Bo to tbe BrookslCole Astronomy Resobrce Ce~tsr (www.brookscols. comlastronomYl tor crltical tblaking exerclses, artlcles, and addl-tional readings trsr Utoirac Collsbe Edition, Broob:ICole's online student Rbranl. Chapter 1 The View from Earth 8) In their search for evidence, modern astronomers have extended their senses with powerful instruments, ellipse semimajor axis (a) open orbit to create a mathematical model that could accurately pre- eccentricity (e) dict the positions of the sun, moon, and planets. escape velocity placed the concentric, Earth-centered heavenly spheres geosynchronous satellite were carried around Earth by great crystalline cus similar? Review Questions or sun centered. One advantage of a heliocentric universe is 1. Why did Greek astronomers conclude that the heavens were made up of perfect crystalline spheres moving at constant speeds? teachings of Aristotle had become part of Church teachings. Classical astronomy was based on the writings of the Greek philosopher Aristotle. He taught that Earth was the immo- bile center of the universe and that the stars and planets This model of the universe was given mathematical form about heliocentric universe AD 140 in the Almagest, the great work of Ptolemy. 3. How did the Ptolemaic model explain retrograde motion? ideas that some would claim were heretical. Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets. preserved the classical belief in geocentrism, but he re- Ptolemy a system of epicycles, deferents, and equants and tried Ptolemy Venus' epicycles are forced to always remain on the Earth/Sun line, they will always be near the Sun and their motions can be explained. This system was fleshed out by Ptolemy in about 150 AD and is therefore called the Ptolemaic system. During the Middle Ages (approx. 500 AD to approx. 1500 AD), Western science didn't make much progress. Middle Eastern astronomers kept Ptolemy's work alive and it remained the dominant theory of how the universe worked for over a thousand years. Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets. 5. Why did the Copernican hypothesis win gradual accep- tance? the Copernican hypothesis by the positivists does not imply that we cannot trust our senses, but that our customary interpretation of experience our intuition may be based on uncritical, fuzzy thinking. 6. Why is it difficult for scientists to replace an old para- digm with a new paradigm? 7, Why did Tycho Brahe expect the new star of 1572 to show parallax? Why was the lack of parallax evidence against the Ptolemaic model? 9. Explain how Kepler's laws contradict uniform circular motion. used the work of Kepler and Galileo to discover three Kepler's first and second laws of planetary motion. The first law: The orbits of the planets are ellipses with the sun at one focus. The second law: A line extending from the sun to a planet sweeps out equal areas in equal Kepler determined the shapes and relative sizes of each planet’s orbit by triangulation not from different points on Earth but from different points on Earth’s orbit, using observations made at many different times of the year. 10. What is the difference between a hypothesis, a theory, and a law? A hypothesis is a reasonable supposition made in describing the results of experiments and observations. It attempts to make predictions of future behavior. Once the evidence for its validity is strong, it becomes a theory. a theory: such validity means that many scientists have repeated and verified confirmed the experi- ments/observations. No theory can be proven to be true. However, data can prove a theory to be false. 11, How did the Alfonsine Tab]es, the Prutenic Tables, and the Rudolphine Tables differ? 1633, Galileo was finally condemned before the Inquisition universal? and predict its path in the future. 13. Galileo was condemned by the Inquisition, but Kepler, also a Copernican, was not. Why not? In contrast to the geocentric universe of classical astron- omy, the universe devised by Copernicus was heliocentric, that retrograde motion, the occasional westward motion of the planets, is easily explained. Copernicus did not publish his book De Revolutionibus until 1543, the year he died. The As a critic of the classical view that Earth is at the center of the universe, Copernicus was exploring controversial ideas, Kepler discovered that the planets follow ellipses with the sun at one focus, that they move faster when near the sun, and that a planet's period squared is proportional to its orbital radius cubed. Galileo Galilei was a great defender of the Copernican hypothesis. Galileo was the first person to use a telescope to observe the heavens and to recognize the significance of what he saw. His discoveries of the phases of Venus, the satellites of Jupiter, the mountains of the moon, and other phenomena helped undermine the Ptolemaic universe. In for refusing to halt his defense of Copernicanism. 14. Why did Newton conclude that gravitation had to be Born in 1642, the same year that Galileo died, Isaac Newton laws of motion and the law of gravity. These laws made it possible to understand the orbital motion of the planets as a consequence of the sun's gravity. In addition, Newton's work made it possible to analyze the motion of any celestial body The 99 years from the death of Copernicus to the birth of Newton marked the birth of modern science. From that time the moon with the statement, "The moon is falling." on, science depended on evidence to support theories and re- lied on the analytic methods first demonstrated by Newton. 2. Why did classical astronomers conclude that Earth had to be motionless? During the Middle Ages (approx. 500 AD to approx. 1500 AD), Western science didn't make much progress. Middle Eastern astronomers kept Ptolemy's work alive and it remained the dominant theory of how the universe worked for over a thousand years. The contributions to astronomy. 4. In what ways were the models of Ptolemy and Coperni- Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets. Ptolemy Venus' epicycles are forced to always remain on the Earth/Sun line, they will always be near the Sun and their motions can be explained. This system was fleshed out by Ptolemy in about 150 AD and is therefore called the Ptolemaic system. Copernicus In the early to mid 1500's, Copernicus, who was familiar with Aristarchus' idea of a Sun-centered universe, began to try to fit the heliocentric model to the observed behavior of the sky. The heliocentric universe explained many observations more simply than the geocentric model. The planets could now be put in order by distance from the Sun, and that information both explained planetary regression and provided a constant increase in sidereal period with distance. The problem with the Copernican theory was inability to abandon the idea that planets could move in anything other than perfect circles. This idea had persisted since Ptolemy, and by keeping it, Copernicus had to add epicycles and deferents to his model to explain existing planetary observations. The new model needed even more epicycles than the Ptolemaic model. The general principle in science (called Occam's razor) is that the simplest explanation that fits the facts is the correct one. More epicycles made this theory more complicated. The theory was published in Latin, a language known only among the educated, in general. Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets. Copernican model, that fit Tycho’s complex mass of detailed observations. In the end, he had to abandon Copernicus’s original simple notion of circular planetary orbits, even greater simplicity emerged as a result. Kepler determined the shapes and relative sizes of each planet’s orbit by triangulation not from different points on Earth but from different points on Earth’s orbit, using observations made at many different times of the year. Noting where the planets were on successive nights, he was able to infer the speeds at which they moved. 8. How was Tycho's model of the universe similar to the Ptolemaic model? How did it resemble the Copernican model? The Danish astronomer Tycho Brahe did not accept the Ptolemaic or the Copernican model but rather developed his own, in which the sun and moon circled Earth, and the plan- ets circled the sun. Although his hypothesis was not correct, Tycho made precise observations of planetary positions that later led to a true understanding of planetary motion. Johannes Kepler, Tycho Brahe's assistant, inherited the Danish astronomer's records in 1601 and used his observa- tions to uncover three laws of planetary motion. 12. Review Galileo's telescope discoveries, and explain why they supported the Copernican model and contra-dicted the Ptolemaic model. Galileo in 1610 Galileo made the first telescope survey of the Milky Way and discovered that it is composed of a multitude of individual stars. 15. Explain why we might describe the orbital motion of 6Y Part 1 The Astronomer's Sky 1. Historian of science Thomas Kuhn has said that De Revo- lutionibus was a revolution-making book, but not a rev- olutionary book. How was it classical? 2. Why might Tycho Brahe have hesitated to hire Kepler? Why do you suppose he appointed Kepler his scientific heir? 3. How does the modern controversy over creationism and evolution reflect two ways of knowing about the physi- 2. It's hard to imagine that an observatory could exist be-fore the invention of the telescope, but Tycho Brahe's observatory at Hveen was a great astronomical center of its day. Search the Web sites on Tycho and his instru- ments and describe what an observing session at Hveen might have involved. 1. If you lived on Mars, which planets would describe ret- rograde loops? Which would never be visible as cres- cent phases? 3. Communications satellites are obvious uses of the geo- 2. Galileo's telescope showed him that Venus has a large synchronous oribt, but can you think of other uses for such orbits? Find an Internet site that uses or displays information gleaned from a geosynchronous orbit that provides a useful service. find the ratio of its maximum distance to its minimum angular diameter (61 seconds of arc) when it is a cres- cent and a small angular diameter (10 seconds of arc) when it is nearly full. Use the small-angle formula to distance. Is this ratio compatible with the Ptolemaic 1. The trial of Galileo is an important event in the history of science. We now know, and the Church now recog- nizes, that Galileo's view was correct, but what were the arguments on both sides of the issue as it was un- folding? Research the Internet for documents chroni- cling the trial, Galileo's observations and publications, and the position of the Church. Use this information to outline cases for and against Galileo in the context of the times in which the trial occurred. cal world? universe shown on page 41? Exploring TheSky 3. Galileo's telescopes were not of high quality by modern standards. He was able to see the moons of Jupiter, but he never reported seeing features on Mars. Use the small angle formula to find the angular diameter of Mars when it is closest to Earth. How does that compare with the maximum diameter of Jupiter? 4. If a planet had an average distance from the sun of 10 AU, what would its orbital period be? 5. If a space probe were sent into an orbit around the sun that brought it as close as 0.5 AU to the sun and as far away as 5.5 AU, what would its orbital period be? 6. Pluto orbits the sun with a period of 247.7 years. What is its average distance from the sun? 7. Calculate the circular velocity of Venus and Saturn around the sun. (Hint: The mass of the sun is 2 x 103° kg.) 8. What is the orbital velocity of an Earth satellite 42,200 km from Earth? How long does it take to circle its orbit once? Chapter 1 2. about 2800 4. The sun is 400,000 times brighter than the full moon. 6. about 190,000 8. approximately 21 minutes of arc 10. After one Saros cycle of 18 yr 113 days, the eclipse occurred on 22 July, 1990, and was visible from Finland and Siberia. Three Saros cycles after July 10, 1972, the eclipse will occur again in Canada on 12 August, 2026. Introduction 2. 3475 km 4. 1.05 X 10° km 6. about 1.2 s 8. about 75,000 yr 10. about 27 Chapter 5 2. It will look 206,265Z = 4.25 X 10'° times fainter, which is about 26.5 magnitudes fainter, so it would be about 22.5 mag. 4. about 3 times the half-life, or 3.9 billion years 6. large amounts of methane and water ices 8. about 133 Chapter 6 2. 210 million years 4. 3% 6. An object cools by radiating from its surface, and the area of a sphere is proportional to its radius squared (rz). But the amount of energy an object contains is proportional to its volume, and the vol- ume of a sphere is proportional to its radius cubed (r3). How fast an object cools is proportional to its area divided by its volume, which equals 1/r. The larger an object is, the slower it cools. 8. From an orbit 200 km above the moon, they would be well over 1000 seconds of arc across. Nop,e were seen. about 32 yr 10. 1.1 seconds of arc 12. No; their angular size was only 0.00054 seconds 3.07 km/s, 24 hours of arc. Chapter 2 2. Ratio of greatest distance to closest distance is 6.1 to 1. No; the ratio in the diagram is about 1.5 to 1. 4. 6. 39.4 AU 8. Chapter 3 2. short radio waves 4. The 10-m Keck telescope has a light-gathering power that is 1.56 million times greater than the human eye. 6. No; his resolving power should have been about 5.8 seconds of arc at best. 8. 0.013 m = 1.3 cm Chapter 8 10. about 50 cm (From 400 km above, a human is about 2. From Callisto, Jupiter would appear to be 4.3°, and 0.25 seconds of arc from shoulder to shoulder.) from Io it would appear to be 19.4°. 4. 16.8 km/s Chapter 4 6. 7.3 s 2. 150 nm 8. 60,000 nm (60 microns) 4. 30 K 10. 1.5 X 101.2 kg 6. by a factor of 34 = 81 8. 31 km/s; receding Chapter 9 2. 21.1 km/s 4. Assuming an orbital distance of 3 AU, the period would be 5.2 years. Chapter 7 2. 47.9 km/s 4. 61 seconds of arc 6. 20 km 8. 962 seconds of arc, or about 16 minutes of arc; 0.074 seconds of arc 8. 2000 nm = 2 micrometers 10. Estimating the absolute magnitude of an 06V star to be -5.6 implies a distance of 1600 pc. The answer is very dependent on the estimate for the absolute large features, since it can resolve structures as magnitude. small as 0.1 second of arc. 6. 0.77 seconds of arc (R = 500 km, D = 1.77 AU). Earth-based telescopes can seldom resolve struc- tures smaller than 0.5 seconds of arc. The Hubble Space Telescope would be able to see some very 8. 9.2 X 106 km. If the tail is not perpendicular to my line of sight, then the tail is longer than 9.2 x 106 km. 10. 0.033 Earth masses 8. about 950 years ago (about 1050 AD) 10. 4 hr; 95 years Chapter 13 2. 300 million years 4. about 18,000 years 6. 8000 Chapter 10 2. 730 km 4. about 3.6 times brighter 6. 400,000 yr 8.9.0 X1016J 10. 0.2 kg Chapter 11 2. 28 m 4. m M d (pc) p (seconds of arc) 7 7 10 11 1 1 -2 40 4 2 25 Chapter 15 0.1 4. 5000 pc 1000 0.001 6. 25 pc 0.025 8. 20 times 0.040 10. 1500 K 6. B2 to B4 2. 16% Chapter 16 2. 2.6 x 106 pc (2.6 Mpc) 4. 93 km/s 6. 28.6 Mpc 8. 165 million years " 12.4.23X1011 m,6.7X1010 m,2.4X1031 kg Chapter 17 2. 7.6 million years 4. 0.024 pc = 4900 AU 6. -28.5 8. 0.3 c 10. AX = 77.8 nm, z = 0.16 of 907 solar luminosities. Chapter 18 2. 17.5 billion years; 11.7 billion years 20,000 AU or 0.097 pc 4. 1.6 x 10-311 gm/cm3 6. 76 km/s/Mpc 8. 16.6 billion years; 11 billion years clei are converted into one 4He nucleus and energy. 8. Using Figure 11-13, M, is estimated to be -7, so d = 400 pc. There is some uncertainty in this num- ber in determining the absolute magnitude of the 08V star from Figure 11-13. 10. a, c, c, c, d (12 solar masses) 14. A 4-solar-mass main-sequence star has a lumi- nosity of approximately 128 solar luminosities. A 9-solar-mass main-sequence star has a luminosity of approximately 2187 solar luminosities, and a 7-solar-mass main-sequence star has a luminosity Chapter 12 2. 4. 100 K 6. In both reactions, the net result is that four 1H nu- In the case of the proton-proton chain, six 1H nu- clei are put into the reaction, and the reaction re- turns two 1H nuclei, one 4 He nucleus, and energy (6'H --~ 2'H + 1 'He + energy, which is equivalent to 4 1H --) 1 4 He + energy). The CNO cycle inputs one 1zC nucleus and four 1H nuclei and returns one 1z C nucleus, one 4 He nucleus, and energy Answers to Even-Numbered Problems 499 Chapter 19 2. 8.9 cm; 0.67 mm 4. about 1.3 solar masses 6. 380 km 8. pessimistic, 2 X 10-s; optimistic, 10'. Answers will vary greatly. Using intermediate values from the table, such as the following numbers: 2 x 10", (1 'ZC + 4'H -~ 1 'ZC + 4 He + energy, which is 0.1, 0.1, 0.1, 0.1, 10-°, yields an answer of 20. equivalent to 4 'H -> 1 4 He + energy). Answers to EvenmHumbered Problems 1. Observe Mars going through its retrograde motion. (Hint: Use Reference Lines under the View menu to turn on the ecliptic. Be sure you are in Free Rotation under the Orientation menu. Locate Mars and use the time skip arrows to watch it move.) 2. Compare the size of the retrograde loops made by Mars, Jupiter, and Saturn. 3. Can you recognize the effects of Kepler's second law in the orbital motion of any of the planets? (Hint: Use 3D Solar System Mode under the View menu.) Kepler's First Law Each planet moves about the Sun in a orbit that ia an ellipse with the Sun at one focus of the ellipse. Kepler's Second Law The straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time. 4. Can you recognize the effects of Kepler's third law in the orbital motion of the planets? Kepler's Third Law The squares of the planet's periods of revolution are in direct proportion to the cubes of the semimajor axis of their orbits. 6o to the Brooks/Cole Astronomy Resource Center (www.krook:eole. comlastronomy) for critical thinking exercises, articles, and addl-tlonal readings from Infoirac College Edition, Brooks/Cole's online stmaat Rbrary. magnifying power alt-azimuth mounting refracting telescopes cannot bring all colors to the same focus, light pollution active optics resulting in color fringes around the images. An achromatic prime focus adaptive optics secondary mirror charge-coupled device (CCD) Newtonian focus false-color image Schmidt-Cassegrain focus spectrograph grating equatorial mounting comparison spectrum radio interferometer P°lar axis mers refer to as seeing. Atop a mountain, the air is steady, and gey~ew uestions dry, and thus it is more transparent, especially in the infrared. sidereal drive Cassegrain focus Chapter 2 The Origin of Modern Astronomy 63 and focus it into an image. Because of chromatic aberration, lens partially corrects for this, but such lenses are expen- sive and cannot be made larger than about 1 m in diameter. Reflecting telescopes use a mirror to focus the light and are less expensive than refracting telescopes of the same diameter. In addition, reflecting telescopes do not suffer from chromatic aberration. Thus, most recently built large telescopes are reflectors. Astronomers build observatories atop high mountains for two reasons. Turbulence in Earth's atmosphere blurs the image in an astronomical telescope, a phenomenon that astrono- the seeing is better. The air at a mountaintop is also thin and The light gathered by an astronomical telescope can be telescope. For many decades, astronomers have used photo- graphic plates to record images at the telescope, but modern 2. electronic systems such as CCDs have now replaced photo- graphic plates in most applications. Spectrographs spread Why? starlight out according to wavelength to form a spectrum. thin? What problems does this cause? 1, Why would you not plot sound waves in the electro-recorded and analyzed by special instruments attached to the magnetic spectrum? If you had unlimited funds to build a large telescope, which type would you choose, a refractor or a reflector? 3. Why do nocturnal animals usually have large pupils in their eyes? How is that related to astronomical tele- scopes? an antenna, an amplifier, and a recorder. Such an instrument 4. Why do optical astronomers sometimes put their tele-scopes at the tops of mountains, while radio astrono-mers sometimes put their telescopes in deep valleys? To observe radio signals from celestial objects, we need a radio telescope, which usually consists of a dish reflector, can measure the intensity of radio signals over the sky and construct radio maps. The poor resolution of the radio telescope can be improved by combining it with another radio telescope to make a radio interferometer. Radio telescopes have three important features-they can detect cool hydrogen, they can see through dust clouds in space, and they can detect certain objects invisible at other wavelengths. Earth's atmosphere admits radiation primarily through two wavelength intervals, or windows-the visual window and the radio window. At other wavelengths, our atmosphere absorbs radiation. To observe in the far infrared, astrono- about astronomical telescopes, how would you improve mers must fly telescopes high in balloons or aircraft, though such advertisements? they can work at some wavelengths in the near infrared from high mountaintops. To observe in the ultraviolet, X-ray, or gamma-ray range and some parts of the infrared, they must send their telescopes into space to get above our atmosphere. 5. Optical and radio astronomers both try to build large telescopes but for different reasons. How do these goals differ? and focus it into an image. Because of chromatic aberration, lens partially corrects for this but such lenses are expen- sive and cannot be made larger than about 1 m in diameter. Reflecting telescopes use a mirror to focus the light and are less expensive than refracting telescopes of the same diameter. In addition, reflecting telescopes do not suffer from chromatic aberration Thus most recently built large Schmidt-Cassegrain focus. radio interferometer in an astronomical telescope, a phenomenon that astrono- mers refer to as seeing. 6. What are the advantages of making a telescope mirror 7. Small telescopes are often advertised as "200 power" or "magnifies 200 times." As someone knowledgeable 8. An astronomer recently said, "Some people think I should give up photographic plates." Why might she change to something else? 9. What purpose do the colors in a false-color image or false-color radio map serve? 10. How is chromatic aberration related to a prism spectro-graph? electromagnetic radiation eyepiece 11, Why would radio astronomers build identical radio telescopes in many different places around the world? photon reflecting telescope 12. Why do radio telescopes have poor resolving power? nanometer (nm) chromatic aberration cooled to low temperature? Hot temperatures among stars. radiation 13. Why must telescopes observing in the far infrared be 14. What might we detect with an X-ray telescope that we could not detect with an infrared telescope? 15. If the Hubble Space Telescope observes at visual wave-lengths, why must it observe from space? 86 Part 1 The AstronomeYs Sky 9. Astronauts observing from a space station need a tele-scope with a light-gathering power 15,000 times that of 1. Why does the wavelength response of the human eye the human eye, capable of resolving detail as small as match so well the visual window of Earth's atmosphere? 0.1 second of arc, and having a magnifying power of 250. Design a telescope to meet their needs. Could you test your design by observing stars from Earth? 2. Basic research in chemistry, physics, biology, and similar sciences is supported in part by industry. How is as-tronomy different? Who funds the major observatories? 3. Most people like beautiful sunsets with brightly glow-ing clouds, bright moonlit nights, and twinkling stars. Most astronomers don't. Why? 10. A spy satellite orbiting 400 km above Earth is suppos-edly capable of counting individual people in a crowd. What minimum-diameter telescope must the satellite carry? (Hint: Use the small-angle formula.) 1. The thickness of the plastic in plastic bags is about 0.001 mm. How many wavelengths of red light is this? 2. Measure the actual wavelength of the wave in Fig-ure 3-1. In what portion of the electromagnetic spec-trum would it belong? 3. Compare the light-gathering powers of a 5-m telescope and a 0.5-m telescope. 4. How does the light-gathering power of one of the Keck telescopes compare with that of the human eye? (Hint: Assume that the pupil of your eye can open to about 0.8 cm.) 1. How do professional astronomers go about making ob-servations at major astronomical facilities? Visit sev-eral observatory Web sites to determine the process an astronomer would go through to secure observing time and make observations at the facility. 2. NASA is in the process of completing a fleet of four space-based "Great Observatories." (The Hubble Space Telescope is one; what are the others?) Examine the cur rent state of these missions by visiting their home pages on the Internet. What advantages would these facilities have over ground-based observatories? 5. What is the resolving power of a 25-cm telescope? What do two stars 1.5 seconds of arc apart look like through this telescope? 6. Most of Galileo's telescopes were only about 2 cm in di-ameter. Should he have been able to resolve the two stars mentioned in Problem 5? 7. How does the resolving power of the 5-m telescope compare with that of the Hubble Space Telescope? Why does the Hubble Space Telescope outperform the 5-m telescope? 8. If we build a telescope with a focal length of 1.3 m, what focal length should the eyepiece have to give a magnification of 100 times? Exploring TWO 1. Astronomical telescopes using equatorial mountings must be aligned precisely with the north celestial pole. Locate Polaris and determine how far it is from the north celestial pole. (Hint: Use Reference Lines under the View menu and check Grid under Equitorial. Be sure the spac-ing is set to auto/fine. Then locate the Little Dipper and zoom in on Polaris.) Chapter 3 Astronomical Tools 81 absorption line Doppler effect emission spectrum (bright- blue shift line spectrum) red shift emission line radial velocity (V,) Kirchhoff's laws or dense .. The hotter stars produces a condense at the wave-.ch depends on the ostly short-wavelength mostly long-wavelength ues to the temperatures of jol stars are red. electrons surrounding an atomic ous permitted orbits. An electron ,ner orbit during a collision between re from one orbit to another by absorb-' of the proper energy. If the energy photon is too large, the atom may lose an jecome ionized. spectra? only certain orbits are permitted, only photons n wavelengths can be absorbed or emitted. Each atom has its own characteristic set of spectral lines. ar telescope, the gas can absorb photons of certain wave- lengths, and we will see dark lines in the spectrum at those positions. Such a spectrum is called an absorption spec- trum. If we look at a low-density gas that is excited to emit tions. Such a spectrum is called an emission spectrum. serve longer wavelengths. This Doppler effect makes it pos- sible for the astronomer to measure a star's radial velocity, 1. Why might we say that atoms are mostly empty space? 2. What is the difference between an isotope and an ion? 3. Why is the binding energy of an electron related to the size of its orbit? 4. Explain why ionized calcium can form absorption lines but ionized hydrogen cannot. 5. Describe two ways an atom can become excited. 6. Why do different atoms have different lines in their 7. Why does the amount of black body radiation emitted depend on the temperature of the object? 8. Why do hot stars look bluer than cool stars? If light passes through a low-density gas on its way to 9. What kind of spectrum does a neon sign produce? 10. Why does the Doppler effect detect only radial velocity? 11, How can the Doppler effect explain shifts in both light and sound? photons, we see bright lines in the spectrum at those posi- When a source of radiation is approaching us, we ob-serve shorter wavelengths, and when it is receding, we ob- 1, In what ways is our model of an atom a scientific model? In what ways is it incorrect? that part of its velocity directed toward or away from Earth. Discussion Questions Problems 1. Human body temperature is about 310 K (98.6°F). At what wavelength do humans radiate the most energy? What kind of radiation do we emit? 2. If a star has a surface temperature of 20,000 K, at what wavelength will it radiate the most energy? 3. Infrared observations of a star show that it is most in-tense at a wavelength of 2000 nm. What is the tempera-ture of the star's surface? ion Kelvin temperature scale 4, Dust orbiting distant stars may be evidence that planets have formed there. If the dust is brightest at the far- black body radiation infrared wavelength of 100,000 run, what is the temper- ature of the dust? molecule absolute zero Coulomb force binding energy wavelength of maximum quantum mechanics intensity (;.m.) 5. If astronomers observe that a liquid flowing out onto the surface of a planet is brightest at a wavelength of page 100 Part I The Astronomer's Sky 1700 nm, what is its temperature? Do you think it is liq-uid water or liquid rock? 6. If we triple the temperature of a black body, by what fac-tor will the total energy radiated per second per square meter increase? 7. If one star has a temperature of 6000 K and another star has a temperature of 7000 K, how much more energy per second will the hotter star radiate from each square meter of its surface? 8. In the laboratory, the Balmer beta line of hydrogen has a wavelength of 486.10 rim. If the line appears in a comet's spectrum at 486.15 nm, what is the comet's ra-dial velocity? 9. The highest velocity stars an astronomer might observe have velocities of about 400 km/s. What change in wave-length would this cause in the spectral line described in Problem 8? 1. The name for the element helium has astronomical roots. Search the Internet for information on the discovery of helium. How and when was it discovered, and how did it get its name? Why do you suppose it took so long for helium to be recognized? 2. How was the model of the atom presented in the text you read developed? Search the Web for information on historical models of the atom and compile a time line of important developments leading to our current under-standing. What evidence exists that supports our model? 6o to the Brooks/Cole Astronomy Resource Center Iwww.broowscole. com/astronomyl Chapter 4 Atoms and Starlight 101 in that chain is planet building. As we explore the solar system in detail in the following chapters, we must stay alert for further clues to the birth of the planets. Summary and allowed it to differentiate into a dense metallic core and a lower-density silicate crust. into larger bodies, the largest of which grew the fastest and eventually became protoplanets. Once a planet had formed from a large number of plan-etesimals, heat from radioactive decay could have melted it 8. Why is almost every solid surface in our solar system Review Questions 1. What produced the helium now present in the sun's at-mosphere? in Jupiter's atmosphere? in the sun's core? 2. What produced the iron in Earth's core and the heavier elements like gold and silver in Earth's crust? 3. What evidence do we have that disks of gas and dust are common around young stars? The solar nebula theory proposes that the solar system be-gan as a contracting cloud of gas and dust that flattened into a rotating disk. The center of this cloud eventually became the sun, and the planets formed in the disk of the nebula. Observational evidence gives astronomers confidence in this theory. Disks of gas and dust have been found around many young stars, so astronomers suspect that planetary systems are common. Planets orbiting other stars are too faint and too close to their star to image directly, but astron-omers have found many of these planets by observing the motion of the star as the star and planet revolve around their center of mass. 4. According to the solar nebula theory, why is the sun's equator nearly in the plane of Earth's orbit? The solar nebula theory explains many of the charac-teristic properties of the solar system. For example, the solar system has a disk shape. The orbits of the planets lie in nearly the same plane, and they all revolve around the sun in the same direction, counterclockwise as seen from the north. With only three exceptions, the planets rotate counterclock-wise around axes roughly perpendicular to the plane of the solar system. This disk shape and the motion of the planets appear to have originated in the disk-shaped solar nebula. Another striking feature of the solar system is the divi-sion of the planets into two families. The terrestrial planets, which are small and dense, lie in the inner part of the system. The Jovian planets are large, low-density worlds in the outer part of the system. In general, the closer a planet lies to the sun, the higher its uncompressed density. The solar system is now filled with smaller bodies such as asteroids, comets, and meteors. The asteroids are small, rocky worlds, most of which orbit the sun between Jupiter and Mars. They appear to be material left over from the for-mation of the solar system. Another important characteristic of the solar system bodies is their similar ages. 5. Why does the solar nebula theory predict that planetary systems are common? In fact, it is possible that the solar nebula cooled as the protoplanets grew so that the first planetesimals were metallic and later additions were sili-cate. It is also likely that the planets grew rapidly enough that the heat of formation released by the in-falling material melted the planets and allowed them to differentiate as they formed. The Jovian planets probably grew rapidly from icy ma-terials and became massive enough to attract and hold vast amounts of nebular gas. The heat of formation raised their temperatures very high when they were young, and Jupi-ter and Saturn still radiate more heat than they absorb from the sun. Once the sun became a luminous object, it cleared the nebula as its light and solar wind pushed material out of the system. The planets helped by absorbing some planetesi mals and ejecting others from the system. Once the solar sys-tem was clear of debris, planet building ended. 6. Why do we think the solar system formed about 4.6 bil- lion years ago? Radioactive dating tells us that Earth, the moon, Mars, and meteorites are no older than 4.6 billion years. it seems our solar system took shape about 4.6 billion years ago. According to the condensation sequence, the inner part of the nebula was so hot that only high-density minerals could form solid grains. The outer regions, being cooler, con densed to form icy material of lower density. The planets grew from these solid materials, with the denser planets forming in the inner part of the nebula and the lower-density Jovian planets forming farther from the sun. Planet building began as dust grains grew by condensa-tion and accretion into planetesimals ranging from a kilome-ter to hundreds of kilometers in diameter. These planetesimals settled into a thin plane around the sun and accumulated 7. If you visited another planetary system, would you be surprised to find planets older than Earth? Why or why not? scarred by craters? 124 Part 2 The Solar System 9. What is the difference between condensation and 8. If you stood on Earth during its formation, as described in Problem 7, and watched a region covering 100 ml, how many impacts would you expect to see in an hour? (Hints: Assume that Earth had its present radius. The surface area of a sphere is 47tr2.) accretion? Why don't terrestrial planets have rings and large satel-lite systems like the Jovian planets? 11. How does the solar nebula theory help us understand the composition of asteroids and comets?  12. How does the solar nebula theory explain the dramatic density difference between the terrestrial and Jovian planets? ~ ~3. If you visited some other planetary system in the act of building planets, would you expect to see the conden--'N. sation sequence at work, or was it unique to our solar system? 14. Why do we expect to find that planets are differentiated? 15. What processes cleared the nebula away and ended planet building? 9. The velocity of the solar wind is roughly 400 km/s. How long does it take to travel from the sun to Pluto? Critical Inquiries 1. How does our solar system compare with the others that have been found? Search the Internet for sites that give information about planetary systems around other stars. What kinds of planets have been detected by these searches so far? Discuss the selection effects (see Win-dow on Science 16-2) that must be considered when in-terpreting these data. 2. The process of protoplanetary accretion is still not well understood. Search the Web for current research in this field. From the results of your search, outline the basic 1. Discuss the history of the atoms in your thumb. Where steps in the formation of a protoplanet through accre- did they come from and what processes brought them together? tion. What specific factors are important in these mod- els of planet building? Do these models produce plane-tary systems similar to the ones we know to exist? 2. If the solar nebula theory is correct, then there are prob-ably more planets in the universe than stars. Do you agree? Why or why not? 1. The nearest star is about 4.2 ly away. If you looked back at the solar system from that distance, what would the maximum angular separation be between Jupiter and the sun? (Hint: 1 ly equals 63,000 AU.) 3. How is radioactive dating carried out on meteorites and rocks from surfaces of various bodies in the solar sys-tem? Look for Web sites on the details of radioactive dating and summarize the methods used to uncover the abundances of radioactive elements in a particular sam-ple. (Hint: Try looking for information on how a par-ticular meteorite-for example, the Martian meteorite ALH84001-was studied, what age range was deter-mined, and what radioactive elements were used to ar-rive at the age.) 2. The brightest planet in our sky is Venus, which is some-times as bright as apparent magnitude -4 when it is at a distance of about 1 AU. How many times fainter would Exploring TheSky it look from a distance of 1 parsec (206,265 AU)? What would its apparent magnitude be? (Hint: Remember the inverse square law, Chapter 2.) 1. Look at the solar system from space. Notice how thin the disk of the solar system is and how inclined the or-bits of Pluto and Mercury are. (Hint: Under the View menu, choose 3D Solar System Mode, and then zoom in or out. Tip the solar system up and down to see it edge-on.) 3. What is the smallest-diameter crater you can identify in the photograph of Mercury on page 112? (Hint: See Ap-pendix A to find the diameter of Mercury in kilometers.) 4. A sample of a meteorite has been analyzed, and the re-sult shows that out of every 1000 nuclei of 4°K origi-nally in the meteorite, only 125 have not decayed. How old is the meteorite? (Hint: See Figure 5-9.) 5. In Table 5-2, which object's density differs least from its uncompressed density? Why? 6. What composition might we expect for a planet that formed in a region of the solar nebula where the tem-perature was about 100 K? 7. Suppose that Earth grew to its present size in 10 million ~-- ® Bo to the Brooks/Cole Astronomy Resource Center Iwww.broehscole. years through the accretion of particles averaging 100 g comtastronomYl for critical Making exercises, articles, and addl- each. On the average, how many particles did Earth cap- ~ tlonal readings from Infoirac CoINpe EdiUOt1. BrooksiCole's online ture per second? (Hint: See Appendix A to find Earth's student library. mass.) 2. Look at the solar system from space and notice how small the orbits of the inner planets are compared to the orbits of the outer planets. They make two distinct groups. (Hint: Use 3D Solar System Mode.) 3. Watch the comets orbiting around the sun. Can you lo-cate the comet C/198 M5 (Linear)? What is its orbit like? (Hint: Use 3D Solar System Mode and set the time step to 30 days (30d).) Chapter 5 The Origin of the Solar System 125 r.against each other to build folded mountain chains, and slide over each other at subduction zones where sections of crust are pushed back down into the interior. This process, plate ttectonics, continually reshapes Earth's surface. `Earth's first atmosphere, its primary atmosphere, was secondary crater rich in carbon dioxide and nitrogen, but carbon dioxide dis- mosphere was left rich in nitrogen, and the evolution of plant large-impact hypothesis solved into the ocean waters and became sediments. The at- Ques~on~ a secondary atmosphere. Today, the greenhouse effect warms 1. What are the four stages in Earth's development? Scientists theorize that the Earth began as a waterless mass of rock surrounded by a cloud of gas. Radioactive materials in the rock and increasing pressure in the Earth's interior gradually produced enough heat to melt the interior of the Earth. Asteroids and Comets colliding with Earth. Particles from the gas dust from the Earth's Shining Star. 2. Why do we expect planets to have differentiated? Impacts from asteroids comets. differentiation The separation of material according to density. cratering From heavy bombardment on solid surface. flooding From interior melting caused by decay of radioactive elements and later, as the atmosphere cooled, from rainfall. surface evolution From erosion, crust movements. 3. How do we know that Earth has a molten core? Liquid Core Molten, thickness: 2400 km; metallic (Iron and Nickel) Inner Core Solid, thickness: 1200 km; metallic (Iron and Nickel). The density is about 14 g/cm3. Temperature 6000 K. 4. How does plate tectonics create and destroy Earth's crust? The crust is fractured into huge plates that float on the mantle. Originally there was one land mass, called Pangaea. about 200 million years ago it broke apart, eventually to form the continents we have today. The plate movement is called plate tectonics (Albert Wegener, 1924). Plates can collide and form folded mountains, like the Himalayas. These mountains were created, and being raised, by the ongoing collision of India with southern Asia. Plates can slide into the mantle in regions called subduction zones. They can split apart and create rift valleys, like the split between Africa and Arabia which formed the Red Sea. 5. Why do we suspect that Earth's primeval atmosphere was rich in carbon dioxide? Carbon dioxide, which is highly soluble in water, was drawn into the oceans. The ensuing chemical reactions with other compounds in the oceans transformed the carbon dioxide into mineral compounds like limestone and silicon dioxide sand. Plants absorb carbon dioxide and generate free oxygen. the Earth's surface was mostly molten rock that gradually cooled through the radiation of heat into space. The primeval atmosphere was composed mostly of water (H2O), carbon dioxide (CO2) and monoxide (CO), molecular nitrogen (N2) and hydrogen (H2), helium (He) and hydrogen chloride (HCl) outgassed from molten rock, with only traces of reactive molecular oxygen (O2). The Earth's second atmosphere that developed after the surface cooled, most likely resembled that of Jupiter's atmosphere. It was formed mostly from the outgassing of such volatile compounds as water vapor, carbon monoxide, methane, ammonia, nitrogen, carbon dioxide, nitrogen, hydrochloric acid and sulfur produced by the constant volcanic eruptions that besieged the Earth. This atmosphere was rich with water vapour released from hydrated minerals and cometary impacts. 6. Why doesn't Earth have as many craters as the moon? and highlands? The Earth is bigger than the Moon. The Moon is smaller in size and in different orbit. 8. Why do we believe that the lunar crater Tycho is a young crater? 9. Why are there no folded mountain ranges on the moon? Because of no plate movement is called plate tectonics. Plates can collide and form folded mountains 10. How do the lunar samples suggest that the moon formed with a molten surface? As the crust solidified, cratering battered it and exca- How do we find the relative ages of the moon's maria maria are dark basalts, whereas the highlands are lighter life added large amounts of oxygen. The air we breathe is Earth's surface, and the ozone layer protects us from ultravio- tlet radiation. The moon is, at first glance, dramatically different from Earth. We see two distinct kinds of terrain on the moon. The maria, named after seas, are lowland plains with few craters. They are great lava flows and are younger than the highlands. The highlands are lighter in color and heavily cratered. Samples returned by the Apollo missions show that the rock such as anorthosite. Many samples are breccias, which show how severely the moon's crust has been fractured. portance of each of these processes in the evolution of a planet and it has passed through all four stages. Studies show that Earth has differentiated into a metallic core and a silicate crust. Currents in the molten portion of the core produce into sections that pull apart along midocean rises, push The ages of the mare samples range from about 3.1 to 3.8 billion years, but the highland samples are about 4.1 to 4.6 billion years old. Few samples are older than 4.3 billion years. This suggests that the moon's surface was molten until about 4.3 billion years ago. vated great basins. Later, molten rock rose through fractures in the crust and flooded the basins to produce the maria. By the time the maria began to form, cratering had declined, so the maria are marked by few craters. The lunar surface now is evolving very slowly. Because thick, and it has never divided into moving plates. Now only meteorites alter the surface. 11. WhY ar'e so many lunar samples breccias? 12. What do the vesicular basalts tell us about the evolution of the lunar surface? 13. What evidence would we expect to find on the moon if it had been subjected to plate tectonics? the moon is small, it has cooled rapidly, its crust has grown 14. Cite objections to the fission, condensation, and cap- ture hypotheses. 15. How does the large-impact hypothesis explain the moon system formed in the collision of two large planetesi- moon's lack of iron? mals. The differentiated bodies formed a single large object The large-impact hypothesis suggests that the Earth- that became the higher-density, iron-rich Earth. Ejected man- tle material, poor in iron, formed a disk and eventually con- densed to form the lower-density, iron-poor moon. subduction zone multiringed basin depends on the planet's mass and temperature. folded mountain range relative age Earth is the largest terrestrial planet in our solar system, rift valley absolute age tidal coupling vesicular basalt terminator anorthosite Earth's magnetic field. limb breccia The rocky crust floats on a plastic mantle and is divided mare regolith Discu~~on Questions 1. If we visited a planet in another solar system and dis-covered oxygen in its atmosphere, what might we guess about its surface? comparative planetology primeval atmosphere - P waves secondary atmosphere S waves greenhouse effect dynamo effect plate tectonics eF mantle basalt plastic midocean rise 2. If liquid water is rare on the surface of planets, then most terrestrial planets must have CO2 rich atmospheres. Why? 3. Old science-fiction paintings and drawings of colonies on the moon often showed very steep, jagged mountains. Why did the artists assume that the mountains would be more rugged than mountains on Earth? Why are lunar mountains actually less rugged than mountains on Earth? Chapter 6 Earth and Its Moon 145 3. Earth's magnetic poles are not coincident with its axis of rotation, nor are their positions fixed. Search for in- formation about the location of the magnetic poles and their motions. Why do the positions change? 4. What are the oldest known rocks on Earth? Where do they come from, and how were their ages determined? Radioactive rocks and radioactive dating. From meteor impacts. Rocks came to earth from outerspace. 5. What evidence can you find about our changing climate? Is the rise in Earth's temperature related to the green- house effect? Scientific, industrial, and political leaders do not agree on this issue, so be prepared to analyze ar- guments with great care. 1. About what percent of Earth's volume is occupied by its iron core? its rocky crust? 90% of rocky crust. 2. If the Atlantic seafloor is spreading at 3 cm/year and is now 6400 km across, how long ago were the continents in contact? 3. The dinosaurs died and mammals arose about 65 mil- lion years ago. For what percent of Earth's history have we mammals been a leading life form? 90% 4. If we estimate that our atmosphere is 200 km deep, what percent of Earth's radius is that? 5. What is the ratio of the volume of the moon to the vol- ume of the Earth? Why is that different from the ratio of the masses? 6. Can you locate an image of the Antarctic ozone hole? Find data for the ozone concentration in the upper atmo- sphere over the place where you live. What is the latest news concerning ozone depletion? 6. Why do small worlds cool faster than large worlds? Com-pare surface area to volume. The less massive the planet is going to release more heat into space. 7. The smallest detail visible through Earth-based tele-scopes is about 1 second of arc in diameter. What size is that on the moon? (Hint: Use the small-angle formula.) 8. The subduction zones on Earth are 1 km or less across. Why are we sure that such features don't exist on the moon? The moon's surface is different. No volcanic activity on the moon. 9. The Imbrium Basin is about 1300 km in diameter. What percentage of the moon's surface is that? 10. If Earth's moon orbited Jupiter, what would its maximum angular diameter be as seen from Earth? (Hint: Use the small-angle formula.) 11. The full moon has an apparent magnitude of -12.5. If it were orbiting Jupiter, how bright would it be? (Hints: Sunlight is dimmer at Jupiter's distance, and the moon would be further from Earth. Use the inverse square law.) 7. What did the Apollo astronauts do while they were on the moon? Search for Web sites that describe the exper-iments performed during the astronauts' extravehicular activities (EVAs). What types of experiments/activities were performed? What did we learn from them? 8. Search for maps of the moon that show geological infor-mation such as chemical composition, elevation, age, and so on. Do the maps confirm your understanding of the lunar surface? 9. What would it be like to walk on the lunar surface? Apollo astronauts visited six different locations on the moon ex-ploring the variations in lunar terrain. Describe the hori zons and general relief of the landing locations of the dif-ferent missions by exploring Web sites that provide lunar surface photography from the missions. What differ-ences do you see between images from landings in high-lands and in maria? 12. An astronaut in a space suit is about 1 meter in diame-ter. Could we have seen the astronauts on the moon with Earth-based telescopes? 13. While two Apollo astronauts went down to the lunar sur-face, a third stayed behind in the command module or-biting 200 km above the surface. Could the astronauts be seen from the command module? CrltICi~l ~'- Exploring The.S~ry 1. Locate the moon in the sky. What constellation is it in? Find the moon's phase as a percentage of full, its angu-lar size diameter, and its distance from Earth. Use Find to locate the moon. 2. Follow the moon through one complete orbit measuring its distance and angular diameter. How do these change? Under Tools, use the Moon Phase Calendar. Change dates by clicking on Data and then Site Information. 1. Search the Web to find the location of the most recent earthquakes, and plot them on the map on page 130. The Earth quakes somewhere each day. Search for seismo graphs that will give you information about the most re-cent earthquakes. 2. Search for information about the arrangement of land masses on Earth before Pangaea. Were there previous su-percontinents? What was Rodinia? What is the evidence? 146 Part Z The Solar System Review Questions Why doesn't Mars have coronae like those on Venus? resonance composite volcano lobate scarp shield volcano intercrater plain corona smooth plain outflow channel subsolar point runoff channel Moon formation theories- there are several theories about the origin of the moon. One is the fission hypothesis, which says that the rapidly spinning Earth ejected the moon from the site of the Pacific ocean billions of years ago. Reasons this hypothesis is unlikely include 1) material thrown from Earth would probably have fallen back to Earth, 2) the moon should've been thrown into the Earth's equatorial plane rather than the ecliptic plane since the rotation of Earth on its axis has nothing to do with its orbit, and 3) connecting the site to the Pacific ocean makes no sense because, due to plate motion, the site of the Pacific ocean didn't exist billions of years ago. A second hypothesis is the capture hypothesis, which suggests the moon was captured by the Earth. While a capture is not impossible (although it is unlikely), a third body has to be involved to get rid of the excess energy. The accretion hypothesis suggests that the moon formed gradually in the same way the Earth did. If it formed at the same time as the Earth, though, it should have essentially the same composition. The chemical composition of moon rocks is noticeably different than those of the Earth, though - it has rocks with high concentrations of potassium, phosphorous, and rare-earth elements. The difference could possibly be explained by assuming that the moon formed at a different time when the conditions around the Earth had changed. A more recent theory that may work even better is the giant impact theory. This theory says that the early Earth was involved in a huge collision with an object approximately the size of Mars and a huge amount of material was thrown into orbit around the Earth. This material eventually assembled itself into the Moon. Advantages of this theory would be the ability to explain the moon's lack of iron it was all in the Earth's core, and the impact was not enough to eject material from Earth's core. and its lack of volatile elements (the incredible temperatures involved in such an impact would boil them all away rapidly). Today, many astronomers favor a scenario often called the impact theory, which postulates a glancing collision between a large, Mars-sized object and a youthful, molten Earth. Computer simulations of such a catastrophic event show that most of the bits and pieces of splattered Earth could have coalesced into a stable orbit, forming the Moon. 1. How does the tidal coupling between Mercury and the sun differ from that between the moon and Earth? Chapter 7 Mercury, Venus, and, Mars 2. Why does Mercury have lobate scarps while Earth, its moon, Venus, and Mars do not? Mercury is a small world, only about 40 percent larger than Earth's moon; and, like the moon, it has lost any permanent atmosphere it might have had and its interior has cooled rap-idly. Debris from the formation of the solar system cratered Mercury's surface as it did the moon's, and lava flows buried parts of that terrain under the intercrater plains. The Caloris Basin, formed near the end of cratering, is the largest crater basin. Soon after the Caloris impact, lava flows created the smooth plains. These lava plains have about the same color as the intercrater plains, so the contrast between plains and highlands is not as obvious on Mercury as it is on the moon. The interior of Mercury must contain a large metallic core to account for the planet's high density. A slight shrink-age in the diameter of the planet at the time of the cooling of the core may have led to wrinkling of the crust. This would account for the lobate scarps that mark all of the photographed parts of Mercury. 0.000133 second before the main echo, how high is the spot above the average surface of Venus? 3. What evidence do we have about the interior of Mercury? 7. The smallest feature visible through an Earth-based telescope has an angular diameter of about 1 second of arc. If a crater on Mars is just visible when Mars is at its closest to Earth, how big is the crater? (Hint: Use the small-angle formula.) had more water than at present? Where did that water 8. What is the maximum angular diameter of Phobos as seen from the surface of Mars? as seen from Earth? 4. Why would we expect Venus and Earth to be similar? Why do they differ? Venus is Earth's twin in size and density, but the planet has evolved along divergent lines because it is slightly closer to the sun. The higher temperature evaporated any early oceans and prevented the absorption of carbon dioxide from the atmosphere. The accumulating carbon dioxide created a greenhouse effect that produces a surface temperature of 745 K (882°F). The crust of Venus, mapped by radar, is marked by low rolling plains and highlands with some very high volcanoes. Spacecraft that have reached the surface have found dark basalts. The coronae on Venus are believed to have been caused by rising currents of molten magma in the mantle push-ing upward under the crust and then withdrawing to eave the circular scars called coronae. Earth, Venus, and Mars have had significant amounts of internal heat, and there is plenty of evidence that they have had rising convection currents of magma under their crusts. Of course, we wouldn't expect to see coronae on Earth because its surface is rapidly modified by erosion and plate tectonics. Furthermore, the mantle convection on Earth seems to produce plate tectonics rather than coronae. Coronae appear to be produced by rising currents of molten rock below the crust. Where the magma breaks through, it builds volcanoes and floods the surface with lava flows. The surface contains about 10 percent as many im-pact craters as the lunar maria, so planetary scientists sus-pect the entire planet has been resurfaced by volcanism within the last billion years. No magnetic field has been detected around Venus, and that suggests that the core is not molten iron generating a magnetic field through the dynamo effect as in Earth's core. The occasional resurfacing of the planet by volcanism may have cooled the interior so it is now solid. Another mystery is the retrograde rotation, which may have been produced by tidal effects or by the off-center impact of a large plan-etesimal during the formation of the planet. 5. What evidence do we have that Venus and Mars once come from, and where did it go? Venus's dense atmosphere is made up almost entirely of a prime greenhouse gas, carbon dioxide. Venus is hot because of the greenhouse effect. Venus extremely high temperatures and are examples of a runaway greenhouse effect. Venus, like Earth and Mars, developed its current atmosphere (the second one) as a result of outgassing or gases coming from the planet itself. Volcanoes are the primary source of this atmosphere, and the gases produced include CO 2 and sulfur compounds. Venus' atmosphere is in fact about 95% CO 2 , and is many times more dense than Earth's. Mars is smaller than Earth and Venus but larger than Mercury. Its southern hemisphere is old and cratered; the northern hemisphere has been resurfaced by lava flows or water. The lack of folded mountains and the vast size of the volcanoes suggest that plate motion does not occur. Mars once had liquid water on its surface, producing riverlike runoff channels and later floodlike outflow chan-nels. The atmosphere must have been thicker in the past to allow liquid water, but the climate changed as gas leaked into space and water froze in permafrost. Layered terrain near the polar caps shows the climate varies in cycles, much of the water may be lost forever. Deimos and Phobos, the two small moons of Mars, may be captured asteroids. Like most small moons, they are irregular in shape and heavily cratered. Tides have locked them to Mars so they keep the same side facing the planet as they follow their orbits. 6. How do we know there is no plate tectonics on Venus? How has the crust of Venus evolved? There aren't really any small craters on Venus less than 3 km across, probably because the atmosphere is so incredibly dense that anything small would burn up on the way down. Apparently absent on Venus is any evidence of plate tectonics. 7. Why doesn't Mars have mountain ranges like those on Earth? Why doesn't Earth have large volcanoes like those on Mars? Mars, however, is a smaller world and must have cooled faster. We see no evidence of plate tectonics, and we do see giant volcanoes that suggest rising plumes of magma erupting up through the crust at the same point over and over. Perhaps we see no coronae on Mars be-cause the crust of Mars rapidly grew too thick to deform easily over a rising plume. On the other hand, perhaps we could think of the entire Tharsis bulge as a single giant corona. Mars is the farthest from the sun. How has that affected the evolution of its atmosphere? Venus seems totally inhospitable, but humans may someday visit Mercury and will probably colonize Mars. 9. Deimos is about 12 km in diameter and has a density of 2 g/cm3. What is its mass? (Hint: The volume of a sphere is 3nr3.) 8. Explain why Venus and Mars both have carbon dioxide-rich atmospheres. How did Earth avoid such a fate? The main reasons are water and life. CO 2 dissolves very easily in the liquid water covering the Earth. Water can help rocks absorb CO 2 and make carbonates. when the plants and animals die, they return nitrogen to the air. Venus was probably too hot for liquid water. The CO 2 and water vapor in the air act to raise the temperature even more through the greenhouse effect and any trapped CO 2 would be released to add to the problem. Venus's dense atmosphere is made up almost entirely of a prime greenhouse gas, carbon dioxide. Venus is hot because of the greenhouse effect. Venus extremely high temperatures and are examples of a runaway greenhouse effect. 9. What evidence do we have that the climate on Mars has changed? The surface of Mars actually shows evidence of huge floods. Although there is apparently no liquid water on Mars now, it is believed that the Martian landscape has water trapped in the form of permafrost. Over long timescales, the climate may change on Mars just as it seems to do on Earth ice ages. When the temperature is high enough, the water can be freed from the permafrost and/or cause landslides in it. Other surface features include volcanoes The volcanoes on Mars are dead now, but how did they get so big? First, plate motion was nonexistent, so upwellings Also, the surface gravity on Mars is about 1/3 of Earth's, so it seems reasonable that volcanoes could grow 3 times larger there, as the force of gravity is what would be expected to drag them back down to lower levels. The current climate on Mars provides a look at the greenhouse effect moving in the other direction. First, due to the smaller size therefore more rapid cooling, Mars "died" geologically long ago. Volcanic eruptions are the likely source for the CO 2 in the atmospheres of the terrestrial planets (except Mercury, of course), so a shorter period of volcanic activity translates into a thinner atmosphere. Mars' low gravity gives a low escape velocity for gases, means Mars' atmosphere is only about 1% as dense as Earth's. This lower amount of CO 2 was much less efficient at trapping heat (and since Mars is both smaller than and twice as far from the Sun as Venus, it receives well under ¼ as much energy from the Sun anyway). Temperatures on Mars would be lower, and that would tend to take water vapor out of the air - we're all familiar with how dry the air is in winter compared to summer. A s the water vapor comes out of the air, the temperature drops even more. The greenhouse effect & runs away in the other direction, with the overall result being that the water on Mars is now apparently frozen into ice. Mars extremely low temperatures are examples of a runaway greenhouse effect. Mar's small size means that any internal heat would have been able to escape more easily than in Venus's dense atmosphere is made up almost entirely of a prime greenhouse gas, carbon dioxide. Venus is hot because of the greenhouse effect. a larger planet like Earth or Venus. It is too cold on Mars. The magnetic field is to weak. Water once flowed on Mars as the evidence is Two types of flow features are seen: runoff channels, and outflow channels. There is no evidence for liquid water anywhere on Mars today. 10. How can we estimate the ages of the erosional features on Mars? How did they form? Radioactive dating of rocks tell the age. cratered surfaces appear on earth, and mars due to asteroid impacts, meteors impacts. Mars Seasonal changes of Martian markings, climatic changes; seasonal winds and summer dust storms. Surface features huge volcanoes, riverbeds, and channels. Evidence of past liquid water. Eroded craters. Very little, if any, plate tectonics. The Tharsis region and Valles Marineris show sign of surface cracking which may be associated with pressures on the crust from below, but no sliding motion as in plate tectonics. 11. Why are Phobos and Deimos nonspherical? Why is Earth's moon not quite spherical? Jon Quitsdons 1. From your knowledge of comparative planetology, de-scribe the view astronauts would have if they landed on the surface of Mercury. 2. From what you know of Earth, Venus, and Mars, do you expect the volcanoes on Venus and on Mars to be active or extinct? Why? 3. If humans colonize Mars, the biggest problem may be finding water and oxygen. With plenty of solar energy beating down through the thin atmosphere, how might colonists extract water and oxygen from the Martian environment? Critical Inquiries for the Web 1. Who decides how planetary features are named? Sur-face features on Venus are (mostly) named after female figures from history and mythology, while figures from the arts and music are used to name features on Mer-cury. Look for information on planetary nomenclature and summarize the way different types of features on Venus are assigned names. 2. "Martians" have fascinated humans for the last century or more. There are many online sources that chronicle the representation of life on Mars throughout history and in literature. Read about the Martians as represented by a particular literary work or nonfiction account and dis-cuss to what extent it is (or is not) based on realistic views of the nature of Mars both in terms of our current understanding and the views of that period. 3. Spacecraft are exploring Mars right now. Search for the latest discoveries and photographs. What spacecraft are on their way to Mars, about to be launched, or being planned right now? 4. What plans are being made to send another spacecraft to Mercury? Problems Exploring TheSlry 1. If we transmitted radio signals to Mercury when it was closest to Earth and waited to hear the radar echo, how long would we wait? 2. Calculate the orbital velocity of Mercury in its orbit around the sun in kilometers per second. 3. Suppose we sent a spacecraft to land on Mercury and it transmitted radio signals to us at a wavelength of 10 cm. If we observed Mercury at its greatest angular distance west of the sun, to what wavelength would we have to tune our radio telescope to detect the signals? (Hint: Use the Doppler effect.) 4. What is the maximum angular diameter of Venus as seen from Earth? (Hint: Use the small-angle formula.) 5. How long did it take for radio signals from the Magel-lan spacecraft orbiting Venus to reach Earth? 6. If you send a radio signal down toward the surface of Venus and you hear an echo from a certain spot Part 2 The Solar System 1. Mercury, Venus, and Mars exhibit phases similar to the moon's phases. a. Use TheSky to sketch the appearance of Mercury, Venus, and Mars at the present time. Also give the phase as a percentage of full. How to proceed: Use Find to give you a highly mag-nified view of a particular planet. Click on the planet to obtain Object Information. b. Explain the observed phase on the basis of geometri-cal relationship between the sun, Earth, and the planet as shown by the 3D Solar System Mode. Bo to the Brooks/Cole AslrouomY Resource Ceuur •www.brooYSCOIe. comlastronomyl for critical thinking exercises. articles, and addi-tional readings from IdoTrac Collego EdIUon, Brooks/Cole's online student IibrarY.  New ®e;;-.,s 2. Why don't the terrestrial planets have rings? If you were to search for a ring among the terrestrial planets, where liquid metallic hydrogen grooved terrain would you look first? 1. What is the maximum angular diameter of Jupiter as gossamer rings ovoid seen from Earth? Repeat this calculation for Saturn and Pluto. (Hints: See Data Files Six, Seven, and Ten and also By the Numbers 1-2.) 2. What is the angular diameter of Jupiter as seen from Callisto? from lo? (Hint: See By the Numbers 1-2.) 1. Why is Jupiter so much richer in hydrogen and helium than Earth? The abundance of hydrogen and helium on these worlds is itself a consequence of the strong jovian gravity. The jovian planets are massive enough to have retained even the lightest gas, hydrogen, and very little of their original stmospheres have escaped since the birth of the solar system 4.6 billion years ago. We do not expect the hydrogen and helium in the interiors of Uranus and Neptune to be compressed as much as in the two larger jovian worlds, yet the average densities of Uranus and Neptune are actually greater than the desity of Saturn, and similar to that of Jupiter. Jupiter is bigger than Earth and Jupiter is farther away from the Star. 3. Measure the photograph in Data File Seven and calcu-late the oblateness of Saturn. 2. How can Jupiter have a liquid interior and not have a liquid surface? the unique temperature and pressure conditions sustain a core whose density is more like liquid or slush. liquid metallic hydrogen is a strange matrix capable of conducting huge electrical currents. The persistent radio noise and wildly strong magnetic field of Jupiter could both come from this layer of metallic liquid. the inner layers of hydrogen in Jupiter's atmosphere, under the pressure of the atmosphere above, may have formed into a layer of what is called liquid metallic hydrogen. Jupiter's massive atmosphere creates tremendous pressures as you move closer to the center of the planet. Jupiter is one of the "gas giants" the others are Saturn, Uranus, and Neptune. The gas planets do not have solid surfaces, their gaseous material simply gets denser with depth 3. How does the dynamo effect account for the magnetic fields of Jupiter and Saturn? All four jovian worlds have strong magnetic fields and emit radiation at radio wavelengths. The combination of rapid overall rotation and an extensive region of highly conductive fluid in its interior gives Jupiter by far the strongest planetary magnetic field in the solar system. 4. Why are the belts and zones on Saturn less distinct than those on Jupiter? similar to Jupiter with belts and zones. However, the features are a lot fainter and appear less distinct. Saturn has no feature like the Great Red Spot on Jupiter. 5. Why do we conclude that neither Jupiter's ring nor Saturn's rings can be left over from the formation of the planets? Jupiter may had been capturing a lot of comets from the Oort Cloud to form the rings. asteroids are one of thousands of very small members of the solar system orbiting the Sun between the orbits of Mars and Jupiter. 6. How can a moon produce a gap in a planetary ring system? Kirkwood gaps Gaps in the spacings of semi-major axes of orbits of asteroids in the asteroid belt, produced by dynamical resonances with nearby planets, Jupiter. 7. Explain how geological activity on Jupiter's moons varies with distance from the planet. 4. If we observe light reflected from Saturn's rings, we should see a red shift at one edge of the rings and a blue shift at the other edge. If we observe a spectral line and see a difference in wavelength of 0.056 nm, and the unshifted wavelength (observed in the laboratory) is 500 nm, what is the orbital velocity of particles at the outer edge of the rings? (Hint: See By the Numbers 4-2.) 5. One way to recognize a distant planet is by its motion along its orbit. If Uranus circles the sun in 84 years, how many seconds of arc will it move in 24 hours? (Hint: Ig-nore the motion of Earth.) 