ࡱ>    !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~Root Entry F`SIWordDocument CompObj^e to actually see planets orbiting other stars. Nonetheless, astronomers have evidence of *35. The planet discovered orbiting the star 70 Virginis, having "seen" an extrasolar planet come between its star 59 light-years from Earth, moves in an orbit with and us. Search the World Wide Web for information semimajor axis 0.47 AU and eccentricity 0.40. The period about this planet, which orbits the star HD 209458. of the orbit is 116.6 days. Find the mass of 70 Virginis. If the planet is too small and faint to be seen, how can Compare your answer with the mass of the Sun. astronomers tell that it came between HD 209458 and us? What have they learned about the planet from *36. Because of the presence of Jupiter, the Sun moves in this event that cannot be learned with the radial velocity a small orbit of radius 742,000 km with a period of method?  Our Solar System 175 176 I CHAPTER 7 2~~pT IO,p ~ 43. Determining Terrestrial Planet Orbital a~ ~ Periods. Access the animation "Planetary Orbits" in Chapter 6 of the Universe web site or CD-ROM. Focus on the motions of the inner planets at the last half of the animation. Using the stop and start buttons, determine how many days it takes Mars, Venus, and Mercury to orbit the Sun once if Earth takes approximately 365 days. nebulae. The table below gives their coordinates (right ascension and declination; see Box 2-1) for the year 2000. Nebula Right ascension Declination Lagoon Omega Trifid Orion 18h 03.8m -24 23' 18 20.8 - -16 11 18 02.3 -23 02 5 35.4 ~ -5 27 O SSF RV 1 N G ('ROJ ECTS ~pRRY Nz~,y 45. Use the Starry Night program to observe 44. There are many young stars still embedded in the h ~ magnified images of at least four of the planets. clouds of gas and dust from which they formed. In the ' First turn off daylight (select Daylight in the Sky winter evening sky, for example, is the famous Orion Display menu) and show the entire celestial sphere (select Nebula (Figure 7-13a). In the summer night sky, the Atlas in the Go menu). Select Planet List in the Window Lagoon, Omega, and Trifid nebulae are found in the Milky menu, and then double-click on the name of the planet you Way. Examine some of these nebulae with a telescope. wish to view. Use the controls at the right-hand end of the Describe their appearance. Can you guess which stars in Control Panel to zoom in until you can see a detailed view of your field of view are actually associated with the the planet. Describe each planet's appearance. From what nebulosity? You can use the Starry Night program that you observe, is there any way of knowing whether you comes with this book to help you find some of these looking at a planet's surface or simply a cloud cover?    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. Plate tectonics, or movement of the plates, is driven by R~ViE1N QL)~STI ONS convection within the asthenosphere. Molten material wells up at oceanic rifts, producing seafloor spreading, and is 1. Describe the various ways in which the Earth is unique returned to the asthenosphere in subduction zones. As one among the planets of our solar system. end of a plate is subducted back into the asthenosphere, it 2, Describe how energy is transferred from the Earth's helps to pull the rest of the plate along. surface to the atmosphere by both convection and radiation. 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). 3. How does the greenhouse effect influence the temperature of the atmosphere? How does this effect differ fromܥe# R&#,l,l  (TgTimes New Roman Symbol ArialTimes New RomanArial Narrow Tahoma VerdanaLucida Console ArialCourier New\ determine the thickness of the disk in AU. (b) Explain why 11.86 years. (a) Calculate the Sun's orbital speed in meters the disk will continue to flatten as time goes by. per second. (b) An astronomer on a hypothetical planet 32. The accompanying infrared image shows IRAS orbiting the star Vega, 25 light-years from the Sun, wants . 04302+2247, a young star that is still surrounded by a to use the astrometric method to search for planets disk of gas and dust. The scale bar at the lower right of the orbiting the Sun. What would be the angular diameter of image shows that at the distance of IRAS 04302+2247, an the Sun's orbit as seen by this alien astronomer? Would the \, angular size of 2 arcseconds corresponds to a linear size of Sun's motion be discernible if the alien astronomer could 0 AU. Find the distance to IRAS 04302+2247. measure positions to an accuracy of 0.001 arcsec? (c) Repeat part (b), but now let the astronomer be located on a hypothetical planet in the Pleiades star cluster, 360 light-years from the Sun. Would the Sun's motion be discernible to this astronomer?  DISCUSSION QIICSTIONS  04 (Courtesy of D. Padgett; W. Brandner, IPAC/Caltech; K. Stapelfeldt, JPL; and NASA) 37. Propose an explanation why the Jovian planets are orbited by terrestrial-like satellites. 38. Suppose that a planetary system is now forming around some protostar in the sky. In what ways might this planetary system turn out to be similar to or different from our own solar system? Explain your reasoning. 39. Suppose astronomers discovered a planetary system in which the planets orbit a star along randomly inclined orbits. How might a theory for the formation of that planetary system differ from that for our own? y~'(ER4CT 1NCf3/CD-ROM QUESTIONS 33. The image accompanying Question 32 shows a dark, opaque disk of material surrounding the young star IRAS 04302+2247. The disk is edge-on to our line of sight, so it appears as a dark band running vertically across this 40. Search the World Wide Web for information about image (north to south in the sky). The material to the left recent observations of protoplanetary disks. What insights and right of this band is still falling onto the disk. (a) Make have astronomers gained from these observations? Is there measurements on this image to determine the diameter of any evidence that planets have formed within these disks? the disk in AU. Use the scale bar at the lower right of this 41. In 2000, extrasolar planets with masses comparable image. (b) If the thickness of the disk is 50 AU, find its to that of Saturn were first detected around the stars volume in cubic meters. (c) The total mass of the disk is HD 16141 (also called 79 Ceti) and HD 46375. (All perhaps 2 x 102' kg (0.01 of the mass of the Sun). How many extrasolar planets discovered prior to these are more atoms are in the disk? Assume that the disk is all hydrogen. massive than Jupiter.) Search the World Wide Web for A single hydrogen atom has a mass of 1.673 x 10-27 kg. information about these "lightweight" planets. Do these (d) Find the number of atoms per cubic meter in the disk. planets move around their stars in the same kind of By comparison, the air that you breathe contains about orbit as Saturn follows around the Sun? Why do you 5.4 x 1025 atoms per cubic meter. Would you describe the suppose this is? How does the discovery of these planets disk material as thick or thin by Earth standards? reinforce the model of planet formation described in Section 7-8? 34. Discuss why the "planet" that orbits the star HD 114762 (see Figure 7-21) could possibly be a brown dwarf, while 42. As of this writing, not even the most powerful the planet that orbits 51 Pegasi probably is not. telescopes have been abl what actually happens in a greenhouse? 4. Describe the interior structure of the Earth. 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 bu the inner core is solid? 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 7. The inner core of the Earth is at a higher temperature surrounds the Earth and blocks the solar wind from hitting than the outer core. Why, then, is the inner core solid and the atmosphere. the outer core molten instead of the other way around? A bow-shaped shock wave, where the supersonic solar 8. Describe the process of plate tectonics. Give specific wind is abruptly slowed to subsonic speeds, marks the examples of geographic features created by plate tectonics. outer boundary of the magnetosphere. 9. Explain how convection in the Earth's interior drives Most of the particles of the solar wind are deflected the process of plate tectonics. around the Earth by the magnetosphere. 10. What is the difference between a rock and a mineral? 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. 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? The Earth's Atmosphere: The Earth's atmosphere differs 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 The Earth's atmosphere evolved from being mostly water Mount St. Helens in Washington State, are usually located vapor to being rich in carbon dioxide. A strong greenhouse in mountain ranges that border on subduction zones? effect kept the Earth warm enough for water to remain 14. Describe the Earth's magnetosphere. If the Earth did liquid and to permit the evolution of life. not have a magnetic field, do you think aurorae would be The appearance of photosynthetic living organisms led to more common or less common than they are today? our present atmospheric composition, about four-fifths 15. How do we know that the Earth's magnetic field is not nitrogen and one-fifth oxygen. due to magnetized iron in the planet's interior? The Earth's atmosphere is divided into layers called the 16. What are the Van Allen belts? troposphere, stratosphere, mesosphere, and thermosphere. Ozone molecules in the stratosphere absorb ultraviolet light. 1~~ Summarize the history of the Earth's atmosphere. What role has biological activity plated in this evolution? Because of the Earth's rapid rotation, the circulation in its atmosphere is complex, with three circulation cells in each hemisphere. 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. The Biosphere: Human activity is changing the Earth's 19, Carbon dioxide and ozone each make up only a biosphere, on which all living organisms depend. fraction of a percent of our atmosphere. Why, then, should Deforestation and the burning of fossil fuels is increasing the we be concerned about small increases or decreases in the greenhouse effect in our atmosphere and warming the planet. atmospheric abundance of these gases?  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 4ttrZ and volume 4/sttr3, 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. J-4 and describe how to solve problems involving blackbody radiation, sccn-, ) discusses precession, and tiection 4-5 describes the properties of elliptical orbits.   20. The total power in sunlight that reaches the top of our atmosphere is 1.75 X 1017 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 20C per kilometer. At what depth would water boil? (Assume the surface temperature is 20C and ignore the effect of the pressure of overlying rock on the boiling point of water.) 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. ADVANCED QUESTIONS I Problem-solving tips and tools 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?  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 rim. (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.  