6. If the ^ ring is 50 km wide and the orbital velocity of Uranus is 6.81 km/s, how long a blink should we ex-pect to see when the ring crosses in front of the star? 8. What makes Saturn's F ring and the rings of Uranus and Neptune so narrow? rings of Saturn are not solid (like the brim of a hat) made of millions of highly reflective, water-ice moonlets which orbit Saturn under gravity since they orbit under gravity they obey Kepler's Laws of planetary motion e.g. the further a particle is from the planet the longer its takes to orbit ring particle sizes range from sand grains to boulders with the average size being that of a typical snowball Axis tilt Saturn has a large axis tilt of 27° and so as the planet orbits the Sun we see different views of rings - sometime we see above the ring plane, sometimes below the ring plane and every so often we see the rings edge-on. We we observe the rings edge on they seem to disappear which suggests they must be much thinner (< 2 km) than they are wide (> 200,000 km) like an LP record 7. What is the angular diameter of Pluto as seen from the surface of Charon? (Hint: See Figure 8-20.) 9. Why is the atmospheric activity of Uranus less than that g, If Pluto has a surface temperature of 50 K, at what wave- length will it radiate the most energy? (Hint: See By the 10. Numbers 4-1.) active more recently than some other moons? Why do we suspect that Enceladus has been geologically of Saturn and Neptune? 9. How long did it take radio commands to travel from Earth to Voyager 2 as it passed Neptune? 11. What are the seasons on Uranus like? Uranus' Extreme Seasons The rotation axis of Uranus is tilted ~98º Uranus is lying on its side in its orbital plane. Get extreme seasonal variations driven by uneven heating between hemispheres: Such extreme seasonal variations may help account for Uranus' lack of weather the strong hemispheric difference may interfere with the processes in bands and zones that are seen on the other Jovian planets. Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus and Neptune are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed. Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane. 12. Why are Uranus and Neptune blue? Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer. Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint. 10. Use the orbital radius and orbital period of Charon to calculate the mass of the Pluto-Charon system. (Hints: 13. What evidence do we have that Triton has been geolog- ically active recently? Express the orbital radius in meters and the period in 14. How do astronomers account for the origin of Pluto? seconds. Then see By the Numbers 2-1.) 15. What evidence do we have that catastrophic impacts have occurred in the solar system's past? Asteroids and comets and influences of gravity 1. Some astronomers argue that Jupiter and Saturn are un-usual, while other astronomers argue that all solar sys-tems should contain one or two such giant planets. What do you think? Support your argument with evidence. 1. If you lived on the surface of Pluto and looked into the sky to observe Charon, what phases would you see? (Hint: Be sure to consider your location on the planet when answering this question.) 2. What factors caused Voyager 2 to see a bland atmosphere when it encountered Uranus in 1986? Given these cir-cumstances, would images of the Uranian atmosphere taken by a space probe arriving at Uranus in 2006 be sim-ilar to those taken in 1986, or would there be significant differences? 204 Part 2 The Solar System 3. Imagine what you'd think if you had been the first per- Exploring TheSky son ever to see Saturn through a telescope. When Gali- leo first observed Saturn in 1610, he did not recognize 1. that it was a ringed planet. It was many years later be- fore the strange apparition of Saturn was finally attrib- uted to a ring structure. Search the Web for information on historical observations of Saturn, summarize the ob- servations of Galileo and others, and determine who was first to recognize what he saw as a ring. Why do you sup- planet? Zoom in on Jupiter and observe the orbital motion of its moons. (Hint: Under Tools, choose Time Skip and then Tracking Setup. Lock on Jupiter. Then use the Time Skip buttons to make the moons move around their orbits.) 2. Calculate the mass of Jupiter from your own observa- tions of the orbital period and orbital radius of Jupiter's moons. pose it took so long to understand that Saturn is a ringed 3. Repeat Activities 1 and 2 above for Saturn. 4. Should Pluto be called a planet or not? Search the Web for news and debate on this issue. What is your opinion? 5. Who was Clyde Tombaugh? What can you find out about his life after he discovered Pluto? Chapter 8 Worlds of the Outer Solar System 205 13. What evidence do we have that cometary nuclei are rich in ices? Comets are frozen while travelling through the cold outerspace. 10. What evidence do we have that some asteroids have had active surfaces? Surface features 11. How is the composition of meteorites related to the for- mation and evolution of asteroids? to 100 kilometers in diameter. In a long, elliptical orbit, the 12 . What is the difference between a type I tail and a type II tail? 14. Why do short-period comets tend to have orbits near the plane of the solar system? 15. How did the bodies in the Kuiper belt and the Oort cloud form? between Mars and Jupiter. The strong gravitational influence of Jupiter could have prevented the material from accumu- lating into a large body. A comet is produced by a lump of dirty ices a few dozen icy body stays frozen until the object draws close to the sun. Then the ices vaporize and release the embedded dust and debris. The gas is caught in the solar wind and blown out- ward to form a type I, or gas, tail. The pressure of sunlight blows the dust away to form a type II, or dust, tail. The coma of a comet can be up to 1,000,000 km in diameter, and it con- tains jets issuing from the nucleus. When spacecraft flew past Comet Halley in 1986, as- tronomers discovered that the nucleus was coated by a dark crust and that jets of vapor and dust were venting from active regions on the sunlit side. The low density of the nucleus showed that it was an irregular mixture of ices and silicates probably containing large voids. Comets are believed to have formed as icy planetesimals in the outer solar system, and some of these objects remain comets. the Jovian planets, and others were ejected to form the Oort loud, the source of the long-period comets. 1. Futurists suggest we may someday mine the asteroids c the formation of kinds of materials could we get from asteroids? (Hint: as the Kuiper belt, the source of some short-period for materials to build and supply space colonies. What What are S-, M-, and C-type asteroids made of?) Many icy planetesimals were swept up by 2 ~ If cometary nuclei were heated by internal radioactive decay rather than by solar heat, how would comets dif- fer from what we observe? Ads Widmanstatten pattern meteor shower chondrite type I, or gas, tail chondrule type II, or dust, tail carbonaceous chondrite coma achondrite Oort cloud stony-iron meteorite Kuiper belt 3. From what you know now, do you think the government should spend money to locate near-Earth asteroids? How serious is the risk? Yes. Since it is possible the near Earth asteroids are out there undetected for now. Problems 1. Large meteorites are hardly slowed by Earth's atmosphere. Assuming the atmosphere is 100 km thick and that a large meteorite falls perpendicular to the surface, how long does it take to reach the ground? (Hint: About how fast do meteoroids travel?) 2. What is the orbital velocity of a meteoroid whose aver-age distance from the sun is 2 AU? (Hint: Find the or-bital period from Kepler's third law. See Table 2-1.) 1. What do Widmanstatten patterns tell us about the history of iron meteorites? 3.If a single asteroid 1 km in diameter were fragmented into meteoroids 1 m in diameter, how many would it yield? (Hint: Volume of sphere = 37cr3.) 2. What do chondrules tell us about the history of chon- drites? Chondrule: Roughly spherical objects found in a type of meteorite called chondrites. Most chondrules are 0.5 to 2 millimeters in size and are composed of olivine and pyroxene, with smaller amounts of glass and iron-nickel metal. The shapes of the mineral grains in them indicate that chondrules were once molten droplets floating freely in space. Chondrites are meteorites that contain rounded objects called chondrules that cooled very rapidly from a molten state. For a long time most scientists thought chondrules formed directly in the solar nebula--the cloud of gas and dust surrounding the primitive Sun. However, chemical and mineralogical properties of chondrules and experiments designed to reproduce the mineral intergrowths in chondrules showed that they could not possibly have condensed from a gas. The condensation idea gave way in the 1980s to the hypothesis that chondrules formed from small aggregations of dust (like those fluffy dust balls that accumulate under your bed) that were melted by some mysterious process in the solar nebula. meteoriticists concluded that chondrules were secondary products. Three chondrites found in Antarctica ALH 85085 and QUE 94411 and the Sahara Hammadah al Hamra 237 are changing that view. Investigators in the U. S. and Europe may have found direct condensates from the solar nebula in those meteorites. Chondrules and grains of metallic iron-nickel chondrules tell the story of heat and wind in the solar nebula. The chemical compositions of the chondrules indicate formation from a cloud that had become enriched in dust before being completely evaporated. When the gas cloud cooled, the tiny droplets condensed, but were blown into much cooler regions far from the Sun before they had a chance to acquire moderately volatile elements such as sodium, potassium, and sulfur. They appear to have accreted into asteroids before other processes affected them, thus preserving the record of heating and jetting in the nebula that surrounded the infant Sun. chance of finding pieces of rock from space that tell stories of creation of planets. The most common meteorites, known as ordinary chondrites, are composed of small grains of rock and appear to be relatively unchanged since the solar system formed. Stony-iron meteorites, on the other hand, appear to be remnants of larger bodies that were once melted so that the heavier metals and lighter rocks separated into different layers. 3. Why are there no chondrules in achondritic meteorites? 4. Why do astronomers refer to carbonaceous chondrites as "unmodified"? About three-quarters of asteroids are extremely dark and are similar to carbon-rich meteorites called carbonaceous chondrites (C-type). About one-sixth of asteroids are reddish, stony-iron bodies (S-type). 5. How do observations of meteor showers reveal one of the sources of meteoroids? meteor is a bright streak in the sky, often referred to as a shooting star, resulting from a small piece of interplanetary debris entering Earth's atmosphere and heating air molecules, which emit light as they return to their ground states. meteorite Any part of a meteoroid that survives passage through the atmosphere and lands on the surface of Earth. 6. How can most meteors be cometary if all meteorites are asteroidal? are made of dust particles and ice. 7. Why do we think the asteroids were never part of a asteroids revolve around the Sun in elliptical orbits, giant Jupiter’s gravity and occasional close encounters with Mars or with another asteroid change the asteroids’ orbits, knocking them out of the Main Belt and hurling them into space across the orbits of the planets. Most of these fragments of ancient space rubble sometimes referred to by scientists as minor planets can be found orbiting the Sun in a belt between Mars and Jupiter. This region in our solar system, called the Asteroid Belt or Main Belt, probably contains millions of asteroids ranging widely in size from Ceres, which at 940 km in diameter is about one-quarter the diameter of our Moon, to bodies that are less than 1 km across. There are more than 20,000 numbered asteroids. 6. What is the maximum angular diameter of Ceres as seen from Earth? Could Earth-based telescopes detect sur-face features? Could the Hubble Space Telescope? (Hint: See By the Numbers 1-2.) planet? 7. If the velocity of the solar wind is about 400 km/s and the visible tail of a comet is 10' km long, how long does 8. What evidence do we have that the asteroids are fragmented? By looking at the concentrations of lighter elements in the outer meter or so of the meteorite (that's about as far as the cosmic rays can penetrate), the time since the meteorite was broken off of a larger body can be found. These ages are generally in the tens of thousands to millions of years. Since we believe the Solar System was essentially finished forming billions of years ago, this would suggest that collisions between asteroids are an important source of meteorites. The surface details on asteroids Asteroids may had been in collisions with other asteroids. it take an atom to travel from the nucleus to the end of 9. What evidence do we have that some asteroids have the visible tail? differentiated? Kirkwood gaps Gaps in the spacings of semi-major axes of orbits of asteroids in the asteroid belt, produced by dynamical resonances with nearby planets, especially Jupiter. 8. If you saw Comet Halley when it was 0.7 AU from Earth and it had a visible tail 5° long, how long was the tail 4. What is the orbital period of a typical asteroid? (Hint: Use Kepler's third law. See Table 2-1.) 5. If half a million asteroids each 1 km in diameter were assembled into one body, how large would it be? (Hint: Volume of sphere = 3~r3.) Chapter 9 Meteorites, Asteroids, and Comets 223 3. Search for the IAU Minor Planet Center Web pages, and find out what asteroids and what comets have come clos est to Earth. 4. Search for information about comets in the sky right now. Are any comets bright enough for you to see? Are Kepler's third law. The circumference of a circular or- any bright comets expected soon? bit = 21cr.) in kilometers? Suppose that the tail was not perpendic- ular to your line of sight. Is your answer too large or too - small? (Hint: See By the Numbers 1-2.) 9. What is the orbital period of a cometary nucleus in the Oort cloud? What is its orbital velocity? (Hints: Use 10. The mass of an average comet's nucleus is about 10'z kg. If the Oort cloud contains 200 x 109 cometary nuclei, Exploring The Sky what is the mass of the cloud in Earth masses? (Hint: Mass of Earth = 6 x 1024 kg.) 1. Of the five brightest asteroids, which has the most in-clined orbit? (Hint: Use Filters in the View menu to turn off everything but the sun, stars, ecliptic, and minor planets. You can use Tracking Setup under Time Skip in the Tools menu to lock onto an object and follow it 1. Some nights are better for looking for meteors than oth- along the ecliptic as time passes.) ers (see Table 9-1). We know these showers are associ- ated with comets, but how are these associations made? There are several sites on the Internet that provide in- formation on meteor showers, including historical data and information on parent comets. Pick a shower whose parent comet is known, and summarize how we came 4. Of the comets shown, which has the smallest orbit? 2. Do asteroids go through retrograde motion? (Hint: Use Filters in the View menu to turn off the stars and turn,Qn the Equatorial Grid. See Activity 1 above.) g, Of the five brightest asteroids, which has the most el- liptical orbit? (Hint: Use 3D Solar System Mode in the View menu and watch objects orbit the sun.) to know that the meteors and the comet are related. (Hint: Use 3D Solar System Mode in the View menu and watch objects orbit the sun.) 2. The chances are small that you will be killed by an aster-oid impact; but, if there are objects out there that astron-omers are not aware of whose orbits intersect Earth, we could be in for a surprise one day. Look for information on the LONEOS project and other investigations into near-Earth asteroids. How many such objects have been discovered? What is the record for closest known pas-sage of an asteroid to Earth? 224 Part 2 The Solar System Ne-W Terms sunspot granulation convection filtergram spicule supergranule magnetic carpet helioseismology Maunder butterfly diagram Maunder minimum Zeeman effect active region differential rotation Babcock model Review Ouestions prominence filament flare reconnection aurora (plural aurorae) coronal hole coronal mass ejection (CME) weak force strong force nuclear fission nuclear fusion Coulomb barrier proton-proton chain neutrino 2. What would the spectrum of an auroral display look like? Why? 3. What observations would you make if you were ordered to set up a system that could warn astronauts in orbit of dangerous solar flares? Such a system exists. 1. The radius of the sun is 0.7 million km. What percent-age of the radius is taken up by the chromosphere? 2. The smallest detail visible with ground-based solar tele-scopes is about 1 second of arc. How large a region does this represent on the sun? (Hint: Use the small-angle formula.) 3. What is the angular diameter of a star like the sun lo-cated 5 ly from Earth? Is the Hubble Space Telescope able to resolve detail on the surface of such a star? 4. If a sunspot has a temperature of 4200 K and the solar surface has a temperature of 5800 K, how many times brighter is the surface compared with the sunspot? (Hint: Use the Stefan-Boltzmann law, By the Numbers 4-1.) 1. Why can't we see deeper than the photosphere? 2. What evidence do we have that granulation is caused by convection? convection Churning motion resulting from the constant upwelling of warm fluid and the concurrent downward flow of cooler material to take its place. Convection Zone Of the three means of transporting radiation convective, radiative, and conductive convective transport is the most efficient. When there is a large temperature gradient meaning when the temperature difference between the bottom of a layer and the top of the layer is very great it takes convective action to transport the hot gas to the next layer for cooling. We are all familiar with convection when we boil water in a pan. Stars like the Sun have a convective layer near their surfaces. Radiation transport and convection are both important in stars like the Sun. Radiation transport occurs in the inner part of the Sun and convection occurs in the outer part. 3. How are granules and supergranules related? How do they differ? 5. A solar flare can release 1025 J. How many megatons of TNT would be equivalent? (Hint: A 1-megaton bomb produces about 4 X 10's J.) 6. The United States consumes about 2.5 X 10'9 J of en-ergy in all forms in a year. How many years could we run the United States on the energy released by the solar flare in Problem 5? 4. How can a filtergram reveal structure in the chromo-sphere? 7. Neglecting energy absorbed or reflected by our atmo-sphere, the solar energy hitting 1 square meter of Earth's surface is 1360 J/s (the solar constant). How long does 6. What heats the chromosphere and corona to high tem- perature? it take a baseball diamond (90 ft on a side) to receive 1 megaton of solar energy? (Hint: See Problem 5.) 5. What evidence do we have that the corona has a very high temperature? where the temperature in- creases rapidly from 104 K to 106 K; and the corona, with temperatures of a few million degrees Kelvin. Solar wind particles stream out into the solar system through coronal 7. How are astronomers able to explore the layers of the sun below the photosphere? We would have to do spectroscopy with a telescope, that can detect ultraviolet radiation, and also one that needs to orbit above the Earth's atmosphere. 8. What evidence do we have that sunspots are magnetic? Probably the most obvious and famous features of the Sun are its sunspots. These dark, cooler spots on the Sun represent regions where the Sun's magnetic field lines are popping out. Sunspots usually come in pairs, with one of the pair representing the north pole of a magnet and the other, the south pole. These sunspots rise and fall in number and size as the solar cycle increases and decreases. the number of sunspots gradually rises and falls in a sunspot cycle with a period of about 11 years the entire magnetic field of the Sun flip-flops every 11 years. These magnetic reversals hint that the sunspot cycle is related to the generation of magnetic fields on the Sun. They also tell us that the complete magnetic cycle of the Sun, called the solar cycle, really averages 22 years, since it takes two 11-year cycles before the magnetic field is back the way it started. 9. How does the Babcock model explain the sunspot cycle? problem differ from Problem 8?) Probably the most obvious and famous features of the Sun are its sunspots. These dark, cooler spots on the Sun represent regions where the Sun's magnetic field lines are popping out. Sunspots usually come in pairs, with one of the pair representing the north pole of a magnet and the other, the south pole. These sunspots rise and fall in number and size as the solar cycle increases and decreases. 10. What does the spectrum of a prominence tell us? What does its shape tell us? The Spectrum Prominences can tell us about how hot the temperature of the gas is. and what kind of gas it is. astronomers noted an unusual emission line while observing a prominence. This line had not been seen on Earth, and could not be reproduced, at that time, in the laboratory. A British civil servant, Norman Lockyer, suggested that it was a new element. The element was, of course, helium, for helios meaning Sun. The element was later found on Earth in 1895. A strange tale of discovery for the second most common element in the Universe. Prominences are huge arches of gas, extending from the active region at the photosphere level high into the chromosphere. The plasma hot, ionized gas is trapped along the magnetic field lines extending from the Sun's surface. Prominences are eruptions of ionized gas flowing along magnetic field lines near sunspots. Hot gas boils out of the Sun's surface. Cools, and falls back down. A prominence may last several weeks. 8. How much energy is produced when the sun converts 1 kg of mass into energy? 9. How much energy is produced when the sun converts 1 kg of hydrogen into helium? (Hint: How does this 10. A 1-megaton nuclear weapon produces about 4 x 10's J of energy. How much mass must vanish when a 5-megaton 11. How can solar flares affect Earth? The solar flares can disrupt radio transmissions, weapon explodes? 12. Why does nuclear fusion require high temperatures? 13. Why does nuclear fusion in the sun occur only near the center? As an object heats up, the thermal energy is transferred to the particles. The particles absorb this energy and convert it to kinetic energy. The more heat that is applied, the faster the particles move. One of the laws of thermal dynamics is that heat flows from hot objects to cold objects. This law explains why your bath water gets cold, and it also explains why huge clouds of gas and dust that will someday form stars cool and contract. 14. How can astronomers detect neutrinos from the sun? the most persistent Neutrinos, whose activities can be measured and photographed as they smack into water-filled receptacles. The lab is the equivalent of a 10-story building constructed 6,800 feet below the Earth's surface. It contains a spherical tank 12 meters in diameter filled with 1,000 metric tons of heavy water water composed of heavy isotopes of hydrogen. The tank is monitored by about 10,000 light sensors. 15. How can neutrino oscillation explain the solar neutrino problem? Making Waves An oscillation in a pool of water creates an expanding disturbance called a wave found that it is equal to the speed of light, which had been measured experimentally. On that basis, he speculated that light was one form of a family of possible electric and mag-netic disturbances called electromagnetic radiation, a conclusion that was again confirmed in laboratory experi-ments. When light (reflected from the pages of an astron-omy textbook, for example) enters a human eye, its changing electric and magnetic fields stimulate nerve end-ings, which then transmit the information contained in these changing fields to the brain. The word radiation will be used frequently in this book, so it is important to understand what it means. In everyday language, radiation is often used to describe cer tain kinds of energetic subatomic particles released by ra-dioactive materials in our environment. (An example is the kind of radiation used to treat cancer.) But this is not what we mean when we speak of radiation in an astronomy text. Radiation, as used in this book, is a general term for light, x rays, and other forms of electromagnetic waves. This ra-diation provides almost our only link with the universe be-yond our own solar system. The Wave-Like Characteristics of Light The changing electric and magnetic fields in radiation are similar to the waves that can be set up on a quiet pool of water. In both cases the disturbance travels rapidly outward from the point of origin and can use its en-ergy to disturb other things farther away. (For example, in the case of water, the expanding ripples moving away from our twitching frog could disturb the peace of a grasshopper sleeping on a leaf in the same pool.) In the case of electro- magnetic waves, the radiation generated by a transmitting antenna full of charged particles at your local radio sta-tion can, sometime later, disturb a group of electrons in your home radio antenna and bring you the news and weather while you are getting ready for class or work in the morning. all electromagnetic waves move at the same speed (the speed of light), which turns out to be the fastest possible speed in the universe. No matter where electromagnetic waves are generated and no matter what other properties they have, when they are moving (and not interacting with matter), they move at the speed of light. Yet you know from ever vday experience that waves like light are not all the same. For example, we per-ceive that light waves differ from one another in a property we call color. 1. What energy sources on Earth cannot be thought of as ( stored sunlight? 1. Do disturbances in one layer of the solar atmosphere pro-duce effects in other layers? We have seen that filter-grams are useful in identifying the layers of the solar atmosphere and the structures within them. Visit a Web site that provides daily solar images, choose today's date (or one near it), and examine the sun in several wave-lengths to explore the relation between disturbances in various layers. Chapter 10 The Sun-Our Star 245 1. Locate the six photos of the sun provided in TheSky and attempt to draw in the sun's equator in each photo. (Hint: In the sun's information box, choose More Infor- mation and then Multimedia. What features are visible in these images that help us recognize the orientation of the sun's equator?) 2. Explore the Web to find out how auroral activity is af- Exploring MAY fected as solar activity rises and falls through the solar cycle. What changes in auroral visibility occur during this cycle? In what other ways can the increased activ- ity associated with a solar maximum affect Earth? 3. Explore the Web to find photos and observations of au- rorae. From what places on Earth are aurorae most often seen? 4. What can you find on the Web about Earth-based efforts to generate energy through nuclear fusion? How do nu-clear fusion power experiments attempt to trigger and control nuclear fusion? So-called "cold fusion" has been largely abandoned as a false trail. How did it resemble nuclear fusion? 6o to the BrooYS/Colo fistrouory Resource Center (imam.hreeksceig. eor/sstrogoryl for critical thinking exercises, articles, aud addi-tional readings from InfoTrse CWqe Edition, Brooks/Cop's inllus student library. 246 Part 3 The Stars low-mass things they are hard to see even if they are near us in space. Why does nature make stars in this peculiar way? To answer that question, we must ex-plore the birth, life, and death of stars. We begin that quest in the next chapter. follow this rule closely, the most massive being the upper-main-sequence stars and the least massive the lower-main-sequence stars. Giants and supergiants do not follow the re-lation precisely, and white dwarfs not at all. A survey in the neighborhood of the sun shows us that lower-main-sequence stars are the most common type. The hot stars of the upper main sequence are very rare. Giants v and supergiants are also rare, but white dwarfs are quite common, although they are faint and hard to find. i Our goal in this chapter was to characterize the stars by find-ing their luminosities, temperatures, diameters, and masses. Before we could begin, we needed to find the distances to stars. Only by first knowing the distance could we find the other properties of the stars. We can measure the distance to the nearer stars by ob-serving their parallaxes. The more distant stars are so far away that their parallaxes are unmeasurably small. To find the distances to these stars, we must use spectroscopic par-allax. Stellar distances are commonly expressed in parsecs. One parsec is 206,265 AU-the distance to an imaginary star whose parallax is 1 second of arc. Once we know the distance to a star, we can find its in-trinsic brightness, expressed as its absolute magnitude or its luminosity. A star's absolute magnitude is the apparent mag nitude we would see if the star were only 10 pc away. The luminosity of the star is the total energy radiated in 1 sec-ond, usually expressed in terms of the luminosity of the sun. We can find the temperatures of stars by studying their spectra to see which atoms produce the strongest spectral lines. To simplify the task, we classify stars in a spectral se quence running from the O stars, which are hot, to the M stars, which are cool. The H-R diagram plots stars according to their intrinsic 1. Why are Earth-based parallax measurements limited to brightness and their surface temperature. In the diagram, roughly 90 percent of all stars fall on the main sequence, the more massive being hotter, larger, and more luminous. The giants and supergiants, however, are much larger and lie above the main sequence. They are more luminous than main-sequence stars of the same temperature. Some of the white dwarfs are hot stars, they fall below the main se quence because they are so small. The large size of the giants and supergiants means their atmospheres have low densities and their spectra have sharper spectral lines than the spectra of main-sequence classes by the widths of their spectral lines. Class V stars are main-sequence stars with broad spectral lines. Giant stars (III) have sharper lines, and supergiants (I) have extremely sharp spectral lines. 4. What does luminosity measure that is different from what absolute visual magnitude measures? 5. Why does the luminosity of a star depend on both its radius and its temperature? The luminosity is beng very dim the temperature is very cool. The luminosity is being very bright the temperature is very hot or warm. stars. In fact, it is possible to assign stars to luminosity. 6. How can we be sure that giant stars really are larger than main-sequence stars? 7. Why do we conclude that white dwarfs must be very small? 8. What observations would we make to classify a star ac- cording to its luminosity? Why does that method work? 9. Why does the orbital period of a binary star depend on its mass? Given the mass and diameter of a star, we can find its average density. 10. What observations would you make to study an eclips- ing binary star? The only direct way we can find the mass of a star is by studying binary stars. When two stars orbit a common cen- ter of mass, we can find their masses by observing the pe- riod and sizes of their orbits. On the main sequence, the stars are about as dense as the sun, but the giants and supergiants are very- stellar parallax (p) main sequence parsec (pc) giant star flux supergiant star absolute visual magnitude white dwarf star ) luminosity class spectroscopic parallax binary stars visual binary system spectroscopic binary system eclipsing binary system light curve luminosity (L) distance modulus spectral class or type spectral sequence L dwarf T dwarf Hertzsprung-Russell (H-R) mass-luminosity relation diagram the nearest stars? 2. Why was the Hipparcos satellite able to make more ac-curate parallax measurements than are ground-based telescopes? 3. What do the words absolute and visual mean in the de-finition of absolute visual magnitude? low-density stars. Some are much thinner than air. The 11. Why don't we know the inclination of a spectroscopic white dwarfs, lying below the main sequence, are tremen- binary? How do we know the incli~ation of an eclips dously dense. ing binary? The mass-luminosity relation says that the more mas- sive a star is, the more luminous it is. Main-sequence stars Y74 Part 3 The Stars 12. How do the masses of stars along the main sequence il- Star Spectral Type m lustrate the mass-luminosity relation? 13. Why is it difficult to find out how common the most lu-minous stars are? The least luminous stars? 14. What is the most common kind of star? 15. If you look only at the brightest stars in the night sky, what kind of star are you likely to be observing? Why? ~iscussion 1. If someone asked you to compile a list of the nearest stars to the sun based on your own observations, what measurements would you make, and how would you analyze them to detect nearby stars? 2. The sun is sometimes described as an average star. Is that true? What is the average star really like? ~roblems a G2 V 5 b B1 V 8 C G2 Ib 10 d M5 III 19 e White dwarf 15 11. If two stars orbit each other with a period of 6 years and a separation of 4 AU, what is their total mass? (Hint: See By the Numbers 11-4.) 12. If the eclipsing binary in Figure 11-20 has a period of 32 days, an orbital velocity of 153 km/s, and an orbit that is nearly edge-on, what is the circumference of the orbit? the radius of the orbit? the mass of the system? 13. If the orbital velocity of the eclipsing binary in Fig-ure 11-20 is 153 km/s and the smaller star becomes completely eclipsed in 2.5 hours, what is its diameter? 14. What is the luminosity of a 4-solar-mass main-sequence star? of a 9-solar-mass main-sequence star? of a 7-solar-mass main-sequence star? 1. If a star has a parallax of 0.050 second of arc, what is its distance in pc? in ly? in AU? 1. The Hertsprung-Russell diagram was named for two fa-mous astronomers. Who were they? What did they do to earn such an honor? 2. If you place a screen of area 1 mz at a distance of 2.8 m i from a 100-watt lightbulb, the light flux falling on the screen will be 1 J/s. To what distance must you move the screen to make the flux striking it equal 0.01 J/s? This , assumes the lightbulb emits all of its energy as light. 2. Algol is a famous binary star. What can you find out about 3. If a star has a parallax of 0.016 second of arc and an ap- the mythology of Perseus, Medusa, and Algol? parent magnitude of 6, how far away is it, and what is 3. An entire class of binary styars is known as the Algol bi- its absolute magnitude? naries. How would you characterize such star systems? 4. Complete the following table. P m M~ d (pc) (seconds of arc) Exploring TheSky 7 10 11 1000 -2 0.025 4 0.040 ;`5. The unaided human eye can see stars no fainter than those with an apparent magnitude of 6. If you can see a bright firefly blinking up to 0.5 km away, what is the absolute magnitude of the firefly? (Hint: Convert the distance to parsecs and use the formula in By the Num-bers 11-2.) 3. Take a survey of the stars. Center on Orion and adjust the field until it is about 100° wide. Click on the ten 6. If a main-sequence star has a luminosity of 400 Lo, what brightest stars and record the spectral types. Now zoom is its spectral type? (Hint: See Figure 11-13.) in until only a few dozen stars are in the frame. Click 7. If a star is 10 times the radius of the sun and half as on the 10 faintest stars and record their spectral types. hot, what will its luminosity be? (Hint: See By the Num- Is there a difference between the brightest and faintest bers 11-3.) stars? Is the result what you expected? Are certain kinds 8. An 08 V star has an apparent magnitude of +1. Use the of stars missing from the data in the computer program? method of spectroscopic parallax to find the distance to 4. Repeat exercise 3 for a region centered on the Big Dipper. the star. Why might this distance be inaccurate? 9. Find the luminosity and spectral type of a 5-Mo main-sequence star. 10. In the following table, which star is brightest in appar-ent magnitude? most luminous in absolute magnitude? largest? least dense? farthest away? 1. Locate the following stars and determine their apparent magnitude, parallax, distance in parsecs and in light-years, and spectral classification: Sirius, Aldebaran, Vega, Deneb, Betelgeuse, Antares, and Altair. (Hint: To center on an object, use Find under the Edit menu and type the object's name followed by a period.) 2. Use the spectral type and parallax of the stars above to estimate their distance from Earth. Compare with dis-tances given in TheSky. Ro ta tbe Brook:/Cole Aatronomy Reaouree l~ater Iwww.brootacole. com/aatrono.yl tor critical tbinklng ewercises, artlclea, and addl-uonai readinba tror utoTras Coua~ Ediuaa, Rraaluacole'a oalW atudeatllbrery. Chapter 11 TYte Family of Stars Y75 5. What observational evidence do we have that star for- mation is a continuous process? 6. How are Herbig-Haro objects related to star formation? its layers by its internal pressure. The fourth says energy can 7. How do the proton-proton chain and the CNO cycle re- semble each other? How do they differ? 8. Why does the CNO cycle require a higher temperature than the proton-proton chain? 9. How does the pressure-temperature thermostat control To keep its pressure high, it must be hot and generate large the nuclear reactions inside stars? amounts of energy. The mass of a star determines its star? 13. Why does a star's life expectancy depend on its mass? 14. That evidence do we have that star formation is hap- pening right now in the Orion Nebula? The Great Nebula in Orion is an active region of star for- mation. The bright stars we see in the center of the nebula formed within the last few million years, and infrared tele- Discussion Questions scopes detect protostars buried inside the molecular cloud 10. Step by step, explain how energy flows from the sun's core to Earth. 11. Why is there a mass-luminosity relation? The first two laws say that mass and energy must be con- served and spread smoothly through the star. The third, hy- drostatic equilibrium, says the star must balance the weight of only flow outward by conduction, convection, or radiation. The mass-luminosity relation is explained by the re- quirement that a star support the weight of its layers by its internal pressure. The more massive a star is, the more weight it must support and the higher its internal pressure must be. luminosity. The massive stars are very luminous and lie along the upper main sequence. The less massive stars are fainter and lie lower on the main sequence. How long a star can stay on the main sequence depends on its mass. The more massive a star is, the faster it uses up its hydrogen fuel. A 25-solar-mass star will exhaust its hydro- gen and die in only about 7 million years, but the sun is ex- pected to last for 10 billion years. 12. Why is there a lower limit to the mass of a main-sequence that lies behind the visible nebula. 1. When we see distant streetlights through smog, they look dimmer and redder than they do normally. But when we see the same streetlights through fog or falling snow, they look dimmer but not redder. Use your knowl-interstellar medium T Tauri star edge of the interstellar medium to discuss the relative sizes of the particles in smog, fog, and snow compared 2. If planets form as a natural by-product of star forma- tion, which do you think are more common-stars or with the wavelength of light. Herbig-Haro object CNO (carbon-nitrogen- planets? triple-alpha process prohlems nebula Bok globule emission nebula HII region bipolar flow reflection nebula dark nebula oxygen) cycle interstellar reddening infrared cirrus conservation of mass molecular cloud conservation of energy shock wave hydrostatic equilibrium association energy transport protostar opacity evolutionary track stellar model 1. The interstellar medium dims starlight by about 1.9 mag-nitudes/1000 pc. What fraction of photons survive a trip of 1000 pc? (Hint: See By the Numbers 1-1.) 2. A small Bok globule has a diameter of 20 seconds of arc. If the nebula is 1000 pc from Earth, what is the diame-ter of the globule? birth line brown dwarf 3. If a giant molecular cloud has a diameter of 30 pc and drifts relative to neighboring clouds at 20 km/s, how long will it take to travel its own diameter? 4. If the dust cocoon around a protostar emits radiation most strongly at a wavelength of 30 microns, what is the temperature of the dust? (Hint: See By the Numbers 4-1.) 1. What evidence do we have that the spaces between the stars are not empty? 2. What evidence do we have that the interstellar medium 3. Why would an emission nebula near a hot star look red, but a reflection nebula near its star looks blue? How long would it take to travel 1 light-year? 5. The gas in a bipolar flow can travel as fast as 300 km/s. contains both gas and dust? 6. Circle all'H and 4H nuclei in Figure 12-8. Explain how both the proton-proton chain and the CNO cycle can be summarized as 4 'H ~ 4He + energy. 4. Why do astronomers rely heavily on infrared observa- tions to study star formation? 7. In the model shown in Figure 12-13, how much of the sun's mass is hotter than 13,000,000 K? 300 Part 3 The Stars Chapter 12 8. If a brown dwarf has a surface temperature of 1500 K, at what wavelength will it emit the most radiation? (Hint: See By the Numbers 4-1.) 9. What is the life expectancy of a 16-solar-mass star? 10. If the 06V star in the Orion Nebula is magnitude 5.4, how far away is the nebula? (Hint: Use spectroscopic parallax.) 11. The hottest star in the Orion Nebula has a surface tem- perature of 40,000 K. At what wavelength does it radi- ate the most energy? (Hint: See By the Numbers 4-1.) Critical Inquiries for the Web 1. Use the Web to supply additional details concerning the evolution of protostars and T Tauri stars. 2. Astronomers continue to study the Orion Nebula and star formation in the molecular cloud behind the neb- ula. What is the latest news from Orion? 3. What is BM Orionis? Where is it located, and what does it do? Why might it be interesting to observe with even a small telescope? Exploring The Sky 1. The following nebulae are all star-formation regions. What kind of nebulae are they? (Hint: To center on an object use Find under Edit. Choose Messier Objects and pick from the list.) M42, M20, M8, M17 Exploring The Sky Chapter 12 The Formation and Structure of Stars 301 2. Locate M8 in TheSky, zoom in, and identify other neb-ulae in the region. Study the photo of NGC6559. Chapter of radiation is produced by rapidly moving electrons spiraling through magnetic fields; in the case of the Crab Nebula, the electrons are so energetic that they also emit visible light. This leaves us with a puzzle. The Crab Nebula is 950 years old, so the electrons should have radiated away their energy long ago. The Crab Nebula must contain a powerful energy source to maintain the synchrotron radiation. REYIEW Critical Ino What causes a type II supernova explosion? A type II supernova occurs when a massive star reaches the end of its usable fuel and develops an iron core. The iron is the final ash produced by nuclear fusion, and it cannot fuse to produce energy because iron is the most tightly bound nucleus. When energy generation begins to fall, the star contracts; but because iron can't ignite, there is no new energy source to stop the contraction. In a fraction of a second, the core of the star falls inward and a shock wave moves outward. Aided by a flood of neutrinos and sudden turbulence, the shock wave blasts the star apart, and we see it brighten as its surface gases expand into space. Type II supernova explosions are easy to recognize because their spectra contain hydrogen lines. Use what you know about type la supernova explosions to ex-plain why their spectra do not contain visible hydro-gen lines. so there is no pressure-temperature thermostat to control the reactions. As a result, the core explodes in a helium flash. All of the energy produced is absorbed by the star. We can see evidence of stellar evolution in the H-R dia-grams of clusters of stars. Beginning their evolution at about the same time, the stars evolve in different ways, depending on their masses. The most massive leave the main sequence first and are followed later by progressively less massive stars. we can estimate the age of a star cluster from the turnoff point in its H-R diagram. New Terms nova Chandrasekhar limit supernova Roche lobe degenerate matter Roche surface helium flash inner Lagrangian point open cluster angular momentum globular cluster accretion disk turnoff point type I supernova horizontal branch type II supernova planetary nebula supernova remnant black dwarf synchrotron radiation When a star's central hydrogen-fusion reactions cease, its core contracts and heats up, and hydrogen fusion begins in a spherical layer around the core-a hydrogen-fusion shell. Energy from this shell swells the star into a cool giant. The contraction of the star's core ignites helium, first in the core 324 Part 3 The Stars Chapter 13 Review Questions 1. Why does helium fusion require a higher temperature than hydrogen fusion? Helium is lighter than hydrogen. core hydrogen burning The energy burning stage for main sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90% of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by core hydrogen burning. 2. How can the contraction of an inert helium core trigger the ignition of a hydrogen-fusion shell? As soon as hydrogen becomes substantially depleted, about 10 billion years after the star arrived on the main sequence, the helium core begins to contract. The shrinkage of the helium core releases gravitational energy, driving up the central temperature and heating the overlying layers, causing the hydrogen there to fuse even more rapidly than before. Figure 12.3 depicts this hydrogen-shell-burning stage, in which hydrogen is burning at a furious rate in a relatively thin layer surrounding the nonburning inner core of helium ash. The hydrogen shell generates energy faster than did the original main-sequence star’s hydrogen-burning core, and the shell’s energy production continues to increase as the helium core contracts. 3. Why does the expansion of a star's envelope make it cooler and more luminous? 4. Why is degenerate matter so difficult to compress? Neutron stars and white dwarfs are similar in that both are at the end points of stellar evolution, have high surface temperatures, no longer produce energy via thermonuclear fusion, are composed of degenerate matter, have very small radii and are very dense. Neutron stars and white dwarfs are different in that neutron stars are the cores of massive stars that went through a supernova, while white dwarfs are the cores of intermediate- mass stars that produced planetary nebulae. 5. How does the presence of degenerate matter in a star trigger the helium flash? helium flash An explosive event in the post-main-sequence evolution of a low-mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again 6. -How can star clusters confirm our theories of stellar evolution? 7. Why don't red dwarfs become giant stars? 8. What causes an aging giant star to produce a planetary nebula? How a star evolves depends on its mass. Stars less mas-sive than about 0.4 solar mass are completely mixed and will have very little hydrogen left when they die. They can not ignite a hydrogen fusion shell, so they cannot become giant stars. They will remain on the main sequence for many times the present age of the universe. Medium-mass stars between about 0.4 and 4 solar masses become giants and fuse helium but cannot fuse carbon. They produce plane-tary nebulae and become white dwarfs. 9: Why can't a white dwarf contract as it cools? What is its fate? White Dwarfs are made up of degenerate material. Energy is transported by conduction in very dense material. The white dwarf has no source of energy other than what was already there. 10. Why can't a white dwarf have a mass greater than 1.4 solar masses? Mass-Radius Relation In white dwarfs, the greater the mass, the smaller the size. However, if the mass is larger than 1.4, the size goes to zero--it completely collapses down to nothing. This is called the Chandrasekhar limiting mass. Stable white dwarfs cannot exist if their mass is greater than 1.4 solar masses. 11. How can a star of as much as 8 solar masses form a white dwarf when it dies? By loosing mass. There are a variety of ways that stars can lose substantial amounts of mass. When stars are red giants, they can have stellar winds that cause the stars to lose large amounts of mass. An 8 solar mass star can decrease to 1.4 solar masses so that it can become a white dwarf after the ejection of its envelope. Depending on the mass of the star, it can go back up through the red giant phase and undergo more nuclear fusion. The most massive stars fuse nuclear fuels up to iron but cannot generate fur-ther nuclear energy because iron is the most tightly bound of all atomic nuclei. When an iron core forms in a massive star, the core collapses and triggers a supernova explosion that expels the outer layers of the star to form an expanding supernova remnant. The first supernova visible to the naked eye since 1604 was seen in February 1987. The study of the deaths of stars has led us to discover astonishing objects of unbelievable density, tempera-ture, and violence-all consequences of the victory of gravity over matter. But we have not considered the strangest circumstance of all. What happens when de-generate matter can't support the weight of a dying star-that is, when the mass of the compact object ex-ceeds the Chandrasekhar limit? The answer is in the title of the next chapter. 12. How can we understand the Algol paradox? Close binary stars evolve in complex ways because they can transfer mass from one star to the other. This explains why some binary systems contain a main-sequence star more massive than its giant companion-the Algol paradox. Also, mass transfer into a accretion disk around a white dwarf can produce X rays from the hot disk and can trigger nova explosions. Stars as massive as 8 solar masses may lose enough mass to eject planetary nebulae and die as white dwarfs, but more-massive stars suffer a different fate. 13. How can the inward collapse of the core of a massive star produce an outward explosion? 14. What is the difference between type Ia and type II super-novae? 15. What is the difference between a supernova explosion and a nova explosion? Discussion Questions 1. How do we know the helium flash occurs if it cannot be observed? Can we accept something as real if we can never observe it? 2. False-color radio images and time-exposure photographs of astronomical images show us aspects of nature we can never see with our unaided eyes. Can you think of common images in newspapers or on television that re-veal phenomena we cannot see? Problems 1. About how long would a 0.4-M star spend on the main sequence? (Hint: See By the Numbers 12-1.) 2. If the stars at the turnoff point in a star cluster have masses of about 4 M, how old is the cluster? 3. The Ring Nebula in Lyrae is a planetary nebula with an angular diameter of 76 seconds of arc and a distance of 5000 ly. What is its linear diameter? (Hint: See By the Numbers 1-2.) 4. If the Ring Nebula is expanding at a velocity of 15 km/s, typical of planetary nebulae, how old is it? 5. Suppose a planetary nebula is 1 pc in radius. If the Doppler shifts in its spectrum show it is expanding at 30 km/s, how old is it? (Hints: 1 pc equals 3 x 10 13 km, and 1 year equals 3.15 X 10 17 seconds.) 6. If a star the size of the sun expands to form a giant 20 times larger in radius, by what factor will its average density decrease? (Hint: The volume of a sphere is 4ttr3.) 3 7. If a star the size of the sun collapses to form a white cooler dwarf the size of Earth, by what factor will its density increase? (Hints: The volume of a sphere is 4ttr3. See Appendix A for the radii of the sun and Earth.) 3 8. The Crab Nebula is now 1.35 pc in radius and is ex-panding at 1400 km/s. About when did the supernova occur? (Hint: 1 pc equals 3 X 10 13 km.) 9. If the Cygnus Loop is 40 pc in diameter and is 20,000 years old, with what average velocity has it been expanding? (Hints: 1 pc equals 3 X 10 13 km, and 1 year equals 3.15 X 10 7 seconds.) 10. Observations show that the gas ejected from SN1987A is moving at about 10,000 km/s. How long will it take to travel one astronomical unit? one parsec? (Hints: 1 AU equals 1.5 X 10 8 km, and 1 pc equals 3 X 10 13 km.) Critical Inquiries for the Web 1. As seen on pages 312 and 313, there is an incredible di star produce an outward explosion? diversity of appearance for planetary nebulae. Browse the Web for images and information on these dying stars and discuss why there is a range of shapes of planetary nebulae that we see. 2. Naked-eye supernovae in our galaxy are rare, but astron-omers have noted supernovae in other galaxies for years. Look for summaries of observations of recent supernovae. Are similar numbers of type la and type II being seen? Compare the number of supernovae seen during the last few years with that of two decades ago. Why are we finding so many more supernovae in recent years than in the past? 3. What can you find out about different kinds of novae? Limit your search to astronomy categories and also search for the use of the word nova on Web pages about variable stars. Exploring The Sky 1. Locate the planetary nebulae M57, M97, and M27. How does their shape distinguish them from the star forma-tion nebulae such as M42 and M8? (Hint: To find an ob ject, use Find under Edit. Choose Messier Objects and pick from the list.) 2. The Crab Nebula is M1. Locate it, zoom in, measure its angular size in seconds of arc, and compute its diameter, assuming it is about 6000 ly from Earth. 3. Locate the supernova remnant called the Cygnus Loop just south of Cygni. How big is this object in angular diameter compared to the full moon? (Hint: Under the View menu, choose Labels and Setup. Check Bayer Designation, go to Cygnus, and zoom in on e Cygni until the Cygnus Loop appears.) v 00 to the Brooks/Cole Astronomy Resource Center (www.broohscole. coo/astronomy) for critical thinking exercises, articles, and addi-tional readings from InfoTrac College Edition, Brooks/Cole's online sudeot library. Chapter 13 The Deaths of Stars 325 What observations would you make of an X-ray binary system to distinguish between a black hole and a neutron star? Compact objects emitting X rays and producing We will see similar phenomena many times more powerful when we explore the galaxies in Chapters 15, 16, and 17. black hole It must collapse to the event horizon collapsar neutron-star stage, with a radius of about 10 km and a den- singularity magnetar Chapter 14 2. 0.00096 AU if the total mass is 2 Me 4.3m;1.1X10-25 m 6. 8.1 minutes 8. 3.0 X 108 m/s; photons Chapter 14 Review Questions 1 .How are neutron stars and white dwarfs similar. How do they differ? Neutron stars are very hot but not very luminous because they have very small surface areas. white dwarf is a dwarf star with a surface temperature that is hot, so that the object glows white. close binary star system, its companion star can transfer mass to it. Material falling gradually onto a white dwarf can explode in a sudden burst of fusion and make a nova. If ma- terial falls rapidly onto a white dwarf, it can push it over the Chandrasekhar limit and cause it to explode completely as a Type I supernova. Material falling onto a neutron star can When a white dwarf or neutron star is a member of a cause powerful bursts of x-ray and gamma-ray radiation. ergy outward, blowing off the outer layers of the star in a 2. Why is there an upper limit to the mass of neutron stars? About a dozen binary systems contain neutrons stars from which we can determine their masses. The table below list these. Some of the binary systems are X-ray sources, (X-ray binaries) and others (binary pulsars) have either 2 neutron stars (NS) or one NS and a white dwarf (WD) companion. All mass are below the 2.0 Msun limit from neutron degeneracy. 3. Why do we expect neutron stars to spin rapidly? If the formation of a neutron star leads to a supernova ex- plosion, explain why only three out of the hundreds of known pulsars are found in supernova remnants. At least some supernovae leave behind a highly mag- netic, rapidly rotating neutron star, which can be observed as a pulsar if its beam of escaping particles and focused radia- tion is pointing toward us. Pulsars emit rapid pulses of radi- ation at regular intervals; their periods are in the range of 0~001 to 10 seconds. The rotating neutron star acts like a lighthouse, sweeping its beam in a circle and giving us a pulse of radiation when the beam sweeps over the Earth. Pulsars lose energy as they age, the rotation slows, and their periods increase. 4. If neutron stars are hot, why aren't they very luminous? Neutron stars are very hot but not very luminous because they have very small surface areas. 5. Why do we expect neutron stars to have a powerful mag- netic field? The gas in the disk develops a spiraling motion, like the water draining out of . It spirals inward from the outer edge, where mass transfer deposits the gas, to the inner edge, where it eventually falls onto the hot, neutron star surface. Because the gas eventually accretes onto the neutron star, we call the disk an accretion disk. The magnetosphere forces matter in the accretion disk to flow onto the neutron star along magnetic field lines. The gas strikes the surface with such force that it reaches temperatures of 100 million K, forming two hot spots on the neutron star, one at each magnetic pole. The X-ray emission emanates from these two locations and sweeps across our line of site with the same period as the rotational period of the neutron star. a pulsar can generate X-ray pulses along with its radio pulses. The collapse of the iron core of a red supergiant generated extremely strong magneitc field around the tiny neutron stars. The resulting magnetosphere can force particles around it as the Earth's magnetosphere does to the solar wind. The neutron star is rapidly rotating neutron star neutron stars and generates pulsed radio signals. The model relies on fast rotation and strong magnetic fields. They are reasonable working hypotheses for neutron stars because rotation and magnetism are properties of all stars and the formation of neutron stars exaggerates both. Rotation will increase as the core shrinks during its final collapse just as the solar nebula spun up as it collapsed. An existing magnetic field will also increase in strength during core collapse because matter anchors the magnetic field to itself. When gravitational collapse compresses the matter, the magnetic field also compresses. 6. Why did astronomers conclude that pulsars could not be pulsating stars? If electrons move at speeds close to the speed of light (relativistic speeds) they emit what is called synchrotron radiation. Sychrotron radiation is very directional and is in the direction of the motion of the electrons. The pulsar mechanism requires the rotating magnetic field to generate an electric field strong enough to lift electrons into the magnetosphere. The electrons will then travel along the magnetic field lines. The strongest concentration of magnetic field lines will be at the two poles. Electrons streaming off neutron stars along the concentration of magnetic field lines can emit a beam of radiation strongly oriented along the magnetic axis. Furthermore, if the pulsarÕs magnetic field axis is not aligned with the rotational axis, the beam rotates with the neutron star like a lighthouse beacon. 7. What does the short length of pulsar pulses tell us? Some pulsars have been observed having extremely short periods, such as 0.003 seconds. These are the millisecond pulsars. These rotate rapidly because, it is thought, material falling onto the neutron star spins it up, just like a ball rotating on your fingertip will speed up if you periodically strike it in the appropriate manner. 8. How does the lighthouse model explain pulsars? At least some supernovae leave behind a highly mag- netic, rapidly rotating neutron star, which can be observed as a pulsar if its beam of escaping particles and focused radia- tion is pointing toward us. Pulsars emit rapid pulses of radi- ation at regular intervals; their periods are in the range of 0~001 to 10 seconds. The rotating neutron star acts like a lighthouse, sweeping its beam in a circle and giving us a pulse of radiation when the beam sweeps over the Earth. Pulsars lose energy as they age, the rotation slows, and their periods increase. 9. What evidence do we have that pulsars are neutron stars? Pulsars are objects later found to be neutron stars that emit very short bursts, or pulses, of radio signals. they were discovered and now observed with radio telescopes. The interval of time between pulses, called the pulsar period, ranged from 0.03 seconds to 4 seconds for the first few discovered. The association of pulsars with neutron stars came as a result of studies of the Crab Nebula, a supernova remanant American Indians and Chinese astronomers observed a supernova in 1054 AD. Modern observations have identified a nebula, the Crab Nebula, at the same location in the sky. In the late 1960's, astronomers discovered a pulsar at the center of the Crab Nebula with a period of 0.033 seconds (33 milliseconds = 33 ms). 10. Why would astronomers at first assume that the first millisecond pulsar was young? Some pulsars have been observed having extremely short periods, such as 0.003 seconds. These are the millisecond pulsars. These rotate rapidly because, it is thought, material falling onto the neutron star spins it up. 11, How can a neutron star in a binary system generate X-rays? Radially accreting white dwarfs are efficient generators of X-rays; post-shock gas for radially accreting neutron stars are too hot to generate copious thermal X-rays, but may produce cyclotron X-rays. a similar evolutionary sequence but of a binary system. Both stars evolve into neutron stars. The letter M with a dot above it (called M-dot) represents mass loss. (In science, we use the dot to represent a change in time; its calculus equivalent is d/dt.) When star B becomes a RSG, its expanding mass falls onto a very small object. The neutron star is such a compact object that the gravitational forces near the star are much larger than that near a main sequence star. Any matter falling onto the neutron star, therefore, accelerates to extremely high speeds where matter violently falls onto the neutron star, produces high temperatures, and generates X-rays. 12. If the sun has a Schwarzschild radius, why isn't it a black hole. Chapter 14 Neutron Stars and Black Holes 345 In a massive star, hydrogen fusion in the core is fol- lowed by several other fusion reactions involving heavier ele- ments. Just before it exhausts all sources of energy, a massive star has an iron core surrounded by shells of silicon and sul- fur, oxygen, neon, carbon, helium, and hydrogen. The fusion of iron requires energy (rather than releasing it). If the mass of a star's iron core exceeds the Chandrasekhar limit (but is less than 3 Ms„a), the core collapses until its density exceeds that of an atomic nucleus, forming a neutron star with a typ- ical diameter of 20 km. The core rebounds and transfers en- Type II supernova explosion. Studies of Supernova 1987A, the core. 13. How can a black hole emit X rays? hypernova not reach stability as a white dwarf. If the remains of a star collapse with a mass greater than the Chandrasekhar limit of 1.4 solar masses, then the object can- sity equal to that of an atomic nucleus. Such a neutron star Schwarzschild radius (Rs) can be supported by the pressure of its degenerate neutrons. But if the mass is greater than 2 to 3 solar masses, then the degenerate neutrons cannot stop the collapse, and the object must become a black hole. should emit X rays. Any X rays emitted before the matter crosses the event horizon will escape, and we can look for black holes by looking for X-ray sources. Of course, an isolated black hole will probably not have much matter falling in, but black holes in binary systems may have large amounts of matter flowing in from the companion star. 14 What evidence do we have that black holes really exist? should emit X rays. Any X rays emitted before the matter crosses the event horizon will escape, and we can look for black holes by looking for X-ray sources. Of course, an isolated black hole will probably not have much matter falling in, but black holes in binary systems may have large amounts of matter flowing in from the companion star. we can search for black holes by looking for X-ray binaries. The search for black holes has succeeded in finding a few strong candidates, but the problem is being sure a particular binary system contains a black hole and not a neutron star. precessing jets of radiation and gas may not be as unusual as they seem. Many stars collapse to form black holes or neutron stars in binary systems, but these are ob-jects of only a few solar masses. 15. How can mass transfer into a compact object produce jets of high-speed gas? X-ray bursts? 16. Describe the possible causes of gamma-ray bursts. Theory predicts that a neutron star should rotate very fast, be very hot, and have a strong magnetic field. Such ob- jects have been identified as pulsars, sources of pulsed radio energy. Pulsars are evidently spinning neutron stars that sweep over Earth, we detect pulses. The spinning neutron ject, gravity is so strong that not even light can escape, and X rays? we term the region a black hole. The surface of this region, spin, they sweep the beams around the sky; if the beams emit beams of radiation from their magnetic poles. As they star slows as it radiates energy into space. Dozens of pulsars have been found in binary systems, which allows astronomers to estimate the masses of the pul- sars. Such masses are consistent with the predicted masses of neutron stars. In some binary systems, such as Hercules mass flows into a hot accretion disk around the neutron If a collapsing star has a mass greater than 2 to 3 solar X-1, star and causes the emission of X rays. Discussion Questions 1. In your opinion, has the existence of neutron stars been sufficiently tested to be called a theory, or should it be called a hypothesis? What about the existence of black holes? 2. Why wouldn't an accretion disk orbiting a giant star get as hot as an accretion disk orbiting a compact object? Problems 1. If a neutron star has a radius of 10 km and rotates 642 times a second, what is the speed of the surface at the neutron star's equator in terms of the speed of light? (Hint: The circumference of a circle is 2ttr.) 2. A neutron star and a white dwarf have been found or-biting each other with a period of 11 minutes. If their masses are typical, what is the average distance between them? (Hint: See By the Numbers 11-4.) 3. If Earth's moon were replaced by a typical neutron star, what would the angular diameter of the neutron star be, as seen from Earth? (Hint: See By the Numbers 1-2.) 4. What is the Schwarzschild radius of Jupiter (mass = 2 x 1. 10 27 kg)? of a human adult (mass = 75 kg)? (Hint: See Appendix A for the values of G and c.) 5. If the inner accretion disk around a black hole has a temperature of 10 6 K, at what wavelength will it radiate the most energy? (Hint: See By the Numbers 4-1.) 6. What is the orbital period of a bit of matter in an accre-tion disk 2 x 10 5 km from a 10-M black hole? (Hint: See By the Numbers 2-1.) 6o to the Brooas/Cole nstronomy Resource Center Iwww.brooYSCOIe. eor/astronomyl lor critical thinhlng eaerclses, article:, and addl-tional readings fror InloTrac College Ed1UOn, Broor:IColo's odlu arudent library. 7. If SS433 consists of a 20-Mo star and a neutron star or-biting each other every 13.1 days, then what is the av-erage distance between them? (Hint: See By the Num-bers 11-4.) 8. What is the orbital velocity at a distance of 7400 meters from the center of a 5 solar mass black hole? What kind of particles could orbit at this distance? Hint See By the Numbers 2-1. 9. Compare the orbit in Problem 8 with an orbit having the same velocity around a 2-solar-mass neutron star. Why is this orbit impossible? (Hint: See By the Numbers 2-1.) Critical Inquiries for the Web 1. Imagine that you are on a mission to explore one of the pulsar planets noted in the chapter. What would you find there? Look for information about pulsars and the known pulsar planets on the Web and describe what you might encounter on such a mission. 2. What would you experience if you were to pilot a space-craft near a black hole? Visit black-hole-related Internet sites to determine what the gravitational effects and general environment would be. Also use the Internet to find the limits of human tolerance to strong gravitational forces. (Hint: Look for information about astronaut train-ing and find out how many g's a human can withstand.) Use these sources to give a brief account about what your voyage would be like. - 3. Search for information about gamma-ray bursters. What is the latest news in this developing story? Exploring The Sky The Crab Nebula pulsar is located in the Crab Nebula, also known as M1. Locate it zoom in, and compare its shape and size with Figure 14-3. The sky does not show the pulsar. The distribution of populations through the galaxy sug- gests a way the galaxy could have formed from a spherical cloud of gas that gradually flattened into a disk. The younger the stars, the more metal rich they are, and the more circu- The very youngest objects lie along spiral arms within the disk. These stars live such short lives they don't have time to move from their place of birth in the spiral arms. Maps of these spiral tracers and cool hydrogen clouds re- veal the spiral pattern of our galaxy. The density wave theory suggests that the spiral arms are regions of compression that move through the disk. When an orbiting gas cloud smashes into the compression wave, page 346 Part 3 The Stars 4. Why is it difficult to specify the thickness or diameter of the disk of our galaxy? : 5, Why didn't astronomers before Shapley realize how large the galaxy is? lar and flat their orbits are. 6. How do we know how old our galaxy is? 7. Why do we conclude that metal-poor stars are older than metal-rich stars? 8. How can astronomers find the mass of the galaxy? 9. What evidence do we have that our galaxy has an ex- tended corona of dark matter? Dark Matter and the Formation of Galaxies even if it is red. So, too, when the early universe was opaque, radiation carried ordinary matter with it, sweeping past the concentrations of dark matter. Now suppose the police leave the motorcade, and the lights all turn red at the same time. The red lights act as traffic traps; approach-ing cars now have to stop, and so they bunch up. Likewise, after the early universe became transparent, ordinary mat-ter interacted with radiation only occasionally and so could fall into the dark-matter traps. The size of the gravitational traps depends on the nature of the dark matter. Suppose it is moving near the speed of light-astronomers call this hot dark matter. If neutri nos really do have mass, then they would be an example of hot dark matter. In this case, small-scale density fluctua-tions are smoothed out by the rapidly streaming particles as they move from high- to low-density regions. In this case, large-scale structure would form first. If, on the other hand, the dark matter moves slowly-we call this cold dark matter-then the particles do not have time to move far enough to smooth out small-scale density fluctuations. In this case, relatively small structures, the size of globular clusters or individual galaxies, are likely to form first. Neither hot nor cold dark matter is entirely successful in explaining the distribution of galaxies discussed in Chapter 19. Hot dark matter models predict that all galaxies should be found in large sheet-like structures, which are not seen. Cold dark matter cannot produce voids, walls, and long structures such as the Great Wall. Now theories are being developed that contain both hot and cold dark matter. Even though current models are not adequate to explain how galaxies form, the important point is that galaxies are difficult to form at all unless a substantial amount of dark matter of some kind is present. The COBE data, however, give us information about density fluctuations only for the type of matter that inter-acts with radiation. Suppose there is a type of matter that does not interact with light at all-namely dark matter. 10. How do the orbits of stars around the Milky Way Galaxy the gas cloud forms stars. Another process, self-sustaining help us understand its origin? star formation, may act to modify the arms as the birth of 11. What evidence contradicts the traditional theory for the massive stars triggers the formation of more stars by com- origin of our galaxy? pressing neighboring clouds. The nucleus of the galaxy is invisible at visual wave- lengths, but radio, infrared, X-ray, and gamma-ray radiation is not fully adequate to explain spiral arms in our galaxy? 1g, What evidence do we have that the density wave theory is a powerful source of energy? can penetrate the dust clouds. These wavelengths reveal crowded central stars and heated clouds of dust. 14. What evidence do we have that the center of our galaxy The very center of the Milky Way is marked by a radio source, Sagittarius A*, that is also a source of infrared radia- 15. Why is the lack of motion of Sgr A* important evidence tion, X rays, and gamma rays. The core must be less than in our study of the center of our galaxy? 4 AU in diameter and must contain about 2.6 million solar masses. Astronomers believe that this central object is a black hole. 12. Why do spiral tracers have to be short-lived? Di !On Q~ variable star differential rotation Cepheid variable star rotation curve instability strip dark matter RR Lyrae variable star dark halo period-luminosity relation galactic corona proper motion population I calibration population II Shapley-Curtis debate metals disk component spiral tracer kiloparsec (kpc) density wave theory spiral arm flocculent galaxy spherical component self-sustaining star halo formation nuclear bulge Sagittarius A* ~ues 1. Why isn't it possible to tell from the appearance of the Milky Way that the center of our galaxy is in Sagittarius? 2. Why is there a period-luminosity relation? 3. How can astronomers use variable stars to find distance? 1. How would this chapter be different if interstellar matter didn't absorb starlight? 2. Are there any observations you could make with the Hubble Space Telescope that would allow you to better understand the nature of Sgr A*? P~°Q ~~~5 1. Make a scale sketch of our galaxy in cross section. In-clude the disk, sun, nucleus, halo, and some globular clusters. Try to draw the globular clusters to scale size. 2. Because of dust, we can see only about 5 kpc into the disk of the galaxy. What percentage of the galactic disk can we see? (Hint: Consider the area of the entire disk and the area we can see.) 3. If the fastest passenger aircraft can fly 1600 km/hr (1000 mph), how many years would it take to reach the sun? the galactic center? (Hint: 1 pc = 3 X 10'3 km.) 4. If the RR Lyrae stars in a globular cluster have apparent magnitudes of 14, how far away is the cluster? (Hint: See By the Numbers 11-2.) 5. If interstellar dust makes an RR Lyrae star look 1 mag-nitude fainter than it should, by how much will we over-estimate its distance? (Hint: See By the Numbers 11-2.) 6. If a globular cluster is 10 minutes of arc in diameter and 8.5 kpc away, what is its diameter? ( Hint: Use the small-angle formula from By the Numbers 1-2.) 310 Part 4 The Universe  7. If we assume that a globular cluster 4 minutes of arc in 2. What if we lived near the center of the galaxy? Search diameter is actually 25 pc in diameter, how far away is the Web for research and information on the distribution it? (Hint: Use the small-angle formula from By the Num- of material near the center of our galaxy. Based on what bers 1-2.) you find, speculate as to how the sky would appear from a planet associated with a star near the galactic center. 8. If the sun is 5 billion years old, how many times has it orbited the galaxy? 9. If the true distance to the center of our galaxy is found to be 7 kpc and the orbital velocity of the sun is 220 km/s, what is the minimum mass of the galaxy? (Hints: Find the orbital period of the sun, and then see By the Num-bers 11-4.) 10. Infrared radiation from the center of our galaxy with a wavelength of about 2 x 10-s m (2000 nm) comes mainly from cool stars. Use this wavelength as Xm. and find the temperature of the stars. (Hint: See By the Num-bers 4-1.) Critical Inquiries for the Web 1. Henrietta Leavitt discovered the period-luminosity re-lation for Cepheids while working on the staff at Har-vard College Observatory under Edward Pickering. She was one of several women "computers" on staff there a century ago. Search the Web for information on Leavitt, her colleagues, and their work at Harvard. List three of the women employed by Pickering and note their con-tributions to astronomy. What was life like for a woman in astronomy at the beginning of the 20th century? Exploring The Sky 1. Locate Sagittarius and examine the shape of the Milky Way there and the profusion of globular clusters. (Hint: To turn on Messier object labels, use Labels and Setup under the View menu.) 2. Locate the following globular clusters: M3, M4, M5, M10, M12, M13, M15, M22, M55, M92. Where are they located in the sky? (Hint: Use Find under the Edit menu.) 3. Compare the distribution of globular clusters with that of open clusters. (Hint: Use Filters under the View menu to turn off everything but globular clusters, the Milky Way, the Galactic Equator, and Constellation Bound-aries. Use the thumbwheel at the bottom of the sky win-dow to rotate the sky. Now repeat with globular clusters off and open clusters on.) °0 6o to the Brook:/Cole Astronomll Resource Center (wenn.brookscole. com/astronorll) for critical thinking exercises, articles, and aAW-flonal readings from lafoirnc College Edition. 8fooks/Colle's OWN student librart Chapter 15 The Milky Way Galaxy 371 there is no star formation there, and halos are made up of older stars. The brightest stars in halos are red giants, which give g, halos a reddish tint. Elliptical galaxies, lacking gas and dust, lack young stars and are consequently slightly reddish be- cause the brightest stars are red giants. 8. Why is it difficult to measure the Hubble constant? How is the rotation curve method related to binary stars and Kepler's third law? 10. What evidence do we have that galaxies contain dark matter? 11. What evidence do we have that galaxies collide and merge? 12. Why are the shells visible around some elliptical galax- ies significant? qg, Ring galaxies often have nearby companions. What does that suggest? can be applied only to nearby galaxies. Both methods sug- 14. Propose an explanation for the lack of gas, dust, and The Hubble law shows that the radial velocity of a galaxy is proportional to its distance. Thus, we can use the Hubble law to estimate distances. The galaxys radial velocity divided by the Hubble constant equals its distance in megaparsecs. The masses of galaxies can be measured in two basic ways-the rotation curve method and the velocity disper- sion method. The rotation curve method is more accurate but gest that galaxies contain 10 to 100 times more dark matter than visible matter. young stars in elliptical galaxies. Galaxies occur in clusters. Our own galaxy is a member 15. How do deep images by the Hubble Space Telescope of the Local Group, a small cluster. A galaxy in a rich clus- confirm our hypothesis about galaxy evolution? ter may collide with other galaxies more often than a galaxy in a poor cluster, and such collisions can force a galaxy to form new stars and use up its gas and dust. Collisions can also strip gas out of a galaxy. This may explain why ellipti- cal and SO galaxies are more common in rich clusters than 1~ Why do we believe that galaxy collisions are likely, but in poor clusters. Spiral galaxies may be star systems that star collisions are not? have not experienced many collisions. 2. Should an orbiting infrared telescope find irregular gal-axies bright or faint in the far infrared? Why? What about elliptical galaxies? elliptical galaxy spiral galaxy barred spiral galaxy irregular galaxy Large Magellanic Cloud Small Magellanic Cloud megaparsec (Mpc) distance indicator look-back time Hubble law Hubble constant (H) rotation curve method cluster method velocity dispersion method rich galaxy cluster poor galaxy cluster galactic cannibalism ring galaxy starburst galaxy 1. If a galaxy contains a type I (classical) Cepheid with a period of 30 days and an apparent magnitude of 20, what is the distance to the galaxy? 2. If you find a galaxy that contains globular clusters that are 2 seconds of arc in diameter, how far away is the gal-axy? (Hints: Assume that a globular cluster is 25 pc in diameter, and see By the Numbers 1-2.) 3. If a galaxy contains a supernova that at its brightest has an apparent magnitude of 17, how far away is the gal-axy? (Hints: Assume that the absolute magnitude of the supernova is -19, and see By the Numbers 11-2.) 4. If we find a galaxy that is the same size and mass as our Milky Way Galaxy, what orbital velocity would a small satellite galaxy have if it orbited 50 kpc from t the center of the larger galaxy? (Hint: See By the Num- bers 2-1.) , 5. Find the orbital period of the satellite galaxy described in Problem 4. (Hint: See By the Numbers 11-4.) 6. If a galaxy has a radial velocity of 2000 km/s and the Hubble constant is 70 km/s/Mpc, how far away is the galaxy? (Hint: Use the Hubble law.) Chapter 16 Review Questions 1. Why didn't astronomers at the beginning of the 20th century recognize galaxies for what they are? 2. How can a classification system aid a scientist? A classification system tells you how hot or cold stars are what the temperature is. 3. What is the difference between an EO galaxy and an E1 galaxy? 4. What is the difference between an Sa and an Sb galaxy? between an SBb and an Sb? 5. Why can't galaxies evolve from elliptical to spiral? Why can't they evolve from spiral to elliptical? 6. How do selection effects make it difficult to decide how common elliptical and spiral galaxies are? 7. Why are Cepheid variable stars good distance indica- tors? What about planetary nebulae? (Hints: 1 pc = 3.08 X 1013 km, and 1 yr = 3.15 X 10' s.) sification. You may be given this information at the site, but examine the images to see if the features of these gal orbital velocities of 150 km/s. If the radius of the galaxy is 4 kpc, what is the orbital period of the outer stars? axies conform to a particular Hubble type. 392 Part 4 The Universe 9. A galaxy has been found that is 5 kpc in radius and whose outer stars orbit the center with a period of 200 million years. What is the mass of the galaxy? On what assumptions does this result depend? (Hint: See By the Numbers 11-4.) Exploring The Sky Critical Inquiries for the Web and its companion galaxies. Zoom in on it and estimate its 1. angular size compared to the full moon. (Hint: Use Find under the Edit menu.) Take a survey of galaxies and see how many are spiral and how many are elliptical. Is there any selection effect in your method? (Hint: Use Filters under the View menu to turn off everything but Galaxies and Mixed Deep Sky using the Labels and Setup under the View menu.) 3, Locate the Sombrero Galaxy (M104). Study the photo- 1. Locate the Andromeda Galaxy, also known as M31, How far out into the universe can we see Cepheid vari- ables? Research sources on the Internet to find other gal- axies whose distances have been found through obser- 2, vation of Cepheids. List the galaxies in which Cepheids have been identified and the distances determined from these data. 2. How does the Milky Way stack up against the other gal- objects. Make sure the Messier labels are switched on axies in the Local Group? Look for information on the other galaxies in our cluster, and rank the top six mem- bers in order of total mass. graphs and discuss this galaxy's special properties. 3. In the early 1900s the nature of the "spiral nebulae" was Zoom in on it and estimate its angular size compared to not well understood. In 1920 a "great debate" was held between Harlow Shapley, who held that these objects 4, were relatively nearby swirling clouds of gas, and Heber Curtis, who saw them as distant "island universes." Use the Internet to find information about the debate, outline the lines of evidence used by the two participants to pre- the moon. Study the distribution of galaxies and notice how they cluster together. Can you find the Virgo cluster? Zoom in until more galaxies appear and then scroll north to find the Coma Cluster. Zoom in on Leo to find the clus- ter there. What other clusters can you find? sent their views, and explain who was right and who 5. Describe the galaxy located near the south celestial pole. was wrong. 4. Locate a Web page dedicated to the Messier Catalogue-a list of galaxies, clusters, and nebulae that is often used as a list of targets for small telescopes. Be sure that your destination includes images of the objects. For each of the galaxies in the Messier list, determine its Hubble clas-  6o to the Brooks/Cole Astronomy Resource Center lwww.broowscole. com/astronomyl for critical thinking exercises, articles, and addl-tional readings from IotoTrac College Edition, Brooks/Coles online student litifart  g. We have found a galaxy in which the outer stars have 7. If you find a galaxy that is 20 minutes of arc in diame-ter, and you measure its distance to be 1 Mpc, what is its diameter? (Hint: See By the Numbers 1-2.) Chapter 16 Galaxies page 393 the bil-lions of galaxies contains billions of stars. Most of those stars probably have fami)ies of planets like our solar system, and on some of those billions of planets liquid-water oceans and a protective atmosphere may have spawned life. It is possible that some other plan-ets in the universe are inhabited by intelligent crea-tures who share our curiosity and our wonder at the scale of the cosmos. 4. Look at Figure I-6. How can you tell that Mercury follows an elliptical orbit? Can you detect the elliptical Our goal has been to preview the scale of astronomical ob- jects. To do so, we journeyed outward from a familiar cam- pus scene by expanding our field of view by factors of 100 meters, or astronomical units for measuring certain Only 12 such steps took us to the largest structures in the 5. Which is the outermost planet in our solar system? Why .does that change? shape of any other orbits in this figure or the next? Pluto. 6, Why are light-years more convenient than miles, kilo- universe. miles and kilometers are for short distances. 7. Why is it difficult to detect planets orbiting other stars? Lots of interstellar dust and scattering of light from the stars. metric system to simplify our calculations and scientific no- 8. What does the size of the star image in a photograph tell us? How big the star is. 9. What is the difference between the Milky Way and the Milky Way Galaxy? 10. What are the largest known structures in the universe? Galaxies, Jovian Planets. to the sun. Of the eight other planets in our solar system, distances? The numbers in astronomy are so large it is not conve- nient to express them in the usual way. Instead, we use the tation to more easily write big numbers. The metric system and scientific notation are discussed in Appendix A. We live on the rotating planet Earth, which orbits a rather typical star we call the sun. We defined a unit of distance, the astronomical unit, to be the average distance from Earth Mercury is closest to the sun, and Pluto is the most distant. The sun, like most stars, is very far from its neighbor- p ing stars, and this leads us to define another unit of distance, the light-year, the distance light travels in 1 year. The near- est star to the sun is Proxima Centauri at a distance of 4.2 ly. As we enlarged our field of view, we discovered that the sun is only one of 100 billion stars in our galaxy and that our in diameter, what is its diameter in kilometers? galaxy is only one of many billions of galaxies in the uni- verse. Galaxies appear to be grouped together in clusters, superclusters, and filaments, the largest structures known. Chapter 2 Review Questions 1, The diameter of Earth is 7928 miles. What is its diame- ter in inches? In yards? 2. If a mile equals 1.609 km and the moon is 2160 miles 3. One astronomical unit is about 1.5 x 108 km. Explain why this is the same as 150 x 106 km. 4. Venus orbits 0.7 AU from the sun. What is that distance in kilometers? 5. Light from the sun takes 8 minutes to reach Earth. How long does it take to reach Mars? 6. The sun is almost 400 times further from Earth than is the moon. How long does light from the moon take to solar system Milky Way scientific notation Milky Way Galaxy astronomical unit (AU) spiral arm light-year (ly) Local Group galaxy 1. What is the largest dimension you have personal knowl-edge of? Have you run a mile? Hiked 10 miles? Run a marathon? 2. In Figure I-4, the division between daylight and dark-ness is at the right on the globe of Earth. How do we know this is the sunset line and not the sunrise line? 3. What is the difference between our solar system, our galaxy, and the universe? reach Earth? Part 1 The AstronomeYs Sky 7. If the speed of light is 3 X 105 km/s, how many kilome-ters are in a light-year? How many meters? 8. How long does it take light to cross the diameter of our Milky Way Galaxy? 9. The nearest galaxy to our own is about 2 million light-years away. How many meters is that? 10. How many galaxies like our own would it take laid edge to edge to reach the nearest galaxy? (Hint: See Problem 9.) 1. Locate photographs of Earth taken from space. What do cities look like? Can you see highways? Is the presence of our civilization detectable from space? 2. Locate photographs of nearby galaxies and compare them with photos of very distant galaxies. What kind of detail is invisible for distant galaxies? 3. One of the biggest clusters of galaxies is the Virgo clus-ter. Find out how many and what kind of galaxies are in the cluster. Is it nearby or far away? Exploring TheSky 1. Locate and center one example of each of three differ-ent types of objects: a. A planet, such as Saturn. Find its rising and setting time. Such objects have distances measured in astro-nomical units (AU). ob How to proceed: Decide on the object you want to lo-cate. Then find and center the object by clicking the Find button on the Object Toolbar. The second method is to press the F key. The third is to click Edit, then Find. Once you have the Object Information win-dow, click the center button. b. A star. All stars in TheSkybelong to our Milky Way Galaxy. Give the star's name, its magnitude, and its distance in light-years. How to proceed: Click on any star, which brings up an Object Information window. c. A galaxy. Give its name and/or its designation. How to proceed: Click on the Galaxies button in the Object Toolbar, then click on any galaxy. Distances to galaxies are millions and billions of light years. 2. Look at the solar system from beyond Pluto by clicking on View and then on 3D Solar System Mode. Tip the solar system edge-on and then face-on. Zoom in to see the inner planets. Under Tools, set the Time Skip Incre-ment to 1 day and then go forward in time to watch the planets move. 3. Identify some of the brightest constellations located along the Milky Way. (Hint: See View, Reference Lines.) r 6o to tbo tiroaRs/COIO ~strosorll Resource Center Inww.Rrookscole. comlasirouors) lor criticsl tRInYtug erercisos, articlss. Introduction: The Scale of the Cosmos 9 corporated into their monument the cycles of the sun away from the moon. As the rotating Earth carries the conti- and moon, nents through these bulges of deeper water, the tides ebb The cycles in the sky are a rich part of our culture, and flow. Friction with the seabeds slows Earth's rotation, but those same motions reveal an astonishing fact- and the gravitational force the bulges exert on the moon Earth is a planet. In the next chapter, we will see how force its orbit to grow larger. humanity made that discovery. When the moon passes through Earth's shadow, sun-light is cut off, and the moon darkens in a lunar eclipse. If the moon only grazes the shadow, the eclipse is partial, or penumbral, and not total. ~e Photosphere completely, resulting in an annular eclipse. - assigned according to brightness within each constellation. The magnitude system is the astronomer's brightness scale. First-magnitude stars are brighter than 2nd-magnitude. the sky in half. The gravitational forces of the moon and sun act on the spinning Earth and cause it to precess like a gyroscope. Earth's axis of rotation swee s around in a conical motion with a points period of 26,000 years, and consequently the celestial poles and celestial e uator move slowly q y g g angular distance of the stars. Because Earth orbits the sun, the sun appears to move eastward along the ecliptic through the constellations. Be- cause the ecliptic is tipped 23.5° to the celestial equator, the sun spends half the year in the northern celestial hemisphere and half the year in the southern celestial hemisphere, pro- ducing the seasons. The seasons are reversed south of Earth's equator. The motion of Earth changes in ways that can affect the vernal equinox climate. Changes in orbital shape, in precession, and in axial tilt can alter the planet's heat balance and may be responsi-ble for the ice ages and glacial periods. Because we see the moon by reflected sunlight, its shape appears to change as it orbits Earth. The lunar phases wax from new moon to first quarter to full moon and wane from full moon to third quarter to new moon. A complete cycle of 1. What is the difference between an asterism and a con- stellation? Give some examples. 2. Do people in other parts of the world see the same con- stellations, asterisms, lunar phases, lunar eclipses, and d lunar phases takes 29.53 days. The moon's gravitational field exerts tidal forces on solar eclipses that you see? Explain. No. 3` Earth that pull the ocean waters up into two bulges, one on the side of Earth facing the moon and the other on the side 3~ What does the word apparent mean in apparent visual magnitude? 36 Part 1 The Astronomer's Sky 4. In what ways is the celestial sphere a scientific model? 8. The average distance of Mars from the sun is 1.52 AU. 5. If Earth did not rotate, could we define the celestial poles and celestial equator? the ecliptic? Use the small-angle formula to calculate the angular di- ameter of the sun as seen from Mars. 9. Draw a diagram showing Earth, the moon, and shadows during (a) a total solar eclipse, (b) a total lunar eclipse, (c) a partial lunar eclipse, (d) an annular eclipse. 6. Where would you go on Earth so you could see both north and south celestial poles at the same time? Where would you go to see a celestial pole at your zenith? 7. What is the difference between the daily and annual mo-tion of the sun? 8. What would our seasons be like if Earth were tipped 35° instead of 23.5°? What would they be like if Earth's axis were perpendicular to its orbit? criticat Inquiries 9. Why are the seasons reversed in the Southern Hemi- 1. Nearly all cultures have populated the sky with gods, 10. Where would you look in the sunset or dawn skies to heroes, animals, and objects. What can you learn on the find the full moon? the first quarter moon? the waning Web about non-Western constellations? crescent moon? the waxing gibbous moon? sphere? 2. Who was Orion? How is he related to the scorpion in 10. A total solar eclipse was visible from eastern Canada on July 10, 1972. When did this eclipse next occur? When will it next be visible from eastern Canada? the sky? 11. What phase would Earth be in if you were on the moon when the moon was full? first quarter? waning crescent? 3. What holidays, rituals, special foods, and beliefs are as- 12. How does the moon slow Earth's rotation? How does sociated with the winter solstice? Earth slow the moon's revolution? 4. What can you find out about Milutin Milankovitch? What is the latest news about the Milankovitch hypothesis? 13. Why isn't there an eclipse at every new and at every full moon? 14. Why is the moon red during a total lunar eclipse? Exploring TheSky 15. Why have most people seen a total lunar eclipse while fewer people have seen a total solar eclipse? 1. As discussed in this chapter, Earth's rotation about its own axis gives us the impression that the whole sky ro-tates around the north celestial pole in a period of one day. This apparent motion of the celestial sphere is dif-ficult to notice because it happens so slowly. 1. All cultures around the world named constellations. TheSky makes it possible to simulate this motion at a Why do you suppose this was such a common practice? pace that is easy to observe by using a feature called 2. Do planets orbiting other stars have ecliptics? Could Time Skip. Observe and describe the apparent motion of they have seasons? Must they have seasons? the sky as you see it looking north> east, south, and west. How to proceed: Set the Time Skip Increment (a drop-down menu on the Time Skip Toolbar) to 1 minute, and click on the Go Forward button to begin the simulation. View the sky from the four cardinal directions, due north, south, east, and west. (You'll find Time Skip under the Tools menu as well.) 3. Why would it be difficult to see prominences if you were on the moon during a total lunar eclipse? 1. If light from one star is 40 times more intense than light from another star, what is their difference in magnitudes? 2. If two stars differ by 8.6 magnitudes, what is their in-tensity ratio? 3. Star A has a magnitude of 2.5; Star B, 5.5; and Star C, 9.5. Which is brightest? Which are visible to the unaided eye? Which pair of stars has an intensity ratio of 16? 4. By what factor is sunlight more intense than moonlight? (Hint: See Figure 1-4.) 5. If you are at a latitude of 35 degrees north of Earth's equator, what is the angular distance from the northern horizon up to the north celestial pole? from the south-ern horizon down to the south celestial pole? 6. If Earth is about 5 billion (5 x 109) years old, how many precessional cycles have occurred? 7. Identify the phases of the moon if on March 21 the moon were located at (a) the vernal equinox, (b) the autumnal equinox, (c) the summer solstice, (d) the winter solstice. 2. In which constellation was the sun located on the date of your birth? (Hint: Click Data and then Site Informa-tion to set the location, date, and time of your birth. Turn on constellation figures, constellation boundaries, and labels. Then find the sun.) 3. Set your location to Earth's North Pole and the date to the summer solstice. Turn on the ecliptic and then step forward in time through a day to see what happens to the sun. Repeat for the autumnal equinox and the win-ter solstice. 4. Repeat Activity 3 above for a location on Earth's equator. Chapter 1 The View from Earth 8 In their search for evidence, modern astronomers have extended their senses with powerful instruments. a system of epicycles, deferents, and equants and tried to create a mathematical model that could accurately pre- In contrast to the geocentric universe of classical astron- omy, the universe devised by Copernicus was heliocentric, that retrograde motion, the occasional westward motion of the planets, is easily explained. Copernicus did not publish his book De Revolutionibus until 1543, the year he died. The As a critic of the classical view that Earth is at the center of the universe, Copernicus was exploring controversial ideas, eccentricity (e) dict the positions of the sun, moon, and planets. Review Questions or sun centered. One advantage of a heliocentric universe is 1~ Why did Greek astronomers conclude that the heavens were made up of perfect crystalline spheres moving at constant speeds? teachings of Aristotle had become part of Church teachings. 3, How did the Ptolemaic model explain retrograde motion? ideas that some would claim were heretical. The 99 years from the death of Copernicus to the birth of Newton marked the birth of modern science. From that time lied on the analytic methods first demonstrated by Newton. the moon with the statement, "The moon is falling." on, science depended on evidence to support theories and re- cus similar? 5. Why did the Copernican hypothesis win gradual accep- tance? 6. Why is it difficult for scientists to replace an old para- digm with a new paradigm? 7, Why did Tycho Brahe expect the new star of 1572 to show parallax? Why was the lack of parallax evidence against the Ptolemaic model? covered that the planets follow ellipses with the sun at one squared is proportional to its orbital radius cubed. 9. Explain how Kepler's laws contradict uniform circular to motion. Kepler's first law States that the orbit of each planet about the Sun is an ellipse with the Sun at one focus. Kepler's second law States that, as a planet moves around its orbit, it sweeps out equal areas in equal times. This tells us that a planet moves faster when it is closer to the Sun (near perihelion) than when it is farther from the Sun (near aphelion) in its orbit. Kepler's third law States that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun (semimajor axis), which tells us that more distant planets move more slowly in their orbits. In its original form, written p2 = a3. See also Newton's version of Kepler's third law Kepler's laws of planetary motion Three laws discovered by Kepler that describe the motion of the planets around the Sun. 10. What is the difference between a hypothesis, a theory, and a law? hypothesis is a tentative model proposed to explain some set of observed facts, but which has not yet been rigorously tested and confirmed. Three basic laws that describe how objects respond to forces. theories of relativity (special and general) Einstein's theories that describe the nature of space, time, and gravity. 11, How did the Alfonsine Tab]es, the Prutenic Tables, and the Rudolphine Tables differ? 1633, Galileo was finally condemned before the Inquisition 13. Galileo was condemned by the Inquisition, but Kepler, also a Copernican, was not. Why not? Galileo Galilei was a great defender of the Copernican hypothesis. Galileo was the first person to use a telescope observe the heavens and to recognize the significance of what he saw. His discoveries of the phases of Venus, the satellites of Jupiter, the mountains of the moon, and other phenomena helped undermine the Ptolemaic universe. In for refusing to halt his defense of Copernicanism. Born in 1642, the same year that Galileo died 14. Why did Newton conclude that gravitation had to be universal? and predict its path in the future. Isaac Newton laws of motion and the law of gravity. These laws made it possible to understand the orbital motion of the planets as a consequence of the sun's gravity. In addition, Newton's work made it possible to analyze the motion of any celestial body used the work of Kepler and Galileo to discover three planets. 2. Why did classical astronomers conclude that Earth had to be motionless? Classical astronomy was based on the writings of the Greek philosopher Aristotle. He taught that Earth was the immo- bile center of the universe and that the stars and planets equant model of the universe was given mathematical form about heliocentric universe AD 140 in the Almagest, the great work of Ptolemy. Ptolemy preserved the classical belief in geocentrism, 4. In what ways were the models of Ptolemy and Coperni- 8. How was Tycho's model of the universe similar to the Ptolemaic model? How did it resemble the Copernican model? The Danish astronomer Tycho Brahe did not accept the Ptolemaic or the Copernican model but rather developed his own, in which the sun and moon circled Earth, and the plan- ets circled the sun. Although his hypothesis was not correct, Tycho made precise observations of planetary positions that later led to a true understanding of planetary motion. Johannes Kepler, Tycho Brahe's assistant, inherited the Danish astronomer's records in 1601 and used his observa- tions to uncover three laws of planetary motion. Kepler dis- focus, that they move faster when near the sun, and that a planet's period 12. Review Galileo's telescope discoveries, and explain why they supported the Copernican model and contra-dicted the Ptolemaic model. 15. Explain why we might describe the orbital motion of 6Y Part 1 The Astronomer's Sky 1. Historian of science Thomas Kuhn has said that De Revo- lutionibus was a revolution-making book, but not a rev- olutionary book. How was it classical? 2. Why might Tycho Brahe have hesitated to hire Kepler? Why do you suppose he appointed Kepler his scientific heir? 3. How does the modern controversy over creationism and evolution reflect two ways of knowing about the physi- 1. The trial of Galileo is an important event in the history of science. We now know, and the Church now recog- nizes, that Galileo's view was correct, but what were the arguments on both sides of the issue as it was un- folding? Research the Internet for documents chroni- cling the trial, Galileo's observations and publications, and the position of the Church. Use this information to outline cases for and against Galileo in the context of the times in which the trial occurred. cal world? 2. It's hard to imagine that an observatory could exist be-fore the invention of the telescope, but Tycho Brahe's observatory at Hveen was a great astronomical center of its day. Search the Web sites on Tycho and his instru- ments and describe what an observing session at Hveen 1. If you lived on Mars, which planets would describe ret- rograde loops? Which would never be visible as cres- might have involved. cent phases? 2. angular diameter (61 seconds of arc) when it is a cres- cent and a small angular diameter (10 seconds of arc) when it is nearly full. Use the small-angle formula to distance. Is this ratio compatible with the Ptolemaic universe shown on page 41? 3. Communications satellites are obvious uses of the geo- Galileo's telescope showed him that Venus has a large synchronous oribt, but can you think of other uses for such orbits? Find an Internet site that uses or displays information gleaned from a geosynchronous orbit that provides a useful service. find the ratio of its maximum distance to its minimum Exploring TheSky 3. Galileo's telescopes were not of high quality by modern standards. He was able to see the moons of Jupiter, but he never reported seeing features on Mars. Use the small angle formula to find the angular diameter of Mars when it is closest to Earth. How does that compare with the maximum diameter of Jupiter? 4. If a planet had an average distance from the sun of 10 AU, what would its orbital period be? 5. If a space probe were sent into an orbit around the sun that brought it as close as 0.5 AU to the sun and as far away as 5.5 AU, what would its orbital period be? 6. Pluto orbits the sun with a period of 247.7 years. What is its average distance from the sun? 7. Calculate the circular velocity of Venus and Saturn around the sun. (Hint: The mass of the sun is 2 x 103° kg.) 8. What is the orbital velocity of an Earth satellite 42,200 km from Earth? How long does it take to circle its orbit once? 1. Observe Mars going through its retrograde motion. (Hint: Use Reference Lines under the View menu to turn on the ecliptic. Be sure you are in Free Rotation under the Orientation menu. Locate Mars and use the time skip arrows to watch it move.) 2. Compare the size of the retrograde loops made by Mars, Jupiter, and Saturn. 3. Can you recognize the effects of Kepler's second law in the orbital motion of any of the planets? (Hint: Use 3D Solar System Mode under the View menu.) 4. Can you recognize the effects of Kepler's third law in the orbital motion of the planets? equatorial mounting comparison spectrum Chapter 2 The Origin of Modern Astronomy 63 and focus it into an image. Because of chromatic aberration, lens partially corrects for this, but such lenses are expen- sive and cannot be made larger than about 1 m in diameter. Reflecting telescopes use a mirror to focus the light Cassegrain focus and are less expensive than refracting telescopes of the same diameter. In addition, reflecting telescopes do not suffer from chromatic aberration. most recently built large telescopes are reflectors. in an astronomical telescope, a phenomenon that astrono- gey~ew questions dry, and thus it is more transparent, especially in the infrared. The light gathered by an astronomical telescope can be 1, Why ~n,ould you not plot sound waves in the electro-recorded and analyzed by special instruments attached to the telescope. For many decades, astronomers have used photo- magnetic spectrum? graphic plates to record images at the telescope, but modern electronic systems such as CCDs have now replaced photo- graphic plates in most applications. Spectrographs spread Why? starlight out according to wavelength to form a spectrum. about astronomical telescopes, how would you improve mers must fly telescopes high in balloons or such advertisements? they can work at some wavelengths in the near infrared from thin? What problems does this cause? aircraft, though 2. If you had unlimited funds to build a large telescope, which type would you choose, a refractor or a reflector? 3. Why do nocturnal animals usually have large pupils in their eyes? How is that related to astronomical tele- scopes? an antenna, an amplifier, and a recorder. Such an instrument 4. Why do optical astronomers sometimes put their tele-scopes at the tops of mountains, while radio astronomers sometimes put their telescopes in deep valleys? Astronomers build observatories atop high mountains for two reasons. Turbulence in Earth's atmosphere blurs the image Atop a mountain, the air is steady, and the seeing is better. The air at a mountaintop is also thin and 5. Optical and radio astronomers both try to build large telescopes but for different reasons. How do these goals differ? To observe radio signals from celestial objects, we need a radio telescope, which usually consists of a dish reflector, can measure the intensity of radio signals over the sky and construct radio maps. The poor resolution of the radio tele-scope can be improved by combining it with another radio telescope to make a radio interferometer. Radio telescopes have three important features-they can detect cool hydrogen, they can see through dust clouds in space, and they can detect certain objects invisible at other wavelengths. Earth's atmosphere admits radiation primarily through two wavelength intervals, or windows-the visual window and the radio window. At other wavelengths, our atmosphere absorbs radiation. To observe in the far infrared, astrono- high mountaintops. To observe in the ultraviolet, X-ray, or gamma-ray range and some parts of the infrared, they must send their telescopes into space to get above our atmosphere. 6. What are the advantages of making a telescope mirror? 7. Small telescopes are often advertised as "200 power" or "magnifies 200 times." As someone knowledgeable 8. An astronomer recently said, "Some people think I should give up photographic plates." Why might she change to something else? 9. What purpose do the colors in a false-color image or false-color radio map serve? 10. How is chromatic aberration related to a prism spectro-graph? electromagnetic radiation eyepiece wavelength refracting telescope 11, Why would radio astronomers build identical radio telescopes in many different places around the world? photon reflecting telescope 12. Why do radio telescopes have poor resolving power? nanometer (nm) chromatic aberration angstrom (t~) achromatic lens cooled to low temperature? atmospheric window light-gathering power focal length resolving power primary lens or mirror diffraction fringe objective lens or mirror seeing 13. Why must telescopes observing in the far infrared be 14. What might we detect with an X-ray telescope that we could not detect with an infrared telescope? 15. If the Hubble Space Telescope observes at visual wave-lengths, why must it observe from space? 86 Part 1 The AstronomeYs Sky 9. Astronauts observing from a space station need a tele-scope with a light-gathering power 15,000 times that of 1. Why does the wavelength response of the human eye the human eye, capable of resolving detail as small as match so well the visual window of Earth's atmosphere? 0.1 second of arc, and having a magnifying power of 250. Design a telescope to meet their needs. Could you test your design by observing stars from Earth? 2. Basic research in chemistry, physics, biology, and similar sciences is supported in part by industry. How is as-tronomy different? Who funds the major observatories? 3. Most people like beautiful sunsets with brightly glow-ing clouds, bright moonlit nights, and twinkling stars. Most astronomers don't. Why? 10. A spy satellite orbiting 400 km above Earth is suppos-edly capable of counting individual people in a crowd. What minimum-diameter telescope must the satellite carry? (Hint: Use the small-angle formula.) 1. The thickness of the plastic in plastic bags is about 0.001 mm. How many wavelengths of red light is this? 2. Measure the actual wavelength of the wave in Fig-ure 3-1. In what portion of the electromagnetic spec-trum would it belong? 3. Compare the light-gathering powers of a 5-m telescope and a 0.5-m telescope. 4. How does the light-gathering power of one of the Keck telescopes compare with that of the human eye? (Hint: Assume that the pupil of your eye can open to about 0.8 cm.) 1. How do professional astronomers go about making ob-servations at major astronomical facilities? Visit sev-eral observatory Web sites to determine the process an astronomer would go through to secure observing time and make observations at the facility. 2. NASA is in the process of completing a fleet of four space-based "Great Observatories." (The Hubble Space Telescope is one; what are the others?) Examine the cur rent state of these missions by visiting their home pages on the Internet. What advantages would these facilities have over ground-based observatories? 5. What is the resolving power of a 25-cm telescope? What do two stars 1.5 seconds of arc apart look like through this telescope? 6. Most of Galileo's telescopes were only about 2 cm in di-ameter. Should he have been able to resolve the two stars mentioned in Problem 5? 7. How does the resolving power of the 5-m telescope compare with that of the Hubble Space Telescope? Why does the Hubble Space Telescope outperform the 5-m telescope? 8. If we build a telescope with a focal length of 1.3 m, what focal length should the eyepiece have to give a magnification of 100 times? Exploring TWO 1. Astronomical telescopes using equatorial mountings must be aligned precisely with the north celestial pole. Locate Polaris and determine how far it is from the north celestial pole. (Hint: Use Reference Lines under the View menu and check Grid under Equitorial. Be sure the spac-ing is set to auto/fine. Then locate the Little Dipper and zoom in on Polaris.) Chapter 3 Astronomical Tools page 81 The hotter ,roduces a con-,ense at the wave-. ch depends on the ostly short-wavelength mostly long-wavelength ues to the temperatures of jol stars are red. electrons surrounding an atomic ous permitted orbits. A electron ,ner orbit during a collision between re from one orbit to another by absorb-' of the proper energy. If the energy photon is too large, the atom may lose an jecome ionized. spectra? ,e only certain orbits are permitted, only photons n wavelengths can be absorbed or emitted. Each ,f atom has its own characteristic set of spectral lines. ar telescope, the gas can absorb photons of certain wave- lengths, and we will see dark lines in the spectrum at those positions. Such a spectrum is called an absorption spec- trum. If we look at a low-density gas that is excited to emit 11, How can the Doppler effect explain shifts in both light and sound? photons, we see bright lines in the spectrum at those posi- tions. Such a spectrum is called an emission spectrum. serve longer wavelengths. This Doppler effect makes it pos- sible for the astronomer to measure a star's radial velocity, 1. Why might we say that atoms are mostly empty space? 2. What is the difference between an isotope and an ion? 3. Why is the binding energy of an electron related to the size of its orbit? 4. Explain why ionized calcium can form absorption lines but ionized hydrogen cannot. 5. Describe two ways an atom can become excited. 6. Why do different atoms have different lines in their 7. Why does the amount of black body radiation emitted depend on the temperature of the object? 8. Why do hot stars look bluer than cool stars? If light passes through a low-density gas on its way to 9. What kind of spectrum does a neon sign produce? 10. Why does the Doppler effect detect only radial velocity? When a source of radiation is approaching us, we ob-serve shorter wavelengths, and when it is receding, we ob- 12. In what ways is our model of an atom a scientific model? In what ways is it incorrect? that part of its velocity directed toward or away from Earth. Discussion QUIU: New Terms nucleus permitted orbit proton energy level neutron excited atom electron ground state isotope heat ionization temperature Problems 1. Human body temperature is about 310 K (98.6°F). At what wavelength do humans radiate the most energy? What kind of radiation do we emit? 2. If a star has a surface temperature of 20,000 K, at what wavelength will it radiate the most energy? 3. Infrared observations of a star show that it is most in-tense at a wavelength of 2000 nm. What is the tempera-ture of the star's surface? 5600 ion Kelvin temperature scale molecule absolute zero Coulomb force binding energy quantum mechanics intensity (;.m.) wavelength of maximum 4. Dust orbiting distant stars may be evidence that planets have formed there. If the dust is brightest at the far- black body radiation infrared wavelength of 100,000 run, what is the temperature of the dust? 5. If astronomers observe that a liquid flowing out onto the surface of a planet is brightest at a wavelength of 100 Part I The Astronomer's Sky 1700 nm, what is its temperature? Do you think it is liq-uid water or liquid rock? 6. If we triple the temperature of a black body, by what fac-tor will the total energy radiated per second per square meter increase? 7. If one star has a temperature of 6000 K and another star has a temperature of 7000 K, how much more energy per second will the hotter star radiate from each square meter of its surface? 8. In the laboratory, the Balmer beta line of hydrogen has a wavelength of 486.10 rim. If the line appears in a comet's spectrum at 486.15 nm, what is the comet's ra-dial velocity? 9. The highest velocity stars an astronomer might observe have velocities of about 400 km/s. What change in wave-length would this cause in the spectral line described in Problem 8? 1. The name for the element helium has astronomical roots. Search the Internet for information on the discovery of helium. How and when was it discovered, and how did it get its name? Why do you suppose it took so long for helium to be recognized? 2. How was the model of the atom presented in the text you read developed? Search the Web for information on historical models of the atom and compile a time line of important developments leading to our current under-standing. What evidence exists that supports our model? 6o to the Brooks/Cole Astronomy Resource Center Iwww.broowscole. com/astronomyl for critical thinking exercises, articles, and addi-tional readings from Igfoirac Collage Edition. Brooks/Cole's online student library. Chapter 4 Atoms and Starlight 101 in that chain is planet building. As we explore the solar system in detail in the following chapters, we must stay alert for further clues to the birth of the planets. Summary into larger bodies, the largest of which grew the fastest and eventually became protoplanets. Once a planet had formed from a large number of plan-etesimals, heat from radioactive decay could have melted it 8. Why is almost every solid surface in our solar system solar nebula theory gravitational collapse extrasolar planet uncompressed density asteroid condensation sequence comet planetesimal terrestrial planet condensation Jovian planet accretion Galilean satellites protoplanet solar wind differentiation meteor outgassing meteoroid heat of formation meteorite radiation pressure half-life heavy bombardment Review s 1. What produced the helium now present in the sun's at-mosphere? in Jupiter's atmosphere? in the sun's core? 2. What produced the iron in Earth's core and the heavier elements like gold and silver in Earth's crust? 3. What evidence do we have that disks of gas and dust are common around young stars? 4. According to the solar nebula theory, why is the sun's equator nearly in the plane of Earth's orbit? The solar nebula theory proposes that the solar system began as a contracting cloud of gas and dust that flattened into a rotating disk. The center of this cloud eventually became the sun, and the planets formed in the disk of the nebula. Observational evidence gives astronomers confidence in this theory. Disks of gas and dust have been found around many young stars, so astronomers suspect that planetary systems are common. Planets orbiting other stars are too faint and too close to their star to image directly, but astron-omers have found many of these planets by observing the motion of the star as the star and planet revolve around their center of mass. The solar nebula theory explains many of the charac-teristic properties of the solar system. For example, the solar system has a disk shape. The orbits of the planets lie in nearly the same plane, and they all revolve around the sun in the same direction, counterclockwise as seen from the north. With only three exceptions, the planets rotate counterclock-wise around axes roughly perpendicular to the plane of the solar system. This disk shape and the motion of the planets appear to have originated in the disk-shaped solar nebula. Another striking feature of the solar system is the divi-sion of the planets into two families. The terrestrial planets, which are small and dense, lie in the inner part of the system. 5. Why does the solar nebula theory predict that planetary systems are common? and allowed it to differentiate into a dense metallic core and a lower-density silicate crust. In fact, it is possible that the solar nebula cooled as the protoplanets grew so that the first planetesimals were metallic and later additions were sili-cate. It is also likely that the planets grew rapidly enough that the heat of formation released by the in-falling material melted the planets and allowed them to differentiate as they formed. The Jovian planets probably grew rapidly from icy ma-terials and became massive enough to attract and hold vast amounts of nebular gas. The heat of formation raised their temperatures very high when they were young, and Jupi-ter and Saturn still radiate more heat than they absorb from the sun. Once the sun became a luminous object, it cleared the nebula as its light and solar wind pushed material out of the system. The planets helped by absorbing some planetesi mals and ejecting others from the system. Once the solar sys-tem was clear of debris, planet building ended. 6. Why do we think the solar system formed about 4.6 bil- lion years ago? Radioactive dating tells us that Earth, the moon, Mars, and meteorites are no older than 4.6 billion years. it seems our solar system took shape about 4.6 billion years ago. According to the condensation sequence, the inner part of the nebula was so hot that only high-density minerals could form solid grains. The outer regions, being cooler, con densed to form icy material of lower density. The planets grew from these solid materials, with the denser planets forming in the inner part of the nebula and the lower-density Jovian planets forming farther from the sun. 7. If you visited another planetary system, would you be surprised to find planets older than Earth? Why or why not? scarred by craters? 124 Part 2 The Solar System 8. If you stood on Earth during its formation, as described in Problem 7, and watched a region covering 100 ml, how many impacts would you expect to see in an hour? (Hints: Assume that Earth had its present radius. The surface area of a sphere is 47tr2.) 9. What is the difference between condensation and accretion? condensation is The formation of solid or liquid particles from a cloud of gas. accretion is the process by which small objects gather together to make larger objects. Planet building began as dust grains grew by condensation and accretion into planetesimals ranging from a kilometer to hundreds of kilometers in diameter. These planetesimals settled into a thin plane around the sun and accumulated Why don't terrestrial planets have rings and large satel-lite systems like the Jovian planets? 11. How does the solar nebula theory help us understand the composition of asteroids and comets? The Jovian planets are large, low-density worlds in the outer part of the system. In general, the closer a planet lies to the sun, the higher its uncompressed density. The solar system is now filled with smaller bodies such as asteroids, comets, and meteors. The asteroids are small, rocky worlds, most of which orbit the sun between Jupiter and Mars. They appear to be material left over from the for-mation of the solar system. Another important characteristic of the solar system bodies is their similar ages. 12. How does the solar nebula theory explain the dramatic density difference between the terrestrial and Jovian planets? ~ ~3. If you visited some other planetary system in the act of building planets, would you expect to see the condensation sequence at work, or was it unique to our solar system? 14. Why do we expect to find that planets are differentiated? 15. What processes cleared the nebula away and ended planet building? 9. The velocity of the solar wind is roughly 400 km/s. How long does it take to travel from the sun to Pluto? Critical Inquries 1. How does our solar system compare with the others that have been found? Search the Internet for sites that give information about planetary systems around other stars. What kinds of planets have been detected by these searches so far? Discuss the selection effects (see Win-dow on Science 16-2) that must be considered when in-terpreting these data. 2. The process of protoplanetary accretion is still not well understood. Search the Web for current research in this field. From the results of your search, outline the basic 1. Discuss the history of the atoms in your thumb. Where steps in the formation of a protoplanet through accre- did they come from and what processes brought them tion. What specific factors are important in these mod- together? els of planet building? Do these models produce plane-tary systems similar to the ones we know to exist? 2. If the solar nebula theory is correct, then there are prob-ably more planets in the universe than stars. Do you agree? Why or why not? 1. The nearest star is about 4.2 ly away. If you looked back at the solar system from that distance, what would the maximum angular separation be between Jupiter and the sun? (Hint: 1 ly equals 63,000 AU.) 3. How is radioactive dating carried out on meteorites and rocks from surfaces of various bodies in the solar sys-tem? Look for Web sites on the details of radioactive dating and summarize the methods used to uncover the abundances of radioactive elements in a particular sam-ple. (Hint: Try looking for information on how a par-ticular meteorite-for example, the Martian meteorite ALH84001-was studied, what age range was deter-mined, and what radioactive elements were used to ar-rive at the age.) 2. The brightest planet in our sky is Venus, which is some-times as bright as apparent magnitude -4 when it is at a distance of about 1 AU. How many times fainter would Exploring TheSky it look from a distance of 1 parsec (206,265 AU)? What would its apparent magnitude be? (Hint: Remember the inverse square law, Chapter 2.) 1. Look at the solar system from space. Notice how thin the disk of the solar system is and how inclined the or-bits of Pluto and Mercury are. (Hint: Under the View menu, choose 3D Solar System Mode, and then zoom in or out. Tip the solar system up and down to see it edge-on.) 3. What is the smallest-diameter crater you can identify in the photograph of Mercury on page 112? (Hint: See Ap-pendix A to find the diameter of Mercury in kilometers.) 4. A sample of a meteorite has been analyzed, and the re-sult shows that out of every 1000 nuclei of 4°K origi-nally in the meteorite, only 125 have not decayed. How old is the meteorite? (Hint: See Figure 5-9.) 5. In Table 5-2, which object's density differs least from its uncompressed density? Why? 6. What composition might we expect for a planet that formed in a region of the solar nebula where the tem-perature was about 100 K? 7. Suppose that Earth grew to its present size in 10 million Bo to the Brooks/Cole Astronomy Resource Center Iwww.broehscole. years through the accretion of particles averaging 100 g comtastronomYl for critical Making exercises, articles, and addl- each. On the average, how many particles did Earth captlonal readings from Infoirac CoINpe EdiUOt1. BrooksiCole's online ture per second? (Hint: See Appendix A to find Earth's student library. mass.) 2. Look at the solar system from space and notice how small the orbits of the inner planets are compared to the orbits of the outer planets. They make two distinct groups. (Hint: Use 3D Solar System Mode.) 3. Watch the comets orbiting around the sun. Can you lo-cate the comet C/198 M5 (Linear)? What is its orbit like? (Hint: Use 3D Solar System Mode and set the time step to 30 days (30d).) Chapter 5 The Origin of the Solar System 125 shape of any other orbits in this figure or the next? 4. Look at Figure I-6. How can you tell that Mercury follows an elliptical orbit? Can you detect the elliptical Our goal has been to preview the scale of astronomical ob- jects. To do so, we journeyed outward from a familiar cam- pus scene by expanding our field of view by factors of 100. Only 12 such steps took us to the largest structures in the universe. meters, or astronomical units for measuring certain The numbers in astronomy are so large it is not conve- distances? nient to express them in the usual way. Instead, we use the metric system to simplify our calculations and scientific no- tation to more easily write big numbers. The metric system and scientific notation are discussed in Appendix A. tell us? We live on the rotating planet Earth, which orbits a rather typical star we call the sun. We defined a unit of distance, the astronomical unit, to be the average distance from Earth Mercury is closest to the sun, and Pluto is the most distant. The sun, like most stars, is very far from its neighbor- p ing stars, and this leads us to define another unit of distance, the light-year, the distance light travels in 1 year. The near- est star to the sun is Proxima Centauri at a distance of 4.2 ly. As we enlarged our field of view, we discovered that the sun is only one of 100 billion stars in our galaxy and that our galaxy is only one of many billions of galaxies in the uni- verse. Galaxies appear to be grouped together in clusters, superclusters, and filaments, the largest structures known. . As we explored, we noted that the universe is evolving. Earth's surface is evolving, and so are stars. Stars form from the gas in space, grow old, and eventually die. We do not yet understand how galaxies form or evolve. Among the billions of stars in each of the billions of galaxies, many have planets. Although astronomers can now 5. Which is the outermost planet in our solar system? Why does that change? Pluto 6. Why are light-years more convenient than miles, kilometers Light years are greater distances while miles kilometers are short distances. 7. Why is it difficult to detect planets orbiting other stars? Interstellar dust between stars and planets. Scattering of dust. 8. What does the size of the star image in a photograph 9. What is the difference between the Milky Way and the Milky Way Galaxy? each of the billions of galaxies contains billions of stars. Most of those stars probably have families of planets like our solar system, and on some of those billions of planets liquid-water oceans and a protective atmosphere may have spawned life. It is possible that some other plan-ets in the universe are inhabited by intelligent crea-tures who share our curiosity and our wonder at the scale of the cosmos. 10. What are the largest known structures in the universe? to the sun. Of the eight other planets in our solar system, is part of a large supercluster-a cluster of clusters. Other galaxies are not scattered at random throughout the universe but lie in clusters within larger super-clusters. To represent the universe at this scale, we use a diagram in which each dot represents the location of a single galaxy. At this scale, we see superclusters linked to form long filaments outlining voids that seem nearly empty of ga)axies. These appear to be the largest structures in the universe. Were we to expand our field of view yet another time, we would probably see a uniform sea of filaments and voids. each of the billions of galaxies contains billions of stars. Most of those stars probably have families of planets like our solar system, and on some of those billions of planets liquid-water oceans and a protective atmosphere may have spawned life. It is possible that some other plan-ets in the universe are inhabited by intelligent crea-tures who share our curiosity and our wonder at the scale of the cosmos. 1. The diameter of Earth is 7928 miles. What is its diame- ter in inches? In yards? 2. If a mile equals 1.609 km and the moon is 2160 miles in diameter, what is its diameter in kilometers? 3. One astronomical unit is about 1.5 x 108 km. Explain why this is the same as 150 x 106 km 4. Venus orbits 0.7 AU from the sun. What is that distance in kilometers? 5. Light from the sun takes 8 minutes to reach Earth. How long does it take to reach Mars? 6. The sun is almost 400 times further from Earth than is the moon. How long does light from the moon take to detect planets orbiting other stars, we know very little about the nature of these planets. Yet we suppose that there must be many planets in the universe and that some are like Earth. We wonder if a few are inhabited by intelligent bei~sv like ourselves. solar system Milky Way scientific notation Milky Way Galaxy astronomical unit (AU) spiral arm light-year (ly) Local Group galaxy 1. What is the largest dimension you have personal knowl-edge of? Have you run a mile? Hiked 10 miles? Run a marathon? 2. In Figure I-4, the division between daylight and dark-ness is at the right on the globe of Earth. How do we know this is the sunset line and not the sunrise line? 3. What is the difference between our solar system, our galaxy, and the universe? Solar System contains a system of 1 bright star with 9 orbiting planets with moons orbiting some of the planets. each of the billions of galaxies contains billions of stars. Most of those stars probably have families of planets like our solar system, and on some of those billions of planets liquid-water oceans and a protective atmosphere may have spawned life. It is possible that some other plan-ets in the universe are inhabited by intelligent crea-tures who share our curiosity and our wonder at the scale of the cosmos. The Universe is Universe (or cosmos) The sum total of all matter and energy. By current understanding, only part of the universe is observable to us even in principle; we call this our observable universe. universe The sum total of all matter and energy. reach Earth? Part 1 The AstronomeYs Sky 7. If the speed of light is 3 X 105 km/s, how many kilome-ters are in a light-year? How many meters? 8. How long does it take light to cross the diameter of our Milky Way Galaxy? 9. The nearest galaxy to our own is about 2 million light-years away. How many meters is that? 10. How many galaxies like our own would it take laid edge to edge to reach the nearest galaxy? (Hint: See Problem 9.) C 1. Locate photographs of Earth taken from space. What do cities look like? Can you see highways? Is the presence of our civilization detectable from space? 2. Locate photographs of nearby galaxies and compare them with photos of very distant galaxies. What kind of detail is invisible for distant galaxies? 3. One of the biggest clusters of galaxies is the Virgo clus-ter. Find out how many and what kind of galaxies are in the cluster. Is it nearby or far away? Exploring TheSky 1. Locate and center one example of each of three differ-ent types of objects: a. A planet, such as Saturn. Find its rising and setting time. Such objects have distances measured in astro-nomical units (AU). ob How to proceed: Decide on the object you want to lo-cate. Then find and center the object by clicking the Find button on the Object Toolbar. The second method is to press the F key. The third is to click Edit, then Find. Once you have the Object Information win-dow, click the center button. b. A star. All stars in TheSkybelong to our Milky Way Galaxy. Give the star's name, its magnitude, and its distance in light-years. How to proceed: Click on any star, which brings up an Object Information window. c. A galaxy. Give its name and/or its designation. How to proceed: Click on the Galaxies button in the Object Toolbar, then click on any galaxy. Distances to galaxies are millions and billions of light years. 2. Look at the solar system from beyond Pluto by clicking on View and then on 3D Solar System Mode. Tip the solar system edge-on and then face-on. Zoom in to see the inner planets. Under Tools, set the Time Skip Incre-ment to 1 day and then go forward in time to watch the planets move. 3. Identify some of the brightest constellations located along the Milky Way. (Hint: See View, Reference Lines.) r 6o to tbo tiroaRs/COIO ~strosorll Resource Center Inww.Rrookscole. comlasirouors) lor criticsl tRInYtug erercisos, articlss, and addl-tianal resdings Iror IutoTrse Collegs EWUog, Rrsob:ICole's sgugo student Ilbrary. Introduction: The Scale of the Cosmos 9 corporated into their monument the cycles of the sun away from the moon. As the rotating Earth carries the conti- and moon, nents through these bulges of deeper water, the tides ebb The cycles in the sky are a rich part of our culture, and flow. Friction with the seabeds slows Earth's rotation, but those same motions reveal an astonishing fact- and the gravitational force the bulges exert on the moon Earth is a planet. In the next chapter, we will see how force its orbit to grow larger. humanity made that discovery. When the moon passes through Earth's shadow, sun-light is cut off, and the moon darkens in a lunar eclipse. If the moon only grazes the shadow, the eclipse is partial, or penumbral, and not total. If the moon passes directly between the sun and Earth, it produces a total solar eclipse. During such an eclipse, the tions. Although the constellations originated in Greek and bright photosphere of the sun is covered, and the fainter corona, chromosphere, and prominences become visible. An observer outside the path of totality sees a partial eclipse. If the moon is in the farther part of its orbit, it does not cover ~e Photosphere completely, resulting in an annular eclipse. Astronomers divide the sky into 88 areas called constella- Middle Eastern mythology, the names are Latin. Even the modern constellations, added to fill in the spaces between the ancient figures, have Latin names. The names of stars usually come from ancient Arabic, though modern astrono- - mers often refer to a star bv constellation and Greek letters assigned according to brightness within each constellation. New Terms The magnitude system is the astronomer's brightness scale. First-magnitude stars are brighter than 2nd-magnitude constellation autumnal equinox stars, which are brighter than 3rd-magnitude stars, and so on. The magnitude we see when we look at a star in the sky asterism summer solstice is its apparent visual magnitude. magnitude scale winter solstice The celestial sphere is a model of the sky, carrying the apparent visual magnitude perihelion celestial objects around Earth. 1. What is the difference between an asterism and a con- stellation? Give some examples. 2. Do people in other parts of the world see the same con- stellations, asterisms, lunar phases, lunar eclipses, and d lunar phases takes 29.53 days. The moon's gravitational field exerts tidal forces on solar eclipses that you see? Explain. 3` Earth that pull the ocean waters up into two bulges, one on the side of Earth facing the moon and the other on the side 3~ What does the word apparent mean in apparent visual magnitude? 36 Part 1 The Astronomer's Sky 4. In what ways is the celestial sphere a scientific model? 8. The average distance of Mars from the sun is 1.52 AU. 5. If Earth did not rotate, could we define the celestial poles Use the small-angle formula to calculate the angular di- ameter of the sun as seen from Mars. and celestial equator? the ecliptic? 9. Draw a diagram showing Earth, the moon, and shadows during (a) a total solar eclipse, (b) a total lunar eclipse, (c) a partial lunar eclipse, (d) an annular eclipse. 6. Where would you go on Earth so you could see both north and south celestial poles at the same time? Where would you go to see a celestial pole at your zenith? 7. What is the difference between the daily and annual mo-tion of the sun? 8. What would our seasons be like if Earth were tipped 35° instead of 23.5°? What would they be like if Earth's axis were perpendicular to its orbit? criticat Inquiries 9. Why are the seasons reversed in the Southern Hemi- 1. Nearly all cultures have populated the sky with gods, 10. Where would you look in the sunset or dawn skies to heroes, animals, and objects. What can you learn on the find the full moon? the first quarter moon? the waning Web about non-Western constellations? crescent moon? the waxing gibbous moon? 2. Who was Orion? How is he related to the scorpion in sphere? 10. A total solar eclipse was visible from eastern Canada on July 10, 1972. The seasons are reversed south of Earth's equator. The motion of Earth changes in ways that can affect the vernal equinox climate. Changes in orbital shape, in precession, and in axial tilt can alter the planet's heat balance and may be responsible for the ice ages and glacial periods. Because we see the moon by reflected sunlight, its shape appears to change as it orbits Earth. The lunar phases wax from new moon to first quarter to full moon and wane from full moon to third quarter to new moon. A complete cycle of When did this eclipse next occur? When will it next be visible from eastern Canada? the sky? 11. What phase would Earth be in if you were on the moon when the moon was full? first quarter? waning crescent? 3. What holidays, rituals, special foods, and beliefs are as- 12. How does the moon slow Earth's rotation? How does sociated with the winter solstice? Earth slow the moon's revolution? 4. What can you find out about Milutin Milankovitch? What is the latest news about the Milankovitch hypothesis? 13. Why isn't there an eclipse at every new and at every full moon? t l 14. Why is the moon red during a total lunar eclipse? Exploring TheSky 15. Why have most people seen a total lunar eclipse while fewer people have seen a total solar eclipse? 1. As discussed in this chapter, Earth's rotation about its own axis gives us the impression that the whole sky ro-tates around the north celestial pole in a period of one day. This apparent motion of the celestial sphere is dif-ficult to notice because it happens so slowly. However, 1. All cultures around the world named constellations. TheSky makes it possible to simulate this motion at a Why do you suppose this was such a common practice? pace that is easy to observe by using a feature called 2. Do planets orbiting other stars have ecliptics? Could Time Skip. Observe and describe the apparent motion of they have seasons? Must they have seasons? the sky as you see it looking north> east, south, and west. How to proceed: Set the Time Skip Increment (a drop-down menu on the Time Skip Toolbar) to 1 minute, and click on the Go Forward button to begin the simulation. View the sky from the four cardinal directions, due north, south, east, and west. (You'll find Time Skip under the Tools menu as well.) 3. Why would it be difficult to see prominences if you were on the moon during a total lunar eclipse? 1. If light from one star is 40 times more intense than light from another star, what is their difference in magnitudes? 2. If two stars differ by 8.6 magnitudes, what is their in-tensity ratio?  3. Star A has a magnitude of 2.5; Star B, 5.5; and Star C, 9.5. Which is brightest? Which are visible to the unaided eye? Which pair of stars has an intensity ratio of 16? 