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 0 U X G Our Living Earth 203 204 I CHAPTER 8 33. Earth's atmospheric pressure decreases by a factor of 2050? In 2100? What effects may the increased ~,, one-half for every 5.5-km increase in altitude above sea temperature have on human health? level. Construct a plot of pressure versus altitude, assuming 40. Use the World Wide Web to investigate the current the pressure at sea level is one atmosphere (1 atm). Discuss status of the Antarctic ozone hole. Is the situation getting the characteristics of your graph. At what altitude is the better or worse? Is there a comparable hole over the North atmospheric pressure equal to 0.001 atm? Pole? Why do most scientists blame the chemicals called CFCs for the existence of the ozone hole? 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 southern hemisphere. Explain why. . D1SClJSSION (ZUESTION.S 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 COZ 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? 41. In 1989 representatives of many nations signed a global treaty called the Montreal Protocol to protect the ozone layer. 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 since it was first signed? When are various substances that destroy the ozone layer scheduled to be phased out? 2~~pTIO,y~ 2~~pTIO,y~ 42. Observing Mountain Range Q ~ Q ~' Formation. Access the two animations , "The Collision of Two Plates: South America" and "The Collision of Two Plates: The Himalayas" in Chapter 8 of the Universe web site or CD-ROM. (a) In which case are the mountains formed by tectonic uplift? In which case are the mountains formed by volcanoes from rising lava? (b) For each animation, describe which plate is moving in which direction. , oRSERV~NC; PRO~~cTs 43. When was the last earthquake near your hometown? How far is your hometown from a plate boundary? What kinds of topography (for example, mountains, plains, seashore) dominate the geography of your hometown area? Does the topography and the frequency of earthquakes seem to be consistent with your hometown's proximity to a plate boundary? y~qERq~~, ~p~,tRY Nr~,y 44. Use the Starry Night program to view the W C ~ / C D - RO M ) lI~ST 1 O N S y ~ Earth from space. First select Viewing Location... in the Go menu and set the viewing 37. The periodic formation and breakup of supercontinents location to your city or town. (You use the list of cities imply that profound environmental changes occur about provided, or you can use the mouse to click on your every 500 million years. What sort of global changes might approximate position on the world map. Then click accompany the formation and breakup of a supercontinent? the Set Location button.) In the Control Panel at the top How might these cycles affect the evolution of life? Use the of the window, set the local time to 12:00:00 P.M. (noon). World Wide Web to research the life-forms that dominated To see the Earth from space, use the elevation buttons the Earth when Pangaea existed 250 million years ago and (the ones that look like a rocket landing or taking off) in when fragments of the preceding supercontinent were as the Control Panel to raise yourself off the surface until you dispersed as today's continents. can see the entire Earth. (You may want to adjust your field of view so that you are looking directly at the Earth.) 38. Because of plate tectonics, the arrangement of continents (a) What evidence can you see that the Earth has an in the future will be different from today. Search the World atmosphere? (b) Using the controls at the right-hand end Wide Web for information about "Pangea Ultima," a of the Control Panel, zoom in to show more detail around supercontinent that may form in the distant future. When is it your city or town. The amount of detail is comparable to predicted to form? How will it compare to the supercontinent the view from a spacecraft a few million kilometers away. that existed 200 million years ago (see Figure 8-11a)? Can you see any evidence of plate tectonics? Of the 39. Use the World Wide Web to research how global presence of life? What does this tell you about the warming may affect the Earth's future surface temperature. importance of sending spacecraft to explore planets at What are some predictions for the surface temperature in close range? 222 I CHAPTER 9 thickness equal to about 80% of the Moon's radius, and a 7. Could you use a magnetic compass to navigate on the small iron core. Moon? Why or why not? The Moon's lithosphere is far thicker than that of the Earth. The lunar asthenosphere probably extends from the base of the lithosphere to 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. The Moon has no global magnetic field today, although 9, Explain why moonquakes occur more frequently it had a weak magnetic field billions of years ago. when the Moon is at perigee than at other locations along '~ Geologic History of the Moon: The anorthositic crust its orbit. , exposed in the highlands was formed between 4.0 and 10. Why is the Earth geologically active while the Moon 4.3 billion years ago, whereas the mare basalts solidified between 3.1 and 3.8 billion years ago. is not? The Moon's surface has undergone very little change over 11. On the basis of moon rocks brought back by the the past 3 billion years. astronauts, explain why the maria are dark colored but the lunar highlands are light colored. Meteoroid impacts have been the only significant "weathering" agent on the Moon. The Moon's regolith, or 12. Briefly describe the main differences and similarities surface 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 All of the lunar rock samples are igneous rocks formed 4.6 billion years old. By contrast, the majority of the largely of minerals found in terrestrial rocks. The lunar Earth's surface is made of oceanic crust that is less than rocks contain no water and also differ from terrestrial 200 million years old, and the very oldest Earth rocks are rocks in being relatively enriched in the refractory elements about 3 billion years old. If the Earth and Moon are and depleted in the volatile elements. essentially the same age, why is there such a disparity in 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 14. Why do most scientists favor the collisional ejection Mars-sized protoplanet and that debris from this collision theory of the Moon's formation? coalesced to form the Moon. This theory successfully explains most properties of the Moon. 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. 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 later by the impact of planetesimals and filled with lava from the lunar interior. Tidal interactions between the Earth and Moon are slowing the Earth's rotation and pushing the Moon away from the Earth. R^VIEW (ZIJESTIONS 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? ADVANC^f7 QIIESTIONS Questions preceded by an asterisk (~') involve topics discussed in Box 9-1. 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/s~tr3, where r is the sphere's radius. The surface area of a sphere of radius r is 4ttr2, while the surface area of a circle of radius r is ttr2. Recall also that the acceleration of gravity on the Earth's surface is 9.8 m/s2. You may find it useful to know that a 1-pound (1-Ib) 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 5rrri>rc 1-7, undergoing a near-collision. During the interactions with other galaxies. By their effect on the inter- millions of years that this close encounter has been taking stellar gas from which stars are formed, tidal interactions can place, the tidal forces of the larger galaxy have pulled an actually trigger the birth of new stars. Our own Sun and solar immense streamer of stars and interstellar gas out of the system anay have been formed as a result of tidal interactions smaller galaxy. of this kind. Hence we may owe our very existence to tidal Many galaxies, including our own Milky Way Galaxy, forces. In this and many other ways, the laws of motion and show signs of having been disturbed at some time by tidal of universal gravitation shape our universe and our destinies. ` KEY WORDS acceleration, p. 78 aphelion, p. 75 conic section, p. 83 conjunction, p. 67 deferent, p. 65 direct motion, p. 65 eccentricity, p. 75 ellipse, p. 74 elongation, p. 67 epicycle, p. 65 focus (of an ellipse; plural foci), p. 74 : force, p. 78 geocentric model, p. 63 gravitational force, p. 80 gravity, p. 80 greatest eastern elongation, p. 67 greatest western elongation, p. 67 heliocentric model, p. 66 hyperbola, p. 83 opposition, p. 67 inferior conjunction, p. 67 parabola, p. 83 inferior planet, p. 67 parallax, p. 73 Kepler's first law, p. 75 perihelion, p. 75 Kepler's second law, p. 76 period (of a planet), p. 68 Kepler's third law, p. 76 Ptolemaic system, p. 65 law of equal areas, p. 76 retrograde motion, p. 65 law of inertia, p. 78 semimajor axis (of an ellipse), p. 74 law of universal gravitation, p. 80 sidereal period, p. 68 major axis (of an ellipse), p. 74 speed, p. 78 mass, p. 80 spring tides, p. 85 neap tides, p. 86 superior conjunction, p. 67 Newtonian mechanics, p. 83 superior planet, p. 67 Newton's first law of motion, p. 78 synodic period, p. 68 Newton's form of Kepler's third law, tidal forces, p. 84 p' 82 universal constant of gravitation, Newton's second law of motion, p. 79 p. 81 Newton's third law of motion, p. 80 velocity, p. 78 Occam's razor, p. 66 weight, p. 80 Gravitation and the Waltz of the Planets 87   21. Suppose that the Earth were moved to a distance of 0.25 AU from the Sun. How much stronger or weaker would the Sun's gravitational pull be on the Earth? Explain.  22. The mass of the Moon is 7.35 X 1022 kg, while that of the Earth is 5.98 x 1024 kg. The average distance from the center of the Moon to the center of the Earth is 384,400 km. What is the size of the gravitational force that the Earth exerts on the Moon? What is the size of the gravitational force that the Moon exerts on the Earth? How do your answers compare with the force between the Sun and the Earth calculated in the text? 12. What are Kepler's three laws? Why are they important? 13. What are the foci of an ellipse? If the Sun is at one focus of a planet's orbit, what is at the other focus? 14. A line joining the Sun and an asteroid is found to I Problem-solving tips and tools sweep out 6.3 AU'- of space during 2004. How much area is swept out during 2005? Over a period of five years? Box 4-1 explains sidereal and synodic periods in detail. The semimajor axis of an ellipse is half the length of the long, or major, axis of the ellipse. For data about the planets and their satellites, see Aopendices l, 2, and -s at the back of this book. If you want to calculate the gravitational force that you feel on the surface of a planet, the distance r to use is the planet's radius (the distance between you and the center of the planet). Boxes 4-2 and 4-4 show how to use Kepler's third law in its original form and in Newton's form. 15. The orbit of a spacecraft about the Sun has a perihelion distance of 0.1 AU and an aphelion distance of 0.4 AU. What is the semimajor axis of the spacecraft's orbit? What is its orbital period? 16. A comet with a period of 125 years moves in a highly elongated orbit about the Sun. At perihelion, the comet comes very close to the Sun's surface. What is the comet's average distance from the Sun? What is the farthest it can get from the Sun? 17. What are Newton's three laws? Give an everyday example of each law. 18. How much force do you have to exert on a 4-kg brick to give it an acceleration of 3 m/s2? If you double this force, what is the brick's acceleration? Explain. 19. What is your weight in pounds and in newtons? What is your mass in kilograms? 20. What is the difference between weight and mass? 23. How far would you have to go from Earth to be completely beyond the pull of its gravity? Explain. 24. What are conic sections? In what way are they related to the orbits of planets in the solar system? 25. Why was the discovery of Neptune an important confirmation of Newton's law of universal gravitation? 26. What is a tidal force? How do tidal forces produce tides in the Earth's oceans? 35. One trajectory that can be used to send spacecraft from 27. What is the difference between spring tides and neap tides? the Earth to Venus is an elliptical orbit that has the Sun at Gravitation and the Waltz of the Planets 89 ADVANCE:D ~tlEST10NS Questions preceded by an asterisk (~'J involve topics discussed in the Boxes. 28. Figure 4-2 shows the retrograde motion of Mars as seen from Earth. Sketch a similar figure that shows how Earth would appear to move against the background of stars during this same time period as seen by an observer on Mars. '~29. The synodic period of Mercury (an inferior planet) is 115.88 days. Calculate its sidereal period in days. 'F30. Table 4-1 shows that the synodic period is greater than the sidereal period for the inferior planets Mercury and Venus, and that the synodic period is less than the sidereal period for all the superior planets. Draw diagrams like the one in Box 4-1 to explain why this is so. *31. A general rule for superior planets is that the greater the average distance from the planet to the Sun, the more frequently that planet will be at opposition. Explain how this rule comes about. 32. In 2000, Mercury was at greatest western elongation on March 28, July 27, and November 15. It was at greatest eastern elongation on February 15, June 9, and October 6. Does Mercury take longer to go from eastern to western elongation, or vice versa? Why do you suppose this is the case? 33. A certain asteroid is 2 AU from the Sun at perihelion and 6 AU from the Sun at aphelion. (a) Find the semimajor axis of the asteroid's orbit. (b) Find the sidereal period of the orbit. , 34. A comet orbits the Sun with a sidereal period of 27.0 years. (a) Find the semimajor axis of the orbit. (b) At aphelion, the comet is 17.5 AU from the Sun. How far is it from the Sun at perihelion? 224 CHAPTER 9 36. How would our theories of the Moon's history have 40. Use a telescope or binoculars to observe the Moon. been affected if astronauts had discovered sedimentary Compare the texture of the lunar surface you see on the rock on the Moon? maria with that of the lunar highlands. How does the 37. Imagine that you are planning a lunar landing mission. visibility 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 full moon. Make sketches of various surface features, such : 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 p~RY Nl~ 42, Use the Starry Night program to observe the World Wide Web for current information about these y~, ~,~y~ changing appearance of the Moon. First turn off missions. When is each mission scheduled to be launched? ~ daylight (select Daylight in the Sky menu) and What investigations will each mission make that have not show the entire celestial sphere (select Atlas in the Go been made before? What scientific issues may these menu). Center on the Moon by using the Find... command missions resolve? in the Edit menu. Using the controls at the right-hand end  WF R/CD-ROM QUCSTIONS A pTI0ry9 39, Determining the Size of the Planetesimal that of the Control Panel, set the time step to 3 hours and click ^' Formed the Moon. Access the animation "The on the Forward button (a triangle that points to the right). Formation of the Moon" in Chapter 9 of the (a) Describe how the phase of the Moon changes. (b) Look Universe web site or CD-ROM. Determine how many carefully at features near the left-hand and right-hand times larger the proto-Earth is than the impacting limbs (edges) of the Moon. Are these features always at the planetesimal. If Mars is about 50% the size of Earth, how same position relative to the limb? Explain in terms of does the planetesimal compare in size with present-day Mars? libration. (c) Does the apparent size of the Moon always stay the same, or does it vary? Explain what this tells you about the shape of the Moon's orbit. PROJ^CTS Observing tips and tools  e`j"k 5, If you do not have access to a telescope, you can learn a lot by observing the Moon through binoculars. Note that the Moon will appear right-side-up through binoculars but inverted through a telescope; if you are using a map of the Moon to aid your observations, you will need to take this into account. Inexpensive maps of the Moon can be purchased from most good bookstores or educational supply stores. You can determine the phase of the Moon either by looking at a calendar (most of which tell you the dates of new moon, first quarter, full moon, and last quarter), by checking the weather page of your newspaper, by consulting the current issue of Sky & Telescope or Astronomy magazine, by using the Starry Night program, or by using the World Wide Web.    Motions of Mercury in the Earth's Sky: At its greatest eastern and western elongations, Mercury is never more than 28 from the Sun, so it can be seen with the naked eye only briefly after sunset or before sunrise. Mercury has a weak magnetic field, indicating that at least part of the iron core is liquid. The magnetic field ~ produces a magnetosphere around Mercury that blocks the solar wind from reaching the surface of the planet. KEY 1 DE1ES Solar transits of Mercury occur about a dozen times per century. Mercury's Rotation: Poor telescopic views of Mercury's surface led to the mistaken impression that the planet always keeps the same side toward the Sun, a configuration called synchronous rotation or 1-to-1 spin-orbit coupling. Radio and radar observations in the 1960s revealed that Mercury in fact has 3-to-2 spin-orbit coupling. For an observer on Mercury, the average time from sunset to sunrise (one-half of a solar day) is equal to the time for a complete orbit around the Sun (one Mercurian year). ` The Surface of Mercury: The Mariner 10 spacecraft made several passes near Mercury in the mid-1970s, providing pictures of its surface. The Mercurian surface is pocked with craters like those of the Moon, but there are extensive smooth plains between these craters. Long cliffs called scarps meander across the surface of Mercury. These scarps probably formed as the planet cooled, solidified, and shrank. The impact of a large object long ago formed the huge Caloris Basin and shoved up jumbled hills on the opposite side of the planet. Mercury's Interior and Magnetic Field: Like the Earth, Mercury has an iron-rich core. The iron core of Mercury has a diameter equal to three-fourths of the planet's diameter, whereas the diameter of the Earth's core is only slightly more than one-half of the Earth's diameter. 5. What is 3-to-2 spin-orbit coupling? How is the rotation period of an object exhibiting 3-to-2 spin-orbit coupling related to its orbital period? What aspects of Mercury's orbit cause it to exhibit 3-to-2 spin-orbit coupling? What telescopic observations proved this? 6. Explain why Mercury does not have a substantial atmosphere. 7. Explain why Mariner 10 was able to photograph only one side of Mercury even though the spacecraft returned to the planet three times. 8. Compare the surfaces of Mercury and our Moon. How are they similar? How are they different? 9. What kind of surface features are found on Mercury? Why are they probably much older than most surface features on the Earth? 10. How could you tell which craters in Figure 10-10 are younger than the others? 11. How do we know that the scarps on Mercury are younger than the lava flows? How can you tell that the scarp in Figure 10-11 is younger than the vertically distorted crater at the center of the figure? 12. Explain why the Sun is directly over the Caloris Basin on Mercury only every other time that the planet is at perihelion. 13. Explain why mountains on Earth can be said to be formed from below, while the mountains that ring the Caloris Basin on Mercury and Mare Orientale on the Moon are best described as having formed from above. 14. Why do astronomers think that Mercury has a very large iron core? 15. Briefly describe at least one possible history that would account for Mercury's large iron core. 16. How is Mercury's magnetosphere similar to that of the Earth? How is it different? Why do you suppose Mercury does not have Van Allen belts? REVIEW QUESTiONS 1. Why can't you see any surface features on Mercury ~when it is closest to the Earth? ~ Why are naked-eye observations of Mercury best made dusk or dawn, while telescopic observations are best ie during the day? . Table 10-2 shows that a greatest western elongation of Mercury is always followed by a greatest eastern elongation, and vice versa. Explain why. F 4. What is a solar transit? Why are solar transits of Mercury relatively infrequent? l~DVhNCED QLIFSTIONS I Problem-solving tips and tools You may need to refresh your memory about the small-angle formula, found in E~ox 1-1; about Kepler's third law, described in ' ; and about Wien's law and the Doppler effect, discussed in Section 5-4 and Section 5-9, respectively. The circumference of a circle of radius r is 2~r. _liox 7-2 discusses the criteria for a planet to be able to retain an atmosphere. Sun-Scorched Mercury 237 238 1 CHAPTER 10 17. Find the largest angular size that Mercury can have as COZ is 12.0 + 16.0 + 16.0 = 44.0, and so the molecule's seen from the Earth. In order for Mercury to have this mass is 44.0 m H.) apparent size, at what point in its orbit must it be? 25, How much would an 80-kg person weigh on Mercury? 18. Suppose you have a superb telescope that can resolve How does that compare with that person's weight on the features as small as 1 arcsec across. What is the size (in Moon? How much does that person weigh on the Earth? kilometers) of the smallest surface features you should be able to see on Mercury? How does your answer compare 26. When an impact crater is formed, material (called with the size of the Caloris Basin? (Hint: Assume that you ejecta) is sprayed outward from the impact. (Look at the choose to observe Mercury when it is at greatest photograph of the Moon on the right-hand side of the elongation, about 25 from the Sun. As Figure 10-1 shows, figure that opens this chapter. At the lower right of this at this point in its orbit, the Earth-Mercury distance is Photograph, you can see the light-colored ejecta about the same as the Earth-Sun distance.) surrounding the crater Stevinus.) While ejecta are found surrounding the craters on Mercury, they do not extend as 19. Figure 10-1 shows Mercury with a greatest eastern far from the craters as do ejecta on the Moon. Explain elongation of 18 and a greatest western elongation of 28. why, using the difference in surface gravity between the (a) Make a drawing like Figure 10-1 that shows how the Moon and Mercury. Earth and Mercury would have to be positioned on their orbits to have Mercury at a greatest eastern elongation 27. The orbital period of Mariner 10 is twice that of of 28 and a greatest western elongation of 18. Mercury. Use this fact to calculate the length of the (b) On June 9, 2000, Mercury was at a greatest eastern semimajor axis of the spacecraft's orbit. elongation of 24. Was Mercury at perihelion, aphelion, 28. Give at least two pieces of evidence that Mercury has or some other point on its orbit? Explain. undergone chemical differentiation. 