4. By what factor is sunlight more intense than moonlight? (Hint: See Figure 1-4.) 5. If you are at a latitude of 35 degrees north of Earth's equator, what is the angular distance from the northern horizon up to the north celestial pole? from the south-ern horizon down to the south celestial pole? 6. If Earth is about 5 billion (5 x 109) years old, how many precessional cycles have occurred? 7. Identify the phases of the moon if on March 21 the moon were located at (a) the vernal equinox, (b) the autumnal equinox, (c) the summer solstice, (d) the winter solstice. 2. In which constellation was the sun located on the date of your birth? (Hint: Click Data and then Site Informa-tion to set the location, date, and time of your birth. Turn on constellation figures, constellation boundaries, and labels. Then find the sun.) 3. Set your location to Earth's North Pole and the date to the summer solstice. Turn on the ecliptic and then step forward in time through a day to see what happens to the sun. Repeat for the autumnal equinox and the win-ter solstice. 4. Repeat Activity 3 above for a location on Earth's equator. Reuiew Questions or sun centered. One advantage of a heliocentric universe is 1. Why did Greek astronomers conclude that the heavens were made up of perfect crystalline spheres moving at constant speeds? teachings of Aristotle had become part of Church teachings. 3. How did the Ptolemaic model explain retrograde motion? ideas that some would claim were heretical.cus similar? geosynchronous satellite were carried around Earth by great crystalline spheres. This model of the universe was given mathematical form about AD 140 in the Almagest, the great work of Ptolemy. Ptolemy preserved the classical belief in geocentrism, but he re- ellipse escape velocity placed the concentric, Earth-centered heavenly spheres with a system of epicycles, deferents, and equants and tried to create a mathematical model that could accurately pre- dict the positions of the sun, moon, and planets. In contrast to the geocentric universe of classical astron- omy, the universe devised by Copernicus was heliocentric. 5. Why did the Copernican hypothesis win gradual accep- tance? 6. Why is it difficult for scientists to replace an old para- digm with a new paradigm? 7. Why did Tycho Brahe expect the new star of 1572 to show parallax? Why was the lack of parallax evidence against the Ptolemaic model? covered that the planets follow ellipses with the sun at one 9. Explain how Kepler's laws contradict uniform circular motion. that retrograde motion, the occasional westward motion of the planets, is easily explained. Copernicus did not publish his book De Revolutionibus until 1543, the year he died. The As a critic of the classical view that Earth is at the center of the universe, Copernicus was exploring controversial ideas, The Danish astronomer Tycho Brahe did not accept the Ptolemaic or the Copernican model but rather developed his own, in which the sun and moon circled Earth, and the plan- ets circled the sun. Although his hypothesis was not correct, Tycho made precise observations of planetary positions that later led to a true understanding of planetary motion. Johannes Kepler, Tycho Brahe's assistant, inherited the Danish astronomer's records in 1601 and used his observa- tions to uncover three laws of planetary motion. Kepler dis- focus, that they move faster when near the sun, and that a planet's period squared is proportional to its orbital radius cubed. 10. What is the difference between a hypothesis, a theory, and a law? 11. How did the Alfonsine Tab]es, the Prutenic Tables, and 13. Galileo was condemned by the Inquisition, but Kepler, also a Copernican, was not. Why not? Galileo Galilei was a great defender of the Copernican hypothesis. Galileo was the first person to use a telescope to observe the heavens and to recognize the significance of what he saw. His discoveries of the phases of Venus, the satellites of Jupiter, the mountains of the moon, and other phenomena helped undermine the Ptolemaic universe. In the Rudolphine Tables differ? 1633, Galileo was finally condemned before the Inquisition for refusing to halt his defense of Copernicanism. Born in 1642, the same year that Galileo died, Isaac 14. Why did Newton conclude that gravitation had to be universal? and predict its path in the future. Newton used the work of Kepler and Galileo to discover three laws of motion and the law of gravity. These laws made it possible to understand the orbital motion of the planets as a consequence of the sun's gravity. In addition, Newton's work made it possible to analyze the motion of any celestial body The 99 years from the death of Copernicus to the birth of Newton marked the birth of modern science. From that time the moon with the statement, The moon is falling. on, science depended on evidence to support theories and re- lied on the analytic methods first demonstrated by Newton. 2. Why did classical astronomers conclude that Earth had to be motionless? 4. In what ways were the models of Ptolemy and Coperni- 8. How was Tycho's model of the universe similar to the Ptolemaic model? How did it resemble the Copernican model? 12. Review Galileo's telescope discoveries, and explain why they supported the Copernican model and contra-dicted the Ptolemaic model. 15. Explain why we might describe the orbital motion of 6Y Part 1 The Astronomer's Sky 1. The trial of Galileo is an important event in the history of science. We now know, and the Church now recog- nizes, that Galileo's view was correct, but what were the arguments on both sides of the issue as it was un- folding? Research the Internet for documents chroni- cling the trial, Galileo's observations and publications, and the position of the Church. Use this information to outline cases for and against Galileo in the context of the times in which the trial occurred. 1. Historian of science Thomas Kuhn has said that De Revo- lutionibus was a revolution-making book, but not a rev- olutionary book. How was it classical? 2.Why do you suppose he appointed Kepler his scientific heir? 3. How does the modern controversy over creationism and evolution reflect two ways of knowing about the physical world? 2. It's hard to imagine that an observatory could exist be-fore the invention of the telescope, Tycho Brahe's observatory at Hveen was a great astronomical center of its day. Search the Web sites on Tycho and his instru- ments and describe what an observing session at Hveen might have involved. synchronous oribt, but can you think of other uses for such orbits? Find an Internet site that uses or displays information gleaned from a geosynchronous orbit that 1. If you lived on Mars, which planets would describe ret- rograde loops? Which would never be visible as cres- cent phases? 3. Communications satellites are obvious uses of the geo- 2. Galileo's telescope showed him that Venus has a large angular diameter (61 seconds of arc) when it is a cres- cent and a small angular diameter (10 seconds of arc) when it is nearly full. Use the small-angle formula to provides a useful service. find the ratio of its maximum distance to its minimum distance. Is this ratio compatible with the Ptolemaic universe shown on page 41? Exploring TheSky 3. Galileo's telescopes were not of high quality by modern standards. He was able to see the moons of Jupiter, but he never reported seeing features on Mars. Use the small angle formula to find the angular diameter of Mars when it is closest to Earth. How does that compare with the maximum diameter of Jupiter? 4. If a planet had an average distance from the sun of 10 AU, what would its orbital period be? 5. If a space probe were sent into an orbit around the sun that brought it as close as 0.5 AU to the sun and as far away as 5.5 AU, what would its orbital period be? 6. Pluto orbits the sun with a period of 247.7 years. What is its average distance from the sun? 7. Calculate the circular velocity of Venus and Saturn around the sun. (Hint: The mass of the sun is 2 x 103° kg.) 8. What is the orbital velocity of an Earth satellite 42,200 km from Earth? How long does it take to circle its orbit once? 1. Observe Mars going through its retrograde motion. (Hint: Use Reference Lines under the View menu to turn on the ecliptic. Be sure you are in Free Rotation under the Orientation menu. Locate Mars and use the time skip arrows to watch it move.) 2. Compare the size of the retrograde loops made by Mars, Jupiter, and Saturn. 3. Can you recognize the effects of Kepler's second law in the orbital motion of any of the planets? (Hint: Use 3D Solar System Mode under the View menu.) 4. Can you recognize the effects of Kepler's third law in the orbital motion of the planets? Chapter 2 The Origin of Modern Astronomy 63 and focus it into an image. Because of chromatic aberration, lens partially corrects for this, but such lenses are expen- sive and cannot be made larger than about 1 m in diameter. Reflecting telescopes use a mirror to focus the light and are less expensive than refracting telescopes of the same diameter. In addition, reflecting telescopes do not suffer from chromatic aberration. The most recently built large telescopes are reflectors. The light gathered by an astronomical telescope can be telescope. For many decades, astronomers have used photo- graphic plates to record images at the telescope, but modern electronic systems such as CCDs have now replaced photo- graphic plates in most applications. Spectrographs spread Why? starlight out according to wavelength to form a spectrum. To observe radio signals from celestial objects, we need a radio telescope, which usually consists of a dish reflector, can measure the intensity of radio signals over the sky and construct radio maps. The poor resolution of the radio tele-scope can be improved by combining it with another radio telescope to make a radio interferometer. Radio telescopes have three important features-they can detect cool hydro-gen, they can see through dust clouds in space, and they can detect certain objects invisible at other wavelengths. Earth's atmosphere admits radiation primarily through thin? What problems does this cause? two wavelength intervals, or windows-the visual window and the radio window. At other wavelengths, our atmosphere absorbs radiation. To observe in the far infrared, astrono- high in balloons or aircraft, though such advertisements? they can work at some wavelengths in the near infrared from high mountaintops. To observe in the ultraviolet, X-ray, or gamma-ray range and some parts of the infrared, they must send their telescopes into space to get above our atmosphere. 1. Why would you not plot sound waves in the electro-recorded and analyzed by special instruments attached to the magnetic spectrum? 2. If you had unlimited funds to build a large telescope, which type would you choose, a refractor or a reflector? 3. Why do nocturnal animals usually have large pupils in their eyes? How is that related to astronomical tele- scopes? an antenna, an amplifier, and a recorder. Such an instrument 4. Why do optical astronomers sometimes put their tele-scopes at the tops of mountains, while radio astrono-mers sometimes put their telescopes in deep valleys? Astronomers build observatories a top high mountains for two reasons. Turbulence in Earth's atmosphere blurs the image in an astronomical telescope, A top a mountain, the air is steady, and the seeing is better. The air at a mountaintop is also thin and dry, and it is more transparent, especially in the infrared. 5. Optical and radio astronomers both try to build large telescopes but for different reasons. How do these goals differ? 6. What are the advantages of making a telescope mirror 7. Small telescopes are often advertised as "200 power" or "magnifies 200 times." As someone knowledgeable about astronomical telescopes, how would you improve mers must fly telescopes 8. An astronomer recently said, "Some people think I should give up photographic plates." Why might she change to something else? 9. What purpose do the colors in a false-color image or false-color radio map serve? 10. How is chromatic aberration related to a prism spectro-graph? electromagnetic radiation eyepiece wavelength refracting telescope 11, Why would radio astronomers build identical radio telescopes in many different places around the world? photon reflecting telescope 12. Why do radio telescopes have poor resolving power? nanometer (nm) chromatic aberration angstrom (t~) achromatic lens cooled to low temperature? atmospheric window light-gathering power focal length resolving power primary lens or mirror diffraction fringe objective lens or mirror seeing 13. Why must telescopes observing in the far infrared be 14. What might we detect with an X-ray telescope that we could not detect with an infrared telescope? 15. If the Hubble Space Telescope observes at visual wave-lengths, why must it observe from space? page 86 Part 1 The AstronomeYs Sky 9. Astronauts observing from a space station need a tele-scope with a light-gathering power 15,000 times that of 1. Why does the wavelength response of the human eye the human eye, capable of resolving detail as small as match so well the visual window of Earth's atmosphere? 0.1 second of arc, and having a magnifying power of 250. Design a telescope to meet their needs. Could you test your design by observing stars from Earth? 2. Basic research in chemistry, physics, biology, and similar sciences is supported in part by industry. How is as-tronomy different? Who funds the major observatories? 3. Most people like beautiful sunsets with brightly glow-ing clouds, bright moonlit nights, and twinkling stars. Most astronomers don't. Why? 10. A spy satellite orbiting 400 km above Earth is suppos-edly capable of counting individual people in a crowd. What minimum-diameter telescope must the satellite carry? (Hint: Use the small-angle formula.) 1. The thickness of the plastic in plastic bags is about 0.001 mm. How many wavelengths of red light is this? 2. Measure the actual wavelength of the wave in Fig-ure 3-1. In what portion of the electromagnetic spec-trum would it belong? 3. Compare the light-gathering powers of a 5-m telescope and a 0.5-m telescope. 4. How does the light-gathering power of one of the Keck telescopes compare with that of the human eye? (Hint: Assume that the pupil of your eye can open to about 0.8 cm.) 1. How do professional astronomers go about making ob-servations at major astronomical facilities? Visit sev-eral observatory Web sites to determine the process an astronomer would go through to secure observing time and make observations at the facility. 2. NASA is in the process of completing a fleet of four space-based "Great Observatories." The Hubble Space Telescope is one; what are the others? Examine the cur rent state of these missions by visiting their home pages on the Internet. What advantages would these facilities have over ground-based observatories? 5. What is the resolving power of a 25-cm telescope? What do two stars 1.5 seconds of arc apart look like through this telescope? 6. Most of Galileo's telescopes were only about 2 cm in di-ameter. Should he have been able to resolve the two stars mentioned in Problem 5? 7. How does the resolving power of the 5-m telescope compare with that of the Hubble Space Telescope? Why does the Hubble Space Telescope outperform the 5-m telescope? 8. If we build a telescope with a focal length of 1.3 m, what focal length should the eyepiece have to give a magnification of 100 times? Exploring TWO 1. Astronomical telescopes using equatorial mountings must be aligned precisely with the north celestial pole. Locate Polaris and determine how far it is from the north celestial pole. (Hint: Use Reference Lines under the View menu and check Grid under Equitorial. Be sure the spac-ing is set to auto/fine. Then locate the Little Dipper and zoom in on Polaris.) 6o to the Brooks/Cole Astronony Resource Center Iwww.broohscole. com/astronowy) Chapter 3 Astronomical Tools page 81 emission line radial velocity (V,) Kirchhoff's laws or dense .. The hotter ,roduces a con-,ense at the wave-.ch depends on the ostly short-wavelength mostly long-wavelength ues to the temperatures of jol stars are red. electrons surrounding an atomic ous permitted orbits. A electron ,ner orbit during a collision between re from one orbit to another by absorb-' of the proper energy. If the energy photon is too large, the atom may lose an jecome ionized. spectra? ,e only certain orbits are permitted, only photons n wavelengths can be absorbed or emitted. Each ,f atom has its own characteristic set of spectral lines. ar telescope, the gas can absorb photons of certain wave- lengths, and we will see dark lines in the spectrum at those positions. Such a spectrum is called an absorption spec- trum. If we look at a low-density gas that is excited to emit tions. Such a spectrum is called an emission spectrum. serve longer wavelengths. This Doppler effect makes it pos- sible for the astronomer to measure a star's radial velocity, 8. Why do hot stars look bluer than cool stars? If light passes through a low-density gas on its way to 9. What kind of spectrum does a neon sign produce? 10. Why does the Doppler effect detect only radial velocity? 11, How can the Doppler effect explain shifts in both light and sound? photons, we see bright lines in the spectrum at those posi- When a source of radiation is approaching us, we ob-serve shorter wavelengths, and when it is receding, we ob- 1, In what ways is our model of an atom a scientific model? In what ways is it incorrect? that part of its velocity directed toward or away from Earth. 1. Why might we say that atoms are mostly empty space? 2. What is the difference between an isotope and an ion? 3. Why is the binding energy of an electron related to the size of its orbit? 4. Explain why ionized calcium can form absorption lines but ionized hydrogen cannot. 5. Describe two ways an atom can become excited. We have described how certain discrete amounts of en-ergy can be absorbed by an atom, raising it to an excited state and moving one of its electrons farther from its nu-cleus. If enough energy is absorbed, the electron can be completely removed from the atom. The atom is then said to be ionized. The mini-mum amount of energy required to remove one electron from an atom in its ground state is called its ionization energy, or ionization potential. Still greater amounts of energy must be absorbed by the now-ionized atom (called an ion) to remove an addi-tional electron deeper in the structure of the atom. Succes sively greater energies are needed to remove the third, fourth, fifth, and so on, electrons from the atom. If enough energy is available (in the form of very short-wavelength photons or from a collision with a very fast-moving electron or another atom), an atom can become completely ionized, losing all of its electrons. A hydrogen atom, having only one electron to lose, can be ionized only once; a helium atom can be ionized twice, and an oxygen atom up to eight times. When we examine regions of the cosmos where there is a great deal of radiation, such as the neighborhoods where hot young stars have recently formed, we see a lot of ion-ization going on. An atom that has become ionized has lost a negative charge-which was carried away by the electron-and thus is left with a net positive charge. It therefore exerts a strong attraction for any free electron. Eventually, one or more electrons will be captured and the atom will become neutral (or ionized to one less degree) again. During the electron capture process, the atom emits one or more pho-tons, depending on whether the electron is captured at once to the lowest energy level of the atom or stops at one 6. Why do different atoms have different lines in their 7. Why does the amount of black body radiation emitted depend on the temperature of the object? Discussion Questions Problems 1. Human body temperature is about 310 K (98.6°F). At what wavelength do humans radiate the most energy? What kind of radiation do we emit? 2. If a star has a surface temperature of 20,000 K, at what wavelength will it radiate the most energy? 3. Infrared observations of a star show that it is most in-tense at a wavelength of 2000 nm. What is the tempera-ture of the star's surface? ion Kelvin temperature scale 4, Dust orbiting distant stars may be evidence that planets molecule absolute zero have formed there. If the dust is brightest at the far- Coulomb force black body radiation infrared wavelength of 100,000 run, what is the temper- binding energy wavelength of maximum ature of the dust? quantum mechanics intensity (;.m.) 5. If astronomers observe that a liquid flowing out onto the surface of a planet is brightest at a wavelength of 100 Part I The Astronomer's Sky 1700 nm, what is its temperature? Do you think it is liq-uid water or liquid rock? 6. If we triple the temperature of a black body, by what fac-tor will the total energy radiated per second per square meter increase? 7. If one star has a temperature of 6000 K and another star has a temperature of 7000 K, how much more energy per second will the hotter star radiate from each square meter of its surface? 8. In the laboratory, the Balmer beta line of hydrogen has a wavelength of 486.10 rim. If the line appears in a comet's spectrum at 486.15 nm, what is the comet's ra-dial velocity? 9. The highest velocity stars an astronomer might observe have velocities of about 400 km/s. What change in wave-length would this cause in the spectral line described in Problem 8? 1. The name for the element helium has astronomical roots. Search the Internet for information on the discovery of helium. How and when was it discovered, and how did it get its name? Why do you suppose it took so long for helium to be recognized? 2. How was the model of the atom presented in the text you read developed? Search the Web for information on historical models of the atom and compile a time line of important developments leading to our current under-standing. What evidence exists that supports our model? Chapter 4 Atoms and Starlight 101 in that chain is planet building. As we explore the solar system in detail in the following chapters, we must stay alert for further clues to the birth of the planets. Summary and allowed it to differentiate into a dense metallic core and a lower-density silicate crust. In fact, it is possible that the solar nebula cooled as the protoplanets grew so that the first planetesimals were metallic and later additions were sili-cate. It is also likely that the planets grew rapidly enough that the heat of formation released by the in-falling material melted the planets and allowed them to differentiate as they formed. The Jovian planets probably grew rapidly from icy ma-terials and became massive enough to attract and hold vast amounts of nebular gas. The heat of formation raised their temperatures very high when they were young, and Jupi-ter and Saturn still radiate more heat than they absorb from the sun. Once the sun became a luminous object, it cleared the nebula as its light and solar wind pushed material out of the system. The planets helped by absorbing some planetesi mals and ejecting others from the system. Once the solar sys-tem was clear of debris, planet building ended. The solar nebula theory proposes that the solar system be-gan as a contracting cloud of gas and dust that flattened into a rotating disk. The center of this cloud eventually became the sun, and the planets formed in the disk of the nebula. Observational evidence gives astronomers confidence in this theory. Disks of gas and dust have been found around many young stars, so astronomers suspect that planetary systems are common. Planets orbiting other stars are too faint and too close to their star to image directly, but astron-omers have found many of these planets by observing the motion of the star as the star and planet revolve around their center of mass. The solar nebula theory explains many of the charac-teristic properties of the solar system. For example, the solar system has a disk shape. The orbits of the planets lie in nearly the same plane, and they all revolve around the sun in the same direction, counterclockwise as seen from the north. With only three exceptions, the planets rotate counterclock-wise around axes roughly perpendicular to the plane of the solar system. This disk shape and the motion of the planets appear to have originated in the disk-shaped solar nebula. Another striking feature of the solar system is the divi-sion of the planets into two families. The terrestrial planets, which are small and dense, lie in the inner part of the system. The Jovian planets are large, low-density worlds in the outer part of the system. In general, the closer a planet lies to the sun, the higher its uncompressed density. The solar system is now filled with smaller bodies such as asteroids, comets, and meteors. The asteroids are small, rocky worlds, most of which orbit the sun between Jupiter and Mars. They appear to be material left over from the for-mation of the solar system. Another important characteristic of the solar system bodies is their similar ages. Radioactive dating tells us that Earth, the moon, Mars, and meteorites are no older than 4.6 billion years. it seems our solar system took shape about 4.6 billion years ago. According to the condensation sequence, the inner part of the nebula was so hot that only high-density minerals could form solid grains. The outer regions, being cooler, con densed to form icy material of lower density. The planets grew from these solid materials, with the denser planets forming in the inner part of the nebula and the lower-density Jovian planets forming farther from the sun. Planet building began as dust grains grew by condensa-tion and accretion into planetesimals ranging from a kilome-ter to hundreds of kilometers in diameter. These planetesimals settled into a thin plane around the sun and accumulated lion years ago? into larger bodies, the largest of which grew the fastest and eventually became protoplanets. Once a planet had formed from a large number of plan-etesimals, heat from radioactive decay could have melted it 8. Why is almost every solid surface in our solar system Critical Inc. 1. How does our solar system compare with the others that have been found? Search the Internet for sites that give information about planetary systems around other stars. What kinds of planets have been detected by these searches so far? Discuss the selection effects (see Win-dow on Science 16-2) that must be considered when in-terpreting these data. 2. The process of protoplanetary accretion is still not well understood. Search the Web for current research in this field. From the results of your search, outline the basic steps in the formation of a protoplanet through accre- tion. 1. Discuss the history of the atoms in your thumb. Where did they come from and what processes brought them together? The star maybe asteroids, comets. What specific factors are important in these mod- els of planet building? Do these models produce plane-tary systems similar to the ones we know to exist? 2. If the solar nebula theory is correct, then there are prob-ably more planets in the universe than stars. Do you agree? Why or why not? 1. The nearest star is about 4.2 ly away. If you looked back at the solar system from that distance, what would the maximum angular separation be between Jupiter and the sun? (Hint: 1 ly equals 63,000 AU.) 3. How is radioactive dating carried out on meteorites and rocks from surfaces of various bodies in the solar sys-tem? Look for Web sites on the details of radioactive dating and summarize the methods used to uncover the abundances of radioactive elements in a particular sam-ple. (Hint: Try looking for information on how a par-ticular meteorite-for example, the Martian meteorite ALH84001-was studied, what age range was deter-mined, and what radioactive elements were used to ar-rive at the age.) 2. The brightest planet in our sky is Venus, which is some-times as bright as apparent magnitude -4 when it is at a distance of about 1 AU. How many times fainter would Exploring TheSky it look from a distance of 1 parsec (206,265 AU)? What would its apparent magnitude be? (Hint: Remember the inverse square law, Chapter 2.) 1. Look at the solar system from space. Notice how thin the disk of the solar system is and how inclined the or-bits of Pluto and Mercury are. (Hint: Under the View menu, choose 3D Solar System Mode, and then zoom in or out. Tip the solar system up and down to see it edge-on.) 3. What is the smallest-diameter crater you can identify in the photograph of Mercury on page 112? (Hint: See Ap-pendix A to find the diameter of Mercury in kilometers.) 4. A sample of a meteorite has been analyzed, and the re-sult shows that out of every 1000 nuclei of 4°K origi-nally in the meteorite, only 125 have not decayed. How old is the meteorite? (Hint: See Figure 5-9.) 5. In Table 5-2, which object's density differs least from its uncompressed density? Why? 6. What composition might we expect for a planet that formed in a region of the solar nebula where the tem-perature was about 100 K? 7. Suppose that Earth grew to its present size in 10 million each. On the average, how many particles did Earth cap- (Hint: See Appendix A to find Earth's student library. mass.) 2. Look at the solar system from space and notice how small the orbits of the inner planets are compared to the orbits of the outer planets. They make two distinct groups. (Hint: Use 3D Solar System Mode.) 3. Watch the comets orbiting around the sun. Can you lo-cate the comet C/198 M5 (Linear)? What is its orbit like? (Hint: Use 3D Solar System Mode and set the time step to 30 days (30d).) Chapter 5 The Origin of the Solar System 125 against each other to build folded mountain chains, and slide sinuous rille jumbled terrain over each other at subduction zones where sections of crust ejecta fission hypothesis are pushed back down into the interior. This process, plate tectonics, continually reshapes Earth's surface. ray condensation hypothesis Earth's first atmosphere, its primary atmosphere, was secondary crater capture hypothesis rich in carbon dioxide and nitrogen, but carbon dioxide dis- micrometeorite large-impact hypothesis solved into the ocean waters and became sediments. The at- mosphere was left rich in nitrogen, and the evolution of plant life added large amounts of oxygen. The air we breathe is ~~~; =,''~ Ques~on~ a secondary atmosphere. Today, the greenhouse effect warms Earth's surface, and the ozone layer protects us from ultraviolet radiation. multiringed basin depends on the planet's mass and temperature. folded mountain range relative age Earth is the largest terrestrial planet in our solar system, terminator anorthosite Earth's magnetic field. limb breccia The rocky crust floats on a plastic mantle and is divided mare 1. What are the four stages in Earth's development? 2. Why do we expect planets to have differentiated? 3. How do we know that Earth has a molten core? Volcanic activity of volcanoe eruptions sending molten lava to the surface. 4. How does plate tectonics create and destroy Earth's crust? 5. Why do we suspect that Earth's primeval atmosphere was rich in carbon dioxide? 6. Why doesn't Earth have as many craters as the moon? maria are dark basalts, whereas the highlands are lighter ~~ How do we find the relative ages of the moon's maria and highlands? 8. Why do we believe that the lunar crater Tycho is a young crater? 9. Why are there no folded mountain ranges on the moon? 10. How do the lunar samples suggest that the moon formed with a molten surface? As the crust solidified, cratering battered it and exca- 11. WhY ar'e so many lunar samples breccias? The moon is, at first glance, dramatically different from Earth. We see two distinct kinds of terrain on the moon. The maria, named after seas, are lowland plains with few craters. They are great lava flows and are younger than the highlands. The highlands are lighter in color and heavily cratered. Samples returned by the Apollo missions show that the rock such as anorthosite. Many samples are breccias, which show how severely the moon's crust has been fractured. portance of each of these processes in the evolution of a planet and it has passed through all four stages. Studies show that Earth has differentiated into a metallic core and a silicate crust. Currents in the molten portion of the core produce into sections that pull apart along midocean rises 12. What do the vesicular basalts tell us about the evolution of the lunar surface? 13. What evidence would we expect to find on the moon if it had been subjected to plate tectonics? the moon is small, it has cooled rapidly, its crust has grown 14. Cite objections to the fission, condensation, and cap- ture hypotheses. 15. How does the large-impact hypothesis explain the moon system formed in the collision of two large planetesi- The ages of the mare samples range from about 3.1 to 3.8 billion years, but the highland samples are about 4.1 to 4.6 billion years old. Few samples are older than 4.3 billion years. This suggests that the moon's surface was molten until about 4.3 billion years ago. vated great basins. Later, molten rock rose through fractures in the crust and flooded the basins to produce the maria. By the time the maria began to form, cratering had declined, so the maria are marked by few craters. The lunar surface now is evolving very slowly. Because thick, and it has never divided into moving plates. Now only meteorites alter the surface. The large-impact hypothesis suggests that the Earth- that became the higher-density, iron-rich Earth. Ejected man- tle material, poor in iron, formed a disk and eventually con- densed to form the lower-density, iron-poor moon. moon's lack of iron? mals. The differentiated bodies formed a single large object Discussion Questions 1. If we visited a planet in another solar system and dis-covered oxygen in its atmosphere, what might we guess about its surface? comparative planetology primeval atmosphere - P waves secondary atmosphere S waves greenhouse effect dynamo effect plate tectonics eF mantle basalt plastic midocean rise 2. If liquid water is rare on the surface of planets, then most terrestrial planets must have COZ rich atmospheres. Why? 3. Old science-fiction paintings and drawings of colonies on the moon often showed very steep, jagged mountains. Why did the artists assume that the mountains would be more rugged than mountains on Earth? Why are lunar mountains actually less rugged than mountains on Earth? Chapter 6 Earth and Its Moon 145 3. Earth's magnetic poles are not coincident with its axis of rotation, nor are their positions fixed. Search for in- formation about the location of the magnetic poles and their motions. Why do the positions change? 4. What are the oldest known rocks on Earth? Where do they come from, and how were their ages determined? 5. What evidence can you find about our changing climate? Is the rise in Earth's temperature related to the green- house effect? Scientific, industrial, and political leaders do not agree on this issue, so be prepared to analyze ar- 6. Can you locate an image of the Antarctic ozone hole? Find data for the ozone concentration in the upper atmo- sphere over the place where you live. What is the latest news concerning ozone depletion? 1. About what percent of Earth's volume is occupied by its iron core? its rocky crust? 2. If the Atlantic seafloor is spreading at 3 cm/year and is now 6400 km across, how long ago were the continents in contact? 3. The dinosaurs died and mammals arose about 65 mil- lion years ago. For what percent of Earth's history have we mammals been a leading life form? 4. If we estimate that our atmosphere is 200 km deep, what arguments with great care. percent of Earth's radius is that? 5. What is the ratio of the volume of the moon to the vol- ume of the Earth? Why is that different from the ratio of the masses? 6. Why do small worlds cool faster than large worlds? Com-pare surface area to volume. 7. The smallest detail visible through Earth-based tele-scopes is about 1 second of arc in diameter. What size is that on the moon? (Hint: Use the small-angle formula.) 8. The subduction zones on Earth are 1 km or less across. Why are we sure that such features don't exist on the moon? 9. The Imbrium Basin is about 1300 km in diameter. What percentage of the moon's surface is that? 10. If Earth's moon orbited Jupiter, what would its maximum angular diameter be as seen from Earth? (Hint: Use the small-angle formula.) 11. The full moon has an apparent magnitude of -12.5. If it were orbiting Jupiter, how bright would it be? (Hints: Sunlight is dimmer at Jupiter's distance, and the moon would be further from Earth. Use the inverse square law.) 7. What did the Apollo astronauts do while they were on the moon? Search for Web sites that describe the exper-iments performed during the astronauts' extravehicular activities (EVAs). What types of experiments/activities were performed? What did we learn from them? 8. Search for maps of the moon that show geological infor-mation such as chemical composition, elevation, age, and so on. Do the maps confirm your understanding of the lunar surface? 9. What would it be like to walk on the lunar surface? Apollo astronauts visited six different locations on the moon ex-ploring the variations in lunar terrain. Describe the hori zons and general relief of the landing locations of the dif-ferent missions by exploring Web sites that provide lunar surface photography from the missions. What differ-ences do you see between images from landings in high-lands and in maria? 12. An astronaut in a space suit is about 1 meter in diame-ter. Could we have seen the astronauts on the moon with Earth-based telescopes? 13. While two Apollo astronauts went down to the lunar sur-face, a third stayed behind in the command module or-biting 200 km above the surface. Could the astronauts be seen from the command module? CrltICi~l ~'- Exploring The.S~ry 1. Locate the moon in the sky. What constellation is it in? Find the moon's phase as a percentage of full, its angu-lar size diameter, and its distance from Earth. Use Find to locate the moon. 2. Follow the moon through one complete orbit measuring its distance and angular diameter. How do these change? Under Tools, use the Moon Phase Calendar. Change dates by clicking on Data and then Site Information. 1. Search the Web to find the location of the most recent earthquakes, and plot them on the map on page 130. The Earth quakes somewhere each day. Search for seismo graphs that will give you information about the most re-cent earthquakes. 2. Search for information about the arrangement of land masses on Earth before Pangaea. Were there previous su-percontinents? What was Rodinia? What is the evidence? 146 Part Z The Solar System REUiEw Why doesn't Mars have coronae like those on Venus? The coronae on Venus are believed to have been caused by rising currents of molten magma in the mantle push-ing upward under the crust and then withdrawing to eave the circular scars called coronae. Earth, Venus, and Mars have had significant amounts of internal heat, and there is plenty of evidence that they have had ris-ing convection currents of magma under their crusts. Of course, we wouldn't expect to see coronae on Earth because its surface is rapidly modified by erosion and plate tectonics. Furthermore, the mantle convection on Earth seems to produce plate tectonics rather than coro-nae. Tides have locked them to Mars so they keep the same side facing the planet as they follow their orbits. resonance composite volcano lobate scarp shield volcano intercrater plain corona smooth plain outflow channel subsolar point runoff channel Chapter 7 Mercury, Venus and, Mars The surface of Mercury has been found to be heavily cratered, as would be expected for a small body without any atmosphere or weathering or plate tectonic activity. The cratering is slightly different from the patterns on the Moon, probably because Mercury's stronger gravity would keep debris from flying as far after an impact. There have been some incredibly strong hits on Mercury. 1. How does the tidal coupling between Mercury and the sun differ from that between the moon and Earth? 2. Why does Mercury have lobate scarps while Earth, its moon, Venus, and Mars do not? 3. What evidence do we have about the interior of Mercury? The interior of Mercury must contain a large metallic core to account for the planet's high density. A slight shrink-age in the diameter of the planet at the time of the cooling of the core may have led to wrinkling of the crust. This would ac-count for the lobate scarps that mark all of the photographed parts of Mercury. Venus is Earth's twin in size and density, but the planet has evolved along divergent lines because it is slightly closer to the sun. The higher temperature evaporated any early oceans and prevented the absorption of carbon dioxide from the atmosphere. The accumulating carbon dioxide created a greenhouse effect that produces a surface temperature of 745 K (882°F). Mercury is a small world, only about 40 percent larger than Earth's moon; and, like the moon, it has lost any permanent atmosphere it might have had and its interior has cooled rap-idly. Debris from the formation of the solar system cratered Mercury's surface as it did the moon's, and lava flows buried parts of that terrain under the intercrater plains. The Caloris Basin, formed near the end of cratering, is the largest crater basin. Soon after the Caloris impact, lava flows created the smooth plains. These lava plains have about the same color as the intercrater plains, so the contrast between plains and highlands is not as obvious on Mercury as it is on the moon. 4. Why would we expect Venus and Earth to be similar? Why do they differ? The crust of Venus, mapped by radar, is marked by low rolling plains and highlands with some very high volcanoes. Spacecraft that have reached the surface have found dark basalts. Coronae appear to be produced by rising currents of molten rock below the crust. Where the magma breaks through, it builds volcanoes and floods the surface with lava flows. The surface contains about 10 percent as many im-pact craters as the lunar maria, so planetary scientists sus-pect the entire planet has been resurfaced by volcanism within the last billion years. No magnetic field has been detected around Venus, and that suggests that the core is not molten iron generating a magnetic field through the dynamo effect as in Earth's core. The occasional resurfacing of the planet by volcanism may have cooled the interior so it is now solid. Another mystery is the retrograde rotation, which may have been produced by tidal effects or by the off-center impact of a large plan-etesimal during the formation of the planet. 5. What evidence do we have that Venus and Mars once come from, and where did it go? Mars is smaller than Earth and Venus but larger than Mercury. Its southern hemisphere is old and cratered; the northern hemisphere has been resurfaced by lava flows or water. The lack of folded mountains and the vast size of the volcanoes suggest that plate motion does not occur. Mars once had liquid water on its surface, producing riverlike runoff channels and later floodlike outflow chan-nels. The atmosphere must have been thicker in the past to allow liquid water, but the climate changed as gas leaked into space and water froze in permafrost. Layered terrain near the polar caps shows the climate varies in cycles, but much of the water may be lost forever. Deimos and Phobos, the two small moons of Mars, may be captured asteroids. Like most small moons, they are ir-regular in shape and heavily cratered. 0.000133 second before the main echo, how high is the spot above the average surface of Venus? 7. The smallest feature visible through an Earth-based telescope has an angular diameter of about 1 second of arc. If a crater on Mars is just visible when Mars is at its closest to Earth, how big is the crater? (Hint: Use the small-angle formula.) had more water than at present? Where did that water 8. What is the maximum angular diameter of Phobos as seen from the surface of Mars? as seen from Earth? 6. How do we know there is no plate tectonics on Venus? How has the crust of Venus evolved? 7. Why doesn't Mars have mountain ranges like those on Earth? Why doesn't Earth have large volcanoes like those on Mars? 9. Deimos is about 12 km in diameter and has a density of 2 g/cm3. What is its mass? (Hint: The volume of a sphere is 3nr3.) 8. Explain why Venus and Mars both have carbon dioxide-rich atmospheres. How did Earth avoid such a fate? 9. What evidence do we have that the climate on Mars has changed? Mars, however, is a smaller world and must have cooled faster. We see no evidence of plate tectonics, and we do see giant volcanoes that suggest rising plumes of magma erupting up through the crust at the same point over and over. Perhaps we see no coronae on Mars be-cause the crust of Mars rapidly grew too thick to deform easily over a rising plume. On the other hand, perhaps we could think of the entire Tharsis bulge as a single giant corona. Planetary astronomers haven't explored enough planets yet to see all the fascinating combinations na-ture has in store for us. Perhaps after we have visited a few hundred terrestrial planets, we will recognize the geology of Mars as typical of medium-size worlds. Of course, Mars is not medium in terms of its location. Of the terrestrial planets, Mars is the farthest from the sun. How has that affected the evolution of its atmosphere? Venus seems totally inhospitable, but humans may someday visit Mercury and will probably colonize Mars. 10. How can we estimate the ages of the erosional features on Mars? How did they form? 11. Why are Phobos and Deimos nonspherical? Why is Earth's moon not quite spherical? Jon Questions 1. From your knowledge of comparative planetology, de-scribe the view astronauts would have if they landed on the surface of Mercury. 2. From what you know of Earth, Venus, and Mars, do you expect the volcanoes on Venus and on Mars to be active or extinct? Why? 3. If humans colonize Mars, the biggest problem may be finding water and oxygen. With plenty of solar energy beating down through the thin atmosphere, how might colonists extract water and oxygen from the Martian environment? Critical Inquiries for the Web 1. Who decides how planetary features are named? Sur-face features on Venus are (mostly) named after female figures from history and mythology, while figures from the arts and music are used to name features on Mer-cury. Look for information on planetary nomenclature and summarize the way different types of features on Venus are assigned names. 2. "Martians" have fascinated humans for the last century or more. There are many online sources that chronicle the representation of life on Mars throughout history and in literature. Read about the Martians as represented by a particular literary work or nonfiction account and dis-cuss to what extent it is (or is not) based on realistic views of the nature of Mars both in terms of our current understanding and the views of that period. 3. Spacecraft are exploring Mars right now. Search for the latest discoveries and photographs. What spacecraft are on their way to Mars, about to be launched, or being planned right now? 4. What plans are being made to send another spacecraft to Mercury? Problems Exploring TheSlry 1. If we transmitted radio signals to Mercury when it was closest to Earth and waited to hear the radar echo, how long would we wait? 2. Calculate the orbital velocity of Mercury in its orbit around the sun in kilometers per second. 3. Suppose we sent a spacecraft to land on Mercury and it transmitted radio signals to us at a wavelength of 10 cm. If we observed Mercury at its greatest angular distance west of the sun, to what wavelength would we have to tune our radio telescope to detect the signals? (Hint: Use the Doppler effect.) 4. What is the maximum angular diameter of Venus as seen from Earth? (Hint: Use the small-angle formula.) 5. How long did it take for radio signals from the Magel-lan spacecraft orbiting Venus to reach Earth? 6. If you send a radio signal down toward the surface of Venus and you hear an echo from a certain spot Part 2 The Solar System 1. Mercury, Venus, and Mars exhibit phases similar to the moon's phases. a. Use TheSky to sketch the appearance of Mercury, Venus, and Mars at the present time. Also give the phase as a percentage of full. How to proceed: Use Find to give you a highly mag-nified view of a particular planet. Click on the planet to obtain Object Information. b. Explain the observed phase on the basis of geometri-cal relationship between the sun, Earth, and the planet as shown by the 3D Solar System Mode. 2. Why don't the terrestrial planets have rings? If you were to search for a ring among the terrestrial planets, where liquid metallic hydrogen grooved terrain would you look first? 1. What is the maximum angular diameter of Jupiter as gossamer rings ovoid seen from Earth? Repeat this calculation for Saturn and Pluto. (Hints: See Data Files Six, Seven, and Ten and also By the Numbers 1-2.) 2. What is the angular diameter of Jupiter as seen from Callisto? from lo? (Hint: See By the Numbers 1-2.) 1. Why is Jupiter so much richer in hydrogen and helium than Earth? Interior: Molecular hydrogen, ice, and rocky core. Featureless cloud tops, unlike the other three Jovian planets. Jupiter is mostly light gas and Earth is mostly rock. Jupiter is only about 315 times the mass of Earth. Jupiter has 1300x the mass of Earth. Jupiter's density is only slightly greater than water, which means it must be largely composed of gases. Careful study of the spectrum of light reflected from Jupiter reveals that it, like the rest of the outer gas giants, is largely composed of hydrogen and helium with a small amount of hydrogen compounds such as methane (CH4) and ammonia (NH 3). This spectral analysis is essentially a plot of intensity of light vs. the frequency (or wavelength) of the light. finding the hydrogen & helium in Jupiter's atmosphere is that they are mainly visible in the ultraviolet region of the spectrum. 3. Measure the photograph in Data File Seven and calcu-late the oblateness of Saturn. 2. How can Jupiter have a liquid interior and not have a liquid surface? Jupiter radiates about twice as much heat as it receives from the Sun the Earth only radiates about half of a percent as much as it absorbs. One other interesting departure from the structure of the inner planets lies underneath the upper atmosphere of Jupiter and Saturn. Pressures are so high there 70,000,000 times Earth's atmospheric pressure near Jupiter's center, and 1,400,000 Earth atmospheres just 7000 km beneath the surface that the hydrogen is thought to be compressed into liquid metallic hydrogen. Saturn's cooler temperatures & lower pressures have caused a liquid helium rain to fall for billions of years. This acts just like differentiation since it's just heavier things falling closer to the center and pushing lighter things out of the way & funnels heat from the center to the surface. The lower potential energy of the helium is exchanged for heat energy. Saturn is almost 10% larger at the equator than pole-to-pole. Careful consideration of the mass, spin, and degree of bulge tells us that Saturn probably has more of its mass tied up in its rocky core. Relative to Jupiter, Saturn has a much smaller region of liquid metallic hydrogen surrounding its core, and a much larger region of ordinary H and He around it. This is reasonable, since it takes mass to generate the pressure needed to form metallic H. Saturn doesn't have much mass compared to Jupiter, so those conditions are only found close to the core. 3. How does the dynamo effect account for the magnetic fields of Jupiter and Saturn? Saturn As the helium sinks toward the center, the planet’s gravitational field compresses it and heats it up. The energy released is the source of Saturn’s internal heating. 4. Why are the belts and zones on Saturn less distinct than those on Jupiter? In a Jovian planet, the light-colored zones form in high-pressure regions where rising gas cools and condenses to form icy crystals of ammonia, which we see as bright clouds. Saturn is twice as far from the sun as Jupiter, so sunlight is weaker and the atmosphere is colder. The rising gas currents don't have to rise as high to reach temperatures cold enough to form clouds. Because the clouds form deeper in the hazy atmosphere, they are not as brightly illuminated by sunlight and look dimmer. A layer of methane haze above the clouds makes the belts and zones look even less distinct. 5. Why do we conclude that neither Jupiter's ring nor Sat-urn's rings can be left over from the formation of the planets? The gravitational influence of Saturn's innermost medium-sized moon, Mimas. Over time, particles orbiting within the Division have been deflected by Mima's gravity into eccentric orbits that cause them to collide with other ring particles, effectively moving them into new orbits at different radii. The net result is that the number of ring particles in the Cassini Division is greatly reduced. Jupiter's faint ring, as photographed (nearly edge-on) by Voyager 2. The ring, made up of dark fragments of rock and dust, possibly chipped off the innermost moons by meteorites, was unknown before the two Voyager spacecraft arrived at the planet. It lies in Jupiter's equatorial plane, only 50,000 km above the cloud tops. 6. How can a moon produce a gap in a planetary ring system? 7. Explain how geological activity on Jupiter's moons varies with distance from the planet. 4. If we observe light reflected from Saturn's rings, we should see a red shift at one edge of the rings and a blue shift at the other edge. If we observe a spectral line and see a difference in wavelength of 0.056 nm, and the unshifted wavelength (observed in the laboratory) is 500 nm, what is the orbital velocity of particles at the outer edge of the rings? (Hint: See By the Numbers 4-2.) 5. One way to recognize a distant planet is by its motion along its orbit. If Uranus circles the sun in 84 years, how many seconds of arc will it move in 24 hours? (Hint: Ig-nore the motion of Earth.) 6. If the ^ ring is 50 km wide and the orbital velocity of Uranus is 6.81 km/s, how long a blink should we ex-pect to see when the ring crosses in front of the star? 8. What makes Saturn's F ring and the rings of Uranus and Neptune so narrow? 7. What is the angular diameter of Pluto as seen from the surface of Charon? (Hint: See Figure 8-20.) 9. Why is the atmospheric activity of Uranus less than that g, If Pluto has a surface temperature of 50 K, at what wave- of Saturn and Neptune? length will it radiate the most energy? (Hint: See By the 10. Why do we suspect that Enceladus has been geologically Numbers 4-1.) active more recently than some other moons? 9. How long did it take radio commands to travel from Earth to Voyager 2 as it passed Neptune? 11. What are the seasons on Uranus like? 12. Why are Uranus and Neptune blue? 10. Use the orbital radius and orbital period of Charon to 13. What evidence do we have that Triton has been geolog- calculate the mass of the Pluto-Charon system. (Hints: ically active recently? Express the orbital radius in meters and the period in 14. How do astronomers account for the origin of Pluto? seconds. Then see By the Numbers 2-1.) 15. What evidence do we have that catastrophic impacts have occurred in the solar system's past? 1. Some astronomers argue that Jupiter and Saturn are un-usual, while other astronomers argue that all solar sys-tems should contain one or two such giant planets. What do you think? Support your argument with evidence. 1. If you lived on the surface of Pluto and looked into the sky to observe Charon, what phases would you see? (Hint: Be sure to consider your location on the planet when answering this question.) 2. What factors caused Voyager 2 to see a bland atmosphere when it encountered Uranus in 1986? Given these cir-cumstances, would images of the Uranian atmosphere taken by a space probe arriving at Uranus in 2006 be sim-ilar to those taken in 1986, or would there be significant differences? 204 Part 2 The Solar System 3. Imagine what you'd think if you had been the first per- Exploring TheSky son ever to see Saturn through a telescope. When Gali- leo first observed Saturn in 1610, he did not recognize 1. Zoom in on Jupiter and observe the orbital motion of its that it was a ringed planet. It was many years later be- moons. (Hint: Under Tools, choose Time Skip and then fore the strange apparition of Saturn was finally attrib- Tracking Setup. Lock on Jupiter. Then use the Time Skip uted to a ring structure. Search the Web for information buttons to make the moons move around their orbits.) on historical observations of Saturn, summarize the ob- 2. Calculate the mass of Jupiter from your own observa- servations of Galileo and others, and determine who was tions of the orbital period and orbital radius of Jupiter's first to recognize what he saw as a ring. Why do you sup- moons. pose it took so long to understand that Saturn is a ringed planet? 3. Repeat Activities 1 and 2 above for Saturn. 4. Should Pluto be called a planet or not? Search the Web for news and debate on this issue. What is your opinion? 5. Who was Clyde Tombaugh? What can you find out about his life after he discovered Pluto? Chapter 8 Worlds of the Outer Solar System page 205 10. What evidence do we have that some asteroids have had active surfaces? 11. How is the composition of meteorites related to the for- mation and evolution of asteroids? to 100 kilometers in diameter. What is the difference between a type I tail and a type II tail? 13. What evidence do we have that cometary nuclei are rich in ices? 14. Why do short-period comets tend to have orbits near the plane of the solar system? 15. How did the bodies in the Kuiper belt and the Oort cloud form? between Mars and Jupiter. The strong gravitational influence of Jupiter could have prevented the material from accumu- lating into a large body. A comet is produced by a lump of dirty ices a few dozen In a long, elliptical orbit, the 12 icy body stays frozen until the object draws close to the sun. Then the ices vaporize and release the embedded dust and debris. The gas is caught in the solar wind and blown out- ward to form a type I, or gas, tail. The pressure of sunlight blows the dust away to form a type II, or dust, tail. The coma of a comet can be up to 1,000,000 km in diameter, and it con- tains jets issuing from the nucleus. When spacecraft flew past Comet Halley in 1986, astronomers discovered that the nucleus was coated by a dark crust and that jets of vapor and dust were venting from active regions on the sunlit side. The low density of the nucleus showed that it was an irregular mixture of ices and silicates probably containing large voids. 1. Futurists suggest we may someday mine the asteroids Comets are believed to have formed as icy planetesimals for materials to build and supply space colonies. What in the outer solar system, and some of these objects remain kinds of materials could we get from asteroids? (Hint: as the Kuiper belt, the source of some short-period comets. What are S-, M-, and C-type asteroids made of?) Many icy planetesimals were swept up by the formation of 2 the Jovian planets, and others were ejected to form the Oort If cometary nuclei were heated by internal radioactive decay rather than by solar heat, how would comets dif- fer from what we observe? loud, the source of the long-period comets. Ads Widmanstatten pattern meteor shower chondrite type I, or gas, tail chondrule type II, or dust, tail carbonaceous chondrite coma achondrite Oort cloud stony-iron meteorite Kuiper belt 3. From what you know now, do you think the government should spend money to locate near-Earth asteroids? How serious is the risk? Prablems 1. Large meteorites are hardly slowed by Earth's atmo-sphere. Assuming the atmosphere is 100 km thick and that a large meteorite falls perpendicular to the surface, how long does it take to reach the ground? (Hint: About how fast do meteoroids travel?) 2. What is the orbital velocity of a meteoroid whose aver-age distance from the sun is 2 AU? (Hint: Find the or-bital period from Kepler's third law. See Table 2-1.) 1. What do Widmanstatten patterns tell us about the history of iron meteorites? 3. If a single asteroid 1 km in diameter were fragmented into meteoroids 1 m in diameter, how many would it yield? (Hint: Volume of sphere = 37cr3.) 2. What do chondrules tell us about the history of chon- drites? 3. Why are there no chondrules in achondritic meteorites? Chondrules, or kleine Kugeln small sphere as they were originally termed (G. Rose, 1864), are mm-sized igneous objects that are the major component of chondritic meteorites. Achondrites are stony meteorites that lack small inclusions called chondrules. These stones have been melted and are very much like terrestrial igneous rocks than other meteorites. Achondrites are interpreted as volcanic rocks from the asteroid belt. 4. Why do astronomers refer to carbonaceous chondrites as "unmodified"? Carbonaceous chondrite is any stony meteorite or asteroid containing material associated with life hydrocarbons, amino acids, and forms resembling microscopic fossils and for which some researchers have postulated an extraterrestrial biological origin. Instead of containing the anhydrous silicates found in most chondrites, the carbonaceous types have claylike hydrous silicate minerals. They also contain carbonate and sulfate minerals, iron oxides, and sulfur. Magnesium sulfate is found in narrow veins; and since it is water soluble, carbonaceous chondrites disintegrate rapidly because of weathering. They comprise about 3 percent of all the meteorites collected after being seen in flight. Their texture, similar to that of the terrestrial rocks called volcanic tuffs, indicates that they have been repeatedly fragmented and re-cemented. 5. How do observations of meteor showers reveal one of the sources of meteoroids? 6. How can most meteors be cometary if all meteorites are asteroidal? 7. Why do we think the asteroids were never part of a planet? 6. What is the maximum angular diameter of Ceres as seen from Earth? Could Earth-based telescopes detect sur-face features? Could the Hubble Space Telescope? (Hint: See By the Numbers 1-2.) 7. If the velocity of the solar wind is about 400 km/s and fragmented? 8. What evidence do we have that the asteroids are the visible tail of a comet is 10' km long, how long does it take an atom to travel from the nucleus to the end of 9. What evidence do we have that some asteroids have the visible tail? differentiated? 8. If you saw Comet Halley when it was 0.7 AU from Earth and it had a visible tail 5° long, how long was the tail 4. What is the orbital period of a typical asteroid? (Hint: Use Kepler's third law. See Table 2-1.) 5. If half a million asteroids each 1 km in diameter were assembled into one body, how large would it be? (Hint: Volume of sphere = 3~r3.) Chapter 9 Meteorites, Asteroids, and Comets 223 in kilometers? Suppose that the tail was not perpendic- 3. Search for the IAU Minor Planet Center Web pages, and ular to your line of sight. Is your answer too large or too find out what asteroids and what comets have come clos- small? (Hint: See By the Numbers 1-2.) est to Earth. 9. What is the orbital period of a cometary nucleus in the Oort cloud? What is its orbital velocity? 4. Search for information about comets in the sky right (Hints: Use now. Are any comets bright enough for you to see? Are Kepler's third law. The circumference of a circular or- any bright comets expected soon? bit = 21cr.) 10. The mass of an average comet's nucleus is about 10'z kg. If the Oort cloud contains 200 x 109 cometary nuclei, Exploring TINNY what is the mass of the cloud in Earth masses? (Hint: Mass of Earth = 6 x 1024 kg.) 1. Of the five brightest asteroids, which has the most in-clined orbit? (Hint: Use Filters in the View menu to turn off everything but the sun, stars, ecliptic, and minor planets. You can use Tracking Setup under Time Skip in the Tools menu to lock onto an object and follow it along the ecliptic as time passes.) 1. Some nights are better for looking for meteors than oth- ers (see Table 9-1). We know these showers are associ- ated with comets, but how are these associations made? There are several sites on the Internet that provide in- formation on meteor showers, including historical data and information on parent comets. Pick a shower whose parent comet is known, and summarize how we came View menu and watch objects orbit the sun.) to know that the meteors and the comet are related. 2. Do asteroids go through retrograde motion? (Hint: Use Filters in the View menu to turn off the stars and turn,Qn the Equatorial Grid. See Activity 1 above.) 3. Of the five brightest asteroids, which has the most el- liptical orbit? (Hint: Use 3D Solar System Mode in the 4. Of the comets shown, which has the smallest orbit? (Hint: Use 3D Solar System Mode in the View menu and watch objects orbit the sun.) 2. The chances are small that you will be killed by an asteroid impact; but, if there are objects out there that astron-omers are not aware of whose orbits intersect Earth, we could be in for a surprise one day. Look for information on the LONEOS project and other investigations into near-Earth asteroids. How many such objects have been discovered? What is the record for closest known pas-sage of an asteroid to Earth? s Go to the Brooks/Cole Astronomy Resource Center Irrww.twooYacole. com/asironomyl for critical thinking exercises, articles, and addi-tional readings from InfoTrac College Edition, Brooks/Cole's online student library. 224 Part 2 The Solar System Ne-W Terms sunspot granulation convection filtergram spicule supergranule magnetic carpet helioseismology Maunder butterfly diagram Maunder minimum Zeeman effect active region differential rotation Babcock model Review Ouestions prominence filament flare reconnection aurora (plural aurorae) coronal hole coronal mass ejection (CME) weak force strong force nuclear fission nuclear fusion Coulomb barrier proton-proton chain neutrino 2. What would the spectrum of an auroral display look like? Why? 3. What observations would you make if you were ordered to set up a system that could warn astronauts in orbit of dangerous solar flares? Such a system exists. 1. The radius of the sun is 0.7 million km. What percent-age of the radius is taken up by the chromosphere? 2. The smallest detail visible with ground-based solar tele-scopes is about 1 second of arc. How large a region does this represent on the sun? (Hint: Use the small-angle formula.) 3. What is the angular diameter of a star like the sun located 5 ly from Earth? Is the Hubble Space Telescope able to resolve detail on the surface of such a star? 4. If a sunspot has a temperature of 4200 K and the solar surface has a temperature of 5800 K, how many times brighter is the surface compared with the sunspot? (Hint: Use the Stefan-Boltzmann law, By the Numbers 4-1.)  1. Why can't we see deeper than the photosphere? 2. What evidence do we have that granulation is caused by convection? 3. How are granules and supergranules related? How do they differ? 5. A solar flare can release 1025 J. How many megatons of TNT would be equivalent? (Hint: A 1-megaton bomb produces about 4 X 10's J.) 6. The United States consumes about 2.5 X 10'9 J of en-ergy in all forms in a year. How many years could we run the United States on the energy released by the solar flare in Problem 5? 4. How can a filtergram reveal structure in the chromo-sphere? 7. Neglecting energy absorbed or reflected by our atmo-sphere, the solar energy hitting 1 square meter of Earth's surface is 1360 J/s (the solar constant). How long does 6. What heats the chromosphere and corona to high tem- it take a baseball diamond (90 ft on a side) to receive perature? 1 megaton of solar energy? (Hint: See Problem 5.) 5. What evidence do we have that the corona has a very high temperature? 7. How are astronomers able to explore the layers of the 8. How much energy is produced when the sun converts sun below the photosphere? 1 kg of mass into energy? 8. What evidence do we have that sunspots are magnetic? 9. How much energy is produced when the sun converts 9. How does the Babcock model explain the sunspot cycle? 1 kg of hydrogen into helium? (Hint: How does this problem differ from Problem 8?) 10. What does the spectrum of a prominence tell us? What does its shape tell us? 10. A 1-megaton nuclear weapon produces about 4 x 10's J of energy. How much mass must vanish when a 5-megaton 11. How can solar flares affect Earth? weapon explodes? 12. Why does nuclear fusion require high temperatures? 13. Why does nuclear fusion in the sun occur only near the center? 14. How can astronomers detect neutrinos from the sun? Neutrinos are so small that they can pass almost unimpeded through significant thicknesses of virtually any material on Earth. When scientists tried to measure them, they found many fewer than calculations based on solar fusion would suggest. This was the origin of the deficit. The neutrino has begun surrendering its secrets in recent years, however, as scientists brought on-line special underground laboratories that block access to all but the most persistent Neutrinos, whose activities can be measured and photographed as they smack into water-filled receptacles. atmospheric Neutrinos "oscillate," or change flavors, as they journey from the sun to the Earth. Physicists agree that oscillation would not be possible unless Neutrinos had mass. The lab is the equivalent of a 10-story building constructed 6,800 feet below the Earth's surface. It contains a spherical tank 12 meters in diameter filled with 1,000 metric tons of heavy water -- water composed of heavy isotopes of hydrogen. The tank is monitored by about 10,000 light sensors. Last year, in its first report since the observatory began operation, the research team showed that the "deficit" existed not because the Neutrinos were mysteriously disappearing, but simply because muon and tau Neutrinos could not be reliably counted. But that experiment was based on tabulations comparing Sudbury data that measured only electron Neutrinos with data from another underground lab that was measuring electron Neutrinos and a bit of something else -- presumably muons and taus. Then, however, the Sudbury scientists dug deeper into their data. By screening out residual radioactivity and other unwanted interference, they isolated the reaction they wanted. When the neutrino enters the tank, it can collide with a heavy water nucleus and knock a neutron loose. Neutrinos have to interact with matter at just the right, infrequent rate. Supernova explosions occur when neutri- nos escape from the cores of collapsing stars, deposit some the cosmic background radiation (CBR). of their energy in the surrounding stellar envelope, and cause it to blow out and away into space. More than a mile deep within the bowels of a Canadian nickel mine, scientists for the first time have counted all the solar Neutrinos that are hitting the Earth, researchers announced yesterday. Neutrinos are elusive, subatomic particles so small that thousands of them pass unimpeded through every human being every second. The research, at the Sudbury neutrino Observatory, near Sudbury, Ontario, demonstrated conclusively that there is no "solar neutrino deficit," scientists said. Neutrinos produced by the sun are reaching the Earth instead of mysteriously vanishing en route, as some scientists had theorized. "Previous experiments have only seen one-third to one-half of what there is," said the University of Pennsylvania's Eugene Beier, one of an army of physicists involved in the experiment. "For the first time, we've been able to measure all of the Neutrinos at once. If neutrinos did not interact with matter at all they would escape from the cores of col- lapsing stars without causing the explosion. If neutrinos in- teracted strongly with matter, they would remain trapped in the stellar core. In either case, the heavy elements would remain locked up inside the collapsing star. If gravity were a much stronger force than it is, stars could form that contain much smaller masses, and their lifetimes would be measured in years rather than billions of years. Chemical processes, on the other hand, would not be speeded up if gravity were a stronger force, and so there would be no time for life to develop while stars were so short-lived. Even if life did develop in a stronger-gravity universe, life forms would have to be tiny or they could not galaxies had been able to form, space would have been stand up or move around. filled with intense x rays and gamma rays, and it would Solar Neutrino Problem Neutrinos are particles that were originally thought to be massless particles, just as photons are massless. (That is, IF photons could be brought to rest, their mass would be zero). Also, neutrinos are without charge. Finally, their interact little with matter. They can travel through light-years worth of solid lead and not interact. The solar model, being a good scientific model, makes prediction of the number of the neutrinos expected from the Sun. We'd like to try to observe neutrinos from the Sun to see how the prediction compares with the observations. If one could detect neutrinos from the reactions in the Sun, one would be looking directly into the center of the sun, where the action is! How to do this? By building a neutrino telescope! Neutrinos sometimes do react with matter. For example, it is possible for a neutrino to interact with an atom of chlorine (Cl37 ) to produce argon (Ar37 ). We can make a telescope out of chlorine and examine it, after a while, to see if there are any argon atoms in the fluid. From this, one could then make a comparison of what is observed with what is expected from the model. Solar Neutrino Experiments Results: Many fewer than expected! This is problem (!) called the solar neutrino problem. It is one of the more important problems in physics and astronomy today (and for the past ~30 years). 