20. (a) Explain why November solar transits of Mercury, which occur near the time of perihelion passage, are more common than May transits. (b) When a transit of Mercury takes place, can it be seen by observers anywhere on Earth? Why or why not? 29. It is thought that the electric currents that generate a planet's magnetic field are aided by the planet's rotation, which helps to keep liquid material inside the planet in motion. Use this idea to argue why Mercury's magnetic field should be much smaller than the Earth's. 21. Find the value of '.ax for blackbody radiation coming from the sunlit side of Mercury. In what part of the electromagnetic spectrum does this lie? 22. If the albedo of Mercury were increased, would the planet's surface temperature go up or down? Explain your answer. 30. Consider the idea that Mercury has a solid iron-bearing mantle that is permanently magnetized like a giant bar magnet. Using the fact that iron demagnetizes at temperatures above 770C, present an argument against this explanation of Mercury's magnetic field. 23. (a) Mercury has a 58.646-day rotation period. What is the speed at which a point on the planet's equator moves due to this rotation? (Hint: Remember that speed is distance divided by time. What distance does a point on Mercury's equator travel as the planet makes one rotation?) (b) Use your answer to (a) to answer the following: As a result of rotation, what difference in wavelength is observed for a radio wave of wavelength 12.5 cm (such as is actually used in radar studies of Mercury) emitted from either the approaching or receding edge of the planet? DISCUSSION QUeSTIONS 31. Before about 350 s.c., the ancient Greeks did not realize that Mercury seen in the morning sky (which they called Apollo) and seen in the evening sky (which they called Hermes) were actually the same planet. Discuss why you think it took some time to realize this. 32. If you were planning a new mission to Mercury, what features and observations would be of particular interest to you? 24. (a) Calculate the minimum molecular weight of a gas 33. What evidence do we have that the surface features on that could in theory be retained as an atmosphere by Mercury were not formed during recent geological history? Mercury if the average daytime temperature were 620 K. ' (b) Are there any abundant gases that meet this minimum ti~cER^cr, criterion? Why doesn't Mercury have an atmosphere of g , yVC(3/C [) _ R O M QUESTIONS molecular weight, g, multiplied by the mass of a hydrogen 34. Search the World Wide Web for the latest informati, atom, mH = 1.67 x 10-27 kg. The molecular weight of a about upcoming missions to Mercury. Which of these molecule equals the sum of the atomic weights of its missions have been given funding so that they can proceed? atoms, which can be looked up in a periodic table of the When will they be launched, and when will they arrive in elements. Thus, for example, the molecular weight of orbit around Mercury? What scientific experiments will  they carry? What scientific issues are these instruments favorable elongation to reduce the chances of being intended to resolve? "clouded out." Select an observing site that has a clear, unobstructed view of the horizon where the Sun sets (or rises). If possible, make arrangements to have a telescope at your disposal. Search for the planet on the dates you have selected, and make a drawing of its appearance through your telescope. ~.PI INZ 35. Elongations of Mercury. Access the animation "Elongations of Mercury" in Chapter 10 of the Universe web site or CD-ROM. (a) View the animation and notice the dates of the greatest eastern and greatest western elongations. Which time interval is greater: from a greatest eastern elongation to a greatest 37. This observing project should be performed only western elongation, or vice versa? (b) Based on what you under the direct supervision of an astronomer who knows observe in the animation, draw a diagram to explain your how to point a telescope safely at Mercury. Make answer to the question in (a). arrangements to view Mercury during broad daylight. This is best done by visiting an observatory where the coordinates (right ascension and declination) of Mercury's position can be used to point the telescope. DO NOT LOOK AT THE SUN! Looking directly at the Sun can cause blindness.  ORSCRVINQ PROJf_CTS Observing tips and tools ----  lO Remember that Mercury is visible in the morning sky when it is at or near greatest western elongation and in the evening sky when at or near greatest eastern elongation. You can consult such magazines as Sky & Telescope and Astronomy, or the web sites for these magazines, for more detailed information about when and where to look for Mercury during a given month. You can also use the Starry Night program on the CD-ROM that accompanies this textbook. 36. Refer to Table 10-2 to determine the dates of the next two or three greatest elongations of Mercury. Consult such magazines as Sky & Telescope and Astronomy, or the web sites for these magazines, to determine if any of these greatest elongations is going to be a favorable one. If so, make plans to be one of those rare individuals who has actually seen the innermost planet of the solar system. Set aside several evenings (or mornings) around the date of the 4pRRY Nry 38. Use the Starry Night program to observe ` ~ solar transits of Mercury. First turn off daylight (select Daylight in the Sky menu) and show the entire celestial sphere (select Atlas in the Go menus). Center on Mercury by using the Find... command in the Edit menu. Using the controls at the right-hand end of the Control Panel, zoom in until the field of view is 1. (a) In the Control Panel, set the date and time to May 7, 2003, at 12:00:00 A.M., and set the time step to 20 minutes. Step backward or forward through time using the single-step buttons (the leftmost and the rightmost buttons) and record the times at which the solar transit begins and ends. What is the total duration of the solar transit? (b) Set the date and time to November 8, 2006, at 12:00:00 A.M. Again step backward or forward through time, record the times when the solar transit begins and ends, and find the total duration of the solar transit. (c) The maximum duration of a solar transit of Mercury is 9 hours. Explain why the transits you observed in (a) and (b) last a substantially shorter time.   Sun-Scorched Mercury 239 256 1 CHAPTER 11 Although Venus is nearly the Earth's twin in size, its sur- atmosphere will experience the same sort of runaway green- face and atmosphere have evolved in radically different ways house effect that occurred on Venus long ago. As temperatures than the Earth. In the long run, however, both planets may drive toward 1000 K, vast amounts of carbon dioxide will be meet the same fate. Our Sun's luminosity will continue to baked out of the Earth's rocks, creating a Venus-like atmo- increase as it ages, and in about a billion years sunlight will be sphere. By studying Venus, we may be looking into the distant bright enough to begin evaporating the Earth's oceans. The future of our own world.  KC1' WORDS equilibrium resurfacing hypothesis, p. 255 prograde rotation, p. 243 shield volcano, p. 248 global catastrophe hypothesis, p. 255 retrograde rotation, p. 243 hot-spot volcanism, p. 248 runaway greenhouse effect, p. 251  KEY 1 DCLES  Motions of Venus in the Earth's Sky: At its greatest eastern The early atmosphere of Venus contained substantial and western elongations, Venus is about 47 from the amounts of water vapor. This caused a runaway greenhouse Sun, and it can be seen for several hours after sunset or effect that evaporated Venus's oceans and drove carbon before sunrise. dioxide out of the rocks and into the atmosphere. Almost Comparison of the Earth and Venus: Venus is similar to the all of the water vapor was eventually lost by the action of Earth in its size, mass, average density, and surface gravity, ultraviolet radiation on the upper atmosphere. but it is covered by unbroken, highly reflective clouds that The Earth has roughly as much carbon dioxide as Venus, conceal its other features from Earth-based observers. but it has been dissolved in the Earth's oceans and chemically bound into its rocks. Venus's Rotation: Venus rotates slowly in a retrograde direction with a solar day of 117 Earth days and a rotation The Surface of Venus: The surface of Venus is surprisingly \ period of 243 Earth days. There are approximately two flat, mostly covered with gently rolling hills. There are a Venusian solar days in a Venusian year. few major highlands and several large volcanoes. Venus's Atmosphere and Clouds: Spacecraft measurements The surface of Venus shows no evidence of the motion of , reveal that 96.5% of the Venusian atmosphere is carbon large crustal plates, which plays a major role in shaping the dioxide. Most of the balance of the atmosphere is nitrogen. Earth's surface. Venus's clouds consist of droplets of concentrated sulfuric acid. Active volcanoes on Venus may be a continual source of this sulfurous material. The density of craters suggests that the entire surface of Venus is no more than a few hundred million years old. According to the equilibrium resurfacing hypothesis, this happens because old craters are erased by ongoing volcanic eruptions. In the alternative global catastrophe hypothesis, all of Venus was resurfaced at essentially the same time. Venus's clouds are confined to altitudes between 48 and 68 km above the planet's surface. A haze layer extends down to an elevation of 30 km, beneath which the atmosphere is clear. The surface pressure on Venus is 90 arm, and the surface j2f VICW QLIC.STI ONS temperature is 460C. Both temperature and pressure decrease as altitude increases. 1. As seen from the Earth, the brightness of Venus The circulation of the Venusian atmosphere is dominated changes as it moves along its orbit. Describe the main by two huge convection currents in the cloud layers, one in factors that determine Venus's variations in brightness as the northern hemisphere and one in the southern hemisphere. seen from the Earth. The upper cloud layers of the Venusian atmosphere move 2. If Venus did not have an atmosphere, how would its rapidly around the planet in a retrograde direction, with a appearance as seen from Earth be different? period of only about four Earth days. 3. Why was it so difficult to determine the rate and direction The Greenhouse Effect: Venus's high temperature is caused of Venus's rotation? How were these finally determined? by the greenhouse effect, as the dense carbon dioxide 4. What roles does the greenhouse effect play in the atmosphere traps and retains energy from sunlight. atmospheres of Venus and the Earth?   20. Before about 350 B.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 Q~ volcanic activity suddenly stopped? How might these ` `i~hanges affect the overall Venusian environment? 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? 22. Venus's sidereal rotation period is 243.01 days and its 11. Why is there so much carbon dioxide in Venus's orbital period is 224.70 days. Use these data to prove atmosphere while very little of this gas ts present tn the that a solar day on Venus lasts 116.8 days. (Hint: Earth's atmosphere? Develop a formula relating Venus's solar day to its sidereal rotation period and orbital period similar to the first formula in Box 4-1.) w~  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?  - hDVI~NC~D (ZIIFSTIONS I- Problem-solving tips and tools    5. Why was it difficult to determine Venus's surface temperature from Earth? How was this finally determined? 6. The Mariner 2 spacecraft did not enter Venus's 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? 8. What is the evidence for active volcanoes on Venus? 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. '~"" -  You should recall that Wien's law ( v--t) relates the temperature of a blackbody to ~maX, its wavelength of maximum emission. Rax .i-4 describes some of the physics of light scattering. Secrio,i 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 circumferencc of ~ circle of radius r is 2ttr. f.  144 days to go from greatest eastern elongation to greatest western elongation. With the aid of a diagram like Figure 11-1, explain why. 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 ! ~ ~ -: I ()-~;,) 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. 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. 32. Water has a density of 1000 kg/m3, so a column of water n meters tall and 1 meter square at its base has a mass of n x 1000 kg. On either the Earth or Venus, which have nearly the same surface gravity, a mass of 1 kg weighs about 9.8 newtons (2.2 Ib). Calculate how deep you would have to descend into the Earth's oceans for the pressure to equal the atmospheric pressure on Venus's surface, 90 atm or 9 X 106 newtons per square meter. 33. Marine organisms produce sulfur-bearing compounds, some of which escape from the oceans into the Earth's atmosphere. (These compounds are largely responsible for the characteristic smell of the sea.) Even more sulfurous gases are injected into our atmosphere by the burning of sulfur-rich fossil fuels, such as coal, in electric power plants. Both of these processes add more sulfur compounds to the atmosphere than do volcanic eruptions. On lifeles Venus, by contrast, volcanoes are the only source for sulfurous atmospheric gases. Why, then, are sulfur compounds so much rarer in our atmosphere than in the Venusian atmosphere? 34. The Hawaiian islands lie along a nearly straight line (see the accompanying figure). A hot spot under the Pacific plate has remained essentially stationary for 70 million years while the plate has moved to the northwest some 6000 km. The upwelling magma has thus produced a long chain of hot-spot volcanoes. (The oldest, at the northwest end of the chain, have eroded so much that they no longer rise above the ocean surface.) Hot-spot volcanoes are also found on Venus, but they do not form long chains. Explain what this tells us about tectonic activity on Venus. D15Ct1S510N ?C.STI ON_S 35. Describe the apparent motion of the Sun during a "day" on Venus relative to (a) the horizon and (b) the background stars. (Assume that you can see through the cloud cover.) 36. If you were designing a space vehicle to land on Venus, what special features would you think necessary? In what ways would this mission and landing craft differ from a spacecraft designed for a similar mission to Mercury? ERqC', WEB/CD-ROM QUfS`f10NS 37. A number of European astronomers traveled to Asia and the Pacific islands to observe the transits of Venus in 1761 and 1769. Search the World Wide Web for information about these expeditions. Why were these events of such interest to astronomers? How definitive were the results of these observations? 38. Search the World Wide Web for the latest information about proposed future missions to Venus. Which of these missions have been given funding so that they can proceed? What scientific experiments will they carry? Which scientific issues are these instruments intended to resolve? 39. Surface Temperature of Venus. Access the Active Integrated Media Module "Wien's Law" in Chapter 5 of the Universe web site or CD-ROM. (a) Using the Wien's Law calculator, determine Venus's approximate temperature if it emits blackbody radiation with a peak wavelength of 3866 run. (b) By trial and error, find the wavelength of maximum emission for a surface temperature of 733 K (for present-day Venus) and a surface temperature of 833 K (as it might be in the event of a global catastrophe that released more greenhouse gases into Venus's atmosphere). In what part of the electromagnetic spectrum do these wavelengths lie?  O}3.5^'RV1NQ PRO_lC('TS Observing tips and tools Venus is visible in the morning sky when it ~ is at or near greatest western elongation and in the evening sky when at or near greatest eastern elongation. These are also the times when Venus can be seen at the highest altitude above the horizon during the hours of darkness. Consult such magazines as Sky & Telescope and Astronomy or their web sites for more detailed information about when and where to look for Venus during a given month. You can also use the Starry Night program on the CD-ROM that accompanies this textbook.       20. Before about 350 B.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 .changes affect the overall Venusian environment? 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? 5. Why was it difficult to determine Venus's surface temperature from Earth? How was this finally determined? 6. The Mariner 2 spacecraft did not enter Venus's 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? 8. What is the evidence for active volcanoes on Venus? 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? 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? 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~elp 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. ~DV~NC~D QUEST10N5 Problem-solving-tips-and tools You should recall that Wien's law ( ~ ) relates the temperature of a blackbody to ~,maX, its wavelength of maximum emission. liox 5--3 describes some of the physics of light scattering. . 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.   144 days to go from greatest eastern elongation to greatest western elongation. With the aid of a diagram like Figure 11-1, explain why. 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 4-1.) 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 Fisvrr ) ()- 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. 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 I 257   , The heavily cratered southern highlands are older and 2. Why is it best to view Mars near opposition? Why are 1 l d d 3. Why did Earth observers report that they had seen about 5 km higher in elevation than the smooth northern some oppositions better than others? lowlands. The origin of this crustal dichotomy is not com ete un erstoo . ~ ; ~ p y straight-line features (` canals ) on Mars?  ~Vlartian volcanoes and the Valles Marineris rift valley were formed by upwelling plumes of magma in the mantle. For reasons that are not understood, the chemical composition of ancient Martian lava is different from that of more recent lava.   No liquid water can exist on the Martian surface today. But Mars's polar caps contain frozen water, a layer of permafrost may exist below the Martian regolith, and there may be liquid water beneath the surface. 7. Giant impact basins like Hellas Planitia are found on The Martian polar caps expand in winter as a thin Mars but not on Earth. Why not? layer of frozen carbon dioxide (dry ice) is deposited from g, ~7hat geologic features (or lack thereof) on Mars have the atmosphere. convinced scientists that plate tectonics did not significantly shape the Martian surface? i Mars's primordial atmosphere was thicker and warmer than the present-day atmosphere. It probably contained enough carbon dioxide and water vapor to support a 4 greenhouse effect that permitted liquid water to exist on .,' the planet's surface. `t i If enough liquid water once existed on the Martian ~' surface, the northern lowlands could have been flooded to form an ocean. 1 ; The Martian Atmosphere: The Martian atmosphere is composed mostly of carbon dioxide. The atmospheric ~ pressure on the surface is less than 1 % that of the Earth and shows seasonal variations as carbon dioxide freezes onto and evaporates from the poles. Great dust storms sometimes blanket Mars. Fine-grained dust in its atmosphere gives the Martian sky a pinkish-orange tint. Seasonal winds blow dust across the face of Mars, covering and uncovering the underlying surface material and causing seasonal color changes. Afternoon dust devils help to transport dust from place to place. 16. What is the current state of knowledge concerning life on Mars? Is it known definitely whether life currently ~ Life on Mars: Chemical reactions in the Martian regolith, exists on Mars or whether it once existed there? ; together with ultraviolet radiation from the Sun, appear to i sterilize the Martian surface. Hence, the Viking Lander 17. What discoveries did Mars Path inder make about ~;pacecraft found no evidence for present-day life on Mars. Martian rocks? Why weren't the Viking Landers able to make these same discoveries 20 years earlier? The Moons of Mars: Mars has two small, football-shaped ~ satellites that move in orbits close to the surface of the 18 What is a dust devil? Why would you feel much less ` planet. They may be captured asteroids or may have breeze from a Martian dust devil than from a dust devil ~formed in orbit around Mars out of solar system debris. on Earth? t 19. Discuss the statement "On Mars, the winters are so cold that the atmosphere freezes." How accurate is this statement?  R~V1EW (?~1~_STIONS ~1ars has no planetwide magnetic field at present but may have had one in the ancient past. Water on Mars: Flash-flood features and dried riverbeds on the Martian surface indicate that water once flowed on Mars. 4. What is the Martian crustal dichotomy? What theories have been proposed to explain its origin? What are the arguments in favor of or against each theory? 5. Compare the volcanoes of Venus, the Earth, and Mars. Cite evidence that hot-spot volcanism is or was active on all three worlds. 6. Olympus Mons and Valles Marineris are respectively the largest volcano and the largest valley known in the solar system. How, then, could the three Mariner spacecraft that flew past Mars in the 1960s have missed seeing these features? 9. If you were an astronaut on Mars, would it be useful to carry a compass for navigation? Why or why not? 10. Why is it impossible for liquid water to exist on Mars today? What was different in the past that allowed liquid water to exist then? 11. Substantial quantities of liquid water have not been present on Mars for a very long time. Explain how we know this. 12. Why is it reasonable to assume that the primordial atmospheres of the Earth and Mars were roughly the same? 13. Why is Mars red? 14. Carbon dioxide accounts for about 95% of the present-day atmospheres of both Mars and Venus. Why, then, is there a strong greenhouse effect on Venus but only a weak greenhouse effect on Mars? 15. Explain why the Martian sky sometimes appears pink.  1. Explain why Mars has the longest synodic period of all 20. In which Martian hemisphere are the summers warmer? the planets, although its sidereal period is only 687 days. Why is there a difference between the hemispheres? As part s Red Planet Mars 279 280 I CHAPTER 12  of your explanation, make a drawing like Figure 12-2 that 29. An amateur astronomer wants to purchase a telescope shows Mars in its orbit around the Sun. that will enable her to resolve Hellas Planitia (diameter 21. A full moon on Earth is bright enough to cast shadows. 