15. How can neutrino oscillation explain the solar neutrino problem? neutrino Virtually massless and chargeless particle that is one of the products of fusion reactions in the Sun. Neutrinos move at close to the speed of light, and interact with matter hardly at all. solar neutrino problem The discrepancy between the theoretically predicted numbers of neutrinos streaming from the Sun as a result of fusion reactions in the core and the numbers actually observed. The observed number of neutrinos is only about half the predicted number. 1. What energy sources on Earth cannot be thought of as ( stored sunlight? Solar Panels. Nuclear Reactors. Water Hydro Dams. 1. Do disturbances in one layer of the solar atmosphere produce effects in other layers? Yes. 10-1 The Solar Atmosphere The sun is 109 times Earth's diameter and 333,000 , times Earth's mass. This seems dramatic, but look at Data File Eleven and notice the sun's density. It is : only a little bit more dense than water. So, although ': the sun is very large and very massive, it must be a : gas from its surface to its center. When we look at the ', sun we see only the outer layers of this vast sphere of gas. In fact, these outer layers, the solar atmosphere, extend high above the visible surface of the sun. Heat Flow in the Sun Simple logic tells us that energy in the form of heat is flowing outward from the sun's interior. The solar spectrum reveals that the temperature of the sun's sur- An image of the sun in visible light shows a few sunspots. The Earth-moon system is added for scale. (Daniel Good) Average distance from Ear Maximum distance from Earth Minimum distance from Earth Average angular diameter seen from Earth Period of rotation Radius Energy Transport Mechanisms Conduction - heat is transmitted by electrons moving in a medium Radiation - heat is transmitted by photons Convection - heat is transmitted by bulk motion of a gas or liquid Which of these mechanisms is important inside the Sun? Conduction is unimportant because the density of the Sun is too low for energy to be moved as rapidly by this mechanism as by the other two. In stars with extreme densities such as white dwarfs, conduction can be important. The relative importance of radiation or convection will depend on several factors and varies with position within the Sun. Radiative Transfer in the Sun's Interior For regions close to the thermonuclear burning core, radiation dominates the outward movement of energy. Photons do not just fly out of the interior of the Sun (what would the Sun's temperature be if they did?). a single visible light photon's worth of energy takes about 30,000 years to escape (as compared to the direct flight time of about 2 sec!). photons get emitted and re-absorbed many times in traveling from the core to the surface. A photon executes a random walk from the core to the surface (or you could say that it diffuses from the center to the surface). Mass Average density Escape velocity at surface Luminosity Surface temperature Central temperature Spectral type Apparent visual magnitude Absolute visual magnitude Data File Eleven 1.00 AU (1.495979 X 108 km) 1.0167 AU (1.5210 x 108 km) 0.9833 AU (1.4710 X 108 km) 0.53' (32 minutes o 5.38 days at equator 0' km (109 Rj 1.989 Y 103' kg (333,000 M®) 1.409 g/anr 617.7 km/s 3.826 x 1026 JIs 5800 K 15 x 106K G2 V -26.74 4.83 13.6 Exploring MAY We have seen that filter-grams are useful in identifying the layers of the solar atmosphere and the structures within them. Visit a Web site that provides daily solar images, choose today's date (or one near it), and examine the sun in several wave-lengths to explore the relation between disturbances in various layers. Chapter 10 The Sun-Our Star 245 2. Explore the Web to find out how auroral activity is af- fected as solar activity rises and falls through the solar cycle. What changes in auroral visibility occur during this cycle? In what other ways can the increased activ- ity associated with a solar maximum affect Earth? 1. Locate the six photos of the sun provided in TheSky and attempt to draw in the sun's equator in each photo. (Hint: In the sun's information box, choose More Infor- mation and then Multimedia. What features are visible in these images that help us recognize the orientation 3. Explore the Web to find photos and observations of au- rorae. From what places on Earth are aurorae most often seen? of the sun's equator?) 4. What can you find on the Web about Earth-based efforts to generate energy through nuclear fusion? How do nu-clear fusion power experiments attempt to trigger and control nuclear fusion? So-called "cold fusion" has been largely abandoned as a false trail. How did it resemble nuclear fusion? page 246 Part 3 The Stars characterize the stars by finding their luminosities, temperatures, diameters, and masses. We can find the temperatures of stars by studying their spectra to see which atoms produce the strongest spectral lines. we classify stars in a spectral sequence running from the O stars, which are hot, to the M stars, which are cool. The H-R diagram plots stars according to their intrinsic roughly 90 percent of all stars fall on the main sequence, . The giants and supergiants, however, are much larger and lie above the main sequence. They are more luminous than main-sequence stars of the same temperature. Some of the white dwarfs are hot stars, but they fall below the main se quence because they are so small. The large size of the giants and supergiants means their atmospheres have low densities and their spectra have sharper spectral lines than the spectra of main-sequence 1. Why are Earth-based parallax measurements limited the more massive being hotter, larger, and more luminous to brightness and their surface temperature. In the diagram, Before we could begin, we needed to find the distances to stars. Only by first knowing the distance could we find the other properties of the stars. We can measure the distance to the nearer stars by observing their parallaxes. The more distant stars are so far away that their parallaxes are unmeasurably small. To find the distances to these stars, we must use spectroscopic par-allax. Stellar distances are commonly expressed in parsecs. One parsec is 206,265 AU-the distance to an imaginary star whose parallax is 1 second of arc. Once we know the distance to a star, we can find its in-trinsic brightness, expressed as its absolute magnitude or its luminosity. A star's absolute magnitude is the apparent magnitude we would see if the star were only 10 pc away. The luminosity of the star is the total energy radiated in 1 second, usually expressed in terms of the luminosity of the sun. 4. What does luminosity measure that is different from what absolute visual magnitude measures? we can find its in-trinsic brightness, expressed as its absolute magnitude or its luminosity. A star's absolute magnitude is the apparent magnitude we would see if the star were only 10 pc away. The luminosity of the star is the total energy radiated in 1 second, usually expressed in terms of the luminosity of the sun. 5. Why does the luminosity of a star depend on both its radius and its temperature? The temperature is keeping the star very bright if the temperature is a very hot temperature from hydrogen gas. stars. In fact, it is possible to assign stars to luminosity 6. How can we be sure that giant stars really are larger than main-sequence stars? classes by the widths of their spectral lines. Class V stars are main-sequence stars with broad spectral lines. Giant stars (III) have sharper lines, and supergiants (I) have extremely sharp spectral lines. small? The only direct way we can find the mass of a star is by studying binary stars. When two stars orbit a common cen- ter of mass, we can find their masses by observing the pe- riod and sizes of their orbits. Given the mass and diameter of a star, we can find its average density. On the main sequence, the stars are about as dense as the sun, but the giants and supergiants are very- 7. Why do we conclude that white dwarfs must be very 8. What observations would we make to classify a star ac- cording to its luminosity? Why does that method work? 9. Why does the orbital period of a binary star depend on its mass? 10. What observations would you make to study an eclips- ing binary star? stellar parallax (p) main sequence parsec (pc) giant star flux supergiant star absolute visual magnitude white dwarf star ) luminosity class spectroscopic parallax binary stars visual binary system spectroscopic binary system eclipsing binary system light curve  luminosity (L) distance modulus spectral class or type spectral sequence L dwarf T dwarf Hertzsprung-Russell (H-R) mass-luminosity relation diagram the nearest stars? 2. Why was the Hipparcos satellite able to make more ac-curate parallax measurements than are ground-based telescopes? 3. What do the words absolute and visual mean in the de-finition of absolute visual magnitude? low-density stars. Some are much thinner than air. The white dwarfs, lying below the main sequence, are tremen- sive a star is, the more luminous it is. Main-sequence stars ing binary? The mass-luminosity relation says that the more mas- Star Spectral Type m lustrate the mass-luminosity relation? 11. Why don't we know the inclination of a spectroscopic binary? How do we know the incli~ation of an eclips dously dense. Y74 Part 3 The Stars 12. How do the masses of stars along the main sequence il- lustrate the mass-luminosity relation? 13. Why is it difficult to find out how common the most luminous stars are? The least luminous stars? 14. What is the most common kind of star? A survey in the neighborhood of the sun shows us that lower-main-sequence stars are the most common type. The hot stars of the upper main sequence are very rare. 15. If you look only at the brightest stars in the night sky, what kind of star are you likely to be observing? Why? Discussion 1. If someone asked you to compile a list of the nearest stars to the sun based on your own observations, what measurements would you make, and how would you analyze them to detect nearby stars? 2. The sun is sometimes described as an average star. Is that true? What is the average star really like? 2. Algol is a famous binary star. What can you find out about Problems 1. If a star has a parallax of 0.050 second of arc, what is its distance in pc? in ly? in AU? 2. If you place a screen of area 1 mz at a distance of 2.8 m i from a 100-watt lightbulb, the light flux falling on the screen will be 1 J/s. To what distance must you move the screen to make the flux striking it equal 0.01 J/s? (This assumes the lightbulb emits all of its energy as light.) 3. If a star has a parallax of 0.016 second of arc and an ap- parent magnitude of 6, how far away is it, and what is its absolute magnitude? 4. Complete the following table. P m M~ d (pc) (seconds of arc) 7 10 11 1000 -2 0.025 4 0.040 5. The unaided human eye can see stars no fainter than those with an apparent magnitude of 6. If you can see a bright firefly blinking up to 0.5 km away, what is the absolute magnitude of the firefly? (Hint: Convert the distance to parsecs and use the formula in By the Num-bers 11-2.) 3. Take a survey of the stars. Center on Orion and adjust the field until it is about 100° wide. Click on the ten 6. If a main-sequence star has a luminosity of 400 Lo, what brightest stars and record the spectral types. Now zoom is its spectral type? (Hint: See Figure 11-13.) in until only a few dozen stars are in the frame. Click 7. If a star is 10 times the radius of the sun and half as on the 10 faintest stars and record their spectral types. hot, what will its luminosity be? (Hint: See By the Num- Is there a difference between the brightest and faintest bers 11-3.) stars? Is the result what you expected? Are certain kinds 8. An 08 V star has an apparent magnitude of +1. Use the of stars missing from the data in the computer program? method of spectroscopic parallax to find the distance to 4. Repeat exercise 3 for a region centered on the Big Dipper. the star. Why might this distance be inaccurate? 9. Find the luminosity and spectral type of a 5-Mo main-sequence star. 10. In the following table, which star is brightest in appar-ent magnitude? most luminous in absolute magnitude? largest? least dense? farthest away? a G2 V 5 b B1 V 8 C G2 Ib 10 d M5 III 19 e White dwarf 15 11. If two stars orbit each other with a period of 6 years and a separation of 4 AU, what is their total mass? (Hint: See By the Numbers 11-4.) 12. If the eclipsing binary in Figure 11-20 has a period of 32 days, an orbital velocity of 153 km/s, and an orbit that is nearly edge-on, what is the circumference of the orbit? the radius of the orbit? the mass of the system? 13. If the orbital velocity of the eclipsing binary in Fig-ure 11-20 is 153 km/s and the smaller star becomes completely eclipsed in 2.5 hours, what is its diameter? 14. What is the luminosity of a 4-solar-mass main-sequence star? of a 9-solar-mass main-sequence star? of a 7-solar-mass main-sequence star? 1. The Hertsprung-Russell diagram was named for two fa-mous astronomers. Who were they? What did they do to earn such an honor? the mythology of Perseus, Medusa, and Algol? 3, An entire class of binary styars is known as the Algol bi- naries. How would you characterize such star systems? 1. Locate the following stars and determine their apparent magnitude, parallax, distance in parsecs and in light-years, and spectral classification: Sirius, Aldebaran, Vega, Deneb, Betelgeuse, Antares, and Altair. (Hint: To center on an object, use Find under the Edit menu and type the object's name followed by a period.) 2. Use the spectral type and parallax of the stars above to estimate their distance from Earth. Compare with dis-tances given in TheSky. Ro ta tbe Brook:/Cole Aatronomy Reaouree l~ater Iwww.brootacole. com/aatrono.yl tor critical tbinklng ewercises, artlclea, and addl-uonai readinba tror utoTras Coua~ Ediuaa, Rraaluacole'a oalW atudeatllbrery. Chapter 11 TYte Family of Stars Y75 The first two laws say that mass and energy must be con- served and spread smoothly through the star. The third, hy- drostatic equilibrium, says the star must balance the weight of g, only flow outward by conduction, convection, or radiation. The mass-luminosity relation is explained by the re- quirement that a star support the weight of its layers by its How are Herbig-Haro objects related to star formation? its layers by its internal pressure. The fourth says energy can How do the proton-proton chain and the CNO cycle re- semble each other? How do they differ? amounts 13. Why does a star's life expectancy depend on its mass? The mass is for how big the star is for how long the star continues to shine until it runs out of hydrogen gas. 14. That evidence do we have that star formation is hap- pening right now in the Orion Nebula? The Great Nebula in Orion is an active region of star for- To keep its pressure high, it must be hot and generate large of energy. The mass of a star determines its internal pressure. The more massive a star is, the more weight , it must support and the higher its internal pressure must be. luminosity. The massive stars are very luminous and lie along the upper main sequence. The less massive stars are fainter and lie lower on the main sequence. How long a star can stay on the main sequence depends on its mass. The more massive a star is, the faster it uses up its hydrogen fuel. A 25-solar-mass star will exhaust its hydro- gen and die in only about 7 million years, but the sun is ex- pected to last for 10 billion years. mation. The bright stars we see in the center of the nebula formed within the last few million years, and infrared tele- scopes detect protostars buried inside the molecular cloud CNO (carbon-nitrogen- that lies behind the visible nebula. T Tauri star edge of the interstellar medium nebula Bok globule Herbig-Haro object HII region bipolar flow reflection nebula dark nebula oxygen) cycle interstellar reddening triple-alpha process infrared cirrus conservation of mass molecular cloud conservation of energy shock wave hydrostatic equilibrium association energy transport protostar opacity evolutionary track stellar model page 300 Review Questions 1. What evidence do we have that the spaces between the stars are not empty? It is dark between bright stars. 2. What evidence do we have that the interstellar medium contains both gas and dust? Telescope observations. 3. Why would an emission nebula near a hot star look red, but a reflection nebula near its star looks blue? hot stars are blue and cool stars are red. A radar beam reflected from a rotating planet yields information about both the planet's overall motion and its rotation rate. The returning pulse bounced off the planet is very much weaker than the outgoing signal. First, the signal as a whole may be redshifted or blueshifted as a consequence of the Doppler effect, depending on the overall radial velocity of the planet with respect to Earth. Let's assume for simplicity that this velocity is zero, so that, on average, the frequency of the reflected signal is the same as the outgoing beam. Second, if the planet is rotating, the radiation reflected from the side of the planet moving toward us returns at a slightly higher frequency than the radiation reflected from the receding side. the two hemispheres as being separate sources of radiation and moving at slightly different velocities, one toward us and one away. The effect is very similar to the rotational line broadening except that in this case the radiation we are measuring was not emitted by the planet but only reflected from its surface. What we see in the reflected signal is a spread of frequencies on either side of the original frequency. By measuring the extent of that spread, we can determine the planet's rotational speed. 4. Why do astronomers rely heavily on infrared observa- tions to study star formation? The detectors can see into stars that our eyes cannot see into the inside of the stars due to too much sun light coming from the stars and sun or star can cause your eyes to go blind if you look at the sun or star directly at it for a long time. 5. What observational evidence do we have that star for- mation is a continuous process? Most stars form in giant moleeular clouds that have star masses as large as 106 times the mass of the Sun and typical diameters of 50 to 200 LY. The best-studied molecular cloud is Orion, where star formation began about 12 million years star formation to our view in Orion. The formation of a star inside a molecular cloud begins with a dense core of mater- ial, which accretes matter and collapses due to gravity The accumulation of material halts when the protostar develops a strong stellar wind. A turbulent cloud will form a rotating with an equatorial disk of material. 6. How are Herbig-Haro objects related to star formation? 7. How do the proton-proton chain and the CNO cycle re- semble each other? How do they differ? In hotter stars, another set of reactions, called the carbon-nitrogen-oxygen (CNO) cycle, accomplishes the same net result. In the CNO cycle, carbon and hydrogen nuclei collide to initiate a series of reactions that form nitrogen, oxygen, and ultimately helium. The nitrogen and oxygen nuclei do not survive but interact to form car-bon again. The outcome is the same as in the proton-proton cycle: four hydrogen atoms disappear, and in their place a single helium atom is created. The CNO cycle plays only a minor role in the Sun 8. Why does the CNO cycle require a higher temperature than the proton-proton chain? The proton-proton chain and CNO cycle are both thermonuclear fusion reactions that convert four hydrogen nuclei to one helium nucleus with the release of energy. Both also occur in the cores of main sequence stars. The two differ in the manner in which the fusion reaction occurs, the temperature at which the reaction occurs, and the mass of the stars where each of these reactions dominate the energy production. The proton-proton chain requires a minimum temperature of 10 million K and involves only nuclei of hydrogen and helium. The proton-proton chain is the dominant reaction in main sequence stars with masses less than about 1.1 solar masses. The CNO cycle requires a minimum temperature of about 16 million K and involves hydrogen, carbon, nitrogen, and oxygen nuclei in the production of energy and the helium nuclei. The CNO cycle is the dominant source of energy for stars on the main sequence with masses greater than about 1.1 solar masses. The CNO cycle requires a greater temperature than the proton-proton chain because the Coulomb barrier is greater in the CNO cycle. The Coulomb force is determined by the charge on the particles that are trying to collide. The greater the charge, the greater the Coulomb barrier that must be overcome. In the proton-proton chain the largest number of positive charges that must be combined is four when the two 3He nuclei are combined. By contrast the reaction in the CNO cycle with the least number of charged particles that need to be combined is seven when the 1 H nucleus is combined with the 12 C nucleus. 9. How does the pressure-temperature thermostat control the nuclear reactions inside stars? 10. Step by step, explain how energy flows from the sun's core to Earth. fusion Mechanism of energy generation in the core of the Sun, in which light nuclei are combined, or fused, into heavier ones, releasing energy in the process. The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface. it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation. In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis. convection Churning motion resulting from the constant upwelling of warm fluid and the concurrent downward flow of cooler material to take its place. Neutrinos eject from the star. 11. Why is there a mass-luminosity relation? mass-luminosity relation The dependence of the luminosity of a main-sequence star on its mass. The luminosity increases roughly as the mass raised to the third power. mass A measure of the total amount of matter contained within an object. mass-luminosity relation The dependence of the luminosity of a main-sequence star on its mass. The luminosity increases roughly as the mass raised to the third power. 12. Why is there a lower limit to the mass of a main-sequence star? 13. Why does a star's life expectancy depend on its mass? The more massive the star is the more energy output of hydrogen gas. 14. What evidence do we have that star formation is hap- pening right now in the Orion Nebula? Discussian Questions 1. When we see distant streetlights through smog, they look dimmer and redder than they do normally. But when we see the same streetlights through fog or falling snow, they look dimmer but not redder. Use your knowledge of the interstellar medium to discuss the relative sizes of the particles in smog, fog, and snow compared with the wavelength of light. 2. If planets form as a natural by-product of star forma- tion, which do you think are more common-stars or planets? Problems 1. The interstellar medium dims starlight by about 1.9 mag-nitudes/1000 pc. What fraction of photons survive a trip of 1000 pc? (Hint: See By the Numbers 1-1.) 2. A small Bok globule has a diameter of 20 seconds of arc. If the nebula is 1000 pc from Earth, what is the diame-ter of the globule? 3. If a giant molecular cloud has a diameter of 30 pc and drifts relative to neighboring clouds at 20 km/s, how long will it take to travel its own diameter? 4. If the dust cocoon around a protostar emits radiation most strongly at a wavelength of 30 microns, what is the temperature of the dust? Hint See By the Numbers 4-1. 5. The gas in a bipolar flow can travel as fast as 300 km/s. How long would it take to travel 1 light-year? 6. Circle all'H and 4H nuclei in Figure 12-8. Explain how both the proton-proton chain and the CNO cycle can be summarized as 4 'H ~ 4He + energy. 7. In the model shown in Figure 12-13, how much of the sun's mass is hotter than 13,000,000 K? 300 Part 3 The Stars 8. If a brown dwarf has a surface temperature of 1500 K, at what wavelength will it emit the most radiation? (Hint: it do? Why might it be interesting to observe with even 3. What is BM Orionis? Where is it located, and what does See By the Numbers 4-1.) a small telescope? 9. What is the life expectancy of a 16-solar-mass star? 10. If the 06V star in the Orion Nebula is magnitude 1. The following nebulae are all star-formation regions. 11. The hottest star in the Orion Nebula has a surface tem- perature of 40,000 K. At what wavelength does it radi- ate the most energy? (Hint: See By the Numbers 4-1.) M42, M20, M8, M17 5.4, how far away is the nebula? (Hint: Use spectroscopic parallax.) What kind of nebulae are they? (Hint: To center on an object use Find under Edit. Choose Messier Objects and pick from the list.) 1. Use the Web to supply additional details concerning the evolution of protostars and T Tauri stars. 2. Astronomers continue to study the Orion Nebula and star formation in the molecular cloud behind the nebula. What is the latest news from Orion? Exploring TheSky Chapter 12 The Formation and Structure of Stars 301 2. Locate M8 in TheSky, zoom in, and identify other neb-ulae in the region. Study the photo of NGC6559. °-* Bo to the Brooks/Cole Astronomy Resource Center Iwww.broowscole. coo/astronomy) for critical thinking exercises, articles, and addi-tional readings from InfoTrac College Edition, Brooks/Cole's online student librart of radiation is produced by rapidly moving electrons spiraling through magnetic fields; in the case of the Crab Nebula, the electrons are so energetic that they also emit visible light. This leaves us with a puzzle. The Crab Nebula is 950 years old, so the electrons should have radiated away their energy long ago. The Crab Nebula must contain a powerful energy source to maintain the synchrotron radiation. We will search for that energy source in the next chapter. What causes a type II supernova explosion? A type II supernova occurs when a massive star reaches the end of its usable fuel and develops an iron core. The iron is the final ash produced by nuclear fusion, and it cannot fuse to produce energy because iron is the most tightly bound nucleus. When energy generation begins to fall, the star contracts; because iron can't ignite, there is no new energy source to stop the contraction. In a fraction of a second, the core of the star falls inward and a shock wave moves outward. Aided by a flood of neutrinos and sudden turbulence, the shock wave blasts the star apart, and we see it brighten as its surface gases expand into space. Type II supernova explosions are easy to recognize because their spectra contain hydrogen lines. Use what you know about type la supernova explosions to ex-plain why their spectra do not contain visible hydro-gen lines. so there is no pressure-temperature thermostat to control the reactions. As a result, the core explodes in a helium flash. All of the energy produced is absorbed by the star. What happens when de-generate matter can't support the weight of a dying star-that is, when the mass of the compact object ex-ceeds the Chandrasekhar limit? Review Questions 324 Part 3 The Stars 1. Why does helium fusion require a higher temperature than hydrogen fusion? When a star's central hydrogen-fusion reactions cease, its core contracts and heats up, and hydrogen fusion begins in a spherical layer around the core-a hydrogen-fusion shell. Energy from this shell swells the star into a cool giant. The contraction of the star's core ignites helium, first in the core Review Questions and later in a shell. If the star is massive enough, it can even- tually fuse carbon and other elements. If a star's mass lies between about 0.4 and 3 solar masses, its helium core becomes degenerate before the helium ignites. In degenerate gas, pressure does not depend on temperature, 2. How can the contraction of an inert helium core trigger the ignition of a hydrogen-fusion shell? 3. Why does the expansion of a star's envelope make it 7. If a star the size of the sun collapses to form a white cooler and more luminous? dwarf the size of Earth, by what factor will its density increase? (Hints: The volume of a sphere is 3nr3. See Appendix A for the radii of the sun and Earth.) 4. Why is degenerate matter so difficult to compress? 5. How does the presence of degenerate matter in a star trigger the helium flash? 6. -How can star clusters confirm our theories of stellar evolution? 7. Why don't red dwarfs become giant stars? 8. What causes an aging giant star to produce a planetary nebula? 9: Why can't a white dwarf contract as it cools? What is its fate? 10. Why can't a white dwarf have a mass greater than 1.4 solar masses? 11. How can a star of as much as 8 solar masses form a white dwarf when it dies? stellar evolution in the H-R dia-grams of clusters of stars. Beginning their evolution at about the same time, the stars evolve in different ways, depending on their masses. The most massive leave the main sequence first and are followed later by progressively less massive stars. we can estimate the age of a star cluster from the turnoff point in its H-R diagram. How a star evolves depends on its mass. Stars less mas-sive than about 0.4 solar mass are completely mixed and will have very little hydrogen left when they die. They can not ignite a hydrogen fusion shell, so they cannot become giant stars. They will remain on the main sequence for many times the present age of the universe. Medium-mass stars between about 0.4 and 4 solar masses become giants and fuse helium but cannot fuse carbon. They produce plane-tary nebulae and become white dwarfs. Close binary stars evolve in complex ways because they can transfer mass from one star to the other. This explains why some binary systems contain a main-sequence star more massive than its giant companion-the Algol paradox. Also, mass transfer into an accretion disk around a white dwarf can produce X rays from the hot disk and can trigger nova explosions. Stars as massive as 8 solar masses may lose enough mass to eject planetary nebulae and die as white dwarfs, but more-massive stars suffer a different fate. The most massive stars fuse nuclear fuels up to iron but cannot generate fur-ther nuclear energy because iron is the most tightly bound of all atomic nuclei. When an iron core forms in a massive star, the core collapses and triggers a supernova explosion that expels the outer layers of the star to form an expanding supernova remnant. The first supernova visible to the naked eye since 1604 was seen in February 1987. The study of the deaths of stars has led us to discover astonishing objects of unbelievable density, tempera-ture, and violence-all consequences of the victory of gravity over matter. 12. How can we understand the Algol paradox? 13. How can the inward collapse of the core of a massive 1. As seen on pages 312 and 313, there is an incredible di star produce an outward explosion? versity of appearance for planetary nebulae. Browse the Web for images and information on these dying stars and discuss why there is a range of shapes of planetary nebulae that we see. 14. What is the difference between type Ia and type II super-novae? 15. What is the difference between a supernova explosion and a nova explosion? Discussion Questions 1. ow do we know the helium flash occurs if it cannot be observed? Can we accept something as real if we can never observe it? 2. alse-color radio images and time-exposure photographs of astronomical images show us aspects of nature we can never see with our unaided eyes. Can you think of common images in newspapers or on television that re-veal phenomena we cannot see? Problems 1. About how long would a 0.4-Mo star spend on the main sequence? (Hint: See By the Numbers 12-1.) 2. If the stars at the turnoff point in a star cluster have masses of about 4 Mo, how old is the cluster? 3. The Ring Nebula in Lyrae is a planetary nebula with an angular diameter of 76 seconds of arc and a distance of 5000 ly. What is its linear diameter? (Hint: See By the Numbers 1-2.) 4. If the Ring Nebula is expanding at a velocity of 15 km/s, typical of planetary nebulae, how old is it? 5. Suppose a planetary nebula is 1 pc in radius. If the Doppler shifts in its spectrum show it is expanding at 30 km/s, how old is it? (Hints: 1 pc equals 3 x 10'3 km, and 1 year equals 3.15 X 10' seconds.) 6. If a star the size of the sun expands to form a giant 20 times larger in radius, by what factor will its average density decrease? (Hint: The volume of a sphere is 37GT3.) 8. The Crab Nebula is now 1.35 pc in radius and is ex-panding at 1400 km/s. About when did the supernova occur? (Hint: 1 pc equals 3 X 1013 km.) 9. If the Cygnus Loop is 40 pc in diameter and is 20,000 years old, with what average velocity has it been ex-panding? (Hints: 1 pc equals 3 X 10'3 km, and 1 year equals 3.15 X 10' seconds.) 10. Observations show that the gas ejected from SN1987A is moving at about 10,000 km/s. How long will it take to travel one astronomical unit? one parsec? (Hints: 1 AU equals 1.5 X 108 km, and 1 pc equals 3 X 10'3 km.) 2. Naked-eye supernovae in our galaxy are rare, but astron-omers have noted supernovae in other galaxies for years. Look for summaries of observations of recent supernovae. Are similar numbers of type la and type II being seen? Compare the number of supernovae seen during the last few years with that of two decades ago. Why are we find-ing so many more supernovae in recent years than in the past? 3. What can you find out about different kinds of novae? Limit your search to astronomy categories and also search for the use of the word nova on Web pages about vari-able stars. Exploring TheSiry 1.Locate the planetary nebulae M57, M97, and M27. How does their shape distinguish them from the star forma-tion nebulae such as M42 and M8? (Hint: To find an ob ject, use Find under Edit. Choose Messier Objects and pick from the list.) 2. he Crab Nebula is M1. Locate it, zoom in, measure its angular size in seconds of arc, and compute its diame-ter, assuming it is about 6000 ly from Earth. 3.Locate the supernova remnant called the Cygnus Loop just south of ^ Cygni. How big is this object in angular diameter compared to the full moon? (Hint: Under the View menu, choose Labels and Setup. Check Bayer Designation, go to Cygnus, and zoom in on e Cygni until the Cygnus Loop appears.) v 00 to the Brooks/Cole Astronomy Resource Center (www.broohscole. coo/astronomy) for critical thinking exercises, articles, and addi-tional readings from InfoTrac College Edition, Brooks/Cole's online sudeot library. Chapter 13 The Deaths of Stars 325 should emit X rays. Any X rays emitted before the mat-ter crosses the event horizon will escape, we can look for black holes by looking for X-ray sources. Of course, an isolated black hole will probably not have much matter falling in, but black holes in binary sys-tems may have large amounts of matter flowing in from the companion star. Thus, we can search for black holes by looking for X-ray binaries. The search for black holes has succeeded in finding a few strong candidates, the problem is being sure a particular binary system contains a black hole and not a neutron star. What observations would you make of an X-ray binary system to distinguish between a black hole and a neutron star? Compact objects emitting X rays and producing pre-cessing jets of radiation and gas may not be as unusual as they seem. Many stars collapse to form black holes or neutron stars in binary systems, but these are ob-jects of only a few solar masses. 1. How are neutron stars and white dwarfs similar. How fast, be very hot, and have a strong magnetic field. Such ob- jects have been identified as pulsars, sources of pulsed radio do they differ? 2. Why is there an upper limit to the mass of neutron stars? energy. 3. Why do we expect neutron stars to spin rapidly? Pulsars are evidently spinning neutron stars that emit beams of radiation from their magnetic poles. As they sweep over Earth, we detect pulses. The spinning neutron star slows as it radiates energy into space. Dozens of pulsars have been found in binary systems, 4.If neutron stars are hot, why aren't they very luminous? spin, they sweep the beams around the sky; if the beams 5.WhY do we expect neutron stars to have a powerful mag- netic field? 6. Why did astronomers conclude that pulsars could not which allows astronomers to estimate the masses of the pul- be pulsating stars? 7. What does the short length of pulsar pulses tell us? 8. How does the lighthouse model explain pulsars? 9. What evidence do we have that pulsars are neutron stars? 10. Why would astronomers at first assume that the first millisecond pulsar was young?' 11, How can a neutron star in a binary system generate X rays? 12. If the sun has a Schwarzschild radius, why isn't it a black hole? X-1, mass flows into a hot accretion disk around the neutron sars. Such masses are consistent with the predicted masses of neutron stars. In some binary systems, such as Hercules star and causes the emission of X rays. If a collapsing star has a mass greater than 2 to 3 solar masses, then it must contract to a very small size-perhaps to a singularity, an object of zero radius. Near such an ob- ject, gravity is so strong that not even light can escape, and we term the region a black hole. The surface of this region, called the event horizon, is the boundary of the black hole. The Schwarzschild radius is the radius of this event hori- zon, amounting to only a few kilometers for black holes of stellar mass. If we were to leap into a black hole, our friends who stayed behind would see two relativistic effects. They would see our clock slow relative to their own clock because of time dilation. Also, they would see our light red-shifted to longer wavelengths. We would not notice these effects, we would feel powerful tidal forces that would deform and heat our mass until we grew hot enough to emit X rays. Any X rays we emitted before crossing the event horizon could escape. To search for black holes, we must look for binary star systems in which mass flows into a compact object and emits X rays. If the mass of the compact object is greater than about 3 solar masses, then the object is presumably a black hole. A number of such objects have been located. Black holes and neutron stars at the center of accretion disks can eject beams of radiation and gas. Gamma-ray burst-ers appear to be related to violent events involving neutron stars and hypernovae, supernovae caused by the collapse of the most massive stars. neutron star time dilation pulsar gravitational red shift millisecond pulsar gamma-ray burster gravitational radiation soft gamma-ray repeater X-ray burster (SGR) Chapter 14 Neutron Stars and Black Holes 345 13. How can a black hole emit X rays? 8.What is the orbital velocity at a distance of 7400 meters 14What evidence do we have that black holes really exist? from the center of a 5-solar-mass black hole? What kind . of particles could orbit at this distance? (Hint: See By 15. How can mass transfer into a compact object produce the Numbers 2-1.) jets of high-speed gas? X-ray bursts? 9. ompare the orbit in Problem 8 with an orbit having the same velocity around a 2-solar-mass neutron star. Why is this orbit impossible? (Hint: See By the Numbers 2-1.) 16. Describe the possible causes of gamma-ray bursts. 1.In your opinion, has the existence of neutron stars been sufficiently tested to be called a theory, or should it be called a hypothesis? What about the existence of black holes? 2.Why wouldn't an accretion disk orbiting a giant star get as hot as an accretion disk orbiting a compact object? Problems 1.If a neutron star has a radius of 10 km and rotates 642 times a second, what is the speed of the surface at the neutron star's equator in terms of the speed of light? (Hint: The circumference of a circle is 2nr.) 2.A neutron star and a white dwarf have been found or-biting each other with a period of 11 minutes. If their masses are typical, what is the average distance between them? (Hint: See By the Numbers 11-4.) 1.Imagine that you are on a mission to explore one of the pulsar planets noted in the chapter. What would you find there? Look for information about pulsars and the known pulsar planets on the Web and describe what you might encounter on such a mission. 2.What would you experience if you were to pilot a space-craft near a black hole? Visit black-hole-related Internet sites to determine what the gravitational effects and general environment would be. Also use the Internet to find the limits of human tolerance to strong gravitational forces. (Hint: Look for information about astronaut train-ing and find out how many g's a human can withstand.) Use these sources to give a brief account about what your voyage would be like. - 3.Search for information about gamma-ray bursters. What is the latest news in this developing story? 3. If Earth's moon were replaced by a typical neutron star, what would the angular diameter of the neutron star be, ~~O~ng Th~Ay as seen from Earth? (Hint: See By the Numbers 1-2.) 4.What is the Schwarzschild radius of Jupiter (mass = 2 x 1.The Crab Nebula pulsar is located in the Crab Nebula, 102' kg)? of a human adult (mass = 75 kg)? (Hint: See also known as M1. Locate it, zoom in, and compare its Appendix A for the values of G and c.) shape and size with Figure 14-3. (TheSky does not show the pulsar.) 5.If the inner accretion disk around a black hole has a temperature of 106 K, at what wavelength will it radiate the most energy? (Hint: See By the Numbers 4-1.) 6.What is the orbital period of a bit of matter in an accre-tion disk 2 x 105 km from a 10-Mo black hole? (Hint: See By the Numbers 2-1.) 6o to the Brooas/Cole nstronomy Resource Center Iwww.brooYSCOIe. eor/astronomyl lor critical thinhlng eaerclses, article:, and addl-tional readings fror InloTrac College Ed1UOn, Broor:IColo's odlu arudent library. 7.If SS433 consists of a 20-Mo star and a neutron star or-biting each other every 13.1 days, then what is the av-erage distance between them? (Hint: See By the Num-bers 11-4.) page 346 Part 3 'ihe Stars The distribution of populations through the galaxy sug- gests a way the galaxy could have formed from a spherical :cloud of gas that gradually flattened into a disk. The younger 4. Why is it difficult to specify the thickness or diameter of the disk of our galaxy? Lots of interstellar dust. 5.Why didn't astronomers before Shapley realize how large the galaxy is? 6.How do we know how old our galaxy is? Radioactive dating tells the age of how old the rock is. most rocks contain trace amounts of radioactive elements such as uranium. By measuring the relative abundances of various radioactive isotopes and their decay products within a rock, scientists can determine the rock’s age. 7.Why do we conclude that metal-poor stars are older than metal-rich stars? the stars, the more metal rich they are, and the more circu- lar and flat their orbits are The very youngest objects lie along spiral arms within the disk. These stars live such short lives they don't have time to move from their place of birth in the spiral arms. Maps of these spiral tracers and cool hydrogen clouds re- veal the spiral pattern of our galaxy. 8. How can astronomers find the mass of the galaxy? 9. What evidence do we have that our galaxy has an ex- pansion rate The density wave theory suggests that the spiral arms tended corona of dark matter? Dark Matter and the Formation of Galaxies even if it is red. So, too, when the early universe was opaque, radiation carried ordinary matter with it, sweeping past the concentrations of dark matter. Neither hot nor cold dark matter is entirely successful in explaining the distribution of galaxies discussed in Chapter 19. Hot dark matter models predict that all galaxies should be found in large sheet-like structures, which are not seen. Cold dark matter cannot produce voids, walls, and long structures such as the Great Wall. Elusive as dark matter may be in the current-day universe, galaxies would probably never have formed without it. As we have seen, galaxies grew from density fluctuations in the early universe. The observations with COBE give us in-formation on the size of those fluctuations. It turns out that the density variations are too small, at least according to our current theories, to have formed galaxies in the first billion years or so after the big bang. Yet observations indi-cate that galaxies were indeed formed that early. The COBE data, however, give us information about density fluctuations only for the type of matter that inter-acts with radiation. Suppose there is a type of matter that does not interact with light at all-namely dark matter. are regions of compression that move through the disk. When an orbiting gas cloud smashes into the compression wave, the gas cloud forms stars. Another process, self-sustaining star formation, may act to modify the arms as the birth of The nucleus of the galaxy is invisible at visual wave- lengths, but radio, infrared, X-ray, and gamma-ray radiation crowded central stars and heated clouds of dust. The very center of the Milky Way is marked by a radio source, Sagittarius A*, that is also a source of infrared radia- masses. Astronomers believe that this central object is a black hole. 10. How do the orbits of stars around the Milky Way Galaxy help us understand its origin? 11. What evidence contradicts the traditional theory for the massive stars triggers the formation of more stars by com- origin of our galaxy? pressing neighboring clouds. 12 What evidence do we have that the density wave theory is not fully adequate to explain spiral arms in our galaxy? can penetrate the dust clouds. These wavelengths reveal 14. What evidence do we have that the center of our galaxy is a powerful source of energy? 15. Why is the lack of motion of Sgr A* important evidence tion, X rays, and gamma rays. The core must be less than in our study of the center of our galaxy? 4 AU in diameter and must contain about 2.6 million solar 12. Why do spiral tracers have to be short-lived? Di !On Q~ variable star differential rotation Cepheid variable star rotation curve instability strip dark matter RR Lyrae variable star dark halo period-luminosity relation galactic corona proper motion population I calibration population II Shapley-Curtis debate metals disk component spiral tracer kiloparsec (kpc) density wave theory spiral arm flocculent galaxy spherical component self-sustaining star halo formation nuclear bulge Sagittarius A* ~ues 1.Why isn't it possible to tell from the appearance of the Milky Way that the center of our galaxy is in Sagittarius? 2. Why is there a period-luminosity relation? 3. How can astronomers use variable stars to find distance? 1.How would this chapter be different if interstellar matter didn't absorb starlight? 2.Are there any observations you could make with the Hubble Space Telescope that would allow you to better understand the nature of Sgr A*? P~°Q ~~~5 1.Make a scale sketch of our galaxy in cross section. In-clude the disk, sun, nucleus, halo, and some globular clusters. Try to draw the globular clusters to scale size. 2.Because of dust, we can see only about 5 kpc into the disk of the galaxy. What percentage of the galactic disk can we see? (Hint: Consider the area of the entire disk and the area we can see.) 3.If the fastest passenger aircraft can fly 1600 km/hr (1000 mph), how many years would it take to reach the sun? the galactic center? (Hint: 1 pc = 3 X 10'3 km.) 4.If the RR Lyrae stars in a globular cluster have apparent magnitudes of 14, how far away is the cluster? (Hint: See By the Numbers 11-2.) 5.If interstellar dust makes an RR Lyrae star look 1 mag-nitude fainter than it should, by how much will we over-estimate its distance? (Hint: See By the Numbers 11-2.) 6.If a globular cluster is 10 minutes of arc in diameter and 8.5 kpc away, what is its diameter? (Hint: Use the small-angle formula from By the Numbers 1-2.) 310 Part 4 The Universe 7.If we assume that a globular cluster 4 minutes of arc in 2.What if we lived near the center of the galaxy? Search the Web for research and information on the distribution diameter is actually 25 pc in diameter, how far away is it? (Hint: Use the small-angle formula from By the Num- of material near the center of our galaxy. Based on what bers 1-2.) you find, speculate as to how the sky would appear from a planet associated with a star near the galactic center. 8.If the sun is 5 billion years old, how many times has it orbited the galaxy? 9.If the true distance to the center of our galaxy is found to be 7 kpc and the orbital velocity of the sun is 220 km/s, what is the minimum mass of the galaxy? (Hints: Find the orbital period of the sun, and then see By the Num-bers 11-4.) 10. Infrared radiation from the center of our galaxy with a wavelength of about 2 x 10-s m (2000 nm) comes mainly from cool stars. Use this wavelength as Xm. and find the temperature of the stars. (Hint: See By the Num-bers 4-1.) Critical Inquiries for the Web 1.Henrietta Leavitt discovered the period-luminosity re-lation for Cepheids while working on the staff at Har-vard College Observatory under Edward Pickering. She was one of several women "computers" on staff there a century ago. Search the Web for information on Leavitt, her colleagues, and their work at Harvard. List three of the women employed by Pickering and note their con-tributions to astronomy. What was life like for a woman in astronomy at the beginning of the 20th century? Exploring MAW 1.Locate Sagittarius and examine the shape of the Milky Way there and the profusion of globular clusters. (Hint: To turn on Messier object labels, use Labels and Setup under the View menu.) 2.Locate the following globular clusters: M3, M4, M5, M10, M12, M13, M15, M22, M55, M92. Where are they located in the sky? (Hint: Use Find under the Edit menu.) 3.Compare the distribution of globular clusters with that of open clusters. (Hint: Use Filters under the View menu to turn off everything but globular clusters, the Milky Way, the Galactic Equator, and Constellation Bound-aries. Use the thumbwheel at the bottom of the sky win-dow to rotate the sky. Now repeat with globular clusters off and open clusters on.) °0 6o to the Brook:/Cole Astronomll Resource Center (wenn.brookscole. com/astronorll) for critical thinking exercises, articles, and aAW-flonal readings from lafoirnc College Edition. 8fooks/Colle's OWN student librart Chapter 15 The Milky Way Galaxy 371 there is no star formation there, and halos are made up of older stars. The brightest stars in halos are red giants, which give halos a reddish tint. Elliptical galaxies, lacking gas and dust, 8. Why is it difficult to measure the Hubble constant? The Hubble law shows that the radial velocity of a galaxy is proportional to its distance. we can use the Hubble law to estimate distances. The galaxys radial velocity divided by the Hubble constant equals its distance in megaparsecs. g,How is the rotation curve method related to binary stars and Kepler's third law? lack young stars and are consequently slightly reddish be- cause the brightest stars are red giants. 10. What evidence do we have that galaxies contain dark matter? The empty space between galaxies. The Hubble law shows that the radial velocity of a galaxy is proportional to its distance. we can use the Hubble law to estimate distances. The galaxys radial velocity divided by the Hubble constant equals its distance in megaparsecs. The masses of galaxies can be measured in two basic ways-the rotation curve method and the velocity disper- sion method. The rotation curve method is more accurate but can be applied only to nearby galaxies. Both methods sug- gest that galaxies contain 10 to 100 times more dark matter. 11. What evidence do we have that galaxies collide and merge? 12. Why are the shells visible around some elliptical galax- ies significant? qg, Ring galaxies often have nearby companions. What does that suggest? 14. Propose an explanation for the lack of gas, dust, and than visible matter. young stars in elliptical galaxies. Galaxies occur in clusters. Our own galaxy is a member 15. How do deep images by the Hubble Space Telescope of the Local Group, a small cluster. A galaxy in a rich clus- confirm our hypothesis about galaxy evolution? ter may collide with other galaxies more often than a galaxy in a poor cluster, and such collisions can force a galaxy to form new stars and use up its gas and dust. Collisions can also strip gas out of a galaxy. This may explain why ellipti- cal and SO galaxies are more common in rich clusters than 1~ Why do we believe that galaxy collisions are likely, but in poor clusters. Spiral galaxies may be star systems that star collisions are not? have not experienced many collisions. 2. Should an orbiting infrared telescope find irregular gal-axies bright or faint in the far infrared? Why? What about elliptical galaxies? elliptical galaxy spiral galaxy barred spiral galaxy irregular galaxy Large Magellanic Cloud Small Magellanic Cloud megaparsec (Mpc) distance indicator look-back time Hubble law Hubble constant (H) rotation curve method cluster method velocity dispersion method rich galaxy cluster poor galaxy cluster galactic cannibalism ring galaxy starburst galaxy 1. If a galaxy contains a type I (classical) Cepheid with a period of 30 days and an apparent magnitude of 20, what is the distance to the galaxy? 30 x20 =60 2. If you find a galaxy that contains globular clusters that are 2 seconds of arc in diameter, how far away is the gal-axy? (Hints: Assume that a globular cluster is 25 pc in diameter, and see By the Numbers 1-2.) 3. If a galaxy contains a supernova that at its brightest has an apparent magnitude of 17, how far away is the gal-axy? (Hints: Assume that the absolute magnitude of the supernova is -19, and see By the Numbers 11-2.) 4. If we find a galaxy that is the same size and mass as our Milky Way Galaxy, what orbital velocity would a small satellite galaxy have if it orbited 50 kpc from t the center of the larger galaxy? (Hint: See By the Num- bers 2-1.) , 5. Find the orbital period of the satellite galaxy described in Problem 4. (Hint: See By the Numbers 11-4.) 6. If a galaxy has a radial velocity of 2000 km/s and the Hubble constant is 70 km/s/Mpc, how far away is the galaxy? (Hint: Use the Hubble law.) 1. Why didn't astronomers at the beginning of the 20th century recognize galaxies for what they are? 2. How can a classification system aid a scientist? 3. What is the difference between an EO galaxy and an E1 galaxy? 4. What is the difference between an Sa and an Sb galaxy? between an SBb and an Sb? 5. Why can't galaxies evolve from elliptical to spiral? Why can't they evolve from spiral to elliptical? 7. If you find a galaxy that is 20 minutes of arc in diame-ter, and you measure its distance to be 1 Mpc, what is its diameter? (Hint: See By the Numbers 1-2.) 6. How do selection effects make it difficult to decide how common elliptical and spiral galaxies are? g. We have found a galaxy in which the outer stars have 7. Why are Cepheid variable stars good distance indica- orbital velocities of 150 km/s. If the radius of the galaxy tors? What about planetary nebulae? is 4 kpc, what is the orbital period of the outer stars? (Hints: 1 pc = 3.08 X 1013 km, and 1 yr = 3.15 X 10' s.) 392 Part 4 The Universe 9. A galaxy has been found that is 5 kpc in radius and sification. You may be given this information at the site, but examine the images to see if the features of these gal million years. What is the mass of the galaxy? On what axies conform to a particular Hubble type. assumptions does this result depend? (Hint: See By the whose outer stars orbit the center with a period of 200 Numbers 11-4.) Exploring TheSky Critical :. .. s for 1. Locate the Andromeda Galaxy, also known as M31, and its companion galaxies. Zoom in on it and estimate its 1. How far out into the universe can we see Cepheid vari- angular size compared to the full moon. (Hint: Use Find ables? Research sources on the Internet to find other gal- under the Edit menu.) axies whose distances have been found through obser- 2, Take a survey of galaxies and see how many are spiral vation of Cepheids. List the galaxies in which Cepheids and how many are elliptical. Is there any selection effect have been identified and the distances determined from in your method? (Hint: Use Filters under the View menu these data. to turn off everything but Galaxies and Mixed Deep Sky 2. How does the Milky Way stack up against the other gal- objects. Make sure the Messier labels are switched on axies in the Local Group? Look for information on the using the Labels and Setup under the View menu.) other galaxies in our cluster, and rank the top six mem- 3. Locate the Sombrero Galaxy (M104). Study the photo- bers in order of total mass. graphs and discuss this galaxy's special properties. 3. In the early 1900s the nature of the "spiral nebulae" was Zoom in on it and estimate its angular size compared to not well understood. In 1920 a "great debate" was held the moon. between Harlow Shapley, who held that these objects were relatively nearby swirling clouds of gas, and Heber Curtis, who saw them as distant "island universes." Use the Internet to find information about the debate, outline the lines of evidence used by the two participants to pre- 4. Study the distribution of galaxies and notice how they cluster together. Can you find the Virgo cluster? Zoom in until more galaxies appear and then scroll north to find the Coma Cluster. Zoom in on Leo to find the clus- ter there. What other clusters can you find? sent their views, and explain who was right and who 5. Describe the galaxy located near the south celestial pole. was wrong. 4. Locate a Web page dedicated to the Messier Catalogue-a list of galaxies, clusters, and nebulae that is often used as a list of targets for small telescopes. Be sure that your destination includes images of the objects. For each of the galaxies in the Messier list, determine its Hubble clas- Chapter 16 Galaxies 393 This implies that its small core contains billions of solar 3. How does the peculiar rotation of NGC5128 help us un- masses, probably in the form of a supermassive black hole derstand the origin of this active galaxy? that has accumulated there since the formation of the galaxy. 4, What statistical evidence suggests that Seyfert galaxies Active galactic nuclei seem to occur in galaxies involved have suffered recent interactions with other galaxies? in collisions or mergers. Seyfert galaxies, for example, are three times more common in interacting galaxies than in iso- 5~ How does the unified model explain the two kinds of lated galaxies. This suggests that an encounter between gal- Seyfert galaxies? axies can throw matter into the central regions of the galax- 6. What observations are necessary to identify the presence ies, where it can feed a black hole and release energy. Galaxies of a supermassive black hole at the center of a galaxy? not recently involved in collisions will not have matter flow- 7. How does the unified model implicate collisions and ing into their central black hole and will not have active ga- mergers in triggering active galaxies? lactic nuclei. 8. Why were quasars first noticed as being peculiar? The unified model of active galaxies supposes that what we see de ends on the orientation of the black hole and its 9. How do the large red shifts of quasars lead us to con- clude they must be very distant? 10. Why do we conclude that quasars are superluminous but must be very small? accretion disk. If we can see into the core, we see broad spec- tral lines and rapid fluctuations. If we see the disk edge-on, we see only narrow spectral lines. If the jet points directly at us, we see a blazar. 11. How do gravitational lenses provide evidence that qua- sars are distant? 12. What evidence do we have that quasars occur in distant galaxies? they are visible at all implies that they are superluminous. The quasars appear to be related objects. Their spectra show emission lines with very large red shifts. Their large red shifts imply that they are very distant, and the fact that Because they fluctuate rapidly, we conclude that they must be very small. A typical quasar can be 100 times more lumi- nous than the entire Milky Way Galaxy but only a few times larger than our solar system. 13. How can our model quasar explain the different radia- tion we receive from quasars? 14. What evidence do we have that quasars must be trig- gered by collisions and mergers? Because quasars lie at great distances, we see them as 15. Why are there few quasars at low red shifts and at high they were long ago. The look-back time to the most distant red shifts but many at red shifts of about 2? quasar is over 10 billion years. They may be young interact- ing galaxies distorting each other and funneling mass into black holes, or they may be young galaxies in the early stages of formation. In any case, heavy mass flow into a central black hole could heat the gas and produce the observed synchro- 1. Why do quasars, active galaxies, SS433, and protostars tron radiation and emission lines. have similar geometry? The highest-resolution images show that quasars are the active cores of distant galaxies. Furthermore, these gal- axies are often distorted, which suggests that quasars are ory 2. By custom, astronomers refer to the unified model of AGN and not to the unified hypothesis or unified the- In your opinion, which of the words seems best? triggered into activity by interactions between galaxies. 3. Do you think that our galaxy has ever been an active gal-axy? Could it have hosted a quasar when it was young? radio galaxy unified model active galaxy BL Lac object active galactic nuclei (AGN) blazar Seyfert galaxy quasar double-lobed radio source relativistic red shift double-exhaust model gravitational lens hot spot 1. What is the difference between the terms radio galaxy and active galaxy? 2. What evidence do we have that the energy source in a double-lobed radio galaxy lies at the center of the galaxy? 4. If a quasar is triggered in a galaxy's core, what would it look like to people living in the outer disk of the galaxy? Could life continue in that galaxy? (Begin by deciding how bright a quasar would look seen from the outer disk of the galaxy, considering both distance and dust.) 1. The total energy stored in a radio lobe is about 1053 J. How many solar masses would have to be converted to energy to produce this energy? (Hints: Use E = mc2. One solar mass equals 2 X 103° kg.) 2. If the jet in NGC5128 is traveling at 5000 km/s and is 40 kpc long, how long will it take for gas to travel from the core of the galaxy to the end of the jet? (Hint: 1 pc equals 3 X 1013 km.) 3. Cygnus A is roughly 225 Mpc away, and its jet is about 50 seconds of arc long. What is the length of the jet in parsecs? (Hint: See By the Numbers 1-2.) Chapter 17 Galaxies with Active Nuclei l11 4. Use the small-angle formula to find the linear diameter of a radio source with an angular diameter of 0.0015 second of arc and a distance of 3.25 Mpc. 1. What object currently holds the distinction as the far-thest known galaxy? Search the Web for information on this distant object and find out its red shift and distance. What is the look-back time for this object? 5. If the active core of a galaxy contains a black hole of 106 solar masses, what will the orbital period be for matter orbiting the black hole at a distance of 0.33 AU? (Hint: See By the Numbers 11-4.) 6. If a quasar is 1000 times more luminous than an entire galaxy, what is the absolute magnitude of such a quasar? (Hint: The absolute magnitude of a bright galaxy is about -21.) 7. If the quasar in Problem 6 were located at the center of our galaxy, what would its apparent magnitude be? (Hints: See By the Numbers 11-2 and ignore dimming by dust clouds.) 8. What is the apparent velocity of recession of 3C48 if its red shift is 0.37? (Hint: See By the Numbers 17-1.) 9. If the Hubble constant is 70 km/s/Mpc, how far away is the quasar in Problem 8? (Hint: Use the Hubble law.) 10. The hydrogen Balmer line HR has a wavelength of 486.1 nm. It is shifted to 563.9 nm in the spectrum of 3C273. What is the red shift of this quasar? (Hint: What is 0)?) 2. Gravitational lenses were first predicted by Einstein in 1936 but were not observed until recently. Search the Web for instances of gravitational lensing of galaxies and quasars. For a particular case, discuss how the lens effect allows astronomers to determine information about the lensing and/or lensed objects that might not have been available without the alignment. w Go to Me Brooks/Cole Astronomy Resource Center tyrana.brookscole. corlasuoooryl for crltleal thinking exercises, articles, and addl-tional readings [fen InteTrac Collaga EWUw. Brooks/We's online student library. 412 Part 4 The Universe filaments, and walls. The size of these fluctuations fits cos-mological theories that are flat. All of the evidence tells us that the universe is flat, accelerating, and no more than 14 bil- 1. Do you think Copernicus would have accepted the cos- lion years old. mological principle? Why or why not? 2. If we reject any model of the universe that has an edge in space becausewe can't comprehend such a thing,shouldn't we also reject any model of the universe that has a beginning or an ending? Are those just edges in time, or is there a difference? Review Questions Chapter 18 1. What does the darkness of the night sky tell us about the universe? There is a lot of empty dark spaces between planets, stars. dark matter is between the stars, and planets. fit a Hubble constant of 50 km/s/Mpc? 2. How can we be located at the center of the observable universe if we accept the Copernican principle? 3. Why can't an open universe have a center? Why can't a closed universe have a center? 4. What evidence do we have that the universe is expanding? that it began with a big bang? 5. Why couldn't atomic nuclei exist when the universe was younger than 2 minutes? The temperatures were to hot. 6. Why is it difficult to determine the present density of the universe? 7. How does the inflationary universe theory resolve the flatness problem? the horizon problem? 20.4 THE BEGINNING OF THE UNIVERSE page 44S Facing the Open Future sity depends on Ho. If the Hubble constant is 20 km/s pe million LY, the critical density is about 10-z9 g/cm3. If the density of the universe is less than the critical density How does this compare to the actual density of the universe? (curve 2 in Figure 20.8), gravity is never important enough to stop the expansion, and so the universe expands forever. This corresponds to the (more difficult to imagine) open geometry discussed earlier and is called an open uni- verse. Such a universe is infinite and always has even more room in it than naive three-dimensional observers would expect. In this case, time and space begin with the big bang, but they have no end; the universe simply continues There are several methods by which we can try to de termine the average density of matter in space. One way is to count all the galaxies out to a given distance and use es- 8. If the Hubble constant is really 100 km/s/Mpc, much of what we understand about the evolution of stars and star clusters must be wrong. Explain why. 9. Why do we conclude that the universe must have been very uniform during its first million years? 