2300 km) during the 2005 opposition of Mars. What must As seen from the Martian surface, would you expect a full be the minimum diameter of the telescope objective? Phobos or full Deimos to cast shadows? Why or why not? Assume that she observes Mars using a yellow filter, so~~ ~ that the wavelength used is SSO nm. Ignore the blurring of the image by the Earth's atmosphere. ~DV1~NC~D QU~STIONS -' Problem-solving tips and tools 30. The Earth's northern hemisphere is 39% land and 61% water, while its southern hemisphere is only 19% - land and 81 % water. Thus, the southern hemisphere could also be called the "water hemisphere." The Moon also has two distinct hemispheres, the near side (which has a number of maria) and the far side (which has almost none). How are these hemispheric differences on Earth and on the Moon similar to the Martian crnst~aY dichotomy? How are they different? You will need to remember the small angle formula ( ` ) and the formula for the angular resolution of a telescope (5ectian G-~). For the relationship between a planet's sidereal and synodic periods, see . You may also need to review the form of Kepler's third law that explicitly includes mass (see Section 4-7 and 13ox 4-4). The volume of a sphere of radius r is 4/3~tr3. 31. For a group of properly attired astronauts equipped with oxygen tanks, a climb to the summit of Olympus Mons would actually be a relatively easy (albeit long) hike rather than a true mountain climb. Give two reasons why. 22. Using Figure 12-2, explain why oppositions of Mars 32, Mars Global Surveyor (MGS) is in a nearly circular are most favorable when they occur in August. orbit with an orbital period of 117 minutes. (a) Using the 23. When Mars is at opposition, what is its phase as seen data in Table 12-1, find the radius of the orbit. (b) What is from the Earth? What is the Earth's phase as seen from the average altitude of MGS above the Martian surface? Mars? Explain with a diagram. (c) The orbit of MGS passes over the north and south 24. Accordin to T~h!~~ '' poles of Mars. Explain how this makes it possible for the g , the synodic period of Mars- spacecraft to observe the entire surface of the planet. that is, the time between successive oppositions-is 780 days. But the dates given in Table 12-2 show that the 33. Suppose Mars Global Surveyor had discovered intervals between successive oppositions vary. Explain this magnetized regions within the Hellas Planitia impact basin. apparent contradiction. How would this discovery have affected our understanding 2S. From one opposition of Mars to the next, how many of Martian history? orbits around the Sun does the Earth complete? How many 34. On Mars, the difference in elevation between the orbits does Mars complete? (Your answers will not be highest point (the summit of Olympus Mons) and the whole numbers.) lowest point (the bottom of the Hellas Panitia basin) is 26. Explain why the image that opens this chapter shows 30 km. On Earth, the corresponding elevation difference much finer detail than Figure 12-1. (from the peak of Mount Everest to the bottom of the deepest ocean) is only 20 km. Discuss why the maximum 27. For a planet to appear to the naked eye as a disk rather elevation difference is so much greater on Mars. than as a point of light, its angular size would have to be 35. (a) The Grand Canyon in Arizona was formed over 1 arcmin, or 60 arcsec. (This is the same as the angular 1S to 20 million years by the flowing waters of the separation between lines in the bottom row of an Colorado River, as well as by rain and wind. Contrast this optometrist's eye chart.) (a) How close would you have to formation scenario to that of Valles Marineris. (b) Valles be to Mars in order to see it as a disk with the naked eye? Marineris is sometimes called "the Grand Can on of Mars." Does Mars ever get this close to Earth? (b) Would the Is this an appropriate description? Why or why not? Earth ever be visible as a disk to an astronaut on Mars? Would she be able to see the Earth and the Moon separately, 36. The classic 1950 science-fiction movie Rocketship X-M' or would they always appear as a single object? Explain. shows astronauts on the Martian surface with oxygen 28. (a) Suppose you have a telescope with an angular masks for breathing but wearing ordinary clothing. Would ' resolution of 1 arcsec. What is the size (in kilometers) of this be a sensible choice of apparel for a walk on Mars? the smallest feature you could see on the Martian surface Why or why not? during the opposition of 2003? (See Table 12-2.) (b) Suppose you had access to the Hubble Space Telescope, which has an angular resolution of 0.1 arcsec. What is the size (in kilometers) of the smallest feature you could see on Mars with the HST during the 2003 opposition? 37. Compare the evolution of the atmospheres of Venus, the Earth, and Mars. Explain how the atmospheres of these three planets turned out so differently from one another, even though they may all have started with roughly the same chemical composition.    38. Suppose Mars were moved into the Earth's orbit and tectonic activity died out on Mars because the planet's the Earth were moved into Mars's orbit. What effect would interior cooled too rapidly, so that convection in the this have on the atmospheres of the two planets? mantle ceased. What sorts of observations of Mars would \ 39. Suppose that the only information you had about Mars You make in an attempt to prove or disprove this was the image of the surface in Figure 12-18. Describe at explanation? least two ways that you could tell from this image that 48. The total cost of the Mars Global Surveyor mission Mars has an atmosphere. was about $154 million. (To put this number into perspective, in 2000 the U.S. Mint spent about $40 million to advertise its new $1 coin. Several recent Hollywood movies have had larger budgets than Mars Global Surveyor.) Does this expenditure seem reasonable to you? Why or why not? 40. Although the Viking Lander 1 and Viking Lander 2 landing sites are 1500 km apart and have different geologic histories, the chemical compositions of the dust at both sites are nearly identical. (a) What does this suggest about the ability of the Martian winds to transport dust particles? (b) Would you expect that larger particles such as pebbles would also have identical chemical compositions at the two Viking Lander sites? Why or why not?  49. Compare the scientific opportunities for long-term exploration offered by the Moon and Mars. What difficulties would be inherent in establishing a permanent base or colony on each of these two worlds? ` 41. The landing site for Viking Lander 1 is in an ancient t flood plain, while the site for Viking Lander 2 is near the 50. Imagine you are an astronaut living at a base on Mars. southernmost extent of the north polar ice cap. The Describe your day's activities, what you see, the weather, j Viking mission planners selected these sites because they the spacesuit you are wearing, and so on. suspected water might lie beneath the surface there. Why { were these good places to search for potential Martian ti`'~ER~c~,' microorganisms? ~ W E lg ~ C. D ~ IZ O M (? UE ST 1 O N S 42. Is it reasonable to suppose that the polar regions of Mars might harbor life forms, even though the Martian regolith is sterile at the Viking Lander sites?  43. The orbit of Phobos has a semimajor axis of 9378 km. Use this information and the orbital period given in the ; text to calculate the mass of Mars. How does your answer ` compare with the mass of Mars given in Table 12-1? 44. Why do you suppose that Phobos and Deimos are not round like our Moon?  45. Calculate the angular sizes of Phobos and Deimos as they pass overhead, as seen by an observer standing on the Martian equator. How do these sizes compare with that of "~ the Moon seen from the Earth's surface? Would Phobos and Deimos appear as impressive in the Martian sky as the Moon does in our sky? 51. Search the World Wide Web for information about ongoing or future Mars missions, including the European Space Agency's Mars Express and the Japanese orbiter Nozomi. What is the current status of these missions? What are their scientific goals? 52. In 1999 two NASA spacecraft-Mars Climate Orbiter and Mars Polar Lander-failed to reach their destinations. Search the World Wide Web for information about these missions. What were their scientific goals? How and why did the missions fail? Which future missions, if any, are intended to pick up where these missions left off? 53. Search the World Wide Web for information about possible manned missions to Mars. How long might such a mission take? How expensive would such a project be? What would be the ~advantages of a manned mission compared to an unmanned one?   46. You are to put a spacecraft into a synchronous circular orbit around the Martian equator, so that its orbital period is equal to the planet's rotation period. Such a spacecraft would always be over the same part of the Martian surface. (a) Find the radius of the orbit and the altitude of a~ the spacecraft above the Martian surface. (b) Suppose Mars had a third moon that was in a synchronous orbit. Would tidal forces make this moon tend to move toward Mars, away from Mars, or neither? Explain. ~~P~I~N.~N 54. Conjunctions of Mars. Access and view the a ~ animation "The Orbits of Earth and Mars" in Chapter 12 of the Universe web site or CD-ROM. (a) Thc animation highlights three dates when Mars is in opposition, so that the Earth lies directly between Mars and the Sun. By using the "Stop" and "Play" buttons in the animation, find two times during the animation when Mars is in conjunction, so that the Sun lies directly between Mars and the Earth (see Fy~, ). For each conjunction, make a drawing showing the positions of the DISCUSSION ~UESTIONS Sun, the Earth, and Mars, and record the month and year when the conjunction occurs. (Hint: See Figure 12-2.) '. One explanation that has been proposed for the (b) When Mars is in conjunction, at approximately what gin of Valles Marineris is that it is a place where plate time of day does it rise as seen from Earth? At what time .tonics started on Mars and where two parts of the crust of day does it set? Is Mars suitably placed for telescopic :gan to move apart like tectonic plates. In this picture, observation when in conjunction? Red Planet Mars 281 282 1 CHAPTER 12   O (3SE RV 1 N Q PROJCCTS zoom in until the field of view is roughly 30 arcseconds (30"). (a) In the Control Panel, set the time step to 30 minutes and Observing tips and tools click on the "Forward" button (a triangle that points to the --- ---- right). Describe what you see. (b) Click on the "stop" button 0"I -k> ~ Mars is most easily seen around an (a black square) in the Control Panel. Change the time step 3~L opposition (see Table 12-2). At other times to 30 days and click again on the Forward button. Describe Mars may be visible only in the early what you see. Using a diagram like , explain the morning hours before sunrise or in the early evening changes in the apparent size of the planet. ' just after sunset. Consult such magazines as Sky & ~PRRY Ntcy 57. Use the Starry Night program to observe the Telescope and Astronomy or their web sites for more y ~ moons of Mars. First turn off daylight (select detailed information about when and where to look Daylight in the Sky menu) and show the entire for Mars during a given month. You can also use the celestial sphere (select Atlas in the Go menu). Center on Starry Night program on the CD-ROM that Mars by using the Find... command in the Edit menu. Using accompanies this textbook. the controls at the right-hand end of the Control Panel, zoom in until the field of view is roughly 1 arcminute (1'). In the Control Panel set the time step to 30 minutes and click 55. If Mars is suitably placed for observation, arrange to on the "Forward" button (a triangle that points to the . view the planet through a telescope. Draw a picture of right). You will see the two moons, Phobos and Deimos, what you see. What magnifying power seems to give you orbiting Mars. (a) Click on the "stop" button (a black the best image? Can you distinguish any surface features? square) in the Control Panel. Record the date and time in Can you see a polar cap or dark markings? If not, can you the display, and note the position of Phobos (the inner offer an explanation for Mars's bland appearance? moon). Step through time using the single-step button (the 'lc y 56. Use the Starry Night program to observe the rightmost time control button) until Phobos returns to IV A appearance of Mars. First turn off daylight (select approximately the same position, then record the date and Daylight in the Sky menu) and show the entire time in the display. From your observations, what is the celestial sphere (select Atlas in the Go menu). Center on orbital period of Phobos? How does your result compare Mars by using the Find... command in the Edit menu. Using with the orbital period given in yuendix ~? (b) Repeat part the controls at the right-hand end of the Control Panel, (a) for Deimos (the outer moon). 298 I CHAPTER 13 KEY WORDS belts, p. 285 brown ovals, p. 289 current sheet, p. 297 decametric radiation, p. 295 decimetric radiation, p. 296 differential rotation, p. 286 Great Red Spot, p. 285 hot spot, p. 294 synchrotron radiation, p. 296 internal rotation period, p. 297 thermal radiation, p. 295 liquid metallic hydrogen, p. 296 white ovals, p. 289 noble gas, p. 294 zonal winds, p. 290 nonthermal radiation, p. 295 zones, p. 285 oblate, oblateness, p. 295 plasma, p. 297  K~Y 1 D~hS Jupiter's Composition and Structure: Jupiter, whose mass is reason may be that the probe fell into an unusually warm equivalent to the mass of 318 Earths, is composed of region, or hot spot. 71% hydrogen, 24% helium, and 5% all other elements. Jupiter has a higher percentage of heavy elements than does the Sun. Jupiter's Magnetic Field and Magnetosphere: Jupiter has ~ strong magnetic field created by currents in the metallic hydrogen layer. Its huge magnetosphere contains a vast Jupiter probably has a rocky core with a mass of about current sheet of electrically charged particles. 8 Earth masses. This core is surrounded by a layer of Charged particles in the densest portions of Jupiter's liquid "ices" (water, ammonia, methane, and associated magnetosphere emit synchrotron radiation at compounds). On top of this is a layer of helium and liquid radio wavelengths. metallic hydrogen that makes up most of the planet's mass. Jupiter's outermost layer is composed primarily of ordinary The Jovian magnetosphere encloses a low-density plasma hydrogen and helium. All of Jupiter's visible features are of charged particles whose temperature is higher than at near the top of this outermost layer. the center of the Sun. The magnetosphere exists in a delicate balance between pressures from the plasma and Jupiter has the shortest rotation period of any planet. from the solar wind. When this balance is disturbed, the The rotation is so rapid that the planet is flattened into a size of the magnetosphere fluctuates drastically. noticeably oblate shape. The rotation of Jupiter's interior is revealed by variations in Jupiter's radio emission. Jupiter emits about twice as much heat as it receives from 1ZFViEW QUESYI ONS the Sun. Presumably the planet's interior is still cooling. l. Jupiter was at opposition on November 28, 2000. On that date Jupiter appeared to be in the constellation Taurus. Approximately when will Jupiter next be at opposition in this same region of the celestial sphere? Explain your answer. Jupiter's Atmosphere: The visible "surface" of Jupiter is actually the tops of its clouds. The planet's rapid rotation twists the clouds into dark belts (where gas is descending and warming) and light zones (where gas is rising and cooling) that run parallel to the equator. Strong zonal winds run along the belts and zones. The outer layers of the Jovian atmosphere show differential rotation: The equatorial regions rotate slightly faster than the polar regions. The polar rotation rate is nearly the same as the internal rotation rate. 2. Which planet, Mars or Jupiter, passes closer to the Earth? On which planet is it easier to see details with an Earth-based telescope? Explain your answers. 3. In what ways are the motions of Jupiter's atmosphere like the motion of water stirred in a pot (see Figure 13-3b)? In what ways are they different? 4. Compare Jupiter's chemical composition with that of ~ the Sun. What does this tell us about Jupiter's formation ~ The colored ovals visible in the Jovian atmosphere represent gigantic storms. Some, such as the Great Red Spot, are quite stable and persist for many years. 5. Astronomers can detect the presence of hydrogen in There are presumed to be three cloud layers in Jupiter's stars by looking for the characteristic absorption lines cyf atmosphere. The reasons for the distinctive colors of these hydrogen in the star's visible spectrum (' ). Tlley different layers are not yet known. The Galileo Probe, so can also detect hydrogen in glowing gas clouds by look.~ng far the only spacecraft to directly explore Jupiter's for hydrogen's characteristic emission lines ( _ ure 5-163), atmosphere, detected only two of these cloud layers. The Explain why neither of these techniques helped  ~~,~ERACTI~ f igure 13-17 f ~ '^ Jupiter's Magnetosphere Jupiter's ~~'~isE y~ magnetosphere is enveloped by a shock wave, where the supersonic solar wind is abruptly slowed to subsonic speeds. Most of the particles of the solar wind are deflected around Jupiter in the turbulent region between the shock wave and magnetopause, shown here in solid purple. Particles trapped inside Jupiter's magnetosphere are spread out into a vast current sheet by the planet's rapid rotation. Jupiter's axis of rotation is inclined to its magnetic axis by about 11 . .   ittracts by a factor of 2, rapidly and frequently. By contrast, are torn off the atoms of the gas. As a result, a plasma is a mix- ~amatic changes in the size of the Earth's magnetosphere are ture of positively charged ions and negatively charged electrons. :;xtremely rare despite occasional "gusts" in the solar wind. The plasma that envelops Jupiter consists primarily of electrons ~0~?"K 1,~~ The inner regions of our magnetosphere are and protons, with some ions of helium, sulfur, and oaygen. dominated by two huge Van Allen belts (recall These charged particles are caught up in Jupiter's rapidly rotat- f`~ Figure 8-20) that are filled with charged particles. ing magnetic field and are accelerated to high speeds. In a similar manner, Jupiter entraps immense quantities of The fast-moving particles in Jupiter's plasma exert a sub- i charged particles in belts of its own. An astronaut venturing stantial pressure that holds off the solar wind. The Voyager 1 into these belts would quickly receive a lethal dose of radia- data suggest that the pressure balance between the solar wind tion. In addition, Jupiter's rapid rotation spews the charged and the hot plasma inside the Jovian magnetosphere is pre- ~lt particles out into a huge current sheet, which lies close to the carious. A gust in the solar wind can blow away some of the plane of Jupiter's magnetic equator (see Figure 13-17). plasma, at which point the magnetosphere deflates rapidly to Jupiter's magnetic axis is inclined 11 from the planet's axis of as little as one-half its original size. Charged-particle detectors rotation, and the orientation of Jupiter's magnetic field is the carried by the Voyager spacecraft recorded several bursts of reverse of the Earth's magnetic field-a compass would point hot plasma that may have been associated with such defla- toward the south on Jupiter. tions of the magnetosphere. However, additional electrons Particles trapped in the Earth's magnetic field stream onto and ions accelerated by Jupiter's rotating magnetic field soon the north and south magnetic poles, creating the aurora (see replenish the plasma, and the magnetosphere expands again. I _S~: ~ ~ :. _s-4). The same effect takes place on Jupiter. Images from the Galileo spacecraft show glowing auroral rings cen-tered on each of the Jovian magnetic poles. As on Earth, the emission comes from a region high in Jupiter's atmosphere, about 300 to 600 km (18~ to 370 mi) above the cloudtops. Figure 13-18 shows radio emissions from charged parti-cles in the densest regions of Jupiter's magnetosphere. These emissions vary slightly with a period of 9 hours, 55 minutes, and 30 seconds, as Jupiter's rotation changes the angle at ~ which we view its magnetic field. Because the magnetic field is anchored deep within the planet, this variation reveals ~ upiter's internal rotation period. This period, indicative of ~e rotation of the bulk of the planet's mass, is slightly slower th: n the atmospheric rotation at the equator (period 9 hours, f ~gure 13-18 0 I V U X G Sa minutes, 28 seconds) but about the same as the atmo- A Radio View of Jupiter The Very Large Array (see Fidure 6-25) spheric rotation at the poles (period 9 hours, 55 minutes, was used to produce this false-color map of synchrotron emission 41 aeconds). The faster motion of the atmosphere at the equa- from Jupiter at a wavelength of 21 cm. The emission comes from a tor telative to the interior is driven by Jupiter's internal heat. region about five Jupiter diameters wide and elongated parallel to 'The Voyager spacecraft discovered that the inner regions the planet's magnetic equator. Jupiter is at the center of the image, of Jupiter's magnetosphere contain a hot, gaslike mixture of indicated by the dashed circle. The strongest emission, shown in charged particles called a plasma. A plasma is formed when a white, comes from electrons trapped in the densest regions of gas ~ heated to such extremely high temperatures that electrons Jupiter's current sheet. (NRAO)  Jupiter: Lord of the Planets 297     Earth-based astronomers to detect hydrogen in Jupiter's atmosphere. 