10. What is the difference between hot and cold dark matter? What difference does it make to cosmology? Dark Matter and the Formation of Galaxies even if it is red. So, too, when the early universe was opaque, radiation carried ordinary matter with it, sweeping past the concentrations of dark matter. Now suppose the police leave the motorcade, and the lights all turn red at the same time. The red lights act as traffic traps; approach-ing cars now have to stop, and so they bunch up. Likewise, after the early universe became transparent, ordinary matter interacted with radiation only occasionally and so could fall into the dark-matter traps. The size of the gravitational traps depends on the nature of the dark matter. Suppose it is moving near the speed of light-astronomers call this hot dark matter. If neutrinos really do have mass, then they would be an example of hot dark matter. In this case, small-scale density fluctua-tions are smoothed out by the rapidly streaming particles as they move from high- to low-density regions. In this case, large-scale structure would form first. If, on the other hand, the dark matter moves slowly-we call this cold dark matter-then the particles do not have time to move far enough to smooth out small-scale density fluctuations. In this case, relatively small structures, the size of globular clusters or individual galaxies, are likely to form first. Neither hot nor cold dark matter is entirely successful in explaining the distribution of galaxies discussed in Chapter 19. Hot dark matter models predict that all galaxies should be found in large sheet-like structures, which are not seen. Cold dark matter cannot produce voids, walls, and long structures such as the Great Wall. Now theories are being developed that contain both hot and cold dark matter. Even though current models are not adequate to explain how galaxies form, the important point is that galaxies are difficult to form at all unless a substantial amount of dark matter of some kind is present. You may have found this brief discussion of dark matter and cosmology a bit frustrating. Elusive as dark matter may be in the current-day universe, galaxies would probably never have formed without it. As we have seen, galaxies grew from density fluctuations in the early universe. The observations with COBE give us in-formation on the size of those fluctuations. It turns out that the density variations are too small, at least according to our current theories, to have formed galaxies in the first billion years or so after the big bang. Yet observations indi-cate that galaxies were indeed formed that early. The COBE data, however, give us information about density fluctuations only for the type of matter that inter-acts with radiation. Suppose there is a type of matter that does not interact with light at all-namely dark matter. This matter could have much greater variations in density, which we would not be able to detect because dark matter COBE. This dark matter might form a kind of gravitational trap that could have begun to attract ordinary matter im- mediately after the universe became transparent. As ordi- nary matter became increasingly concentrated, it could have turned into galaxies more quickly thanks to these traps. For an analogy, imagine a boulevard with traffic lights every half-mile or so. Suppose you are part of a motorcade 11. What evidence do we have that the expansion of the uni- verse is accelerating? that counts; rather, the ultimate judge must be whether ex-periments support the theory. What evidence do we have that gravity can warp or bend spacetime? We have already discussed a number of experiments confirming this view in Chapters 15 and 18, including the deflection of light and the advance in the per-ihelion of Mercury, as well as the existence of gravitational lenses in the realm of the galaxies. Today, scientists have learned to accept both the curvature of spacetime and their inability to picture it. Unfortunately, it is very difficult to estimate whether the rate of expansion is changing with time and in what way. One method might be to measure the distances and speeds of very distant galaxies. We are seeing these as they Every model of the universe must include the expansion we observe. Furthermore, the cosmological principle tells us that the universe is homogeneous. As a result, the ex- pansion rate must be uniform (the same everywhere dur- ing any epoch of cosmic time). If so, we don't need to think about the entire universe when we think about the expan- sion, we can just look at a portion of it. were long ago and so we could see how much faster (or slower) they were moving when the universe was young. galaxy clusters (including in the great voids) where e difficult for observers in any of them to see the others (see Making Connections box). we At the critical density (curve 3) the universe can just barely expand forever. The critical-density universe has an age of exactly two-thirds The ages of stars also suggest that we live in an open universe. We saw that the best estimate of How is 20 km/s per million LY, which corresponds to a Hubble time of 15 bil- lion years. If we lived in a critical-density (or flat) universe, the actual age would be only two thirds of 15 billion, which is 10 billion years. It seems highly unlikely that our models of how stars evolve, which give ages of at least 13 billion Years for the oldest stars, are inaccurate enough to give stel- lar ages that are wrong by this amount. The older the stars are found to be, the more time needs to have elapsed since the big bang, and thus the less dense the universe must be. But now suppose the observations of the supernovae are correct and that the expansion is actually accelerating. If this is the case, then the universe will not only continue to expand forever but will do so at an ever-accelerating rate. Taking up the ideas in our Making Connections box, this would mean the universe would get colder and lonelier even sooner. of all of the model universes illustrated in Figure 20.8. It is LookbaekTime In Chapter 17 we discussed how we can use the Hubble law to measure the distance to a galaxy. Hubble law ~'. Who's Winning the Tug of War? fast (that is, are not too far away). Once we get to large dis-tances, we are looking so far into the past that we must take What kind of universe do we live in? Is the amount of mass changes in the rate of the expansion of the universe into ac-count. Since we do not know how big these chan larger or smaller than the critical density? The critical density Over the past 20 years, estimates of the Hubble con- stant have ranged from about 15 to 35 km/s per million LY. Since the Hubble constant is the reciprocal of the age, the bigger the constant, the faster the universe expands, and the younger it must be. The Hubble time implied by these values ranges from 20 billion years on the old end to about 10 billion years on the young end. In just the past 5 years, several new techniques for estimating distances have all converged on a Hubble constant of about 20 kmls per million LY, which makes the Hubble time about 15 billion years. (In units used by professional astronomers and often quoted in the press, 20 km/s per million LY corresponds to 65 km/s per million parsecs.) gun using type Ia supernovae to try to measure the of expansion of the universe. Remember that this tyl supernova occurs when a white dwarf gathers eno matter to be pushed over the Chandrasekhar limit ano- explode. All of these supernovae have very nearly the sam, brightness at maximum light (and astronomers know how to correct for the slight differences among them). such supernovae can serve as standard bulbs (as di~ cussed in our chapter on galaxies). If we detect a type Ia supernova in a distant galaxy we therefore immediately know its distance very accurately. From its spectrum, we can measure how fast that galaxy is moving away from us. Substituting into Hubble's law, we can get 12. What evidence do we have that the universe is flat? Critical Inquiries for the Web 1. Will the universe go on expanding forever? Search the Web for information on recent investigations that shed light on the question of the density of matter in the uni verse. What predictions do these studies make about the fate of the universe? What kinds of observations were necessary to make these predictions? 2. The steady-state theory was once a rival cosmology of the big bang. Search for Web sites that provide informa- tion on steady-state cosmology. (Be careful to locate le- gitimate sites that discuss the theory, rather than sites scientific arguments on cosmology.) What were the key predictions of steady-state cosmology? How has recent evidence led to its decline? where individuals use steady-state ideas as part of non- Problems 1. Use te data on page 418 to plot a velocity-distance di-agram, find H and determine the approximate age of the universe. 2. If a galaxy is 8 Mpc away from us and recedes at 456 km/s, how old is the universe, assuming that gravity is not slowing the expansion? How old is the universe if it is flat? 3. If the temperature of the big bang had been 106 K at the time of recombination, what maximum wavelength would the primordial background radiation have as seen from Earth? 4. If the average distance between galaxies is 2 Mpc and the average mass of a galaxy is 10'1 solar masses, what is the average density of the universe? (Hints: The volume of a sphere is 3arr3. The mass of the sun is 2 X 1033 g.) 7, what answer should he have obtained? 5. Figure 18-9 is based on an assumed Hubble constant of 70 km/s/Mpc. How would you change the diagram to 6. Hubble's first estimate of the Hubble constant was 530 Chapter 18 Cosmology 4317. What is the maximum age of the universe predicted by Hubble's first estimate of the Hubble constant? 8. If the value of the Hubble constant were found to be 60 km/s/Mpc, how old would the universe be if it were not slowed by gravity? if it were flat? km/s/Mpc. If his distances were too small by a factor of 3. Search for the Web pages of large surveys of galaxies, 40° or smaller and the fainter galaxies appear. Search such as the 2dF Deep Field Survey and the Sloan Digital for clusters of galaxies in Ursa Major, Canes Venatici, Sky Survey. What are they discovering about the largest Lynx, Virgo, and Coma Bereneces.) and most distant objects in the universe? 2. Locate small clusters of galaxies and compare the bright- 4. What is the latest news concerning observations of the ness of the galaxies with those in large clusters. Can you tell that the smaller clusters tend to be farther away, or cosmic microwave background radiation? NASA plans to launch a satellite to make detailed measurements, and is the range of galaxy luminosities too great? (Hint: Click further balloon observations are planned by a number of on a galaxy to find its magnitude.) research teams. 3. Can you find galaxies and clusters of galaxies along the Milky Way? Turn on the Milky Way and search for gal Exploring TheSky axy clusters within its outline. 1. Search for galaxy clusters. (Hint: Use the View menu to turn off everything but galaxies, deep sky objects, con-stellations, and labels. Zoom in until the field of view is page 438 Part 4 The Universe Are we the only thinking race? If we are, we bear the sole responsibility to understand and admire the uni- verse. We now have the technology to search for other intelli-gent life in the universe. Although such searches are contro To discuss life on other worlds, we must first understand something about life in general, life on Earth, and the origin of life. In general, we can identify two properties of living stable long enough for life to develop. Saturn's moon Titan may have organic materials on its surface, but it may be too cold for chemical reactions to lead to life. To find life, we must look beyond our solar system. Be- cause we suspect that planets form from the leftover debris of star formation, we suspect that most stars have planets. The rise of intelligence may take billions of years, however, so short-lived massive stars and binary stars with unstable planetary orbits must be discarded. The best candidates are G and K main-sequence stars. The distances between stars are too large to permit travel, but communication by radio could be possible. A certain intelligent life. wavelength range called a radio window is suitable, and a small range between the radio signals of H and OH , the so-called water hole, is especially likely. versial, a number of radio astronomers are now searching for radio signals from extraterrestrial civilizations. If life is common in the universe, success seems inevitable. things: a physical basis and a controlling unit of information. maintain the organism, and modify the surroundings to promote the organism's survival. The Miller experiment suggests that energy sources of itself, . Though natural selection Drake equation to fit its environment. such as lightning could have caused amino acids and other complex molecules Chapter 19 Review Questions 1. If life is based on information, what is that information? 2. What would happen to a life form if the information handed down to offspring was always the same? How would that endanger the future of the life form? The life form may become extinct. 3. How does the DNA molecule produce a copy of itself? The process of life must extract energy from the surround-ings, arrangement of matter and energy that implements the life process. On Earth, all life is based on carbon chemistry. The controlling information is the data necessary to maintain the organism's function. Data for Earth life are stored in long carbon-chain molecules called DNA. The DNA molecule stores information in the form of chemical bases linked together like the rungs of a ladder. When these patterns are copied by RNA molecules, they can direct the manufacture of proteins and enzymes. The DNA information is the chemical formulas the cell needs to function. When a cell divides, the DNA molecule splits lengthwise and duplicates itself so that each of the new cells mutant has a copy of the information. Errors in the duplication or damage to the DNA molecule can produce mutants, organ- isms that contain new DNA information and have new prop- erties. 4. Give an example of natural selection acting on new DNA patterns to select the most advantageous characteristics. Natural selection determines which of these new or- ganisms are best suited to survive, and the species evolves to form. Chemical evolution would have connected these together in larger and more complex, but not yet living, molecules. When a molecule acquired the ability to produce copies natural selection began im-proving the organism through biological evolution 5. Why do we believe that life on Earth began in the sea? Water has been around for a long time. 6. Why do we think that liquid water is necessary for the origin of life? Water contains molecules 7. What is the difference between chemical evolution and biological evolution? Chemical is a liquid 8. What was the significance of the Miller experiment? 9. How does intelligence make a creature more likely to survive? 10. Why are upper-main-sequence stars unlikely sites for intelligent civilizations? Problems It seems unlikely that there is now life on other planets in our solar system. Most of the planets are too hot or too cold. Mars may have been suitable in the past, but it is now a cold, dry desert, and any liquid water is trapped below the crust. Jupiter's moon Europa has a liquid-water ocean below its icy crust, but conditions there may not have remained few million years ago. lithis may have happened in the first billion years, life did not become diverse and complex until the Cambrian period, about 0.5 billion years ago. Life emerged from the oceans about 0.4 billion years ago, and humanity developed only a 458 Part 5 Life 11. Why do we suspect that travel between stars is nearly 6. Mathematician Karl Gauss suggested planting forests impossible? and fields in a gigantic geometric proof to signal to pos- 12. How does the stability of technological civilizations af- sible Martians that intelligent life exists on Earth. If Mar- fect the probability that we can communicate with them? tians had telescopes that could resolve details no smaller than how large would the smallest ele- 13. What is the water hole, and why would it be a good place 1 second of arc,ment of Gauss's proof have to be? (Hint: See By the Num-bers 1-2.) Discussion Questions 1. What would you change in the Arecibo message if hu-manity lived on Mars instead of Earth? 2. What do you think it would mean if decades of careful searches for radio signals for extraterrestrial intelligence turned up nothing? 7. If we detected radio signals with an average wavelength of 20 cm and suspected that they came from a civiliza-tion on a distant planet, roughly how much of a change in wavelength should we expect to see because of the orbital motion of the distant planet? (Hint: See By the Numbers 4-2.) 8. Calculate the number of communicative civilizations per galaxy from your own estimates of the factors in Table 19-1. 1. A single human cell encloses about 1.5 m of DNA con-taining 4.5 billion base pairs. What is the spacing be-tween these base pairs in nanometers? how far apart are the rungs on the DNA ladder? 2. If we represent the history of the Earth by a line 1 m long, how long a segment would represent the 400 million years since life moved onto the land? How long a seg ment would represent the 3-million-year history of hu-man life? 3. If a human generation, the time from birth to childbear-ing, is 20 years, how many generations have passed in the last million years? 4. If a star must remain on the main sequence for at least 5 billion years for life to evolve to intelligence, how massive could a star be and still harbor intelligent life on one of its planets? (Hint: See By the Numbers 12-1.) 5. If there are about 1.4 x 10-' stars like the sun per cubic light-year, how many lie within 100 light-years of Earth? (Hint: The volume of a sphere is 3Ycr~'.) 1. The popular movie Contact focused interest on the SETI program by profiling the work of a radio astronomer dedicated to the search for extraterrestrial intelligence. Visit Web sites that give information about the movie, SETI programs, and radio astronomy, and discuss how realistic the movie was in capturing how such research is done. 2. Where outside the solar system would you look for hab-itable planets? NASA has increasingly focused its inter-est on this question. Look for information online about programs dedicated to detecting which planets might support life as we know it. What criteria are used to choose targets for the planned searches? What methods will be used to carry out the searches? 6o to the BroobslCole Astronomy Bosonreo Center (own.breeknele. corlastronomy) for critical thinking exercises, articles, and addr-tional readingslror IafoTraC College EdIUOn, Broor:ICON': oYlno studon IIbrorY. Chapter 19 Life on Other Worlds 45! TABLE 2.2 Oribtal Data for the Planets Planet Semimajor Axis (AU) Period (yr) Eccentricity Mercury 0.39 0.24 0.21 Venus 0.72 0.62 0.01 Earth 1.00 1.00 0.02 Mars 1.52 1.88 0.09 Ceres 2.77 4.60 0.08 Jupiter 5.20 11.86 0.05 Saturn 9.54 29.46 0.06 Uranus 19.19 84.07 0.05 Neptune 30.06 164.80 0.01 Pluto 39.60 248.60 0.25 Figure 4-13 The Hydrogen Atom This is a schematic diagram of a hydrogen atom in its lower energy state also called the ground state. The proton and electron have equal but opposite charges which exert an electromagnetic force that binds the hydrogen atom together. Electron in lower left corner. Peoton in center 5 x 10 -11 m the Hubble constant, and in this way determine if the expansion rate of the universe (which is what the Hub ble constant tells us) at the time the supernova explosion occurred is the same as the rate today. Supernovae of type Ia are extremely bright, and we can detect them at distances of several billion LY. The most distant of the supernovae measured so far emitted their light when the universe was about half its current age. Two groups of astronomers have been searching for these su-pernovae and have observed more than a hundred of them so far. Both groups conclude that the universe is expanding at a faster rate today than it was at the time the supernova (such as 80 km/h). In this case, given that you were driving explosions occurred. The universe, according to these re- faster at the beginning, the trip home would have taken sults, is accelerating. This means that some kind of "anti- less than a half-hour. gravity" force is pushing galaxies apart at an increasing rate-just the effect that Einstein's cosmological constant would add to our recipe for the universe. The energy associated with the cosmological constant is not possessed by matter or radiation, but by "empty" space. Pushing things apart at an increasing rate requires energy If the universe is expand-ing faster now than in the past, our motion away from the A Universal Aeeeleration? distant supernovae has speeded up since the light left them, sweeping us farther away The light then has to The Role of Deceleration The Hubble time is the right age for the universe only if the expansion rate has been constant throughout the time since the expansion of the universe began. Continuing with i our end-of-the-semester-party analogy, this is equivalent to i assuming that you traveled home from the part,y at a constant rate, when in fact this may not have been the case. At first, angry about having to leave, you may have driven fast, I but then as you calmed down-and thought about police ; cars on the highway-you may have begun to slow down until you were driving at a more socially acceptable speed In the same way, in calculating the Hubble time, we have assumed that H has been constant throughout all of time. This may not be a good assumption. Matter creates gravity, whereby all objects pull on all other objects. This mutual attraction will slow the expansion as time goes on, which means that, if gravity were the only force acting (i.e., if the cosmological constant is zero), then the rate of expansion must have been faster in the past than it is to-day. 2, Do people in other parts of the world see the same con- stellation? Give some examples. stellations, asterisms, lunar phases, lunar eclipses, and solar eclipses that you see? Explain. Earth that pull the ocean waters up into two bulges, one on the side of Earth facing the moon and the other on the side 3~ What does the word apparent mean in apparent visual magnitude? page 36 Part 1 The Astronomer's Sky Figure 1-19 The moon's orbit is tipped about 5° to Earth's orbit. The nodes N and N' are the points where the moon passes through the plane of Earth's orbit. If the line of nodes does not point at the sun, the shadows miss, and there are no eclipses at new moon and full moon. At those parts of Earth's orbit where the line of nodes points toward the sun, eclipses are possible at new moon and full moon. stones and markers there that seem to align with the most northern and most southern moonrises. With-out knowing what the sun and moon are and without understanding what an eclipse really is, an observer could use those markers to find the eclipse seasons and guess which full moons were in danger of being eclipsed. That we could use Stonehenge to predict eclipses does not mean that ancient people could do the same, but it would not be too surprising if they could. The cycles of the sun and moon are regular and predictable, and the drama of the lunar phases and lunar eclipses has drawn the attention of every cul-ture around the world. REYIEW What would solar eclipses be like if the moon's orbit were not tipped to the plane of Earth's orbit? There would be a solar eclipse every month at new moon. Because the ecliptic is inclined 23.5° to the celes-tial equator, the sun can never be farther than 23.5° from the celestial equator. That means the moon's shadow would sweep over Earth from west to east somewhere in a band extending 23.5° north or south of Earth's equa-tor. The shadow would never sweep over those of us who live too far north of Earth's equator. We would see the moon pass south of the sun at new moon, and we would never see a solar eclipse. If we lived too far south, the moon would pass north of the sun, and, again, we would see no eclipse. Analyzing a problem with one parameter slightly al-tered is a good way to study a complicated process. We could repeat this analysis for lunar eclipses, it will be more interesting to change a different factor. What would lunar eclipses look like if Earth had no atmosphere? This question asks about a geometry that does not exist, so we must use our imagination. As we struggle to ana- lyze this imaginary situation, we will better understand the geometry of real eclipses. If the moon's orbit were not inclined, then the moon would follow the ecliptic, and it would cross in front of the sun at every new moon. The elegant cycles of the sun and moon produce beau- tiful phenomena to decorate Earth's sky, they control our environment through the seasons and tides, and they may even influence Earth's climate. It is not surprising that the people who built Stonehenge in- Chapter 1 The View from Earth Photographs taken from the Shuttle and would dominate Western thinking for nearly two millenia. the same stars overhead. The only possible explanation is that 2. The Space Shuttle circles the Earth once every 90 min-utes or so. As a second argument, Aristotle explained that travelers parallax shift was immeasurably small. A cosmos of such enormous extent required a leap of imagination that most ancient thinkers were not prepared to make, so thev re- treated to the safety of the Earth-centered view, which who go south a significant distance are able to observe stars that are not visible farther north. And the height of the North Star-the star nearest the north celestial pole-decreases as a traveler moves south. On a flat Earth, everyone would see the traveler must have moved over a curved surface on the Earth, showing stars from a different angle. (See the box ideas on proving the Earth is round.) Measurement of the Earth b Eratosthenes How Do We Know the Earth Is Round? other satellites show that the Earth is round from every above for more In addition to the two ways (from Aristotle's writings) dis- cussed in this chapter, you might also reason as follows: perspective. 3. Suppose you made a friend in each time zone of the Earth. You could call all of them in the same hour and ask, "Where is the Sun?" On a flat Earth, each caller would give you roughly the same answer. on a round Earth you would find that for some, the Sun would be high in the sky, while for others it would be rising, or set-ting, or completely out of sight and this last group of friends would be upset with you for waking them up. I . Let's watch a ship leave its port and sail into the distance on a clear day. On a flat Earth, you would just see the ship get smaller and smaller as it sailed away. that's not what we actually observe. Instead, ships sink below - the horizon, with the hull disappearing first and the mast remaining visible for a while longer. Eventually only the top of the mast can be seen, as the ship sails around the curvature of the Earth; then finally the ship disappears. The Greeks not only knew the Earth was round, they were also able to measure its size. The first fairly accurate deter- mination of the Earth's diameter was made about 200 s.c. by Eratosthenes, a Greek living in Alexandria, Egypt. His method was a geometrical one, based on observations of the Sun. The Sun is so distant from us that all the light rays that strike our planet approach us along essentially parallel lines. To see why, look at Figure 1.9. Take a source of light near the Earth, say at position A. Its rays strike different parts of the Earth along diverging paths. From a light source at B, or at C, still farther away, the angle between rays that strike opposite parts of the Earth is smaller. The more distant the source, the smaller is the angle between the rays. For a source infinitely distant, the rays travel along parallel lines. Of course, the Sun is not infinitely far away, but light rays striking the Earth from a point on the Sun diverge from one another by an angle far too small to be observed with the unaided eye. As a consequence, if people all over the Earth who could see the Sun were to point at it, their fingers would all be essentially parallel to one another. (The same is also true for the planets and stars, an idea we will use in our discussion of how telescopes work.) Eratosthenes noticed that on the first day of summer at Syene, Egypt (near modern Aswan), sunlight struck the bottom of a vertical well at noon. This indicated that the Sun was right over the well (that Syene was on a direct line One brave Greek thinker, Aristarchus of Samos (310-230 s.C.), even suggested, long before Copernicus, that the Earth was moving around the Sun, but Aristotle and most of the ancient Greek scholars rejected this idea. One of the reasons for their conclusion was their under- standing that if the Earth moved about the Sun, they would be observing the stars from different places along Earth's orbit. This would mean that the apparent directions of nearby stars in the sky would change during the year rela- tive to more distant stars. (In a similar way, we see fore- ground objects appear to move against a more distant background whenever we are in motion. When we ride on a train, the trees in the foreground appear to shift their position relative to distant hills as the train rolls by Un- consciously, we use this phenomenon all of the time to estimate distances around us.) The apparent shift in the direction of an object as a re- sult of the motion of the observer is called parallax. We call the shift in the apparent direction of a star due to the Earth's orbital motion stellar parallax. The Greeks made dedicated efforts to observe stellar parallax, even enlisting the aid of Greek soldiers with the clearest vision, but to no avail. The brighter (and presumably nearer) stars just did not seem to shift as they observed them in the spring and then again in the fall. This meant either that the Earth was not moving or that the stars had to be so tremendously far away that the Light Rays from Space The more distant an object, the more nearly parallel are the rays of light coming from it. catalog with about 850 entries. He designated celestial co- ordinates for each star, specifying its position in the sky, just as we can specify the position of a point on the Earth by giving its latitude and longitude. He also divided the stars into magnitudes, according to their apparent bright- ness. He called the brightest ones "stars of the first mag- nitude," the next brightest group "stars of the second magnitude," and so forth. This system, in modified form, Earth meant that "straight up" was not the same in the two still remains in use today cities. This, he realized, could be used to measure how big the Earth is. 1.2 ANCIENT ASTRONOMY 23 FIGURE 1.9 from the center of the Earth to the Sun). At the corre- sponding time and date in Alexandria, he observed that the Sun was not directly overhead but slightly south of the zenith, so that its rays made an angle with the vertical equal to about 1/50 of a circle (7°). Since the Sun's rays striking the two cities are parallel to one another, why would the two rays not make the same angle with the Earth's surface? Eratosthenes reasoned that the curvature of the round. Alexandria, he saw, must be U50 of the Earth's cir-cumference north of Syene (Figure 1.10). Alexandria had been rneasured to be 5000 stadia north of Syene the stadium was a Greek unit of length, derived from the length at noon on of the racetrack in a stadium. Eratosthenes thus found June 22 that the Earth's circumference must be 50 X 5000, or To zenith 250,000 stadia. at Alexandria To Sun It is not possible to evaluate precisely the accuracy of Eratosthenes' solution because there is doubt about which of the various kinds of Greek stadia he used as his unit of distance. If it was the common Olympic stadium, his result was about 20 percent too large. According to another in-terpretation, he used a stadium equal to about 1/6 km, in which case his figure was within one percent of the correct value of 40,000 km. Even if his measurement was not ex-act, his success at measuring the size of our planet by using only shadows, sunlight, and the power of human thought was one of the greatest intellectual achievements in history. Hipparchus Perhaps the greatest astronomer of pre-Christian antiquity was Hipparchus, born in Nicaea in what is present-day Turkey. He erected an observatory on the island of Rhodes in the period around 150 s.C., when the Roman Republic was increasing its influence throughout the Nlediterranean region. There he measured as accurately as possible the di-rections of objects in the sky, compiling a pioneering star FIGURE 1.10 How Eratosthenes Measured the Size of the Earth The Sun's rays come in parallel, but because the Earth's surface curves, a ray ac Syene comes scraight down, while a ray at Alexandria makes an angle of 7° with the verticaLThat means, in effect, that at Alexandria the Earth's surface has curved away from Syene by 7° out of 360°, or I l50 of a full circle.Thus the distance between the two cities must be I/50 the circumference of the Earth. Discovery of Neptune The discovery of the eighth planet, Neptune, was one of the high points in the development of gravitational the-ory. In 1781, William Herschel, a musician and unpaid astronomer, accidently discovered the seventh planet, Uranus. It happens that Uranus had been observed a century before, but in none of those earlier sightings was it recognized as a planet; rather, it was simply recorded as a star. By 1790, an orbit had been calculated for Uranus us-ing observations of its motion in the decade following its discovery. Even after allowance was made for the perturb ing effects of Jupiter and Saturn, however, it was found that Uranus did not move on an orbit that exactly fit the earlier observations of it made since 1690. By 1840, the discrep-ancy between the positions observed for Uranus and those predicted from its computed orbit amounted to about 0.03°-an angle barely discernible to the unaided eye but still larger than the probable errors in the orbital calcula-tions. Uranus just did not seem to move on the orbit predicted from Newtonian theory. In 1843, John Couch Adams (Figure 2.15), a young Englishman who had just completed his studies at Cam-bridge, began a detailed mathematical analysis of the irreg- ularities in the motion of Uranus to see if they might be produced by the pull of an unknown planet. His calcula-tions indicated the existence of a planet more distant than Uranus from the Sun. In October 1845, The tremendously hot gas inside stars has such a high pressure that the stars would surely explode were it '- not for their own gravity. Another reason to study the sun is that life on Earth depends critically on the sun. Should the sun's energy output vary by even a small amount, life on Earth might vanish. In addition, we get nearly all our energy from the sun-oil and coal are merely stored sunlight and our pleasant climate is maintained by energy from the sun. the sun's atmosphere of very thin gas reaches out past Earth's orbit, and any change in the sun, such as an eruption or a magnetic storm, can have a direct effect on Earth. Finally, we study the sun because it is beautiful. Our analysis of sunlight will reveal that the sun is both powerful and delicate. 1~~111 Lyman series an absorption line spectrum can be seen superimposed on the continuous spectrum (b). If we look only at a cloud of excited gas atoms (w~th no continuous source seen behind it), we see that the excited atoms give off an emission line spectrum (c). In both b and c the atoms of thin gas only ab-sorb or give off certain colors, which can be "read off ' to tell us what elements are present in the gas. Atoms in a hot gas are moving at higlo speeds and con-tinually colliding with one another and with any loose elec-trons. They can be excited and de-excited by these collisions, as well as by absorbing and emitting light. FIGURE 4. 7 7 Energy-Level Diagram for Hydrogen As you get to higher and higher energy levels, they get more and more crowded together, approaching a Iimit. The shaded region represents energies at which the atom is ionized (the electron is no longer attached to the atom). page 98 CHAPTER 4 RADIATION AND SPECTRA FIGURE 4.17 Energy-Level Diagram for Hydrogen As you get to higher and higher energy levels, they get more and more crowded together, approaching a Iimit. The shaded region represents energies at which the atom is ionized (the electron is no longer attached to the atom). Each series of arrows represents electrons falling from higher levels to lower ones, releasing photons or waves of energy in the process. that the transitions to or from the ground state, called the Lyman series of lines, result in the emission or absorption of ultraviolet photons. But the transitions to or from the first excited state (labeled n = 2 in Figure 4.16), called the Balmer series, produce emission or absorption in visible light. In fact it was to explain this Balmer series that Bohr first suggested his model of the atom. We mentioned that atoms that have absorbed specific photons from a passing beam of white light and have become excited generally de-excite themselves and emit that light again in a very short time. You might therefore wonder why dark spectral lines are ever produced. In other words, why doesn't this re-emitted light quickly "fill in" the darker absorption lines? Rising and Setting of the Sun 1.1 THE $KY ABOVE 1 9 in fine detail. Let's examine how we can do this and what 106/T X m~x = 3 X 106/T Optical Properties of Light Visible light and other forms of electromagnetic energy exhibit certain behaviors that are important to the design of telescopes and other instruments. For example, light is re-flected from a surface. If the surface is smooth and shiny, as in a mirror, the direction of the reflected light beam can be accurately calculated from a knowledge of the shape of the reflecting surface. Light is also bent, or refracted, when it passes from one kind of transparent medium into another, say from the air into a glass lens. The reflection and refraction of light are the basic properties that make possible all optical instruments-from eyeglasses to giant astronomical telescopes. Such in struments are generally combinations of glass lenses, which bend light according to the principles of refraction, and curved mirrors, which depend on the properties of re-flection. Small optical devices, such as eyeglasses or binoc-ulars, generally use lenses, while large telescopes depend almost entirely on mirrors for their main optical elements. In Chapter 5 we will discuss a number of astronomical in- E = vT4 struments and their uses. For now, we turn to another be-havior of light, one that is essential for the decoding of light. When light passes from one transparent medium to another, an interesting effect occurs in addition to simple refraction. Because the bending of the beam depends on the wavelength of the light as well as the properties of the medium, different wavelengths or colors of light are bent by different amounts and separated. This phenomenon is called dispersion. Figure 4.7 shows how light can be separated into different colors with a prism-a piece of glass in the shape of a triangle. Upon entering one face of the prism, light is re- fracted once, the violet light more than the red, and upon SPECTROSCOPY IN leaving the opposite face, the light is bent again and further where the wavelength is in nanometers and the tempera-ture is in Kelvins. This relationship is called Wien's law. For the Sun, the wavelength at which the maximum energy is emitted is 520 nm, which is near the middle of that portion of the electromagnetic spectrum called visible light. If the Sun, for example, were twice as hot as it is today-that is, if it had a temperature of 11,600 K-it would radiate 24, or 16, times more energy than it does now. Tripling the tempera-ture would raise the energy output 81 times! ASTRONOMY This figure shows the thermometer for measuring the temperatures of stars. noting the spectrum of waves they give off. red is often called a "hot" color and blue a "cool" color, but in nature, it's the other way around.) measuring how much energy a star gives off at each wave- length and by constructing diagrams like Figure 4.8. turns out to be 5800 K. water freezes at 273 K and boils at 373 K. the shorter the wavelength at which the maximum energy is emitted. As you might expect, hot objects give off their gies) than do cool objects. 4.2 THE ELECTROMAGNETIC $PECTRUM 89 FORMATION OF electron goes to a higher level, the energy difference must be obtained from somewhere else. Each jump (or transi- tion) to a different level has a fixed and definite energy The HydrogenSpectrum Now we can use Bohr's model of the atom to understand how spectral lines are formed. Suppose a beam of white light (which consists of photons of all wavelengths) shines through a gas of atomic hydrogen. It turns out that a pho- ton of wavelength 656 nm has just the right energy to raise an electron in a hydrogen atom from the second to the third orbit. as all the photons of different energies (waves of different wavelengths or colors) stream by the hydrogen atoms, photons with this particular wavelength can be absorbed by those atoms whose electrons are orbit- ing on the second level. When they are absorbed. the elec- trons on the second level will now be on the third level, and a number of the photons of this wavelength and energy will be missing from the general stream of white light. the hydrogen atoms absorb light only at certain wavelengths and produce dark lines at those wavelengths in the spectrum we see. Now suppose we have a container of hydrogen gas SPECTRAL LINES 96 CHAPTER 4 RADIATION AND SPECTRA 9. Astronauts observing from a space station need a tele-scope with a light-gathering power 15,000 times that of 1. Why does the wavelength response of the human eye the human eye, capable of resolving detail as small as match so well the visual window of Earth's atmosphere? 0.1 second of arc, and having a magnifying power of 250. Design a telescope to meet their needs. Could you test your design by observing stars from Earth? 2. Basic research in chemistry, physics, biology, and similar sciences is supported in part by industry. How is as-tronomy different? Who funds the major observatories? 3. Most people like beautiful sunsets with brightly glow-ing clouds, bright moonlit nights, and twinkling stars. Most astronomers don't. Why? 10. A spy satellite orbiting 400 km above Earth is suppos-edly capable of counting individual people in a crowd. What minimum-diameter telescope must the satellite carry? (Hint: Use the small-angle formula.) 1. The thickness of the plastic in plastic bags is about 0.001 mm. How many wavelengths of red light is this? 2. Measure the actual wavelength of the wave in Fig-ure 3-1. In what portion of the electromagnetic spec-trum would it belong? 3. Compare the light-gathering powers of a 5-m telescope and a 0.5-m telescope. 4. How does the light-gathering power of one of the Keck telescopes compare with that of the human eye? (Hint: Assume that the pupil of your eye can open to about 0.8 cm.) 1.How do professional astronomers go about making ob-servations at major astronomical facilities? Visit sev-eral observatory Web sites to determine the process an astronomer would go through to secure observing time and make observations at the facility. 2.NASA is in the process of completing a fleet of four space-based "Great Observatories." (The Hubble Space Telescope is one; what are the others?) Examine the cur rent state of these missions by visiting their home pages on the Internet. What advantages would these facilities have over ground-based observatories? 5. What is the resolving power of a 25-cm telescope? What do two stars 1.5 seconds of arc apart look like through this telescope? 6.ost of Galileo's telescopes were only about 2 cm in di-ameter. Should he have been able to resolve the two stars mentioned in Problem 5? 7. How does the resolving power of the 5-m telescope compare with that of the Hubble Space Telescope? Why does the Hubble Space Telescope outperform the 5-m telescope? Exploring TheSky 1,Astronomical telescopes using equatorial mountings must be aligned precisely with the north celestial pole. Locate Polaris and determine how far it is from the north celestial pole. (Hint: Use Reference Lines under the View menu and check Grid under Equitorial. Be sure the spac-ing is set to auto/fine. Then locate the Little Dipper and zoom in on Polaris.) 8.If we build a telescope with a focal length of 1.3 m, what focal length should the eyepiece have to give a magnification of 100 times? Chapter 3 Astronomical Tools 81 Because only certain orbits are permitted, only photons ~, of certain wavelengths can be absorbed or emitted. Each kind of atom has its own characteristic set of spectral lines. our telescope, the gas can absorb photons of certain wave- lengths, and we will see dark lines in the spectrum at those positions. Such a spectrum is called an absorption spec- trum. If we look at a low-density gas that is excited to That is, the radial velocity divided by the speed of light, c, is equal to AA divided by Xo. In astron- omy, radial velocities are almost always given in kilometers per second, so we will express c as 300,000 km/s. For example, suppose the laboratory wave- length of a certain spectral line is 600.00 nm, and the line is observed in a star's spectrum at a wave- length of 600.10 nm. Then AX is +0.10 nm, and the velocity is 0.10/600 multiplied times the speed of light. The radial velocity equals 50 km/s. Because AX is positive, we know the star is reced- ing from us. bell at each clang is shown by the black bells with the We can see that the clangs are squeezed together ahead of the fire truck and stretched apart behind. R E Y I E W i Why do astronomers need to know the precise wave- length of spectral lines? First, a spectrum usually contains many spectral lines, and the only way to identify lines of a certain chemical element is to know exact wavelengths. Second, if we want to find the Doppler shift in the spectrum, we need to know the laboratory wavelengths so we can compare them with the observed wavelengths. For these two rea- sons, astronomers use long lists of precise laboratory wavelengths for the spectral lines that appear in astro- nomical spectra. Measuring wavelength is one thing, but why would astronomers want to know the wavelength at which an we can measure the velocity by measuring the object emits the most electromagnetic energy? amount by which the spectral lines are shifted in wave- length. Astronomical spectra are filled to burting clues Chapter 4 Atoms and Starlight 101 in that chain is planet building. As we explore the solar and allowed it to differentiate into a dense metallic core and system in detail in the following chapters, we must a lower-density silicate crust. In fact, it is possible that the stay alert for further clues to the birth of the planets. solar nebula cooled as the protoplanets grew so that the first planetesimals were metallic and later additions were sili-cate. It is also likely that the planets grew rapidly enough that the heat of formation released by the in-falling material melted the planets and allowed them to differentiate as they formed The Jovian planets probably grew rapidly from icy ma- terials and became massive enough to attract and hold vast amounts of nebular gas. The heat of formation raised their temperatures very high when they were young, and Jupi- ter and Saturn still radiate more heat than they absorb from the sun. center of mass Once the sun became a luminous object, it cleared the nebula as its light and solar wind pushed material out of the system. The planets helped by absorbing some planetesi- mals and ejecting others from the system. Once the solar system was clear of debris, planet building ended. The solar nebula theory proposes that the solar system be- . gan as a contracting cloud of gas and dust that flattened into a rotating disk. The center of this cloud eventually became the sun, and the planets formed in the disk of the nebula. Observational evidence gives astronomers confidence in this theory. Disks of gas and dust have been found around many young stars, so astronomers suspect that planetary systems are common. Planets orbiting other stars are too faint and too close to their star to image directly, but astron- omers have found many of these planets by observing the motion of the star as the star and planet revolve around their extrasolar planet solar nebula theory gravitational collapse uncompressed density wise around axes roughly perpendicular to the plane of the asteroid condensation sequence solar system. This disk shape and the motion of the planets appear to have originated in the disk-shaped solar nebula. Another striking feature of the solar system is the divi- sion of the planets into two families. The terrestrial planets, which are small and dense, lie in the inner part of the system. Galilean satellites protoplanet The Jovian planets are large, low-density worlds in the outer part of the system. In general, the closer a planet lies to the sun, the higher its uncompressed density. meteor The solar system is now filled with smaller bodies such as asteroids, comets, and meteors. The asteroids are small, meteorite radiation pressure rocky worlds, most of which orbit the sun between Jupiter and Mars. They appear to be material left over from the for- mation of the solar system. Another important characteristic of the solar system bodies is their similar ages. half-life heavy bombardment Why are there two kinds of planets in our solar system? Planets begin forming from solid bits of matter, not from gas. Consequently, the kind of planet that forms at a given distance from the sun depends on the kind of compounds that can condense out of the gas to form solid particles. In the inner parts of the solar nebula, the temperature was so high that most of the gas could the hydrogen- and helium-rich Jovian worlds. The condensation sequence combined with the solar nebula hypothesis gives us a way to understand the for- mation of the planets. Can we extend our insight to other stars? What would a planetary system be like if it formed a star that was slightly hotter than the sun? magnifying power alt-azimuth mounting refracting telescopes cannot bring all colors to the same focus, light pollution active optics resulting in color fringes around the images. An achromatic prime focus adaptive optics , secondary mirror charge-coupled device grating Newtonian focus false-color image spectrograph (CCD) REYIEW (a) The Hubble Space Telescope is the largest orbiting telescope ever launched. It was carried into orbit by the space shuttle in 1990. (b) This image of Mars recorded by the Hubbie Space Telescope reveals thin clouds drifting around high volcanoes at the left and details of the polar caps at the top. (NASA) REYIEW Why can infrared astronomers observe from high mountaintops, while X-ray astronomers must observe from space? In this analysis, we find similar consequences spring-ing from different causes. Although both infrared and X-ray telescopes have been put into orbit, they differ dramatically. Infrared radiation is absorbed mainly by water vapor in Earth's atmosphere, which is confined to the lower atmospheric layers. Summary Electromagnetic radiation is an electric and magnetic disturbance that transports energy at the speed of light. The electromagnetic spectrum includes gamma rays, X rays, ultraviolet radiation, visible light, infrared radiation, and radio waves. ranges from 400 nm to 700 run. Infra-red and radio photons have longer wavelengths and carry less energy. Ultraviolet, X-ray, and gamma-ray photons have shorter wavelengths and carry more energy. To obtain data, astronomers use telescopes to gather light, see fine detail, and magnify the image. The first two __ of these three powers of the telescope depend on the tele scope's diameter; Chapter 3 Astronomical Tools 7. If the speed of light is 3 x 105 km/s, how many kilome-ters are in a light-year? How many meters? 8. How long does it take light to cross the diameter of our Milky Way Galaxy? 9. The nearest galaxy to our own is about 2 million light-years away. How many meters is that? 10. How many galaxies like our own would it take laid edge to edge to reach the nearest galaxy? (Hint: See Problem 9.) Billion Galaxies. 1. Locate photographs of Earth taken from space. What do cities look like? Can you see highways? Is the presence of our civilization detectable from space? 2. Locate photographs of nearby galaxies and compare them with photos of very distant galaxies. What kind of detail is invisible for distant galaxies? 3. One of the biggest clusters of galaxies is the Virgo clus-ter. Find out how many and what kind of galaxies are in the cluster. Is it nearby or far away? How to proceed: Decide on the object you want to lo-cate. Then find and center the object by clicking the Find button on the Object Toolbar. The second method is to press the F key. The third is to click Edit, then Find. Once you have the Object Information win-dow, click the center button. b. A star. All stars in TheSky belong to our Milky Way Galaxy. Give the star's name, its magnitude, and its distance in light-years. How to proceed: Click on any star, which brings up an Object Information window. c. A galaxy. Give its name and/or its designation. How to proceed: Click on the Galaxies button in the Object Toolbar, then click on any galaxy. Distances to galaxies are millions and billions of light years. 2. Look at the solar system from beyond Pluto by clicking on View and then on 3D Solar System Mode. Tip the solar system edge-on and then face-on. Zoom in to see the inner planets. Under Tools, set the Time Skip Incre-ment to 1 day and then go forward in time to watch the planets move. 3. Identify some of the brightest constellations located along the Milky Way. (Hint: See View, Reference Lines.l E~ploring TheSky 1. Locate and center one example of each of three differ-ent types of objects: a. A planet, such as Saturn. Find its rising and setting time. Such objects have distances measured in astro-nomical units (AU). 1, What is the largest dimension you have personal knowl- edge of? Have you run a mile? Hiked 10 miles? Run a marathon? 2, In Figure I-4, the division between daylight and dark- ness is at the right on the globe of Earth. How do we know this is the sunset line and not the sunrise line? scale of the cosmos. 3. What is the difference between our solar system, our galaxy, and the universe? 5. Which is the outermost planet in our solar system? Why does that change? 7.Why is it difficult to detect planets orbiting other stars? metric system to simplify our calculations and scientific no- tation to more easily write big numbers. The metric system 8. What does the size of the star image in a photograph solar system, and on some of those billions of planets liquid-water oceans and a protective atmosphere may have spawned life. It is possible that some other plan- ets in the universe are inhabited by intelligent crea- tures who share our curiosity and our wonder at the detect planets orbiting other stars, we know very little about the nature of these planets. Yet we suppose that there must be many planets in the universe and that some are like Earth. We wonder if a few are inhabited by intelligent bei4gs like ourselves. 4. Look at Figure I-6. How can you tell that Mercury fol-lows an elliptical orbit? Can you detect the elliptical shape of any other orbits in this figure or the next? Our goal has been to preview the scale of astronomical ob- jects. To do so, we journeyed outward from a familiar cam- pus scene by expanding our field of view by factors of 100. Only 12 such steps took us to the largest structures in the g, Why are light-years more convenient than miles, kilo- meters, or astronomical units for measuring certain The numbers in astronomy are so large it is not conve- nient to express them in the usual way. Instead, we use the and scientific notation are discussed in Appendix A. We live on the rotating planet Earth, which orbits a rather typical star we call the sun. We defined a unit of distance, the astronomical unit, to be the average distance from Earth LO the sun. Of the eight other planets in our solar system, distances? tell us? Mercury is closest to the sun, and Pluto is the most distant. The sun, like most stars, is very far from its neighbor-ing stars, and this leads us to define another unit of distance, the light-year, t he distance light travels in 1 year. The near- est star to the sun is Proxima Centauri at a distance of 4.2 ly. 9. What is the difference between the Milky Way and the Milky Way Galaxy? the sun is only one of 100 billion stars in our galaxy and that our verse. Galaxies appear to be grouped together in clusters, superclusters, and filaments, the largest structures known. the universe is evolving. Among the billions of stars in each of the billions galaxies, many have planets. Earth's surface is evolving, and so are stars. Stars form from the gas in space, grow old, and eventually die. We do not yet understand how galaxies form or evolve long does it take to reach Mars? 10. What are the largest known structures in the universe? 1. The diameter of Earth is 7928 miles. What is its diame- ter in inches? In yards? 2. If a mile equals 1.609 km and the moon is 2160 miles in diameter, what is its diameter in kilometers? galaxy is only one of many billions of galaxies in the universe 3.One astronomical unit is about 1.5 x 108 km. Explain why this is the same as 150 x 106 km. 4. Venus orbits 0.7 AU from the sun. What is that distance in kilometers? 5,Light from the sun takes 8 minutes to reach Earth. How 6. The sun is almost 400 times further from Earth than is the moon. How long does light from the moon take to roble reach Earth? 8 Part 1 The Astronomer's Sky 12. A car accident occurs around midnight on the night of a full moon. The driver at fault claims he was blinded momentarily by the Moon rising on the eastern horizon. Should the police be- lieve him? a. How often would the Sun rise? b. How often would the Earth set? e. During what fraction of the time would you be able to see the stars? 13. The secret recipe to the ever-popular veggie burgers in the college cafeteria is hidden in a drawer in the director's office. Two students decide to break in and get their hands on it, but they want to do it a few hours before dawn on a night when. there is no Moon, so they are less likely to be caught. What phases of the Moon would suit their plans? 14. Your granduncle, who often exaggerates events in his own 16. Suppose you lived in the crater Copernicus on the side of the Moon facing the Earth. 17. In a lunar eclipse, does the Moon enter the shadow of the Earth from the east or west side? Explain why 18. Describe what an observer at the crater Copernicus would see while the Moon is eclipsed. What would the same observer 19. The day on Mars is 1.026 Earth days long. The martian year I 5. One year, when money is no object, you enjoy your birth- day so much that you want to have another one right away. see during what would be a total solar eclipse as viewed from life, tells you about a terrific adventure he had on February 29, the Earth? 1900. Why would this story make you suspicious? lasts 686.98 Earth days. The two moons of Mars take 0.32 Earth You days (for Phobos) and 1.26 Earth days (for Deimos) to circle the get into your supersonic jet. Where should you and the people planet. You are given the task of coming up with a martian cal celebrating with you travel? From what direction should you endar for a new Mars colony. What might you do? approach? Explain. 20. a. If a star rises at 8:30 P.M. tonight, approximately what time will it rise two months from now? b. What is the altitude of the Sun at noon on December 22, as seen from a place on the Tropic of Cancer? 22. Consider a calendar based entirely on the day and the month then, would be the difference in latitude between the Arctic of 23°? (the Moon's period from full phase to full phase). How many days are there in a month? Can you figure out a scheme analogous to leap year to make this calendar work? Can you also 21. Suppose the tilt of the Earth's axis were only 16°. What, incorporate the idea of a week into your lunar calendar? 23. Show that the Gregorian calendar will be in error by one Circle and the Tropic of Cancer? What would be the effect on day in about 3300 years. the seasons compared with that produced by the actual tilt 'TABLE 8.'1 Spectral Classes for Stars Approximate pectral lass Ca1or Temperature (K) , Principal Features Examples K Orange to red Red M >28,000 Relatively few absorption lines. Lines of doubly ionized nitrogen, 10 Lacertae triply ionized silicon, and lines of other highly ionized atoms. Blue 10,000-28,000 Lines of neutral helium, singly and doubly ionized silicon, singly Rigel ionized oxygen, and magnesium. Spia Hydrogen lines more pronounced than in O-type stars. 7500-10,000 Strong lines of hydrogen. Lines of singly ionized magnesium,silicon, Sirius iron, titanium, calcium, and others. Lines of some neutral metals Vega show weakly. Blue to 6000-7500 Hydrogen lines weaker than in A-type stars, but still conspicuous. Lines Canopus white of singly ionized calcium, iron, and chromium, plus lines of neutra) Procyon iron and chromium, are present, as are lines of other neutral metals. White to 5000-6000 Lines of ionized calcium are most conspicuous spectral features. Many Sun yellow lines of ionized and neutral metals are present. Hydrogen lines are Capella weaker than in F-type stars. Bands of the moVecule CH are strong. tines of neutral metals predominate.The CH bands are still present: Arcturus Aldebaran Strong lines of neutral metals and molecular bands of titanium oxide Betelgeuse dominate. Antares Violet Blue 3500-5000 <3500 1 76 CHAPTER 8 ANALYZING STARLIGHT The second goal in our quest is to find the tempera- tures of the stars. In Chapter 4, we learned that hot stars are blue and cool stars are red, so a measurement of the color of a star can give us a good hint to its tem- perature. A star's spectrum, however, is a rich mine of information, and the most important nugget in the mine is an accurate temperature. We will need those magnitudes can tell us the luminosities of the stars. second each star is emitting. We can use absolute mag- "-3 Stellar Spectra nitude to compare a star with the sun, but a small cor- rection is necessary. Absolute visual magnitude refers to light, but we want to know the luminosity, which includes all energy. Hot stars emit a great deal of ultra- violet radiation that we can't see, and cool stars emit infrared. To add in the energy we can't see, astrono- mers make a small correction that depends on the tem- perature of the star. With that correction, the absolute accurate temperatures later in this chapter when we Astronomers know the luminosity of the sun be- discuss the diameters of the stars. . cause they can send satellites above Earth's atmosphere and measure the amount of energy Earth receives from The Star Thermometer the sun in 1 second. The luminosity of the sun is about 4 x 1026 J/s. The dark lines in a star's spectrum tell us about the gases of the star's photosphere, its visible surface. Although astron-omers look at all of the spectral lines in a star's spec-trum, the most important lines in our discussion are the Balmer lines of hydrogen. Recall that the Balmer lines are produced by hy-drogen atoms that have their electrons in their second orbit. Only three Balmer lines, a red line, a blue line, and a violet line, are visible to our eyes, but astronom-ical spectra record many Balmer lines extending down into the ultraviolet part of the spectrum. Astronomers can calculate the strength of spec-tral lines from theory. In the hottest stars, some hydrogen atoms are ionized, and those atoms can't absorb any photons. In contrast, most of the hydrogen atoms in cool stars have their electrons in the ground state, and those atoms can't absorb Balmer-wavelength photons. temperature dependence of other atoms, we get a pow- erful tool to study the stars. Our graph (Figure 11-6c) found the luminosities of the stars. When we reach is a precision star thermometer. We can find a star's the next goal, we will know even more about the stars. Although we detoured to find the distances to the stars, we have now reached our first goal. We have REYIEW might expect from their luminosities and distances? Stars like the sun radiate most of their energy at visible wavelengths, and we can see those wavelengths, so they look about right for their luminosities and distances. But the coolest stars radiate the vast majority of their pho-tons in the infrared, which we can't see. The luminosity includes all radiated energy. If we use that luminosity and a cool star's distance to predict how bright it should look, we might be disappointed. Most of the energy that cool stars radiate is invisible to our eyes, so they will look fainter than we might expect. Of course, the same thing happens for very hot stars, which radiate most of their energy in the ultraviolet part of the spectrum. The luminosity is a very important piece of informa-tion about any star, we must remember all the de-tails that go into the measurement of luminosity. Explain exactly how errors in measuring the positions of blurred star images could introduce errors into our measure-ments of luminosity. Chapter 11 The Family of Stars 253 temperature by comparing its spectrum to our graph. For example, if we see in a star's spectrum very weak Balmer lines and strong lines of ionized helium, we know the star is very hot. If we see medium-strength Balmer lines and weak lines of ionized calcium, we know the star is about 7000 K, a bit hotter than the sun. If a stellar spectrum contains dark bands pro-duced by the titanium oxide molecule (Ti0) we know immediately that the star is very cool. Only in the coolest stars can these molecules avoid being broken up by collisions with other atoms. Stellar spectra can give us accurate measurements of the temperatures of stellar surfaces. The hottest stars have surface temperatures of 40,000 K and the coolest 2000 K. Compare these with the surface temperature of the sun, about 5800 K. Spectral Classiflcadon High Hydrogen seven spectral classes, or types, still used today: O, B,A,F,G,K,M.* This sequence of spectral types, called the spec-tral sequence, is important because it is a temperature sequence. The O stars are the hottest, and the temper ature continues to decrease down to the M stars, the coolest. For maximum precision, astronomers divide each spectral class into 10 subclasses. For example, *Generations of astronomy students have remembered the spectral sequence using the mnemonic "Oh, Be A Fine Girl (Guy), Kiss Me." More recent suggestions from students include, "Oh Boy, An F Grade Kills Me," and "Only Bad Astronomers Forget Generally Known Mnemonics." Hydrogen Balmer lines are strongest for medium-temperature stars. We have seen that the strengths of spectral lines depend on the surface temperature of the star. From this we can predict that all stars of a given temperature should have similar spectra. If we learn to recognize the pattern of spectral lines produced by a 6000-K star. 10,000 6000 4000 Temperature (K) Hydrogen 10,000 6000 4000 Temperature (K) Hydrogen Lines of ionized calcium are strongest at lower temperatures than the hydrogen Balmer lines_. The lines of each atom a ~ molecule are strongest at I a particular temperature. lonized helium lonized FIgU~C 11-6 calcium The strength of spectral lines can tell us the temperature of a star. (a) Balmer hydrogen lines alone are not enough b ecause Balmer lines of a certain strength could be produced by a hotter star or a cooler star. (b) Adding another atom to the diagram helps, and (c) adding many atoms and molecules to the diagram gives us a precise tool to find the 10,000 6000 4000 temperatures of stars. c . Temperature (K) Low Y54 Part 3 The Stars . . 9. What is the difference between condensation and 8. If you stood on Earth during its formation, as described accretion? in Problem 7, and watched a region covering 100 m2, how many impacts would you expect to see in an hour? (Hints: Assume that Earth had its present radius. The surface area of a sphere is 4nrz.) 10. Why don't terrestrial planets have rings and large satel-lite systems like the Jovian planets? 11. How does the solar nebula theory help us understand the composition of asteroids and comets? 12. How does the solar nebula theory explain the dramatic density difference between the terrestrial and Jovian planets? 13. If you visited some other planetary system in the act of building planets, would you expect to see the conden-sation sequence at work, or was it unique to our solar system? l4. Why do we expect to find that planets are differentiated? 15. What processes cleared the nebula away and ended planet building? 2.The process of protoplanetary accretion is still not well understood. Search the Web for current research in this field. From the results of your search, outline the basic 1. Discuss the history of the atoms in your thumb. Where steps in the formation of a protoplanet through accre- did they come from and what processes brought them tion. What specific factors are important in these mod together? els of planet building? Do these models produce plane-tary systems similar to the ones we know to exist? 2.If the solar nebula theory is correct, then there are prob-ably more planets in the universe than stars. Do you agree? Why or why not? 1.The nearest star is about 4.2 ly away. If you looked back at the solar system from that distance, what would the maximum angular separation be between Jupiter and the sun? (Hint: 1 ly equals 63,000 AU.) 2.The brightest planet in our sky is Venus, which is some-times as bright as apparent magnitude -4 when it is at a distance of about 1 AU. How many times fainter would Exploring TheRy it look from a distance of 1 parsec (206,265 AU)? What would its apparent magnitude be? (Hint: Remember the inverse square law, Chapter 2.) 1.Look at the solar system from space. Notice how thin the disk of the solar system is and how inclined the or-bits of Pluto and Mercury are. (Hint: Under the View menu, choose 3D Solar System Mode, and then zoom in or out. Tip the solar system up and down to see it edge-on.) 3.What is the smallest-diameter crater you can identify in the photograph of Mercury on page 112? (Hint: See Ap-pendix A to find the diameter of Mercury in kilometers.) 4. sample of a meteorite has been analyzed, and the re-sult shows that out of every 1000 nuclei of 4°K origi-nally in the meteorite, only 125 have not decayed. How old is the meteorite? (Hint: See Figure 5-9.) 5.In Table 5-2, which object's density differs least from its uncompressed density? Why? 6.What composition might we expect for a planet that formed in a region of the solar nebula where the tem-perature was about 100 K? 9.The velocity of the solar wind is roughly 400 km/s. How long does it take to travel from the sun to Pluto? Chapter 5 The Origin of the Solar System 125 5. nfrared observations are made with telescopes aboard largest aperture instrument in space is the Hubble Space aircraft and in space, as well as from ground-based facilities Telescope (HST). The Compton Gamma-Ray Observatory on dry mountain peaks. Ultraviolet, x-ray, and gamma-ray (GRO) is the most sophisticated instrument for obsen~ing observations must be made from above the atmosphere. high-energy gamma rays ever built. Many additional astro- Many orbiting observatories have been flown to observe in nomical satellites are planned, although most are smalleX diese bands of the spectrum in the last few decades. The (and less expensive) than HST. INTER-ACTIVITY Most large telescopes get many more proposals for ob- i,# Make a list of all the ways that an observing session at a serving projects than there is night observing time avail- large optical telescope and a large radio telescope might able in the course of a year. Suppose your group is the differ. (One hint: Bear in mind that because the Sun is not telescope time allocation committee reporting to an ob- especially bright at many radio wavelengths, observations servatory director. What criteria would you use in decid- ing how to give out time on the telescope? What steps could you take to make sure all your colleagues thought with radio telescopes can often be done during the day.) the process was fair and people would still talk to you at future astronomy meetings? Another "environmental threat" to astronomy (besides light pollution) comes from the encroachment of terres-trial communications into the "channels"-wavelengths and frequencies-previously reserved for radio astron-omy For example, the demand for cellular phones means that there could be more and more radio channels, used for this purpose. The faint signals from cosmic radio sources will be drowned in a sea of earthly conversation (translated and sent as radio waves). Assume your group is a Congressional committee being lobbied by both radio astronomers who ~~ant to save some clear channels for doing astronomy and the companies that stand to make a lot of money from expanding cellular phone use. What ar-guments would sway you to each side? [For a real-world example of where this issue is being debated, see Science magazine, Nov. 28, 1997, p. 1569.] Your group is a committee of nervous astronomers, about to make a proposal to the government of a smaller Euro-pean country to chip in to build the world's largest telescope in the high dry desert of the Chilean Andes mountains. You expect the government ministers to be very skeptical about supporting this project. What argu-ments would you make to convince them to participate? The same government ministers we met in activity B ask you to draw up a list of the pros and cons of having the world's largest telescope in the mountains of Chile (in-stead of a mountain in Europe). What would your group list as a pro and as a con? REVIEW 'Q`UESTIONS'' 1. Name the two spectral windows through which electromag- netic radiation reaches the surface of the Earth, and describe the largest aperture telescope currently in use for each window 2. List the six bands into which we commonly divde the elec- tromagnetic spectrum, and list the largest-aperture telescope currently in use in each band. 3. When astronomers discuss the apertures of their telescopes, are the advantages for different spectral regions? they say bigger is better Explain why. 4. What are the properties of an image, and what factors deter- mine each? 5. Compare the eye, photographic film, and CCDs as detectors for light. What are the advantages and disadvantages of each? 6. Radio and radar observations are often made with the same antenna, but otherwise they are very different techniques. Compare and contrast radio and radar astronomy in terms of the equipment needed, the methods used, and the kind of re- sults obtained. 7. Why do astronomers place telescopes in Earth orbit? What Less light pollution and no atmospheric turbulence. 8. What was the problem with the Hubble Space Telescope ~d how was it solved? 9. Describe the techniques radio astronomers use to obtain a resolution comparable to what astronomers working with vis- ible light can achieve. 10. What happens to the image produced by a lens if the lens is "stopped dow~n" with an iris diaphragm-a device that covers its periphery? I I. What would be the properties of an ide~d astronomical de- tector? How closely do the actual properties of a CCD ap- proach this ideal? 12. Fifty years ago, the astronomers on the staff of Mount Wil- son and Palomar Observatories each received about 60 nights per year for their observing programs. Today an astronomer feels fortunate to get 10 nights per year on a large telescope. Can you suggest some reasons for this change? 126 CHAPTER 5 ASTRONOMICAL INSTRUMENTS 5. I The astronomical telescope collects light and forms an clear weather, dark skies, low water vapor, and excellent at- image, using a convex lens in a refractor or a concave mir- mospheric seeing (low atmospheric turbulence). A new ror in a reflector to bring it to a focus. The distance from generation of large instruments has been constructed in the the lens or mirror to the focus is called the focal length. 1990s, including the 10-m Keck telescopes at Mauna Kea The diameter, or aperture, of the telescope determines the and the four 8-m instruments that constitute the European brightness and resolution of the image. Resolution is usu- Very Large Telescope in Chile. ally expressed in units of arcseconds. 5.4 In the 1930s, radio astronomy was pioneered by Jansky 5.2 Optical (visible light) detectors include the eye, photo- and Reber. A radio telescope is basically a radio antenna (of- graphic film, and the charge-coupled device (CCD) de- ten a large curved dish) connected to a receiver. Significantly tector. Detectors sensitive to infrared radiation must be enhanced resolution can be obtained with interferometers, cooled to very low temperatures. A spectrometer disperses including interferometer arrays like the 27-element VIA. the light into a spectrum to be recorded for detailed analysis. Expanding to very long baseline interferometers, radio as-tron40 Supernova, explosion that leaves a black hole * Stars in this mass range may produce a type of supernova different from the one we have discussed. each zone, all places keep the same .standard time. along the 180° meridian of longitude. The International Date Line Where the Date Changes The international date line is an arbitrarily drawn line on the Earth where the date changes. 3.4 THE CALENDAR 65 the Moon about the Earth; and the year,. Among other purposes, it was useful for various ways by different cultures. predicting astronomical events-for example, the posi-tions of Venus in the sky (Figure 3.12). Stonehenge (Figure 3.11), about 13 km from Salisbury in southwest England. It is a complex array of stones, ditches, and holes arranged in concentric circles. Carbon dating and other studies show that Stonehenge was built during three periods ranging from about 2800 B.c. to 1500 s.c. Some of thevtones are aligned with the direc- The ancient Chinese developed an especially complex calendar, largely limited to a few privileged hereditary court astronomer-astrologers. In addition to the motions of the Earth and Moon, they were able to fit in the approx- imately 12-year cycle of Jupiter, which was central to their system of astrology. The Chinese still preserve some as- pects of this system in their cycle of 12 "years"-the Year of the Dragon, the Year of the Pig, and so on-that are de- fined by the position of Jupiter in the zodiac. Our Western calendar derives from Greek calendars dating from at least the 8th century B.C. Observing the Planets Figure 9-13 (a) The theory that the impact of one or more comets altered Earth's dimate and drove dinosaurs to extinction has become so popular it appeared on this Hungarian stamp. Climate change, global warming. The spacecraft shown (ICE) flew through the tail of Comet Giacobini-Zinner in 1985. Note the dead dinosaurs in the background. (b) The giant impact scar buried in Earth's crust near the village of Chicxulub in the northern Yucatan was formed about 65 million years ago. Such a large impact would have altered Earth's dimate dramatically, and the Chicxulub impact is suspected of being the trigger that ended the Cretaceous period and destroyed the dinosaurs. United States Chicxulub crater Nearly all of the mass of a comet is in the nucleus, but the light we see comes from the coma and the tail. What kind of spectra do we get from comets, and what does that tell us about the process that converts dirty ice into a comet? Our solar system is an elegant planetary system, but we must remember that it is small. The universe contains billions of stars more or less like our sun. We begin our Stony meteorites can be classified as chondrites, which contain small, glassy particles called chondrules. If such a meteorite is rich in volatiles and carbon, it is called a carbona ceous chondrite. An achondrite is a stony meteorite that con-tains no chondrules. Achondrites appear to have been melted after they formed, but chondrites were never melted. Carbona-ceous chondrites were never even warm enough to drive off volatiles. This evidence suggests that the different kinds of mete-orites were formed in bodies roughly 100 km in diameter. Such objects could have melted, differentiated, and cooled very slowly, and when they were later broken by collisions. Mexico. INTER-ACTIVITY (This section in each chapter will be devoted to activities that so many fewer people know the night sky today than at small groups of students can collaborate on, either during the time of the ancient Greeks? What reasons can you class, in a discussion section, or as independent projects. think of why a person may want to be acquainted with Your instructor may assign some of these or you can use the night sky? them in a study group to extend your understanding of as- tronomy.) Constellations commemorate great heroes, dangers, or events in the legends of the people who name them. Suppose we had to start from scratch today, naming the patterns of stars in the sky What would you choose to commemorate by naming a constellation after it/him/her and why? Can the members of your group agree on any choices? You can begin by considering the question with which we began this chapter. How many ways can you think of to prove to a rabid member of the "Flat Earth Society" that our planet is indeed round? Have your group make a list of ways in which a belief in astrology (the notion that your life path or personality is controlled by the position of the Sun, Moon, and planets at the time of your birth) might be harmful to an indi-vidual or to society at large. Members of the group should compare their experi-ences with the night sky Have you seen the Milky Way? Can you identify any constellations? Why do you think How many can your group come up with? (Think of things like Milky Way" candy bars, Sat-TM 1. From where on Earth could you observe all of the stars dur- ing the course of a year? What fraction of the sky can be seen 2. Describe a practical way to determine in which constellation the Sun is found at any time of the year. does it mean when an astronomer says, "I saw a comet in Orion last night"? 4. Give four ways to demonstrate that the Earth is round. 5. Explain why we see retrograde motion of the planets, ac- cording to both geocentric and heliocentric cosmologies. from the North Pole? 6. Draw a picture that explains why Venus goes through phases the way the Moon does, according to the heliocentric cosmol-ogy. Does Jupiter also go through phases as seen from the 3. What is a constellation as astronomers define it today? What Earth? Why? 7. In what ways did the work of Copernicus and Galileo differ from the traditional views of the ancient Greeks and of the Catholic Church? 8. Show with a simple diagram how the lower parts of a ship disappear first as it sails away from you on a spherical Earth. and their motions influence human behavior. 10. Design an experiment to test whether or not the planets Use the same diagram to show why lookouts on old sailing ships could see farther from the masthead than from the deck. Would 11. Why do you think so many people believe in astrology? What psychological needs does such a belief system satisfy? there be any advantage to posting lookouts on the mast if the Earth were flat? (Note that these nautical arguments for a spherical Earth were quite familiar to Columbus and other mariners of his time.) 12. Consider three cosmological perspectives: (1) the geocen- tric perspective, (2) the heliocentric perspective, and (3) the modern perspective in which the Sun is a minor star on the out-skirts of one galaxy among billions. Discuss some of the cultural 9. Prallaxes of stars were not observed by ancient as- tric hypothesis? and philosophical implications of each point of view. tronomers. How can this fact be reconciled with the heliocen- 36 CHAPTER I OBSERVING THE SKr. THE BIRTH OF ASTRONOMY PROBLEMS I 3. The Moon moves relative to the background stars. Go out- side at night and note the position of the Moon relative to Earth's circumference? nearby stars. Repeat the observation a few hours later. How far has the Moon moved? (For reference, the diameter of the Moon is about'/2 .) Based on your estimate of its motion, how to the stars in which you first observed it? of 30° with the vertical. What then would he have found for the 16, Suppose Eratosthenes' results for the Earth's circumfer- ence were quite accurate. If the diameter of the Earth is 12,740 km, evaluate the length of his stadium in kilometers. long will it take for the Moon to return to the position relative 17. Suppose you are on a strange planet and observe, at night, that the stars do not rise and set but circle parallel to the hori- zon. Now you walk in a constant direction for 8000 miles, and at 14. The north celestial pole appears at an altitude above the horizon that is equal to the observer's latitude. Identify Polaris, the North Star, which lies very close to the north celestial pole. Measure its altitude. (This can be done with a protractor. Al- ternatively your fist, extended at arm's length, spans a distance approximately equal to 10°.) Compare this estimate with your latitude. (Note that this experiment cannot be easily per- formed in the Southern Hemisphere because Polaris itself is not visible in the south and no bright star is located near the what you suggest? your new location on the planet you find that all stars rise straight up in the east and set straight down in the west, per- pendicular to the horizon. a, How could you determine the circumference of the planet without any further observations? b, What evidence is there that the Greeks could have done ~, What is the circumference, in miles, of that planet? south celestial pole.) 15. Suppose Eratosthenes had found that in Alexandria at noon on the first day of summer, the line to the Sun makes an angle Q The Constellations and Their Stars SUMMARY 1.1 The direct evidence of our senses supports a geo- centric perspective, with the celestial sphere pivoting on the celestial poles and rotating about a stationary Earth. We see only half of this sphere at one time, limited by the horizon; the point directly overhead is our zenith. The Sun's annual path on the celestial sphere is the ecliptic, a line that runs through the center of the zodiac, the 18°-wide strip of sky within which we always find the Moon and plan- 1.3 The ancient religion of astrology, whose main contri- bution to civilization was a heightened interest in the heav- ens, began in Mesopotamia. It reached its peak in the Greco-Roman world, especially as recorded in the Tetrabib- Natal astrology is based on the assumption that the positions of the planets at the time of our birth, as described by a horoscope, determines our future. How- ever, modern tests clearly show that there is no evidence for ets. The celestial sphere is organized into 88 constellations, this, even in a broad statistical sense. or sectors. 1.4 Nicolaus Copernicus introduced the heliocentric 1.2 Ancient Greeks such as Aristotle recognized that the cosmology to Renaissance Europe Earth and Moon are spheres, and understood the phases tionibus. of the Moon, but because of their inability to detect stellar parallax, they rejected the idea that the Earth moves. Era- tosthenes measured the size of the Earth with surprising precision. Hipparchus carried out many astronomical obser- vations, making a star catalog, defining the system of stellar magnitudes, and discovering precession from the apparent shift in the position of the north celestial pole. Ptolemy of Alexandria summarized classical astronomy in his Almagest; he explained planetary motions, including retrograde motion, with remarkably good accuracy using a model cen- tered on the Earth. This geocentric model, based on combi- nations of uniform circular motion using the epicycles, was accepted as authority for more than a thousand years. SUMMARY 35 ER-ACTIVITY Constellations commemorate great heroes, dangers, or events in the legends of the people who name them. Suppose we had to start from scratch today, naming the patterns of stars in the sky. What would you choose to commemorate by naming a constellation after it/him/her and why? Can the members of your group agree on any choices? You can begin by considering the question with which we began this chapter. How many ways can you think of to prove to a rabid member of the "Flat Earth Society" that our planet is indeed round? Have your group make a list of ways in which a belief in astrology (the notion that your life path or personality is controlled by the position of the Sun, Moon, and planets at the time of your birth) might be harmful to an indi-vidual or to society at large. Members of the group should compare their experi-ences with the night sky Have you seen the Milky Way? Can you identify any constellations? Why do you think Although astronomical mythology no longer holds a powerful sway over the modern imagination, we still find proof of the power of astronomical images in the number of products in the marketplace that have astro-nomical names. How many can your group come up with? (Think of things like Milky Way TM candy bars, Sat-TM Urn cars, or Comet'`" cleanser.) REVIEW QUESTIONS Catholic Church? Earth? Why? 1. From where on Earth could you observe all of the stars dur- ing the course of a year? What fraction of the sky can be seen from the North Pole? 2. Describe a practical way to determine in which constellation the Sun is found at any time of the year. 3. What is a constellation as astronomers define it today? What does it mean when an astronomer says, "I saw a comet in Orion last night"? 4. Give four ways to demonstrate that the Earth is round. 5. Explain why we see retrograde motion of the planets, ac- cording to both geocentric and heliocentric cosmologies. 6. Draw a picture that explains why Venus goes through phases the way the Moon does, according to the heliocentric cosmol- ogy. Does Jupiter also go through phases as seen from the THOUGHT QUESTIONS 7. In what ways did the work of Copernicus and Galileo differ from the traditional views of the ancient Greeks and of the Although he retained the Aristotelian idea of uni- form circular motion, Copernicus suggested that the Earth is a planet and that the planets all circle about the Sun, de- throning the Earth from its position at the center of the uni- verse. Galileo Galilei was the father of both modern experi- mental physics and telescopic astronomy. He studied the acceleration of moving objects, and in 1610 began tele- scopic observations, discovering the nature of the Milky Way, the large-scale features of the Moon, the phases of Venus, and four satellites of Jupiter. Although he was ac- cused of heresy for his support of the heliocentric cosmol- ogy, Galileo's observations and brilliant writings convinced most of his scientific contemporaries of the reality of the Copernican theory. 8. Show with a simple diagram how the lower parts of a ship disappear first as it sails away from you on a spherical Earth.. Use the same diagram to show why lookouts on old sailing ships could see farther from the masthead than from the deck. Would Earth were flat? (Note that these nautical arguments for a spherical Earth were quite familiar to Columbus and other mariners of his time.) 10. Design an experiment to test whether or not the planets and their motions influence human behavior 11, Why do you think so many people believe in astrology? What psychological needs does such a belief system satisfy? there be any advantage to posting lookouts on the mast if the 12. Consider three cosmological perspectives: (1) the geocen- tric perspective, (2) the heliocentric perspective, and (3) the modern perspective in which the Sun is a minor star on the out-skirts of one galaxy among billions. Discuss some of the cultural 9. Parallaxes of stars were not observed by ancient as- tric hypothesis? and philosophical implications of each point of view. tronomers. How can this fact be reconciled with the heliocen- TABLE 5.1 Large Optical Telescopes Being Built or in Operation Aperture (m) Telescope Name Location Status Web Address 16.4 Very Large Telescope Cerro Paranal, Chile* First telescope www.eso:org/vIU (four 8.2-m telescopes) completed 1998 11.8 Large Binocular Telescope Mount Graham, Arizona First light 2002-2003 medusa.as.arizona.edu/btwww/ (two 8.4-m telescopes) tech/Ibtbook.htm! 10.0 Keck I Mauna Kea, Hawaii Completed 1993 astro.caltech.edu/mirrorlkeck/index.html 10.0Keck II Mauna Kea, Hawaii Completed 1996 astro.caltech.edu/mirrorlkeck/index.html 9.9 Hobby-Eberly (HET) Mount Locke,Texas Completed 1997 www.astro.psu.edu/het/overview.html 8.3 Subaru (Pleiades) Mauna Kea, Hawaii First light 1998 www.naoj.org/ 8.0 Gemini (North) Mauna Kea, Hawaiij- First light 1994 www.gemini.edu 8.0 Gemini (South) Cerro Pachon, Chile j- First light 2000 www.gemini.edu 6.5 Multi-Mirror (MMT) Mount Hopkins, Arizona First light 1998 sculptor.as.arizona.edu.edu/foltz/www/ 6.5 Magellan Las Campanas, Chile First light 1997 www.ociw.edul--johns/magellan.html 6:0 Large Alt-Azimuth Mount Pastukhov, Russia Completed 1976 - 5.0 Hale Mount Palomar, California Completed 1948 astro.catteeh.edulobservatories/ palomar/pubiic/index.html William Herschel Canary Islands, Spain Completed 1987 www.ast.cam.ac.uk/INGIPR/pr.html SOAR Cerro Pachon, Chile First light 2002 www.noao.edu/ BlancoTelescope (NOAO) CerroTololo,Chilej- Completed 1974 www.ctio.noao.edu/ctio.html Anglo-Australian (AAT) Siding Spring, Australia Completed 1975 www.aao.gov.au/index.html NOAO Mayall Kitt Peak,Arizonat Completed 1973 www.noao.edu/noao.html United Kingdom Infrared (UKIRT) Mauna Kea, Hawaii Completed 1979 www.jach.hawaii.edu/UKIRT/home.html Canada-France-Hawaii (CFHT) Mauna Kea, Hawaii Completed 1979 www.cfht.hawaai.edu/ ESO Cerro La Silla, Chile* Completed 1976 www.ls.eso.orgl ESO NewTechnology Cerro La SiIla,Chile* Completed 1989 www.ls.eso.orgl Max Planck Institut Calar Alto, Spain Completed 1983 www.mpia-hd.mpg.de/CAHA/ WIYN Kitt Peak,Arizonaj- Completed 1993 www.noao.edu/wiyn/wiyn:html Astrophysical Research Corp. Apache Point, New Mexico Completed 1993 www.apo.nmsu.edu/ Shane (Lick Observatory) Mount Hamilton, California Completed 1959 www.ucolick.org/ NASA Infrared (IRTF) Mauna Kea, Hawaii Completed 1979 irtf.ifa.hawaii.edu 4.2 4.2 4.0 3.9 3.8 3.8 3.6 3.6 3.6 3.5 3.5 3.5 3.0 3.0 Another lucky accident is that the universe is so finely balanced between expansion and contraction. It is expand-ing, but very slowly. If the expansion had been at a much higher rate, all of the matter would have thinned out be-fore galaxies could form. If it were at a much slower rate, then the expansion would have reversed and all of the mat-ter would have recollapsed, probably into a black hole-again, no stars, no planets, no life. The development of life forms on Earth depends on still more lucky coincidences. According to the Anthropic Principle, any universe in which creatures like us exist must have rules that give us enough time, and the right conditions, to evolve. (NASA) page 484 CHAPTER 20 THE BIG BANG 20. THE ANTHROPIC PRINCIPLE 453 THE ANTHROPIC PRINCIPLE does not affect the intensity of the radiation measured by The picture we have developed about the evolution of our universe is a remarkable one. With new telescopes we have begun to collect enough observational evidence that we can describe how the universe evolved from a mere frac- tion of a second after the expansion began. all the deuterium would have been converted to helium. amount of deuterium we see today thus gives us a clue to the density of the universe when it was about 3 min old. When the universe was about 3 min old and its tempera- ture was down to about 900 million K, protons and neu- trons could combine without being immediately disrupted The deuterium measurements indicate that the pre- by high-energy photons. They began to form the simplest sent-day density is about 5 X 10-31 g/cm3. (heavy hydrogen), helium, and lithium. Atoms Form Learning From Deuterium 446 CHAPTER 20 THE BIG BANG F~ruRE 20.12 Farticle Interactions in the Early Universe (a) In the first fractions of a second, when the universe was very hot, energy was converted to particles and antiparticles. The reverse reaction also happened: a particle and antiparticle could collide and produce energy (b) As the temperature of the universe decreased, the energy of typical photons became too low to create matter. Instead, existing particles fused to create such nuclei as deuterium and helium. (c) Later, it became cool enough for electrons to settle down with nuclei and make neutral atoms. Most of the universe was still hydrogen. Among the particles created in the early phases of the universe was the ghostly neutrino (see Chapter 7), which ~ today interacts only very rarely with ordinary matter. In the ; crowded conditions of the very early universe, however, ~ neutrinos ran into so many electrons and positrons that ~ they experienced frequent interactions despite their "anti-= social" natures. By the time the universe was a little more than 1 s old, the density had dropped to the point where neutrinos no longer interacted with matter, but simply traveled freely through space. In fact, these neutrinos should now be all around us. Since they have been traveling through space unimpeded (and hence unchanged) since the universe was 1 s old, measurement of their properties would offer one of the best tests of the big bang model. Unfortunately, the timates of their masses, including dark matter, to calculate the average density. Such estimates indicate a density of about 1 to 2 X 10-3° g/cm3 (10 to 20 percent of critical) and suggest that the universe is open. The emission produced by infalling matter comes from a small volume of space closely surrounding the black hole. The small size of the radiating region is exactly what we need to explain the fact that quasars vary on time scales of weeks to months. 18.3 THE POWER BEHIND THE QUASARS 395 380 Arc Seconds 88.000 Liqht-Years Figure 18.18 shows observations of an elliptical galaxy galaxy, there is a ring of dust and gas about 400 LY in di- ameter, surrounding a 1.2-billion-Ms„„ black hole. observations show that two jets emerge in a direction per- might it be replenished? pendicular to the ring, just as the model predicts. One possibility for the original fuel source is the very dense star clusters that form near the centers of galaxies. Evolution of Quasars There is one other important fact about quasars that our black hole model can explain. Recall that when first intro- ducing quasars, we mentioned that they generally tend to 1.5 x 1 0-6 be far away-and are, in fact, the most distant objects we can detect in the universe. This result is very interesting, because when we see extremely distant objects, we are see- ing them as they were long ago. Radiation from a quasar 8 billion LY away is telling us what that quasar was like C2 10-6 8 billion years ago, much closer to the time the galaxy formed. If there are more quasars far away, there must have been more quasars long ago-when the universe was Q younger. o Recently completed counts of quasars at different red- a r ~ 1 ' I / shifts show us how dramatic this trend really is (Figure Z 5 x 10-~ 18.19). The number of quasars was the greatest at the time  when the universe was only 20 percent of its present age. Our model says that quasars are black holes with enough fuel to make a brilliant accretion disk right around them. Why were there more quasars long ago (far away) than there are today (nearby)? Perhaps in the days of its Age of the universe (current age = 1.0) youth, a black hole at the center of a galaxy could find abundant fuel. But it may be that later in its life, much of FIGURE 18.19 The Number of Quasars as We Look Back in Time This the available fuel has been used up, and the hungry black graph shows the number of quasars at earlier and earlier times (that hole has very little left with which to light up the galaxy's is, farther and farther away). An age of 0 corresponds to the central regions. beginning of the universe; an age of I corresponds to the present In other words, if matter in the accretion disk is con- • time. Note that quasars were most abundant when the universe was tinually being depleted by falling into the black hole or about 20 percent of its current age. page 396 CHAPTER 18 QUASARS AND ACTIVE GALA)CIES FIGURE 19.4 FIGURE 19.$ Galaxy in a Nearby Group This member of a neighboring group Central Region of the Virgo Cluster Virgo is the nearest rich cluster and is at a distance of about 50 million LY With its hundreds of bright galaxies, Virgo is the dominant feature of the Local of galaxies, called Dwingeloo 1 (after the location of the Dutch telescope that found it), was discovered in 1994. It is about 10 million LY away. The spiral galaxy is in the middle of the picture. The Coma cluster, with a diameter of at least 10 million LY (Figure 19.6). Lying about 250 to 300 million LY away, this cluster is centered on two giant ellip- ticals whose luminosities equal about 400 billion Suns. Thousands of galaxies have been observed in Coma, cluster. Although M87 is not shown in Figure 19.5, two 20, or 30 thousand galaxies, and each galaxy has billions other giant ellipticals in the cluster are. and billions of stars.,.If you were traveling at the speed 19.1THE DISTRIBUTION Of GALAXIES IN SPACE 409 FIGURE 19.6 The Central Part of the Rich Coma Cluster of Galaxies light, it would still take you more than 10 million years (longer than the history of the human species) to cross this giant swarm of galaxies. And if you lived on a planet on the outskirts of one of these galaxies, many other members of the cluster would be close enough to be noteworthy sights in your nighttime sky. Really rich clusters such as Coma usually have a high concentration of galaxies near the center. Giant elliptical galaxies occur in these central regions, but few if any spiral galaxies are found there. The spirals that do exist generally occur on the outskirts of clusters. We might say that ellipti-cals are highly "social": They are often found in groups, and very much enjoy "hanging out" with other ellipticals in crowded situations. Spirals, on the other hand, are more "shy": They are more likely to be found in poor clusters or on the edges of rich clusters. Superclusters and Voids The best-studied of the superclusters is the Local Supercluster. page 410 CHAPTER 19 MAPPING THE UNIVERSE FIGURE 19.7 The Local Supercluster The local supercluster of galaxies covers a volume of space that is over 100 million LY across, and these images show where the galaxies are located. Using a computer, astronomer BrentTully has mapped the concentration of galaxies from two different perspectives. The Local Group is at the left wall in each case, and theVirgo cluster is in the center. The green areas show where the galaxies are found; the regions colored red and yellow have the highest density of galaxies. Note that the galaxies are found in clumps and small groups, while much of space is empty. (Brent Tullg University o( Hawaii) old stars or new stars. If a galaxy is at a distance of 10 billion light years, then we are seeing it as it was 10 bil- lion years ago. If we think the age of the universe is 13-15 billion years, then that galaxy must have formed not more shape. spiral galaxies contain young stars than 3-5 billion years after the universe began. elliptical also find stars in that galaxy that are already 3 billion years galaxies have mostly old stars and very little interstellar old, then the galaxy must have formed very close to the be- ginning of time. our own Galaxy contains globu- lar clusters with stars that are at least 13 billion years old, and some may be even older than that. The Milky Way must be at least as old as the oldest stars in it. 19.2 THE EVOLUTION OF GALAXIES: THE OBSERVATIONS 415 224 I CHAPTER 9 36. How would our theories of the Moon's history have been affected if astronauts had discovered sedimentary rock on the Moon? 40. Use a telescope or binoculars to observe the Moon. Compare the texture of the lunar surface you see on the maria with that of the lunar highlands. How does the 37. Imagine that you are planning a lunar landing mission. visty of details vary with distance from the terminator What type of landing site would you select? Where might (the boundary between day and night on the Moon)? you land to search for evidence of recent volcanic activity? 41. Observe the Moon through a telescope every few nights over a period of two weeks between new moon and ,~~ERQCT, full moon. Make sketches of various surface features, such ~'- A W.1212/CD120 M Ql / F ST 1() N S as craters, mountain ranges, and maria. How does the appearance of these features change with the Moon's 38. Several unmanned missions to the Moon were under phase? Which features are most easily seen at a low angle development as of this writing. These include LUNAR-A of illumination? Which features show up best with the Sun and SELENE (Institute of Space and Astronautical Science, nearly overhead? Japan) and SMART-1 (European Space Agency). Search the Moon gradually cooled, low-density lava floating on the Moon's surface began to solidify into the anorthositic crust that exists today. The barrage of rock fragments that ended about 3.8 billion years ago produced the craters that cover the lunar highlands. At the end of this crater-making era, more than a dozen ' asteroid-size objects, each measuring at least 100 km across, .r struck the Moon and blasted out the vast mare basins. Mean-while, heat from the decay of long-lived radioactive elements like uranium and thorium began to melt the inside of the , Moon. From 3.8 to 3.1 billion years ago, great floods of mol-ten rock gushed up from the lunar interior, filling the impact basins and creating the basaltic maria we see today. Very little has happened on the Moon since those ancient times. A few relatively fresh craters have been formed, but the figure 9-21 astronauts visited a world that has remained largely un- changed for more than 3 billion years. During that same The Moon-Site of Future Industry? Volcanic material on the lunar surface contains trapped oxygen, which could be released by per period on the Earth, by contrast, the continents have been `t totally transformed time and time again (see ), heating lunar material along with hydrogen gas. Oxygen and water could also be extracted from ice deposits at the lunar poles. These Many questions and mysteries still remain. The six Ameri- ,can and three Soviet lunar landings have brought back samples essentials could support a permanent lunar colony and long-term exploration of the Moon. The Moon's weaker gravity and lack of an We still know very little about the Moon's far side and poles. atmosphere also make it a useful launching pad for sending payloads farther into the solar system. The rocket fuel for this enterprise would use oxygen "mined" from the Moon. (NASA) Are there really vast ice deposits at the poles? How much of the Moon's interior is molten? Just how large is its iron core? How old are the youngest lunar rocks? Did lava flows occur over western Oceanus Procellarum only 2 billion years ago, as crater densities there suggest? Could there still be active volca- be extracted, making the Moon a springboard for human noes on the Moon? Are there mineral deposits on the lunar sur- exploration of the solar system (Figure 9-21)? Such questions face from which oxygen for life support and rocket fuel could can be answered only by returning to the Moon. Analysis of seismic waves and other data indicates that striking the surface. There is no evidence of plate tectonic activity on the Moon. the Moon has a crust thicker than that of the Earth (and thickest on the far side of the Moon), a mantle with a Our Barren Moon 1 221 ~f the Moon's radius, and a 7. Could you use a magnetic compass to navigate on the Moon? Why or why not? s far thicker than that of the ~phere probably extends from the co the core. 8. Describe the evidence that (a) the Moon has a more solid interior than the Earth and (b) the Moon's interior is not completely solid. olobal magnetic field today, although 9, Explain why moonquakes occur more frequently ~etic field billions of years ago. when the Moon is at perigee than at other locations along ~ of the Moon: The anorthositic crust its orbit. highlands was formed between 4.0 and and 3.8 billion years ago. 10. Why is the Earth geologically active while the Moon ars ago, whereas the mare basalts solidified is not? ~on's surface has undergone very little change over c 3 billion years. 11. On the basis of moon rocks brought back by the astronauts, explain why the maria are dark colored but the lunar highlands are light colored. aeoroid impacts have been the only significant eathering" agent on the Moon. The Moon's regolith, or 12. Briefly describe the main differences and similarities arface layer of powdered and fractured rock, was formed between Moon rocks and Earth rocks. by meteoritic action. 13. Rocks found on the Moon are between 3.1 and 4.6 billion years old. By contrast, the majority of the Earth's surface is made of oceanic crust that is less than 200 million years old, and the very oldest Earth rocks are about 3 billion years old. If the Earth and Moon are essentially the same age, why is there such a disparity in 14. Why do most scientists favor the collisional theory of the Moon's formation? 15. Some people who supported the fission theory proposed that the Pacific Ocean basin is the scar left when the Moon pulled away from the Earth. Explain why this idea is probably wrong. later by the impact of planetesimals and filled with lava • All of the lunar rock samples are igneous rocks formed largely of minerals found in terrestrial rocks. The lunar rocks contain no water and also differ from terrestrial rocks in being relatively enriched in the refractory elements and depleted in the volatile elements. Origin of the Moon: The collisional ejection theory of the the ages of rocks on the two worlds? Moon's origin holds that the proto-Earth was struck by a ejection Mars-sized protoplanet and that debris from this collision coalesced to form the Moon. This theory successfully explains most properties of the Moon. • The Moon was molten in its early stages, and the anorthositic crust solidified from low-density magma that floated to the lunar surface. The mare basins were created from the lunar interior. ADVLlN C^ D ~II~STI O NS • Tidal interactions between the Earth and Moon are slowing the Earth's rotation and pushing the Moon away from the Earth. Review Questions 1. Explain why liquid water cannot exist on the surface of the Moon. 2. What kind of features can you see on the Moon with a small telescope? 3. Describe the differences between the maria and the lunar highlands. Which kind of terrain is more heavily cratered? Which kind of terrain was formed later in the Moon's history? How do we know? 4. What does it mean to say the Moon is a "one-plate world"? What is the evidence for this statement? Questions preceded by an asterisk ('`) involve topics discussed in Box 9-1. L Problem-solving tips and tools Recall that the average density of an object is its mass divided by its volume. The volume of a sphere is 4/37cr3, where r is the sphere's radius. The surface area of a sphere of radius r is 47cr'-, while the surface area of a circle of radius r is ~trZ. the acceleration of gravity on the Earth's surface is 9.8 m/s'-. You may find it useful to know that a 1-pound (1-lb) weight presses down on the Earth's surface with a force of 4.448 newtons. You might want to review Newton's universal law of gravitation in Section 4-7. Consult Table 9-1 and the Appendices for any additional data. 5. Why was it necessary to send unmanned spacecraft to ` 16. In a whimsical moment during the Apollo 14 mission, astronaut Alan Shepard hit two golf balls over the lunar surface. Give two reasons why they traveled much farther than golf balls do on Earth. 6. What is the evidence that ice exists at the lunar poles? Is this evidence definitiye? land on the Moon before sendmg humans there? l 1 ~° ADVANCED QUESTIONS Problem-solving tips and tools You can find most of the Earth data that you need in Table 8-1. You'll need to know that a sphere has surface area 4nr2 and volume 4/3nr3, where r is the sphere's radius. You may have to consult an atlas to examine the geography of the South Atlantic. Also, remember that the average density of an object is its mass divided by its volume. Sections 5-3 and 5-4 and describe how to solve problems involving blackbody radiation, discusses precession, and Section 4-3 describes the properties of elliptical orbits. The wavelength at which a blackbody emits its maxi- mum energy can be calculated according to the equation we can learn. If the light leaving the prism is focused on a screen, white light are lined up side by side (Figure 4.9) as in a the different wavelengths or colors that compose As we hinted at the beginning of the chapter, electromag- netic radiation carries a tremendous amount of infor- rainbow-which is formed by the dispersion of light mation about the nature of stars and other astronomical through raindrops (see "Making Connections: The Rain- objects. To extract this information, however, astronomers bow"). Because this array of colors is a spectrum of light, must be able to study the amounts of energy we receive at the instrument used to disperse the light and form the different wavelengths of visible light (and other radiation) spectrum is called a spectrometer. The blackbody will radiate electromagnetic waves. By absorbing radiation, the blackbody heats up until it is emitting energy at the same rate that energy is being absorbed. orange-red (shorter wavelength). 1000 - 1500 2000 2500 3000 Wavefength (nm) approximations of blackbodies. can appear brilliant yellow or even blue-white. The radiation from a blackbody has several characteristics. 20. The total power in sunlight that reaches the top of our atmosphere is 1.75 X 1011 W (a) How many watts of power are reflected back into space due to the Earth's albedo? (b) If the Earth had no atmosphere, all of the solar power that was not reflected would be absorbed by the Earth's surface. In equilibrium, the heated surface would act as a blackbody that radiates as much power as it absorbs from the Sun. How much power would the entire Earth radiate? (c) How much power would one square meter of the surface radiate? (d) What would be the average temperature of the surface? Give your answer in both the Kelvin and Celsius scales. (e) Why is the Earth's actual average temperature higher than the value you calculated in (d)? 21. On average, the temperature beneath the Earth's crust increases at a rate of 20°C per kilometer. At what depth would water boil? (Assume the surface temperature is 20°C and ignore the effect of the pressure of overlying rock on the boiling point of water.) 22. What fractions of Earth's total volume are occupied by the core, the mantle, and the crust? 23. What fraction of the total mass of the Earth lies within the inner core? 24. (a) Using data for the mass and size of the Earth listed in Table 8-1, verify that the average density of the Earth is 5500 kg/m3. (b) Assuming that the average density of material in the Earth's mantle is about 3500 kg/m3, what must the average density of the core be? Is your answer consistent with the values given in Table 8-2? 25. Africa and South America are separating at a rate of about 3 centimeters per year, as explained in the text. Assuming that this rate has been constant, calculate when these two continents must have been in contact. Today the two continents are 6600 km apart. 26. When the crust under an ocean is pushed toward a continent, the oceanic crust is pushed under the continental crust and undergoes subduction (see the right-hand side of Figure 8-15). What does this tell us about the density of oceanic crust compared to continental crust? (Courtesy of H. D. Holland) R I in U X G 27. The surfaces of Mercury, the Moon, and Mars are riddled with craters formed by the impact of space debris. Many of these are billions of years old. By contrast, there are only a few conspicuous craters on the Earth's surface, and these are generally less than 500 million years old. What do you suppose explains the difference? 28. Fast-moving charged particles can damage living organisms in a number of ways. For example, they can cause mutations if they collide with a DNA molecule within a cell. Explain how the fact that the Earth's interior is molten helps minimize the effect of the solar wind on the biosphere. 29. Most auroral displays have a green color dominated by emission from oxygen atoms at a wavelength of 557.7 nm. (a) What minimum energy (in electron volts) must be imparted to an oxygen atom to make it emit this wavelength? (b) Why is your answer in (a) a minimum value? 30. Describe how the present-day atmosphere and surface temperature of the Earth might be different (a) if carbon dioxide had never been released into the atmosphere; (b) if carbon dioxide had been released, but life had never evolved on Earth. 31. The Earth's primordial atmosphere probably contained an abundance of methane (CH4) and ammonia (NH3), whose molecules were broken apart by ultraviolet light from the Sun and particles in the solar wind. What happened to the atoms of carbon, hydrogen, and nitrogen that were liberated by this dissociation? 32. The photograph below shows the soil at Daspoort Tunnel near Pretoria, South Africa. The whitish layer that extends from lower left to upper right is 2.2 billion years old. Its color is due to a lack of iron oxide. More recent soils typically contain iron oxide and have a darker color. Explain what this tells us about the history of the Earth's atmosphere. Our Living Earth 203 Red Dwarfs that these red dwarfs should use up nearly all of their hydrogen and live very long lives on the lower main sequence. They could survive for a hundred billion years or more. Of course, we can't test this part of our theories because the universe is only about 14 billion years old, so not a single red dwarf has died of old here in the universe age anywhere of the life expectancies of stars. Stars less massive than about 0.4 solar mass-the red dwarfs-have two advantages over more massive stars. First, they have very small masses, and thus they have very little weight to support. Their pressure- temperature thermostats are set low, and they con-. sume their hydrogen fuel very slowly. Our discussion dicted that the red dwarfs could live very long lives. The red dwarfs have a second advantage because they are totally convective. That is, they are stirred by circulating currents of hot gas rising from the interior and cool gas sinking inward. This means the stars are mixed like a pot of soup that is constantly stirred as it cooks. Hydrogen is consumed and helium accumu- lates uniformly throughout the star, which means the star is not limited to the fuel in its core. It can use all of its hydrogen to prolong its life on the main sequence. Because a red dwarf is mixed by convection, it can- not develop an inert helium core surrounded by un- a hydrogen shell and thus cannot become a giant star. Rather, nuclear fusion converts hydrogen into helium, which cannot fuse because the star cannot get hot enough. What we know about stellar evolution tells Medium-Mass Stars Stars with masses between roughly 4 solar masses and 0.4 solar mass,' including the sun, evolve in the same They can ignite hydrogen and helium and be- way. come giants, but they cannot get hot enough to ignite carbon, the next fuel after helium. When they reach that impasse, they collapse and become white dwarfs. There are two keys to the evolution of these sunlike stars: the lack of complete mixing, and mass loss. The interiors of medium-mass stars are not com- pletely mixed (Figure 13-6). Stars with a mass of 1,1 solar masses or less have no convection near their processed hydrogen. Consequently it can never ignite This mass limit is uncertain, as are many of the masses quoted here. The evolution of stars is highly complex, and such parameters are difficult to specify. Figure 13-6 Inside stars. The more-massive stars have small convective interiors and radiative envelopes. Stars like the sun have radiative interiors and convective envelopes. The lowest-mass stars are convective through-out. The 'cores' of the stars where nuclear fusion occurs (not shown) are smaller than the interiors. (illustration design by author) 310 Part 3 The Stars the universe was young. How these first clouds of stars formed soon after the universe began is a mystery that astronomers are now exploring. Larger telescopes now under construction will carry us to greater distances, greater look-back times, and ever closer to the birth of galaxies. REYIEW elliptical galaxies now contain little star-making material. The beautiful disk typical of spiral galaxies is very orderly, with all the stars following sim- , ilar orbits. When galaxies collide, the stellar orbits get ,_. scrambled, and an orderly d isk galaxy could be con-~ erted into the chaotic swarm of stars typical of ellipti-al galaxies. it seems likely that elliptical galax-js have had much more complex histories than spiral ;alaxies have had. What evidence do we have to support the story in , ~he preceding paragraph? :ore we can consider the universe as a whole, we rst examine the galaxies from a different perspective. me galaxies are suffering tremendous eruptions in eir centers. We can divide galaxies into three classes-elliptical, spiral, and irregular-with subclasses specifying the galaxy's shape. The elliptical galaxies contain little gas and dust and few bright, young stars. Spiral and irregular galaxies have large , amounts of gas and dust and are actively making new stars. To measure the properties of galaxies, we must first find out how far away they are. For the nearer galaxies, we can judge distances using distance indicators, objects whose lu minosity or diameter is known. The most accurate distance indicators are Cepheid variable stars. Other distance indica-tors are globular clusters, planetary nebulae, and supernovae. In addition, we can estimate the distance to the farthest galaxy clusters using the average luminosity of the brightest galaxies. The disks of spiral galaxies are tinted blue by the hot mas-sive stars formed by active star formation in the spiral arms. Because the halos of galaxies contain so little gas and dust, Chapter 16 Galaxies page 891 there is no star formation there, and halos are made up of older stars. The brightest stars in halos are red giants, which give halos a reddish tint. elliptical galaxy Hubble constant (H) spiral galaxy rotation curve method Small Magellanic Cloud 1. Why didn't astronomers at the beginning of the 20th century recognize galaxies for what they are? 2. How can a classification system aid a scientist? 3. What is the difference between an EO galaxy and an E1 galaxy? 4. What is the difference between an Sa and an Sb galaxy? between an SBb and an Sb? 5. Why can't galaxies evolve from elliptical to spiral? Why can't they evolve from spiral to elliptical? How did elliptical galaxies get that way? A growing body of evidence suggests that elliptical galaxies have been subject to collisions in their past and that spiral galaxies have not. During collisions, a galaxy can be driven to use up its gas and dust in a burst of star formation or may be stripped of gas and dust by an en-counter. 6. How do selection effects make it difficult to decide how common elliptical and spiral galaxies are? 7. Why are Cepheid variable stars good distance indica-tors? What about planetary nebulae? page 392 Part 4 The Universe 8. Why is it difficult to measure the Hubble constant? 9. How is the rotation curve method related to binary stars and Kepler's third law? The Hubble law shows that the radial velocity of a galaxy is proportional to its distance. we can use the Hubble law to estimate distances. The galaxy's radial velocity divide by the Hubble constant equals its distance in megaparsecs The masses of galaxies can be measured in two basic ways-the rotation curve method and the velocity dispersion method. 10. What evidence do we have that galaxies contain dark matter? The rotation curve method is more accurate bull can be applied only to nearby galaxies. Both methods suggest that galaxies contain 10 to 100 times more dark matter than visible matter. 11. What evidence do we have that galaxies collide and merge? Galaxies occur in clusters. Our own galaxy is a member of the Local Group, a small cluster. A galaxy in a rich cluster may collide with other galaxies more often than a galaxy in a poor cluster, and such collisions can force a galaxy tt form new stars and use up its gas and dust. Collisions can also strip gas out of a galaxy. This may explain why elliptical and SO galaxies are more common in rich clusters than in poor clusters. Spiral galaxies may be star systems t} have not experienced many collisions. Astronomers are just beginning to understand which factors are im-portant in the birth of galaxies. Observations with the largest and most sophisti-cated telescopes are taking us back to the age of galaxy formation. At great distances the look-back time is so great that we see the galaxies as they were long ago, and we discover that there were more spirals then and fewer ellipticals. Also, the ellipticals were smaller than they are now. We can even see that galaxies were closer together long ago; about a third of all distant galaxies are in close pairs, but only 7 percent of nearby galax-ies are in pairs. The observational evidence clearly supports the hypothesis that galaxies have evolved by merger. Look again at the Hubble Deep Field in Figure 16-2 and notice the smallest, faintest objects. These objects, enlarged in Figure 16-15, are small, blue, distorted clouds of stars and gas each containing about 10 times the mass of a large globular cluster. These may be the objects that fell together to form the first galaxies when 12. Why are the shells visible around some elliptical galax-ies significant? 13. Ring galaxies often have nearby companions. What does that suggest? 14. Propose an explanation for the lack of gas, dust, and young stars in elliptical galaxies. Elliptical galaxies, lacking gas and dust, lack young stars and are consequently slightly reddish because the brightest stars are red giants. 15. How do deep images by the Hubble Space Telescope confirm our hypothesis about galaxy evolution? 1. Why do we believe that galaxy collisions are likely, but star collisions are not? 2. Should an orbiting infrared telescope find irregular gal-axies bright or faint in the far infrared? Why? What about elliptical galaxies? 1. If a galaxy contains a type I (classical) Cepheid with a period of 30 days and an apparent magnitude of 20, what is the distance to the galaxy? 2. If you find a galaxy that contains globular clusters that are 2 seconds of arc in diameter, how far away is the gal-axy? (Hints: Assume that a globular cluster is 25 pc in diameter, and see By the Numbers 1-2.) 3. If a galaxy contains a supernova that at its brightest has an apparent magnitude of 17, how far away is the gal-axy? (Hints: Assume that the absolute magnitude of the supernova is -19, and see By the Numbers 11-2.) 4. If we find a galaxy that is the same size and mass as our Milky Way Galaxy, what orbital velocity would a small satellite galaxy have if it orbited 50 kpc from the center of the larger galaxy? (Hint: See By the Num-bers 2-1.) 5. Find the orbital period of the satellite galaxy described in Problem 4. (Hint: See By the Numbers 11-4.) 6. If a galaxy has a radial velocity of 2000 km/s and the Hubble constant is 70 km/s/Mpc, how far away is the galaxy? (Hint: Use the Hubble law.) 7. If you find a galaxy that is 20 minutes of arc in diame-ter, and you measure its distance to be 1 Mpc, what is its diameter? (Hint: See By the Numbers 1-2.) 8. We have found a galaxy in which the outer stars have orbital velocities of 150 km/s. If the radius of the galaxy is 4 kpc, what is the orbital period of the outer stars? (Hints: 1 pc = 3.08 x 1013 km, and 1 yr = 3.15 X 10' s.j a axies do not form in isolation. Cooler gas clouds may fall into galaxies and add material for star formation. Also, we need to consider differences in the initial cloud of gas from which a galaxy forms. Figure 16-15 The most distant objects in the Hubble Deep Fields are irregular blue clouds of gas and stars smaller than all but the smallest galax-ies. These may be the precursors of galaxies. Each of these frames shows a region about one-third the diameter of our Milky Way Galaxy. (Rogier Windhorst and Sam Pascarelle, Arizona State University, and NASA) 8. Why is it difficult to measure the Hubble constant? 9. How is the rotation curve method related to binary stars and Kepler's third law? 10. What evidence do we have that galaxies contain dark matter? A lot of empty dark space between stars. 11. What evidence do we have that galaxies collide and merge? Astronomers are just beginning to understand which factors are im-portant in the birth of galaxies. Observations with the largest and most sophisti-cated telescopes are taking us back to the age of galaxy formation. At great distances the look-back time is so great that we see the galaxies as they were long ago, and we discover that there were more spirals then and fewer ellipticals. Also, the ellipticals were smaller than they are now. We can even see that galaxies were closer together long ago; about a third of all distant galaxies are in close pairs, but only 7 percent of nearby galax-ies are in pairs. The observational evidence clearly supports the hypothesis that galaxies have evolved by merger. Look again at the Hubble Deep Field in Figure 16-2 and notice the smallest, faintest objects. These objects, enlarged in Figure 16-15, are small, blue, distorted clouds of stars and gas each containing about 10 times the mass of a large globular cluster. These may be the objects that fell together to form the first galaxies when 12. Why are the shells visible around some elliptical galax-ies significant? 13. Ring galaxies often have nearby companions. What does that suggest? 14. Propose an explanation for the lack of gas, dust, and young stars in elliptical galaxies. 15. How do deep images by the Hubble Space Telescope confirm our hypothesis about galaxy evolution? 1. Why do we believe that galaxy collisions are likely, but star collisions are not? 2. Should an orbiting infrared telescope find irregular gal-axies bright or faint in the far infrared? Why? What about elliptical galaxies? 1. If a galaxy contains a type I (classical) Cepheid with a period of 30 days and an apparent magnitude of 20, what is the distance to the galaxy? 2. If you find a galaxy that contains globular clusters that are 2 seconds of arc in diameter, how far away is the gal-axy? (Hints: Assume that a globular cluster is 25 pc in diameter, and see By the Numbers 1-2.) 3. If a galaxy contains a supernova that at its brightest has an apparent magnitude of 17, how far away is the gal-axy? (Hints: Assume that the absolute magnitude of the supernova is -19, and see By the Numbers 11-2.) 4. If we find a galaxy that is the same size and mass as our Milky Way Galaxy, what orbital velocity would a small satellite galaxy have if it orbited 50 kpc from the center of the larger galaxy? (Hint: See By the Num-bers 2-1.) 5. Find the orbital period of the satellite galaxy described in Problem 4. (Hint: See By the Numbers 11-4.) 6. If a galaxy has a radial velocity of 2000 km/s and the Hubble constant is 70 km/s/Mpc, how far away is the galaxy? (Hint: Use the Hubble law.) 7. If you find a galaxy that is 20 minutes of arc in diame-ter, and you measure its distance to be 1 Mpc, what is its diameter? (Hint: See By the Numbers 1-2.) 8. We have found a galaxy in which the outer stars have orbital velocities of 150 km/s. If the radius of the galaxy is 4 kpc, what is the orbital period of the outer stars? (Hints: 1 pc = 3.08 X 10'3 km, and 1 yr = 3.15 X 107S.) 2050? In 2100? What effects may the increased temperature have on human health? 204 I CHAPTER 8 33. Earth's atmospheric pressure decreases by a factor of one-half for every 5.5-km increase in altitude above sea level. Construct a plot of pressure versus altitude, assuming the pressure at sea level is one atmosphere (1 atm). Discuss the characteristics of your graph. At what altitude is the atmospheric pressure equal to 0.001 atm? 34. The Earth is at perihelion on January 3 and at aphelion on July 4. Because of precession, in 13,000 years the amount of sunlight in summer will be more than at present in the northern hemisphere but less than at present in the ozone layer. southern hemisphere. Explain why. 40. Use the World Wide Web to investigate the current status of the Antarctic ozone hole. Is the situation getting better or worse? Is there a comparable hole over the North Pole? Why do most scientists blame the chemicals called CFCs for the existence of the ozone hole? 41. In 1989 representatives of many nations signed a global treaty called the Montreal Protocol to protect the Use the World Wide Web to investigate the current status of this treaty. How many nations are signatories to this treaty? Has the treaty been amended D 1 S C U SS 1 O N ~UF ST 1 O N S since it was first signed? When are various substances that destroy the ozone layer scheduled to be phased out? 35. The human population on Earth is currently doubling about every 30 years. Describe the various pressures placed on the Earth by uncontrolled human population growth. Can such growth continue indefinitely? If not, what natural and human controls might arise to curb this growth? It has been suggested that overpopulation problems could be solved by colonizing the Moon or Mars. Do you think this is a reasonable solution? Explain your answer. 36. One scientific study suggests that the continued burning of fossil and organic fuels by humans is releasing enough CO2 to stimulate the greenhouse effect and eventually melt the polar icecaps. Antarctica has an area of 13 million square kilometers and is covered by an icecap that varies in thickness from 300 meters near the coast to 1800 meters in the interior. Estimate the volume of this icecap. Assuming that water and ice have roughly the same density, estimate the amount by which the water level of the world's oceans would rise if Antarctica's ice were to melt completely. What portions of the Earth's surface would be inundated by such a deluge? The Earth's Energy Sources: All activity in the Earth's atmosphere, oceans, and surface is powered by three sources of energy. solid inner core surrounded by a liquid outer core. The outer core is surrounded by the dense mantle, which in turn is surrounded by the thin low-density crust. • Solar energy is the energy source for the atmosphere. In the greenhouse effect, some of this energy is trapped by infrared-absorbing gases in the atmosphere, raising the Earth's surface temperature. • Seismologists deduce the Earth's interior structure by studying how longitudinal P waves and transverse S waves travel through the Earth's interior. Tidal forces from the Moon and Sun help to power the motion of the oceans. The internal heat of the Eaxth is the energy source for Both temperature and pressure steadily increase with geologic activity. depth inside the Earth. Our Living Earth 201 page 202 I CHAPTER 8 The lithosphere is divided into huge plates that move ' • Industrial chemicals released into the atmosphere are about over the plastic layer called the asthenosphere in threatening the ozone layer in the stratosphere. the upper mantle. The Earth's Magnetic Field and Magnetosphere: Electric currents in the liquid outer core generate a magnetic field. This magnetic field produces a magnetosphere that surrounds the Earth and blocks the solar wind from hitting the atmosphere. • A bow-shaped shock wave, where the supersonic solar wind is abruptly slowed to subsonic speeds, marks the outer boundary of the magnetosphere. • Most of the particles of the solar wind are deflected . around the Earth by the magnetosphere. • Some charged particles from the solar wind are trapped in two huge, doughnut-shaped rings called the Van Allen belts. An excess of these particles can initiate an auroral display. The appearance of photosynthetic living organisms led to our present atmospheric composition, about four-fifths nitrogen and one-fifth oxygen. The Earth's atmosphere is divided into layers called the troposphere, stratosphere, mesosphere, and thermosphere. Ozone molecules in the stratosphere absorb ultraviolet light. Because of the Earth's rapid rotation, the circulation in its atmosphere is complex, with three circulation cells in each hemisphere. REVIEW Questions 1. Describe the various ways in which the Earth is unique among the planets of our solar system. Earth allows life to live with oxygen and water. The Earth's Atmosphere: The Earth's atmosphere differs The Earth's atmosphere evolved from being mostly water vapor to being rich in carbon dioxide. 2. Describe how energy is transferred from the Earth's surface to the atmosphere by both convection and radiation. 3. How does the greenhouse effect influence the temperature of the atmosphere? How does this effect differ from what actually happens in a greenhouse? A strong greenhouse effect kept the Earth warm enough for water to remain liquid and to permit the evolution of life. Solar energy is the energy source for the atmosphere. In the greenhouse effect, some of this energy is trapped by infrared-absorbing gases in the atmosphere, raising the Earth's surface temperature. 4. Describe the interior structure of the Earth. The Earth's Interior: Studies of seismic waves vibrations Plate Tectonics: The Earth's crust and a small part of its produced by earthquakes show that the Earth has a small, upper mantle form a rigid layer called the lithosphere. The Earth's inner and outer cores are composed of almost pure iron with some nickel mixed in. The mantle is composed of iron-rich minerals. 5. How do we know that the Earth was once entirely molten? 6. The deepest wells and mines go down only a few kilometers. What, then, is the evidence that iron is abundant in the Earth's core? That the Earth's outer core is molten but the inner core is solid? 7. The inner core of the Earth is at a higher temperature than the outer core. Why, then, is the inner core solid and the outer core molten instead of the other way around? 8. Describe the process of plate tectonics. Give specific examples of geographic features created by plate tectonics. Plates can go underneath each other during a earthquake. Plate tectonics is responsible for most of the major features of the Earth's surface, including mountain ranges, volcanoes, and the shapes of the continents and oceans. Plate tectonics is involved in the formation of the three major categories of rocks: igneous rocks cooled from molten material, sedimentary rocks formed by the action of wind, water, and ice, and metamorphic rocks altered in the solid state by extreme heat and pressure. 9. Explain how convection in the Earth's interior drives the process of plate tectonics Plate tectonics, or movement of the plates, is driven by convection within the asthenosphere. Molten material wells up at oceanic rifts, producing seafloor spreading, and is returned to the asthenosphere in subduction zones. As one end of a plate is subducted back into the asthenosphere, it helps to pull the rest of the plate along. 10. What is the difference between a rock and a mineral? A rock is a hard object. A mineral is like a small element like uranium. 11. What are the differences among igneous, sedimentary, and metamorphic rocks? What do these rocks tell us about the sites at which they are found? 12. Why do some geologists suspect that Pangaea was the from those of the other terrestrial planets in its chemical most recent in a succession of supercontinents? composition, circulation pattern, and temperature profile. 13. Why do you suppose that active volcanoes, such as Mount St. Helens in Washington State, are usually located in mountain ranges that border on subduction zones? 14. Describe the Earth's magnetosphere. If the Earth did not have a magnetic field, do you think aurorae would be more common or less common than they are today? 15. How do we know that the Earth's magnetic field is not due to magnetized iron in the planet's interior? 16. What are the Van Allen belts? 17. Summarize the history of the Earth's atmosphere. What role has biological activity plated in this evolution? 18. Describe the structure of the Earth's atmosphere. Explain how heat from the Sun and the Earth's rotation affect the circulation of the Earth's atmosphere. 19. Carbon dioxide and ozone each make up only a fraction of a percent of our atmosphere. Why, then, should we be concerned about small increases or decreases in the atmospheric abundance of these gases? The Biosphere: Human activity is changing the Earth's biosphere, on which all living organisms depend. • Deforestation and the burning of fossil fuels is increasing the greenhouse effect in our atmosphere and warming the planet. 144 days to go from greatest eastern elongation to greatest western elongation. With the aid of a diagram like Cloud-Covered Venus page 259 5. Why was it difficult to determine Venus's surface temperature from Earth? How was this finally determined? A lot of clouds on Venus. Venus’s atmosphere is much deeper and denser. In the 1950s, however, when radio observations of the planet penetrated the cloud layer and gave the first indication of conditions near the surface, they revealed a temperature exceeding 600 K! Almost overnight, the popular conception of Venus changed from lush tropical jungle to arid, uninhabitable desert. 6. The Mariner 2 spacecraft did not enter Venus's Figure 11-1, explain why. atmosphere, but it was nonetheless able to determine that the atmosphere is very dry. How was this done? 7. Why is it hotter on Venus than on Mercury? Mercury is 0.4 AU from the Sun Venus, 0.7 AU 8. What is the evidence for active volcanoes on Venus? Far fewer impact craters on Venus than on Mercury. 20. Before about 350 s.c. it was not generally understood that Venus seen in the morning sky (that is, at greatest western elongation) and Venus seen in the evening sky (at greatest eastern elongation) were actually the same planet. Construct a geocentric model of the planets (like that shown in ) in which the "morning Venus" and the "evening Venus" are two distinct planets. 9. How might Venus's cloud cover change if all of Venus's volcanic activity suddenly stopped? How might these ~hanges affect the overall Venusian environment? Venus may have ended up in its weird rotational state due to tidal interactions with the Sun of the same kind that have locked the Moon's rotation around the Earth so that it always keeps the same face towards us. 21. During what time of the day or night was the photograph in Figure 11-2 made? How can you tell? 10. Why are there no oceans on Venus? Where has Venus's water gone? Outgassed. 11. Why is there so much carbon dioxide in Venus's atmosphere while very little of this gas is present in the Earth's atmosphere? Venus have more carbon dioxide in its atmosphere Venus is hotter than Earth. 12. What is the difference between the greenhouse effect as it exists on Venus today and the runaway greenhouse effect that existed in Venus's early atmosphere? Venus's dense atmosphere is made up almost entirely of a prime greenhouse gas, carbon dioxide. Venus is hot because of the greenhouse effect. Venus probably has a molten iron-rich core like Earth. Venus's atmosphere is much more massive than our own, and it extends to a much greater height above the planet's surface. The dominant constituent (96.5 percent) of Venus's atmosphere is carbon dioxide. Almost all of the remaining 3.5 percent is nitrogen. Earth When Earth formed, any atmosphere it might have had-sometimes called the primary atmosphere-would have consisted of the gases most common in the early solar system: light gases such as hydrogen, helium, methane, ammonia, and water vapor. 13. Describe the Venusian surface. What kinds of features would you see if you could travel around on the planet? 14. In what ways does the surface topography of Venus differ from that of the Earth? 15. Why do scientists think that Venus's surface was not molded by the kind of tectonic activity that shaped the Earth's surface? 16. Describe how Venus's lack of water and high surface temperatures may help explain the absence of plate tectonic activity. 17. Compare and contrast the kinds of geologic activity that occur on Venus with those that occur on Earth. 18. Describe two competing hypotheses that attempt to explain why Venus's surface is only a few hundred million years old. 22. Venus's sidereal rotation period is 243.01 days and its orbital period is 224.70 days. Use these data to prove that a solar day on Venus lasts 116.8 days. (Hint: Develop a formula relating Venus's solar day to its sidereal rotation period and orbital period similar to the first formula in Box 4- .) 23. In Section 11-2 we described the relationship between the length of Venus's synodic period and the length of an apparent solar day on Venus. Using this and a diagram, explain why at each inferior conjunction the same side of Venus is turned toward the Earth. 24. If you aim microwaves of wavelength 1.9 cm at the entire face of Venus, what will be the spread in wavelength of the reflected waves caused by the planet's rotation? (See 1 y-,uwy ~_;.) 25. At what wavelength does Venus's surface emit the most radiation? Do astronomers have telescopes that can detect this radiation? Why can't we use such telescopes to view the planet's surface? 26. The Mariner 2 spacecraft detected more microwave radiation when its instruments looked at the center of Venus's disk than when it looked at the edge, or limb, of the planet. (This effect is called limb darkening.) Explain how these observations show that the microwaves are emitted by the planet's surface rather than its atmosphere. ~DV1~NC~D QUCSTIONS Problem-solving tips and tools You should recall that Wien's law (Sw, ticm S-4) relates the temperature of a blackbody to maX, its wavelength of maximum emission. : ~ ~x 5--+ describes some of the physics of light scattering. Secti~n 5-9 and _ - ±- explain the Doppler effect and how to do calculations using it. The linear speed of a point on a planet's equator is the planet's circumference divided by its rotation period; recall that the circumference of a circle of radius r is 2nr. The wavelength at which a blackbody emits its maxi- mum energy can be calculated according to the equation we can learn. If the light leaving the prism is focused on a screen, the different wavelengths or colors that compose As we hinted at the beginning of the chapter, electromag- netic radiation carries a tremendous amount of infor- mation about the nature of stars and other astronomical objects. To extract this information, however, astronomers must be able to study the amounts of energy we receive at different wavelengths of visible light (and other radiation) spectrum is called a spectrometer. The blackbody will radiate electromagnetic waves. By absorbing radiation, the blackbody heats up until it is emitting energy at the same rate that energy is being absorbed. orange-red (shorter wavelength). 1000 - 1500 2000 "~ 2500 3000 Wavefength (nm) approximations of blackbodies. can appear brilliant yellow or even blue-white. The radiation from a blackbody has several characteristics. 27. Explain how you could estimate the size of the droplets that make up Venus's clouds by beaming radio waves of different wavelengths through the clouds to a spacecraft on the planet's surface. 28. When the Galileo spacecraft flew past Venus in 1990 while on its way to Jupiter, it used its infrared camera to view lower-level clouds in the Venusian atmosphere. Why was it necessary to use infrared light to see these clouds? 29. In the classic Ray Bradbury science-fiction story "All Summer in a Day," human colonists on Venus are subjected to continuous rainfall except for one day every few years when the clouds part and the Sun comes out for an hour or so. Discuss how our understanding of Venus's atmosphere has evolved since this story was first published in 1954. 19. Venus takes 440 days to move from greatest western 30. A hypothetical planet has an atmosphere that is opaque elongation to greatest eastern elongation, but it needs only to visible light but transparent to infrared radiation. How Cloud-Covered Venus 257 258 1 CHAPTER 11 would this affect the planet's surface temperature? Contrast and compare this hypothetical planet's atmosphere with the greenhouse effect in Venus's atmosphere. 31. Suppose that Venus had no atmosphere at all. How would the albedo of Venus then compare with that of Mercury or the Moon? Explain your answer. The surface of Mercury has been found to be heavily cratered, as would be expected for a small body without any atmosphere or weathering or plate tectonic activity. The cratering is slightly different from the patterns on the Moon, probably because Mercury's stronger gravity would keep debris from flying as far after an impact. There have been some incredibly strong hits on Mercury. The Caloris basin, over 1300 km in diameter, is evidence of one. This impact was so great that the ground on the opposite side of the planet is jumbled as a result of it called weird terrain. When the crater formed, shock waves were sent all through Mercury and they converged on the opposite side in a kind of super-earthquake activity. While there is evidence of lava flowing on Mercury billions of years ago, it is essentially dead geologically - no mountains, active volcanoes, plate tectonic activity, or even plates. Mercury is hard to see because its orbit is much smaller than ours, and therefore it is always relatively close in angular terms to the Sun. At its maximum elongation the point where it is as far from the Sun as possible, seen from Earth, it is only about 27° away from the Sun. 222 CHAPTER 10 CELESTIAL DISTANCES with one important exception. The light elements lithium, beryllium, and boron are far more abundant in cosmic rays than in the Sun and stars. These light elements are formed when high-speed cosmic ray nuclei of carbon, nitrogen, and oxygen collide with protons in interstellar space and break apart. (By the way; if you, like most readers, have not memorized all the elements and want to see how any of those we mention fit into the sequence of elements, you electromagnetic fields in interstellar space are strong enough to keep all but the most energetic cosmic rays from escaping the Galaxy. It therefore seems likely that they are produced somewhere inside the Galaxy. The only likely exceptions are those with the very highest energy. Such cosmic rays move so rapidly that they are not significantly influenced by inter- stellar magnetic fields, and they could escape our Galaxy. 242 CHAPTER I I BETWEEN THE STARS: GAS AND DUST IN SPACE Interactive Radio Map of the Galactic Center The International Dark-SkyAssociation [rsd-www.nrl.navy.mil/7213/lazio/GC/] [www.darksky.org/ida/ida 2/index.html] This large, detailed radio map of the central region of our As discussed in Making Connections: Light Pollution and the Galaxy allows you to click on different parts and see close-ups Milky Way, a major environmental concern for astronomers is and more infonnation, and to appreciate how many different the encroachment of human lighting on the dark skies we need types of objects radio telescopes reveal near the center. at observatories to see distant objects. The IDA is the main organization for disseminating information and encouraging political activity about light pollution. Their site has lots of Infrared Sky Map Generator [poe.ipae.ealtech.edu:8080/astronomy/skymap.html] background information, news, links, and technical material on The Center at JPL and Caltech that stores and processes in- ing for the whole map or specific objects or regions. light fixtures. (This is a terrific paper topic for students with an frared data from space has produced an intriguing site where anterest in the environment.) you can see the sky or any portion at infrared wavelengths, ask- On the maps, the disk of our Milky Way Galaxy stands out clearly SUMMARY I6.1 The Sun is located in the outskirts of the Milky Way Galaxy. The Galaxy consists of a disk containing dust, gas, and young stars; a nuclear bulge containing old stars; and a spherical halo containing very old stars, globular clusters, and RR Lyrae variables. Analysis by Shapley of the distribu- tion of globular clusters gave the first indication that the Sun is not located at the center of the Galaxy. Radio observations at 21 cm show that cold hydrogen is confined to a flat disk with a thickness near the Sun of only 400 LY Dust is also re- stricted to this same thin disk; very little dust exists outside star formation is active, are concentrated in the spiral arms. old. The bulge stars do have substantial amounts of heavy elements, presumably because there were many massive first-generation stars in this dense region and these quickly contaminated the next generations of stars with the prod- ucts of nucleosynthesis. The mass of the Galaxy can be determined by mea- suring the orbital velocities of stars or interstellar matter. The Sun revolves completely around the galactic center in about 225 million years (sometimes called a galactic year.) 90 percent of this mass consists of dark matter that emits no electromagnetic radiation and can be detected only be- 16.2 The Galaxy has four main spiral arms and several short spurs; the Sun is located on one of these spurs. Mea- surements show that the Gala~cy does not rotate as a solid body but instead its stars follow Kepler's laws; those closer to the galactic center complete their orbits more quickly The spiral density wave theory is one way to account for the spiral arms. Calculations show that the gravitational forces within the Galaay cause stars and gas clouds to slow down in the vicinity of the spiral arms, thereby leading to higher densities of material. When molecular clouds attempt to pass through these regions of higher density star forma- 16.6 The Galaxy formed about 13 billion years ago. Mod- els suggest that the stars in the halo and globular clusters formed first, while the Galaay was spherical. The gas, some- what enriched in heavy elements by the first generation of stars, then collapsed from a spherical distribution to a disk- shaped distribution. Stars are still forming today from the gas and dust that remain in the disk. Star formation occurs most rapidly in the spiral arms, where the density of inter- stellar matter is highest. There is evidence that the Galaxy captured (and is still capturing) additional stars and globular clusters from small,galaxies that ventured too close to the Milky Way 16.3 We can roughly divide the stars in the Galaxy into two categories. Old stars with few heavy elements are re- ferred to as population II stars and are found in the halo and in globular clusters. Population I stars contain more heavy elements than globular cluster and halo stars, are younger and found in the disk, and are especially concen- trated in the spiral arms. The Sun is a member of popula- tion I. Population I stars formed after previous generations of stars produced heavy elements and ejected them into the- interstellar medium. The stars in the nuclear bulge are also often referred to as population II because, they are all very SUMMARY page 359 You are captured by space aliens who take you inside a complex cloud of interstellar gas, dust, and a few newly formed stars. To escape, you need to make a map of the cloud. Luckily, the aliens have a complete astronomical observatory with equipment for measuring all the bands of the electromagnetic spectrum. Use what you have learned in this chapter and have your group discuss what kinds of maps you would make of the cloud to plot your most effective escape route. The diagram that Herschel made of the very irregular outer boundary (Figure 16.3). Can your group think of a reason for this? routine part of textbooks like ours. Can your group make a list of earlier astronomical observations that began as a surprise and mystery, but wound up (with more observa- tions) as well-understood parts of introductory textbooks? physicist Gregory Benford has written a whole series of science fiction novels that take place near the center of the Milky Way Galaxy in the far future. Suppose your group were writing such a story. How would the environment near the galactic center differ from the Milky Way has a environment in the "galactic suburbs," where the Sun is located? Would life as we know it have an easier or harder time surviving on planets orbiting stars near the center (and why)? Suppose that for your final exam in this course, your group is assigned telescope time to observe a star selected for you by your professor. The professor tells you the po-sition of the star on the sky its right ascension and decli-nation nothing else. You can make any observations you wish. How would you go about determining whether the star is a member of population I or population II? The existence of dark matter comes as a great surprise and its nature is a great mystery today. Someday, astronomers will know a lot more about it, and it will be a Following up on the material in the box on light pollu-tion, your group has been appointed a special task force by the mayor of the city where you live on how to help astronomers maintain dark skies locally and yet not inter-fere with safety and commerce in the city. Discuss what witnesses you would call for public hearings, what factors you might weigh in your decision, and how important you think preserving dark skies is for your community. REVIEW QUES OISIS I . Explain why we see the Milky Way as a faint band of light 4. Briefly describe the two main parts of our Galaxy-the disk stretching across the sky. and the halo. 2. Explain where (and why) in a spiral galaxy you would expect 5. Describe the evidence indicating that a black hole may be to find globular clusters, molecular clouds, and atomic hydrogen. at the center of our Galaxy. 3. Describe several characteristics that distinguish population I 6. Explain why the abundances of heavy elements in stars cor- from population 11 stars. relate with their positions in the Galaxy. 7. Suppose the Milky Way were a band of light extending only halfway around the sky (that is, in a semicircle). What, then, 11. The dwarf galaxy in Sagittarius is the one closest to the 8. The globular clusters revolve around the Galaxy in highly elliptical orbits. Where would you expect the clusters to spend most of their time? (Think of Kepler's laws.) At any given time, would you expect most globular clusters to be moving at high or c. Which are thought to be very young? d. Which are thought to be very old? would you conclude about the Sun's location in the Galaxy? e. Which have the hottest stars? Give your reasoning. Milky Way, yet it was discovered only in 1994. Can you think of a reason it was not discovered earlier? (Hint: Think about what else is in its constellation.) 9. Shapley used the positions of globular clusters to deter- mine the location of the Galactic center. Could he have used open clusters? Why or why not? center of the Galaxy. 10. Consider the following five kinds of objects: (1) open clus- ter, (2) giant molecular cloud, (3) globular cluster, (4) group of O and B stars, and (5) planetary nebulae. of 20,000 LY, 25,000 LY, and 30,000 LY from the Galactic cen- ter, and suppose that all three are lined up in such a way that it is possible to draw a straight line through them and on to the How will the relative positions of these three stars change with time? Assume that their orbits are all circular and lie in the plane of the disk. 12. Suppose three stars lie in the disk of the Galaxy at distances low speeds with respect to the center of the Galaxy? Why? 13. Why does star formation occur primarily in the disk of the a. Which occur only in spiral arms? Galaxy? b. Which occur only in the parts, of the Galaxy other than the spiral arms? Astronomers can measure the wavelengths of lines in a star's spectrum to find the e- locity of the star. The Doppler effect is the apparent change in the wavelength of radia- tion caused by the motion of the source. If a star is moving toward Earth the lines in its spectrum will be shifted slightly toward shorter wave- lengths toward the blue end of the spectrum. If a star is moving away from Earth the lines are shifted slightly toward the red end of the spectrum. a red shift. The Doppler effect tells us how rapidly the dis- tance between us and the source of light is increasing or decreasing. Paschen series IR 954.6 nm 1005.0 nm 1093. nm The Astro 150 Tidal Heating Tutorial Jupiter's moon Io is about the same mass and size as the Earth's Moon. Based on this we would expect Io to have about the same inventory of radioactive elements and the same cooling rate as the Moon. We should the expect Io to have the level geological activity as the Moon, namely none. However, Io is the most geologically active surface in the Solar system. This means that the heating mechanism working on Io is very different that what is happening on the Moon. The mechanism responsible for heating the interior of Io is called Tidal Heating. This little tutorial is my attempted to explain a rather simplified version of the tidal heating of Io. The force of gravity between two objects (M) and (m) depends of their respective mass and the square of the distance (d) between them. The force is very strongly dependent on the distance between the objects. Astronomy 150 0 - 14 The Planets Comparison of Ring Systems Jupiter Saturn Uranus Neptune Size Albedo Particle Size Thickness Total Mass Astronomy 150 0 - 15 The Planets CHAPTER 2 Labs Without this mostly blank page, the following text would be on the wrong side of the paper. Lab #1 Lunar Geological Mapping Introduction Few things are as important to explorers as good maps. For a planetary explorer, one of the most important types of maps is a geological map. A geological map shows what types of terrains or rocks one is likely to encounter on a planetary surface, and in what order they were created. Since our ability to go to di erent planetary surfaces is limited, planetary scientists must do most of their exploring and mapping by studying images from spacecraft. Anybody can look at a pretty picture of a planetary surface and say “That’s Mars”, or “That’s the Moon”. A planetary scientist can take that picture and interpret what is there and reconstruct the history of that surface. Recognizing various landforms on spacecraft images and being able to determine their relative ages is an important component of this class. The Identity of the various landforms can be ascertained by comparing known images with what you see (and by asking your TA), while the relative ages of the various features can be determined by using the Principle of Superposition. Nicholas Steno, a seventeenth century physician, is first credited with stating this simple but powerful geological principle. He wrote that “... at the time when the lowest stratum was being formed, none of the upper strata existed.” or in its modern restatement: the youngest formations are found at the top of a vertical sequence, with the oldest on the bottom. (young formations overlap old formations.) The Lab In this lab we will act as planetary scientists and interpret what we see by creating a geological map and writing a description of a selected region of the Moon. Our materials will be the same primary data the Apollo program scientists used when they were first trying to both understand the lunar surface and find interesting landing spots. The photographs are from the Lunar Orbiter missions that were flown around the Moon prior to the Apollo landings. A reader of your map should be able to understand what the various surface features are in the photograph, and in what order they were formed. Often it is not clear what a surface feature is or when it was formed. Be bold and make a guess, we are not going to go the Moon in the near future, so it is hard to tell if you are wrong. The pictures on the following page give you some idea as to what we want to do. On the left is an image taken from the command module of Apollo 15. The image is of the area surrounding Hadley Rille, the Apollo 15 landing site. On the right is my geological map of the region identifying the di erent landforms, and at the bottom is the key to my geological map with a timeline indicating the relative ages of the various features. Pick an interesting region on your Lunar Orbiter image. You do not have to map the entire photograph. The region you do pick should have enough di erent features to impress your TA (i.e. do NOT pick a small piece of mare and turn in a blank map), but not so many that it would take you a week to draw the map. Astronomy 150 1 - 1 The Planets Mountains (Highlands) Crater Ghost Lava Flow (Mare) Impact Lava Channel Crater Old Young Key Astronomy 150 1 - 2 The Planets What to Turn in You need to turn in your geological map and a paragraph describing the features you mapped. Make sure you have a key to your map that shows the stratigraphic order of the various features. Your paragraph should both describe the features and indicate their relative ages. For example, a description of the example images might go along the lines of: “The oldest features in this image are the highland mountains and the ghost crater. It is not possible to tell the relative ages of the two features. They may actually be part of the same feature. A lava flow (mare surface) fills in much of the image and overlays and abuts the mountains and ghost crater. Lava channels overlay the lava flow in numerous places. One of the lava channels bisects the mountains. All of the lava channels seem to have the same relative age since they overlay the same features. Simple impact craters dot the mare surface. A few craters seems to partially cover lava channels, indicating they are younger. The rest of the impact craters on the mare do not intersect the channels, so their relative ages cannot be determined. My guess is that they are probably younger than the lava channels based on their fresh appearance.” Astronomy 150 1 - 3 The Planets This page is not blank Nothing important is here Just taking up room1 1email me your blank page haiku, it has to better than mine - TS Astronomy 150 1 - 4 The Planets Lab #2 Planetarium - The Orbit of Mars Introduction For thousands of years, humans have been observing the night sky, trying to make sense of what they saw. One of the most important contributions these first astronomers made was when they noticed that while the stars rise and set with precisions depending on the season, some “stars” wandered through the sky. They named these stars planets (from the Greek word   o meaning wanderers). The path that these wanderers take is not a simple one. As seen from the Earth, the motions of the planets are confined to a band around the sky called the ecliptic. Within this band, the planets oscillate about their mean trajectories, and sometimes they even seem to travel backwards for a time. For the purposes of this lab, it is important to try to visualize the solar system as seen from the Earth. As can be seen in the figure on the left, the orbits of the planets all lie in a plane. This plane is called the ecliptic. Now imagine you are standing on Earth, looking out at the sky. Since the planets all lie in the ecliptic plane, they will appear to follow a narrow path that run around the sky. The path of the ecliptic passes through 12 constellations, the signs of zodiac. A similar e ect occurs when we look at the sky through the plane of our Milky Way galaxy. We see a band of white, which are the other stars and gas that orbit in our flattened disk. From our vantage point on the Earth, the Sun also appears to move along the path of the ecliptic (in reality it is the Earth that does the moving). It takes the Sun one year to make a complete circuit of the ecliptic. The figure on the right shows the position of the ecliptic and the celestial equator. It also shows the position of the Sun on June 21 (the first day of northern summer) and December 21 (the first day of northern winter). In this lab we are going to use the position of the Sun on the ecliptic as a coordinate. Understanding the underlying order of the motion of the planets ranks as one of the greatest scientific discoveries of all time. The discovery of this underlying order was made by Johannes Kepler (1571-1630) between the years 1605 and 1618. Kepler’s analysis of Tycho Brahe’s observational data of the position of Mars, found that the orbit of Mars (as well as all of the planets in the Solar system) followed three relations. These relations are now known as Kepler’s three laws of planetary motion. Kepler knew this was a paradigm-shifting discovery. He opened his book Haromonices Mundi (1618) with the preface “...The die is cast; I will write my book, and little does it matter whether it is read now or has to await prosperity. It may well wait one hundred years for a reader, since God himself had waited six thousand years for someone to behold His work.” Astronomy 150 2 - 1 The Planets The most important aspect of Kepler’s laws was that it allowed him to accurately determine the position of planets for any date in past and the future. In Kepler’s day, this was of great practical importance. As the court mathematician to the Holy Roman Emperor, Kepler had to cast horoscopes for the court. To do his he had to be able to determine the position of planets in the past (i.e., on a King’s birthday) and predict their future position. Kepler’s laws were also used to determine the date for Easter. † Just a random note I want to fit it - The position of the Sun against the background zodiacal constellation, is supposed have some importance in astrology. For example, on October 9th, the Sun is in the constellation Virgo, according to the boundaries of this constellation as set by the International Astronomical Union. However, October 9th falls in the astrological sign of Libra. The Earth precesses as it orbits the Sun, so that in a few thousand years, Polaris won’t be the North Star anymore. The spin axis of the Earth is tracing out a cone with an angle of 23.5 degrees, at a rate of one full precession cycle every 26,000 years. In the couple thousand years since astrology was invented, we’ve gone through about 2,000/26,000 (roughly 1/12) of the cycle, the span of one sign of the zodiac on the sky. Two thousand years ago, the Sun used to be in Libra on October 9th, now it’s in the next sign over, Virgo. The Lab In the planetarium today, we are going see how the complicated motions of Mars in the sky can be understood using Kepler’s laws. And how these laws allow us to predict the future position of Mars on the ecliptic. Once your eyes have adjusted to the planetarium, your TA will identify Mars. Using the data sheet provided, write down Mars’ position on the ecliptic using the date as your “coordinate.” For example, if the date is Sept 1 (the Sun is located on Sept 1 on the ecliptic) and Mars is near May 13, write May 13 on your data sheet. Your TA will advance the planetarium in one-month increments. Each month, locate Mars and write down its position on the data sheet (again, using the date on the ecliptic as the coordinate). Continue until 1 Earth year has elapsed. Sun Mars Sun Mars Sun Mars Jan 1 May 1 Sep 1 Feb 1 Jun 1 Oct 1 Mar 1 Jul 1 Nov 1 Apr 1 Aug 1 Dec 1 Astronomy 150 2 - 2 The Planets Determining the Orbit of Mars On the back of this lab is a drawing of the orbits of the Earth and Mars about the Sun with two small date circles below it. You should think of this drawing as a view of the solar system if you were above it looking down. The Sun is located at the center of the solar system, and the inner circle represents the orbit of Earth. The outer circle is the orbit of Mars. The date-circles will represent the ecliptic in this case (an ecliptic circle). 1. Cut out one of the small ecliptic circles. I printed two in case one gets lost. Cut out the small inner circle on the ecliptic circle. 2. Place the ecliptic circle so that the center hole is on the orbit of the Earth and the outside of the circle points to the Sun at the Jan 1 mark. Make sure that the ecliptic circle is in the correct orientation (i.e., Jul 1 pointing up). from the point of view of a person on Earth, the Sun will appear to lie on Jan 1 on the ecliptic, so it is January 1 on Earth. 3. Place a mark at the position of the Earth (the center of the hole) and a mark just outside the ecliptic circle at the “coordinate” of Mars on Jan 1. [For example, if Mars’ “coordinate” for Jan 1 is May 20, make a mark just outside the ecliptic circle by May 20.] 4. Remove the ecliptic circle. 5. Draw a line from the position of the Earth on Jan 1 through the mark of Mars’ “coordinate” until it intersects the orbit of Mars. Mark and label the position of Mars for Jan 1. 6. Place the ecliptic circle so that the center hole is on the orbit of the Earth and the outside of the circle points to the Sun at the Feb 1 mark (i.e. it will be Feb 1 on Earth). Again, make sure that the ecliptic circle is in the correct orientation (i.e., Jul 1 pointing up). 7. Repeat steps 4-6 for Feb 1. 8. Repeat for the rest of the year. You can now see how far along Mars has moved in its orbit during each month. Based on your plots, determine the period for Mars. Show your work on the plots. Mars Period = days = Earth years. We have learned that Kepler’s 3rd law relates the period of a planet’s orbit (in years) to the average distance of the planet from the Sun (in astronomical units, AU). That is, P2 = D3. Average Distance [AU] = Period2/3 [years] Mars Distance = AU Astronomy 150 2 - 3 The Planets The actual period and distance of Mars is Period = 687 days, Distance = 1.524 AU. Explain why your numbers may be di erent from these. In particular, how have we used Kepler’s 1st law in this lab? Since you have now determined the orbit of Mars, you can make predictions about its position into the future. In two Earth-years from your first observation on Jan 1, what will be the position of Mars as seen on the ecliptic on Earth? [Give the coordinate.] Mars’ coordinate in two years = You are now able to even make predictions of what time of day Mars will be visible. The drawing on the left is an Earth-centered view that shows the position of the Sun at Noon. Draw and label where you are standing at Sunset, Sunrise, and Midnight. Earth rotates counter-clockwise. Determine at what time of the day (approximately) Mars would appear directly overhead on January 1, July 1 and December 1. Jan 1 Jul 1 Dec 1 Time Overhead The best time to observe Mars is when it is directly overhead at midnight. When will this occur? Astronomy 150 2 - 4 The Planets Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Astronomy 150 2 - 5 The Planets lack of words dwell here as I ponder while writing oh well, blank it is! 2 2Thanh Tran - Fall 2002 Astronomy 150 2 - 6 The Planets Lab #3 Crater Counting Introduction Impact craters are the dominate landforms on most of the solid surfaces in our solar system. These impact craters have accumulated on the surfaces over the age of our solar system. The number of craters on a surface increases with the length of time that surface has been exposed to space. These rather simple ideas are the basis for a very powerful tool, called crater counting, that planetary scientists use to unravel the history of a planetary surface. The basic idea is that an old surface will have more impact craters than a younger surface. By counting the number of craters in some defined area on a world (determining its crater density) and comparing it to the number of craters on a same-sized area on another part of that world, you can determine the relative ages of the two surfaces (e.g. one area is twice as old as another). If you want to find out the absolute age of the surface you are studying, you need a sample from that surface. Fortunately for us, the Apollo mission brought back lots of rocks from six sites on the Moon. By measuring the ages of rocks from these six sites, and measuring the crater density at these sites, we can determine how the crater density is related to the absolute age at these sites. Now, at least for the Moon, if we can measure the crater density of any part of the Moon, we can compare it to the crater density at the Apollo sites to determine their relative ages. Since we now know the absolute ages of the rocks at the Apollo sites, we can determine the absolute age of any part of the Moon. In this class, we make the assumption that the cratering rate measured by Apollo on the Moon is typical of the cratering rate in the inner solar system. We can now extend our measurements of the crater density on the Moon to estimate the ages of various regions on the surface of Mars. Materials The materials we will use for this lab are images taken by the Viking 1 and 2 orbiters. The Viking project consisted of launches of two separate spacecraft to Mars. Viking 1, launched on 20 August 1975, and Viking 2 launched on 9 September 1975. Each spacecraft consisted of an orbiter and a lander. After orbiting Mars and returning images used for landing site selection, the orbiter and lander detached. The lander entered the Martian atmosphere and soft-landed at the selected site in the summer of 1976 . The orbiters continued imaging and conducting other scientific operations from orbit, while the landers deployed instruments on the surface. The Viking 1 orbiter was turned o on 17 August 1980, after returning more than 30,000 images in 1485 orbits around Mars. The Viking 2 orbiter was turned o on 25 July 1978, after returning almost 16,000 images in 706 orbits around Mars. Astronomy 150 3 - 1 The Planets Procedure a) Determine the crater density. On each of the images are 5 white bars that represent 128, 64, 32, 16 and 8 km in size. Use these bars to create a scale bar that is divided into several di erent size ranges (0-8 km, 8-16 km, 16-32 km, etc.). It should looks like this: 0 8 16 32 64 128 Now use your scale bar (NOT the example one above) to determine how many craters are in each size range. You may not be able to use all the di erent size ranges. There may be no craters in some of the larger ranges or too many craters in the smallest ranges. Try to fill in as many of the size ranges as you can. Record the numbers in the Crater Density Data Table. b) The data for the crater density of the Apollo sites was determined over 1,000,000 km2. The total area of the images you used are shown at the bottom of the image. Using your numbers from the table and the formula below, determine how many craters of each size range are found in 1,000,000 km2. Record this number in the table. Number of craters per 1,000,000 km2 = Number of craters × 1, 000, 000 Image Size [km2] [km2] c) Plot your data points from the table on the Crater Density Graph. Put your points on the graph in the middle of your size range. For example, if you had 200 craters in the 0-8 km size range, you should put your point at the intersection of 200 on the y-axis, and 6 on the x-axis. (Note: the y-axis of this graph has a logarithmic scale. If you are unfamiliar with how to plot points on a logarithmic scale ask your TA). d) Determine the age of your surface. Once you have your points plotted, fit a straight line through the points, or as close to them as you can. Your line should be roughly parallel to the age lines on the graph. The line you have drawn represents the average age of the cratered surface you have been examining. Estimate the age by interpolating the location of the line you have drawn with the age lines already on the graph. Martian Northern Hemisphere Surface Age = billion years old Martian Southern Hemisphere Surface Age = billion years old Astronomy 150 3 - 2 The Planets Martian Crater Density Data Table Northern Hemisphere Southern Hemisphere Crater size Number of Craters Number of Craters Number of Craters Number of Craters range (km) in Image in 1,000,000 km2 in Image in 1,000,000 km2 < 8 8 - 16 16 - 32 32 - 64 64 - 128 Astronomy 150 3 - 3 The Planets Questions 1) How accurate do you believe your estimate of the age of the surfaces are - for each surface, what are the oldest and youngest ages that fit your data? Be quantitative (i.e. ± 1 billion years). 2) What do you believe was your greatest source of errors? 3) Consider these two facts: (a) The Earth has been hit by as many impactors as the Moon and Mars. (b) The state of Washington has a total land area of about 177,000 km2. Calculate how many 5-km-sized craters have been formed in Washington state over the last 4 billion years. [Show your work.] 4) Currently the state of Washington has zero 5 km impact craters. What happened to them? Astronomy 150 3 - 4 The Planets Martian Northern Hemisphere - Image Size = 812,250 km2 Astronomy 150 3 - 5 The Planets Martian Southern Hemisphere - Image Size = 774,250 km2 Astronomy 150 3 - 6 The Planets Lab #4 Reflectance Spectroscopy I Introduction Since our direct samples of materials from other worlds is very limited, most of our knowledge about planetary surfaces comes from analyzing the light that they reflect. This is the idea behind the science of remote sensing. One way to do this is to simply examine images taken in visible light, images much like you would produce with an ordinary camera. Most of the objects you see in such an image (trees, soil, sand, snow, water, concrete . . . ) do not emit their own light. Instead, we see them because they reflect sunlight (or moonlight, or light from light bulbs). However, when the light bounces o of them, some colors are reflected and others are absorbed. Things with di erent compositions will absorb di erent parts of the spectrum (i.e. di erent colors). Much more information can be obtained by carefully analyzing how much of each di erent type of light is reflected by a surface. It is particularly informative if you look at light outside of the visible spectrum. Reflectance Spectroscopy - the analysis of the spectrum of reflected light - is a fundamental tool of planetary science. It allows us to determine the properties of planetary surface from which we have no direct samples, or even from places on Earth where sampling would be dicult or time consuming. The Ground Truth In this lab we will learn how reflectance spectroscopy can be used to determine the properties of a planetary surface. The world we will explore will be our own: the Earth. We choose the Earth because it is the world we are most familiar with, and it is a place where samples can easily be collected. Collecting samples is a very important part of remote sensing, because it allows us to establish what is called the “ground truth.” Before we can recognize a lava flow on the Moon, we have to know the characteristics of light reflected from lava. In order to do that, we need a chunk of lava to bounce light o of and measure. In order to establish the “ground truth,” we are going to be using reflectance spectrometers to measure how much light of various colors in the spectrum is absorbed and reflected by objects we might see in images of the Earth taken from space. Instead of shining the whole continuous spectrum at the same time, these spectrometers shine one color of light at a time. Take a minute to look at your spectrometer - DO NOT put anything into the hole in the back! Each of the 9 buttons on the front makes a di erent light shine. You will not be able to see the last two lamps since they are emitting light beyond the visible spectrum. In the middle of the circle of lights is a small photodetector. The display on the front of the spectrometer gives a measure of how much light it detects. Astronomy 150 4 - 1 The Planets Calibration - Making the Numbers Make Sense 1. Set the spectrometer on a flat surface so that no light is allowed in. Notice that the detector does not read zero; rather, it reads a small number. This is called the dark current. The detector has been set so that even with NO light, a small number will be displayed on the readout. This number is important, because all your measurements will need to have this o set subtracted out in order to find out how much light the instrument actually measured. 2. Next we need to calibrate our instrument so that the numbers have some meaning. We are going to be measuring things in terms of how much light they reflect at each wavelength. Luckily, photographers also need to do the same type of calibration, so they have made a standard gray card that reflects 18% of the light at each wavelength. We will be comparing our measurements of other objects to our measurements of this gray card. 3. Position your reflectance spectrometer flat on the gray card. Turn on the lights one at a time and hold down the button until the numbers in the display stabilize. Record the number on the display in the first column of the calibration chart. Make sure no stray light is entering the hole on the back of the spectrometer. 4. This is not the actual amount of light being reflected by the gray card - remember the dark o set! To determine the actual amount of light being reflected by the gray card, subtract the dark o set from each measurement and record the result in the second column. 5. Next we want to find out how much light each of the bulbs is actually emitting (and thus to figure out how much light our samples are reflecting). But we know that the gray card reflects 18% (or 0.18) of the light hitting it, so we can use our previous measurements to calculate this: Light reflected by gray card = 0.18 × Light emitted from bulb 6. Therefore, the light emitted is the measurement in the second column divided by 0.18. Record your calculations in the third column. Those are all the necessary calibrations we need in order to measure the reflectance of any object. In the next part of the lab, we are going to establish the “ground truth” by measuring the reflectance spectrum of various stu found all over the Earth. Because you will be comparing your plots with photographs in the lab room, you should make your calculations and plot your results as you go along. Collecting Data • To get a feel for what it is that we are really measuring, observe the green card at each of the wavelengths on the reflectance spectrometer. In the first column of the data table, record the numbers you observe (obs) in the first column. For the second column, calculate and record the reflectance (refl) of the green card. First subtract the dark o set, then divide this value by the amount of light the bulb is actually emitting (you calculated this in the last column of previous table). The numbers in the ”refl” column should be between 0.0 (i.e. reflecting no light) to 1.0 (reflecting all the light). Plot your results and connect the points. • Measure the reflectance of live vegetation (i.e. the green leaf). Again, the observed measurements go in the “obs” column and the calculated reflectance values go in the “refl” column. Plot your results on the same graph. Please use a di erent symbol, color, or line style to distinguish each of the lines on your plot. • Observe the dead leaf and soil. Calculate their reflectance, record them in the table, and plot your results on the graph. Astronomy 150 4 - 2 The Planets Questions If you were to measure a red card, what colors would be absorbed? How does the spectrum of the green leaf compare to the spectrum of the green card? Most modern digital cameras are sensitive to near-infrared light (wavelengths > 800 nm). However, most cameras have an internal filter that blocks this light from reaching the camera. Imagine you took a digital image of the green card and green leaf: For a camera with an internal filter, how would the brightness of the green card compare to the green leaf? For a camera without an internal filter, how would the brightness of the green card compare to the green leaf? Astronomy 150 4 - 3 The Planets Calibration Table Observed Amount of Reflected Light, Gray Card Actual Amount of Reflected Light (Observed - Dark) Amount of Light Emitted by Bulb (Obs - Dark)/0.18) 470 nm 555 nm 585 nm 605 nm 635 nm 660 nm 695 nm 880 nm 940 nm Data Table Green Card (obs) Green Card (refl) Green Leaf (obs) Green Leaf (refl) Dead Leaf (obs) Dead Leaf (refl) Soil (obs) Soil (refl) 470 nm 555 nm 585 nm 605 nm 635 nm 660 nm 695 nm 880 nm 940 nm Astronomy 150 4 - 4 The Planets Astronomy 150 4 - 5 The Planets Landsat are a series of Earth-observation satellites that have fundamentally changed how we look at our world. The first Landsat was launched in July of 1972 and the seventh in the series was launched April 15, 1999. The Landsat satellites image Earth at many di erent wavelengths, including wavelengths in the infrared. When spacecraft takes images of other worlds they most often take those images through filters. Filters allow only certain range of wavelengths of light to enter the camera. The range of wavelengths allowed by a filter is called its bandpass. An example of a set of filters can been seen in your plot of the reflectance of the Earth samples. The three cross-hatched area represent the bandpass of the filters the Landsat satellite uses. The most common way to look at Landsat images is to combine images taken through di erent filters into one color image. The colors in the resulting image do not represent “real” colors; rather, they are usually arranged so that some colors represent wavelengths outside the visible spectrum. For example, for our spectral data let us call Filter #1 a “blue” filter, Filter #2 a“yellow” filter, and Filter #3 a “red” filter. Using a Color Wheel, we can then determine the “color” of combining the filters. For example, if a spectra is equally bright in the “yellow” filter and the “blue” filter, combining the two filters would give a “green” color. If a spectra is very bright in the “red” and dark in the other filters, combining the filters would give a “red” color. If the spectra is equally bright in all filters, the resulting “color” would be a shade of grey. What color would a leafy tree appear in the Landsat image? What color would a dead tree appear in the Landsat image? What color would soil appear in the Landsat image? Look at the Landsat images of the Mt. St. Helens regions. (The images are also available on the class web page.) It is easy to see the the dramatic change between the pre- and post-eruption images. What happened to change the colors? Several years after the eruption, the images again look di erent. Describe what is di erent and give an explanation as to its cause. Astronomy 150 4 - 6 The Planets Lab #5 Impactors and the Impacted Introduction Meteorites are fragments of other worlds that have survived the entry into the Earth’s atmosphere. Most meteorites originate in the asteroid belt from bodies that formed very early in the history of the solar system. Almost all of the information we have learned about the solar system, such as its age, history, and chemical composition, is due to the detailed study of meteorites. From the point of view of origin, there are three basic types of meteorites: stony, stony-iron, and iron. Meteoriticists recognize many more types of meteorites and have reconstructed a marvelously detailed history of the solar system from their subtle di erences. When a large meteorite strikes the Earth, the kinetic energy of the meteorite is converted into thermal, mechanical, and acoustic energy that creates a shockwave that passes through the ground and distorts, fractures, and ejects pieces of the target. This modified target material is often all that remains of a crater after millions of years of geological activity. Therefore, the recognition of this material plays an important role in understanding impact events. The most common types of impact-modified material we will see are: impact breccia, shatter cones, and tektites. Iron Meteorites Iron meteorites are the most easily recognizable meteorites. Since even a casual examination shows that they are not ordinary rocks, they tend to be very common in collections although they are rare in space. They are very dense and, except for a thin crust (made by the melting of the skin during the passage through the atmosphere), they look and feel like metal. Chemically, they are composed mostly of iron with a few percent nickel and a little cobalt. When sawed in half, polished, and etched, they display a geometrical pattern called a Widmanst¨atten pattern (see figure). This pattern is actually crystals of iron and nickel that form as the result of the meteorite having cooled very slowly (about 1 per million years) under very high pressure. The existence of the Widmanst ¨atten pattern is our best evidence that iron meteorites were once the cores of larger, di erentiated bodies. Buried deep in a body, the mass of the overlying rocks provide the high pressure and the insulation for slow cooling. [The image shows a polished and etched cross-section of an iron meteorite from the Henbury impact craters in Australia.] Astronomy 150 5 - 1 The Planets Stony Meteorites Stony meteorites are the most common meteorites that fall to Earth. Since they tend to have a similar appearance and density as Earth rocks, stony meteorites are dicult to recognize in the field. Unless someone sees them fall, they usually go uncollected. Therefore, although stony meteorites are the most common type out in space, they are more rare than iron meteorites in collections on Earth. Stony meteorites show a wide variety of appearances: some light, some dark, some coarse grained, some fine-grained, but almost all stony meteorites contain some metallic iron. Chemically they are also diverse, thought they all have a telltale composition that tells us they are not from the Earth. Most stony meteorites are from the outer parts of an asteroid that su ered destruction by collision. Some are pieces of lava flows from the surface, some are pieces of impact breccia, and some are pieces of material that apparently never existed in a much larger body. Meteorites that come from such a small, undi erentiated body are called primitive meteorites. [This image shows a slice of a type of stony meteorite called an ordinary chondrite. This sample fell to the Earth in Homestead, Iowa on February 12, 1876.] Stony-Iron Meteorites Stony-Iron meteorites are the rarest class of meteorites, comprising only about 1% of meteorites that fall to Earth. There are two broad classes of stony-iron meteorites: Pallasites ( composed primarily of iron with crystals of a rock mineral called olivine embedded in it) and mesosiderites (that look like stony meteorites with lots of metallic iron veins running through them). Pallasites are thought to be material from the boundary zone between the iron cores and the stony outer mantles of the now-destroyed asteroids, while mesosiderites are theorized to be formed when an impact on an asteroid mixes material from the rocky mantle with iron from the core. [The image shows a polished slice of a pallasite stony-iron meteorite. The dark roundish inclusions are the rocky mineral olivine, and the lighter surrounding material is metallic iron. This sample is from the Brenham meteorite crater in Kansas.] Astronomy 150 5 - 2 The Planets Carbonaceous Chondrite Meteorites An especially important type of meteorite is the carbonaceous chondrite, a specific type of stony meteorite that originates from primitive asteroids. They are black to dark gray in color, rich in the element carbon (thus their black color), and contain small spherical droplet-like inclusions called chondrules. They are among the most primitive objects in the solar system, having survived almost unchanged for 4.6 billion years. Carbonaceous chondrites were the first place amino acids were found outside of the Earth, and it has been recently learned that some of the materials in these meteorites were formed outside of our solar system before our solar system was even formed, so they are not only an important probe into our early solar system history, but they may supply us with samples of materials from beyond our solar system. Although carbonaceous chondrites are fairly abundant among meteorites that fall to the Earth, they look enough like Earth rocks that they are rare in collections. They also weather very easily and do not survive long on the surface of the Earth. [This image shows a slice of a type of carbonaceous chondrite meteorite that fell to the Earth in Allende, Mexico, on February 9, 1969.] Impact Breccia Impact breccias form when a crater-forming meteorite shatters, pulverizes, and melts the target material. They are composed of rock and mineral fragments embedded in a matrix of fine-grained material. The fragments are usually sharp and angular, and vary greatly in size and shape. The composition of the fragments depends on the target material. Impact breccias often have the appearance of poorly mixed concrete (see figure). On airless, impact-covered worlds like the Moon, impact breccias are a very common type of rock. The most common type of sample returned by the Apollo lunar mission was impact breccia. Unfortunately, rocks that look a lot like impact breccias can be formed by volcanic and tectonic processes, so finding a breccia is not always a clear indication of an impact event. [This images shows a piece of impact breccia from the Ries crater in Germany.] Astronomy 150 5 - 3 The Planets Shatter Cones Shatter cones form when the shockwave from a meteorite impact event passes through the target rocks and modifies them. The resulting rocks have distinctive, curved, striated fractures that typically form partial to complete cones [see figure]. Shatter cones can form in all types of target rocks. The better-looking shatter cones form in very fine-grained rocks like sandstones. They can range in size from centimeters to many tens of meters. Shatter cones are now accepted as a unique identifier of a meteorite impact event. This means that if you find a shatter cone, you have found a place where a meteorite has hit. Since the Earth is such an dynamic world, it will erase impact craters over a relatively short time period. Often, shatter cones are all that is left to identify an impact crater. An interesting feature of shatter cones is that the tips point toward the origin of the shockwave. This means that you can use shatter cones to reconstruct the size and shape of ancient impact craters that have subsequently been modified by other processes. [This image shows a shatter cone from the Steinheim Basin in Germany.] Tektites Tektites have been controversial objects since their discovery, with both their origin and source being a subject of hot debate for more than a century. Tektites are small, glassy objects with shapes like spheres, ellipsoids, dumbbells, and other forms characteristics of isolated molten blobs. They are typically black, but can be brown, gray, or even green. Tektites look a lot like volcanic glass (e.g., obsidian) but are chemically distinct. The most telling chemical di erence is that unlike volcanic glasses, tektites contain virtually no water. Current scientific consensus is that tektites are terrestrial material that has been melted and ejected from an impact event. Their shape is derived from cooling, aerodynamically, during flight from the impact. Tektites are fairly common all over the Earth. (You can almost always find them for sale in “New Age” crystal shops.) However, linking them with particular impact events has proven problematic. When exactly the tektites are formed during an impact event, and why they are found at only a few craters are two of the more obvious problems that have yet to be satisfactorily solved. [This image shows a collection of tektites from Thailand.] Astronomy 150 5 - 4 The Planets Astronomy 150 Name: The Planets Section: The tables in the lab have a number of di erent types of samples. Examine them carefully, with the idea that afterwards you will be identifying samples for which the types are not going to be given. [You will probably also see one of these on the final exam.] Sample Type Specific Characteristics Density Color & Textures Other Iron Meteorite Stony Meteorite Stony-Iron Meteorite Carbonaceous Chondrite Impact Breccia Shatter Cone Tektite Astronomy 150 5 - 5 The Planets Meteorite Quiz On the table are a number of unknown samples. Write down what type of meteorite or impact rock you think it is and why. One or more of the rocks might not be meteorites or impact rocks at all. If you think one of the samples is not a meteorite or impact rock, just write down ROCK as its type. Unknown Sample Type Reason for this Identification 1. 2. 3. 4. 5. 6. 7. 8. Astronomy 150 5 - 6 The Planets Lab #6 Satellites of Saturn 1. Describe the surface. Note the amount of cratering, unusual terrain, etc. Is it uniform? World #1 World #2 2. Is the surface young or old compared to the surface of the Moon? Explain your reasoning (and make sure you indicate whether you are talking about the lunar highlands or mare)! World #1 World #2 3. Based on the physical characteristics of your satellites (see data table), what would you conclude they are made of? How do the data support this? World #1 World #2 Astronomy 150 6 - 1 The Planets 4. Compare your satellites to the Moon. If the moon had the albedo of the satellite you chose, what would it look like in the sky? How would it compare in brightness to the Sun? World #1 World #2 5. Now take a look at the image of Saturn’s small satellites. Describe the level of geologic activity you would expect to see on these satellites? Why aren’t they spherical? 6. Look at the image of Titan. Do you see any surface features? What might be an explanation for this? 7. Based on the physical characteristics of Titan (see data table), what would you expect the level of geological activity to be on its surface compared to the Earth’s Moon? Astronomy 150 6 - 2 The Planets Lab #7 Volcanoes of Io Introduction Io is the most geologically active world in our solar system. In fact, the surface of Io is so active that is the only solid surface in our solar system without any impact craters. Io is resurfaced in under a million years and the entire crust is recycled in about 10 million years. That means that over the age of the solar system the crust of Io has been completely recycled over 400 times. The mechanism of this intense amount of resurfacing is volcanism. Io’s volcanism dwarfs all other worlds in our solar system. Volcanoes were first discovered on Io when the Voyager spacecraft flew past in 1979 and detected eight volcanoes in eruption. The Galileo spacecraft now in orbit around Jupiter has discovered more volcanoes and continues to monitor the eruptions on Io. The size of the eruptions is immense, some hurtling material higher and farther than terrestrial volcanoes. Part of the reason for the di erence is the lower gravity of Io, and another part is the lack of an atmosphere. The Lab Prometheus - 1 mm = 5400 m Ra Patera - 1 mm = 9100 m (Scale refers to small inset picture) On the left is an image taken by Voyager 2 of the volcano Prometheus, and on the right is an image of the Ra Patera volcano taken by the Galileo spacecraft. For each of the images, determine the maximum height and range of the volcano from the surface of Io. Height of Prometheus: mm = meters Range of Prometheus: mm = meters Height of Ra Patera: mm = meters Range of Ra Patera: mm = meters Astronomy 150 7 - 1 The Planets The graph below displays the relationship between the height and range (distance along the ground from the vent), the velocity, and the angle of ejection of the material thrown out from the surface of Io. The graph allows you to determine two unknown quanties if you know the other two. The graph takes into account the lower gravity and lack of atmosphere on Io. 90 30 35 40 45 50 55 60 65 70 75 80 85 1.1 1.05 1.0 0.9 0.8 0.7 0.6 0.5 0.2 km/sec 0 0 100 300 400 500 600 700 200 o o o o o o o o o o o o o km/sec km/sec 200 300 400 100 Range (kilometers) Maximum Height (kilometers) Using the graph above and the measurements you took on the first page, determine the speed and ejection angle for Prometheus and Ra Patera. Prometheus: Velocity: km/sec Angle: degrees Ra Patera: Velocity: km/sec Angle: degrees Kazuhiro Sasaki can throw a fastball about 150 km/hr. How does the velocity Prometheus compare to his fastball? (Be quantitative: how many times faster or slower?) - Show your work. Astronomy 150 7 - 2 The Planets The Yellowstone geyser Old Faithful (which in some respects resembles Io’s volcanoes more than some other terrestrial volcanoes like Mt. St. Helens) spews steam to a height of about 200 feet with a velocity of about 50 m/s. If Old Faithful were transported to Io it would spew steam at about 350 m/s. Use the graph to determine how high and how far would Old Faithful would reach on Io (assume an ejection angle of 80) . Height of Old Faithful: km. Range of Old Faithful: km. The largest volcano on Io is named Pele (named after the Hawaiian goddess, not the soccer player). It is one of the most conspicuous objects visible on the surface of Io. Voyager 1 took this image from above Pele while it was in eruption. If the scale on this image is 1mm = 9000 m, how far is material being thrown from the central vent of Pele? Range of Pele: mm = meters If you assmume the height of Pele is 300 km, use the graph to determine the velocity and and ejection angle of Pele. Pele: Velocity: km/sec Angle: degrees How does this speed compare to Old Faithful on Earth? [show your work] Astronomy 150 7 - 3 The Planets Emptiness abounds Nothing scripted in black ink Void, desolate, bare 3 3Nona Davenport - Winter 2003 Astronomy 150 7 - 4 The Planets Lab #8 Reflectance Spectroscopy II - Rocks Introduction One of the best pieces of evidence we have that the majority of meteorites come from the asteroid belt is that the reflectance spectra of some asteroids matches the reflectance spectra of meteorites. Why is this important? Rocks are made up of a collection of minerals, and these minerals are made up of a specific collection and structure of elements (usually in a crystal structure). For example, the “Genesis Rock” brought back from Apollo 15 is a type of rock called an Anorthosite. This rock is composed primarily of mineral called plagioclase feldspar. This mineral has the chemical formula [CaAl2Si2O8] meaning that it is made up of the elements Calcium (Ca), Aluminum (Al), Silicon (Si), and Oxygen (O) all arranged in a specific crystal structure. When light is reflected o a rock, the various crystals in the rocks absorb di erent wavelengths of light. The specific wavelengths of light they absorb depends on the composition and structure of the crystal. By looking at the reflectance spectrum of a rock we can get an idea of the minerals that make up the rock. This turns out to be a very powerful tool in planetary astronomy. It allows us to determine the composition of a rock by looking at the reflected light. We do not need the have a sample in hand. When we look at the reflectance spectra of asteroids, we are in essence determining their mineral composition. Since we have meteorites in our labs, we can easily determine their mineral composition. By comparing these compositions we can determine that they are consistent with most meteorites coming from asteroids in the asteroid belt. The Samples Before we can determine what the reflectance spectra of other worlds tell us, we need to see what the reflectance spectra of known materials look like. While we cannot grind up meteorites (it would be way too expensive) to compare them to asteroids, we can look at Earth rocks that are very similar to rocks found on other worlds. The rock samples we will analyze are: Basalt - We have seen basalt on the surface of nearly all of the worlds we have explored in this class (i.e. the lunar maria, Mars’ Tharsis ridge, the Hawaiian islands, the lowlands of Venus). Basalt is formed by the cooling of volcanic lava, both above ground and below ground. This particular sample of basalt came from the Columbia Flood basalts of Eastern Washington. Olivine - Olivine is a very common rock-building mineral on Earth. However, since it is dense, most of it exists in the Earth’s mantle under our feet. Volcanic activity can bring it to the surface. The gem-quality form of olivine is called peridot. This sample came from a rock found in a pile behind Johnson Hall. Anorthosite - See above. Astronomy 150 8 - 1 The Planets Like the first reflectance lab, the first thing we have to do is calibrate our spectrometers. Calibration - Making the numbers make sense. 1. Set the spectrometer on a flat surface so that no light is allowed in. Notice that the detector does not read zero; rather, it reads a small number. This is called the dark current. The detector has been set so that even with NO light, a small number will be displayed on the readout. This number is important, because all your measurements will need to have this o set subtracted out in order to find out how much light the instrument actually measured. 2. Next we need to calibrate our instrument so that the numbers have some meaning. We are going to be measuring things in terms of how much light they reflect at each wavelength. Luckily, photographers also need to do the same type of calibration, so they have made a standard gray card that reflects 18% of the light at each wavelength. We will be comparing our measurements of other objects to our measurements of this gray card. 3. Position your reflectance spectrometer flat on the gray card. Turn on the lights one at a time and hold down the button until the numbers in the display stabilize. Record the number on the display in the first column of the calibration chart. Make sure no stray light is entering the hole on the back of the spectrometer. 4. This is not the actual amount of light being reflected by the gray card-remember the dark o set! To determine the actual amount of light being reflected by the gray card, subtract the dark o set from each measurement and record the result in the second column. 5. Next we want to find out how much light each of the bulbs is actually emitting (and thus to figure out how much light our samples are reflecting). But we know that the gray card reflects 18% (or 0.18) of the light hitting it, so we can use our previous measurements to calculate this: Light reflected by gray card = 0.18 × Light emitted from bulb 6. Therefore, the light emitted is the measurement in the second column divided by 0.18. Record your calculations in the third column. Collecting Data • Observe each of the three rock samples at each of the wavelengths on the reflectance spectrometer. • In the first column of the data table, record the numbers you observe (obs). • For the second column, calculate and record the reflectance (refl) of the samples. First subtract the dark o set, then divide this value by the amount of light the bulb is actually emitting (you calculated this in the last column of previous table). The numbers in the ”refl” column should be between 0.0 (i.e. reflecting no light) to 1.0 (reflecting all the light). • Plot your results on the same graph. Please use a di erent symbol, color, or line style to distinguish each of the lines on your plot. Astronomy 150 8 - 2 The Planets Questions When spacecraft take images of other worlds they most often take those images through filters. Filters allow only a certain range of wavelengths of light to enter the camera. The range of wavelengths allowed by a filter is called its bandpass. An example of a set of filters can been seen in your plot of the reflectance of the rock samples. The three cross-hatched areas represent the bandpass of three di erent filters. Which rock sample would be darkest as seen through filter #3 How much darker is the darkest sample in #3 from the next darkest sample? [Be quantitative] As you can see it can be dicult to distinguish the rock type by looking through only one filter. However, much more information can be gained by combining filter information. One easy way to combine information is to make “color” images. For our spectral data let us call: • Filter #1 a “blue” filter • Filter #2 a“yellow” filter • Filter #3 a “red” filter. Using a Color Wheel we can then determine the “color” of combining the filters. For example if a spectra is equally bright in the “yellow” filter and the “blue” filter, combining the two filters would give a “green” color. If a spectra is very bright in the “red” and dark in the “yellow” combining the filters would give a “red-orange” color. For each of the following filter combinations, determine what the resulting “color” would be for the three samples. Filters #1 - #2 Filters #2 - #3 Filters #1 - #3 Basalt Olivine Anorth. Astronomy 150 8 - 3 The Planets As you can see from your table, two di erent samples may appear to be the same “color” using one combination of filters, but a very di erent color using another combination. Your choice of filters depends on what you are looking for. What filter combination would you use to distinguish Basalt from Olivine? [Explain your answer.] What filter combination would you use to distinguish Olivine from Anorthosite? [Explain your answer.] The Clementine spacecraft orbited the Moon in 1994 equipped with a camera that had many di erent filters. Look at the images of the complex impact crater Tycho taken by Clementine. The image on the left (a) was taken through Filter #3, while the image on the right (b) is a “normal” photograph of the region. Based on image (a), describe the distribution of Anorthosite in the impact crater. Prior to the impact that created the crater Tycho, was the Anorthosite deep underground or at the surface? [Explain your answer.] Astronomy 150 8 - 4 The Planets Calibration Table Observed Amount of Reflected Light, Gray Card Actual Amount of Reflected Light (Observed - Dark) Amount of Light Emitted by Bulb (Obs - Dark)/0.18) 470 nm 555 nm 585 nm 605 nm 635 nm 660 nm 695 nm 880 nm 940 nm Data Table Basalt (obs) Basalt (refl) Olivine (obs) Olivine (refl) Anorth. (obs) Anorth. (refl) 470 nm 555 nm 585 nm 605 nm 635 nm 660 nm 695 nm 880 nm 940 nm Astronomy 150 8 - 5 The Planets Astronomy 150 8 - 6 The Planets Lab #9 51 Peg - The Discovery of a New World Introduction In just the past few years, astronomers have announced discoveries of at least 74 planets in 66 planetary systems orbiting nearby stars. These discoveries seem to finally answer the question of whether or not our solar system is unique. We should note, however, that when astronomers state that they have discovered a new planet, what they are really saying is that their data can best be interpreted as a planet orbiting a star. One cannot ”prove” that these other planets exist (short of actually going there to explore!); one can only state that, until the hypothesis is disproved, a planet orbiting the star best explains the observations. We cannot see these planets. We can only measure indirectly the influence each one has on its parent star as the star and planet orbit their common center of mass. The planet makes the star ”wobble.” We enter this realm of discovery by working with actual data from observations of the star 51 Pegasi (51 Peg) made at the Lick Observatory in California. The table on the next page lists the measured radial velocities (RV) as a function of time (recorded in days). These data are the measurements of the Doppler shift of the wavelengths of the absorption lines seen in the spectra of 51 Peg. These were obtained by using the formula: / = v/c. Solving for the radial velocity v of the star: v = c(/). Here, c is the speed of light,  is the laboratory wavelength of the absorption line being measured, and  is the di erence between the measured wavelength of the line and the laboratory value. As you can see, the radial velocities are sometimes positive and sometimes negative indicating that sometimes the star is moving away from (the light is redshifted) and sometimes approaching (the light is blueshifted) our frame of reference. This wobble of the star was the first indication that the star 51 Peg had an invisible companion. Procedure Plot the 32 data points on the graph. Use the observed radial velocities (in m/s) versus the day of the observation. Draw a smooth curve (do not simply connect-the-dots) through the data. The curve is a sine curve (ask if you don’t know) and thus will always reach the same maximum and minimum values and have the same ”number of days” between each ”peak” and ”valley”. You should interpolate between data where points are missing. Thought questions: Why are there data missing? Why are there sizable gaps in the data? (Hint, some gaps are a little over 1/2 day long and these are observations from the ground.) Astronomy 150 9 - 1 The Planets 51 Pegasi Radial Velocity Data Day RV Day RV Day RV Day RV (m/s) (m/s) (m/s) (m/s) 0.6 -20.2 4.7 -27.5 7.8 -31.7 10.7 56.9 0.7 -8.1 4.8 -22.7 8.6 -44.1 10.8 51 0.8 5.6 5.6 45.3 8.7 -37.1 11.7 -2.5 1.6 56.4 5.7 47.6 8.8 -35.3 11.8 -4.6 1.7 66.8 5.8 56.2 9.6 25.1 12.6 -38.5 3.6 -35.1 6.6 65.3 9.7 35.7 12.7 -48.7 3.7 -42.6 6.7 62.5 9.8 41.2 13.6 2.7 4.6 -33.5 7.7 -22.6 10.6 61.3 13.7 17.6 Astronomy 150 9 - 2 The Planets Exercise 1) A period is defined as one complete cycle; that is, where the radial velocities return to the same position on the curve (but at a later time). a) How many cycles did the star go through? Number of cycles = b) What is the period, P, in days? Period = [days] c) What is P in years? P = [years] (Hint: divide the period in days by the number of days in a year; the answer will be a decimal smaller than 1.) d) What is the uncertainty in your determination of the period? That is, by how many days or fractions of a day could your value be wrong? Uncertainty = [days] e) What is the amplitude, K? K = [m/s] (Take 1/2 of the value of the full range of the velocities.) f) How accurate is your determination of this value? Uncertainty = [m/s] 2) We will make some simplifying assumptions for this new planetary system: • The orbit of the planet is circular (e = 0) • The mass of the star is 1 solar mass • The mass of the planet is much, much less that of the star • We are viewing the system nearly edge on • We express everything in terms of the mass and period of Jupiter We make these assumptions to simplify the equations we have to use for determining the mass of the planet. The equation we must use is: Mplanet = P 121/3 K 13 [MJupiter] P should be expressed in [years] (or fractions of a year), and K in [m/s]. 12 [years] is the approximate orbital period for Jupiter and 13 [m/s] is the magnitude of the ”wobble” of the Sun due to Jupiter’s gravitational pull. Put in your values for P and K and calculate the mass of this new planet in terms of the mass of Jupiter. (For example: Mplanet = 4 [MJupiter].) Show all work. Astronomy 150 9 - 3 The Planets 3) Assume that the parent star is 1 solar mass, and that the planet is much less massive than the star. We can then calculate the distance this planet is away from its star, in astronomical units (AU’s) by using Kepler’s third law: a3/P2 = 1 Again, P is expressed in [years], and a represents the average distance in [AU’s]. Solve for a: a = (P2)1/3 a = [AU] 4) Compare this planet to those in our solar system. For example, Mercury is 0.4 AU from the Sun; Venus, 0.7 AU; Earth, 1.0 AU; Mars, 1.5 AU; Jupiter, 5.2 AU. Jupiter is more massive than all the rest of the matter in the solar system combined, excluding the Sun. What is unusual about this new planet? 5) Science is based upon the ability to predict outcomes. However, nothing prepared astronomers for the characteristics of this ”new” solar system. Why was it such a surprise? 6) Explain why it would be difficult to detect a planet the same size and distance as the the Earth using this technique. Astronomy 150 9 - 4 The Planets Homework #4 Tidal Forces We have learned in class that the gravitational force between two bodies is proportional to the mass of the two objects and inversely proportional to the square of the distance between them. This can be express mathematically as: FGravity / M1M2 d2 If you wanted to gure out the ratio between the force of gravity the Earth feels from the Sun to that of the Moon, you only need to do a little simple algebra: FGravity(Sun) / MSunMEarth (dSun) 2 FGravity(Moon) / MMoonMEarth (dMoon) 2 FGravity(Sun) FGravity(Moon) = MSunMEarth (dSun) 2  (dMoon) 2 MMoonMEarth = MSun(dMoon) 2 MMoon(dSun) 2 Now if you plug the number from the box on the right into the above equation you will nd that the gravitational force of the Sun on the Earth is 176 times that of the Moon. MSun = 2.0  1030 kg MMoon = 7.3  1022 kg dSun = 1.5  108 km dMoon = 3.8  105 km Tidal Forces Tidal forces operate slightly di erently from gravitational forces. The tidal force from a perturbing body is proportional to the mass of that body and inversely proportional to the cube of the distance. Mathematically, this is: FTidal / M d3 1. Write down the equation that would allow you to gure out the ratio between the tidal forces the Earth feels from the Sun to that of the Moon. 2. Plug in the numbers from the box on the other side of this sheet. Does the Earth feel a stronger tidal force from the Sun or the Moon? What is the ratio? The Earth feels a stronger tidal force from the Moon with the ability of the Moon;s tidal force on the lowering the tides and the rising of the tides of the ocean water. 3. Jupiter's moon Io feels a tidal force from Jupiter in much the same way our Moon feels a tidal force from the Earth. Using the data in the table to the right, gure out the ratio of the tidal force Io feels from Jupiter to the tidal force the Moon feels from Earth. [We will assume that the Moon and Io have the same mass. This is a pretty good assumption.] MJupiter = 1.9 x 1027 kg MEarth = 6.0 x 1024 kg dJupiter Io = 4.2 x 105 km dEarth Moon = 3.8 x 105 km lab4.pdf
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