6. Is the chemical composition of Jupiter as a whole the same as that of its atmosphere? Explain any differences in terms of chemical differentiation. 7. What are the belts and zones in Jupiter's atmosphere? Is the Great Red Spot more like a belt or a zone? Explain your answer. 8. Give one possible explanation why weather systems on Jupiter are longer-lived than weather systems on Earth. 9. What are white ovals and brown ovals? What can we infer about them from infrared observations? ADVANCED QUESTIONS I Problem-solving tips and tools describes how to use a very useful form of Kepler's third law. Newton's universal law of gravitation, discussed in ticwtion -i -, is the basic equation from which you can      !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ calculate a planet's surface gravity. Box 7--, discusses escape speed, and Sections 5-3 and 5-4 discuss the properties of thermal radiation (including the Stefan-Boltzmann law, which relates the temperature of a body to the amount of thermal radiation that it emits).  10. Compare and contrast the source of energy for motions in our atmosphere with the energy source for motions in Jupiter's atmosphere. 11. What is thought to be the source of Jupiter's excess internal heat? 12. Did the Comet Shoemaker-Levy 9 impacts on Jupiter provide unambiguous information about the composition of Jupiter's atmosphere? Why or why not? 13. Comets like Comet Shoemaker-Levy 9 are much more likely to hit Jupiter than they are to hit Earth. Give two justifications for this statement, one in terms of Jupiter's diameter, the other in terms of Jupiter's mass. i 14. Which data from the Galileo Probe were in agreement ! with astronomers' predictions? Which data were surprising? 15. Fewer than one in every 105 atoms in Jupiter's atmosphere is an argon atom, and fewer than one in 108 is ; an atom of krypton or xenon. If these atoms are so rare, why are scientists concerned about them? How do the abundances of these elements in Jupiter's atmosphere compare to the abundances in the Sun? What hypotheses have been offered to explain these observations? 23. The angular diameter of Jupiter at opposition varies little from one opposition to the next (see Table 13-2). By contrast, the angular diameter of Mars at opposition is quite variable (see 1 -'-1). Explain why is there a difference between these two planets. 24. Jupiter's equatorial diameter and the rotation period at Jupiter's equator are both given in Table 13-1. Use these data to calculate the speed at which an object at the cloudtops along Jupiter's equator moves around the center of the planet. 25. Using orbital data for a Jovian satellite of your choice (see Appendix 3), calculate the mass of Jupiter. How does your answer compare with the mass quoted in Table 13-1? 26. Roughly speaking, Jupiter's composition (by mass) is three-quarters hydrogen and one-quarter helium. The mass of a single hydrogen atom is given in ; ~ ; the mass of a single helium atom is about 4 times greater. Use these numbers to calculate how many hydrogen atoms and how many helium atoms there are in Jupiter. 27. Estimate the wind velocities in the Great Red Spot, which rotates with a period of about six days.   28. If Jupiter emitted just as much energy (as infrared radiation) as it receives from the Sun, the average temperature of the planet's cloudtops would be about 107 K. Given that Jupiter actually emits twice this much energy, find what the average temperature must actually be. 18. Describe Jupiter's internal structure, and compare it 29, (a) From Figure 13-10, what is the temperature in Jupiter's atmosphere at the depth where the pressure is 19. What data and what techniques have been used to 1 atmosphere, the same as at the Earth's surface? Give determine the internal structure of Jupiter? your answer in the Kelvin, Celsius, and Fahrenheit scales. (b) At the depth where the temperature in Jupiter's : - 20. Compare and contrast Jupiter's magnetosphere with atmosphere is the same as room temperature (20C = 68F), the magnetosphere of a terrestrial planet like Earth. Why is what is the atmospheric pressure? (c) The pressure at the the size of the Jovian magnetosphere highly variable, while surface of the Earth's oceans is 1 atmosphere, and increases that of the Earth's magnetosphere is not? . by 1 atmosphere for every 10 meters that you descend below the surface. At what depth in the Earth's oceans is the pressure the same as the value you found in (b)? 21. If Jupiter does not have any observable solid surface and its atmosphere rotates differentially, how are astronomers ~ able to determine the planet's internal rotation rate?  16. Why is Jupiter oblate? What do astronomers learn from the value of Jupiter's oblateness? 17. What is liquid metallic hydrogen? What is its significance for Jupiter? with the internal structure of the Earth. 22. What is a plasma? Where are plasmas found in the vicinity of Jupiter? 30. Consider a hypothetical future spacecraft that would float, suspended from a balloon, for extended periods in Jupiter's upper atmosphere. If it is desired to have this  Jupiter: Lord of the Planets 299 300 1 CHAPTER 13    spacecraft return to the Earth after completing its mission, ~,qERACT to what speed would the spacecraft's rocket motor have to accelerate it in order to escape Jupiter's gravitational pull? Compare with the escape speed from the Earth, equal to 38. Search the World Wide Web for recent information 11.2 km/s. about the orbiting Galileo spacecraft. What new 31. The Galileo Probe had a mass of 339 kg. On Earth, its observations of Jupiter has it conducted? What discoveries weight (the gravitational force exerted on it by Earth) was has it made? 3320 newtons, or 747 lb. What was the gravitational force 39, On Jupiter, the noble gases argon, krypton, and xenon that Jupiter exerted on the Galileo Probe when it entered ~ provide important clues about Jupiter's past. On Earth, Jupiter's clouds. xenon is used in electronic strobe lamps because it emits a very white light when excited by an electric current. Argon, by contrast, is one of the gases used to fill ordinary incandescent lightbulbs. Search the World Wide Web for information about why argon is used in this way. Why do premium, long-life lightbulbs use krypton rather than argon? WeRICO-R()M `l1CST10NS  32. From the information given in Section 13-5, calculate the average density of Jupiter's rocky core. How does this compare with the average density of the Earth? With the average density of the Earth's solid inner core? (See Table 8-I and (able 8-2 for data about the Earth.) 33. In the outermost part of Jupiter's outer layer (shown in yellow in Figure 13-16), hydrogen is principally in the form of molecules (HZ). Deep within the liquid metallic hydrogen layer (shown in orange in Figure 13-16), hydrogen is in the form of single atoms. Recent laboratory experiments suggest that there is a gradual transition between these two states, and that the transition layer overlaps the boundary between the ordinary hydrogen and liquid metallic hydrogen layers. Use this information to redraw Figure 13-16 and to label the following regions in Jupiter's interior: (i) ordinary (nonmetallic) hydrogen molecules; (ii) nonmetallic hydrogen with a mixture of atoms and molecules; (iii) liquid metallic hydrogen with a mixture of atoms and molecules; (iv) liquid metallic hydrogen atoms. ,a9EO i~~, 40. Moving Weather Systems on Jupiter. Access ~ and view the video "The Great Red Spot" in Chapter 13 of the Universe web site or CD-ROM. (a) Near the bottom of the video window you will see a white oval moving from left to right. By stepping through the video one frame at a time, estimate how long it takes this oval to move a distance equal to its horizontal dimension. (Hint: You can keep track of time by noticing how many frames it takes a feature in the Great Red Spot, at the center of the video window, to move in a complete circle around the center of the spot. The actual time to move in such a circle is about six days.) (b) The horizontal dimension of the white oval is about 4000 km. At what approximate speed (in kilometers per hour) does the white oval move? DiSCUSSION QtIESTIONS ORSCRVINC, PROJCCTS 34. Describe some of the semipermanent features in Jupiter's atmosphere. Compare and contrast these long-lived features with some of the transient phenomena seen in Jupiter's clouds. 35. Suppose you were asked to design a mission to Jupiter involving an unmanned airplanelike vehicle that would spend many days (months?) flying through the Jovian clouds. What observations, measurements, and analyses should this aircraft be prepared to make? What dangers might the aircraft encounter, and what design problems would you have to overcome? 36. What sort of experiment or space mission would you design in order to establish definitively whether Jupiter has a rocky core? I Observing tips and tools ~O\,SNK 1~ ~ Like Mars, Jupiter is most easily seen around opposition. The dates of Jupiter's oppositions are listed in Table 13-2. There ? is an opposition of Jupiter every 13 months, and the planet is visible in the night sky for several months before or after opposition. At other times, Jupiter may be visible in either the predawn morning sky or the early evening sky. Consult such magazines as Sky & Telescope and Astronomy or their web sites for more detailed information about when and where to look for Jupiter during a given month. You can also use the Starry Night program on the CD-ROM that . accompanies this textbook. A relatively  37. The classic science-fiction films 2001: A Space Odyssey I telescope with an objective diameter of 15 to 20 cm and 2010: The Year We Ma ke C b ontact oth involve (6 to 8 inches), used with a medium-power eyepiece What kinds of I to give a magnification of 25X or so, shoul~enable manned spacecraft in orbit around Jupiter. observations could humans make on such a mission that you to see some of the dark belts. small - cannot be made by robotic spacecraft? What would be the I risks associated with such a mission? Do you think that a 41. If Jupiter is visible in the night sky, make arrangements manned Jupiter mission would be as worthwhile as a to view the planet through a telescope. What magnifying manned mission to Mars? Explain your answers. power seems to give you the best view? Draw a picture of what you see. Can you see any belts and zones? How many? Can you see the Great Red Spot? ~0,1"K 42. Make arrangements to view Jupiter's Great "' Red Spot through a telescope. Consult the Sky & Telescope web site, which lists the times when the center of the Great Red Spot passes across Jupiter's central meridian. The Great Red Spot is well placed for viewing for 50 minutes before and after this ' happens. You will need a refractor with an objective of at least 15 cm (6 in.) diameter or a reflector with an objective of at least 20 cm (8 in.) diameter. Using a pale blue or ; --#*,green filter can increase the color contrast and make the ! spot more visible. For other useful hints, see the article "Tracking Jupiter's Great Red Spot" by Alan MacRobert (Sky & Telescope, September 1997).   ,~pRRY NrCy 43. Use the Starry Night program to observe the appearance of Jupiter. First turn off daylight (select Daylight in the Sky menu) and show the entire celestial sphere (select Atlas in the Go menu). Center on Jupiter by using the Find... command in the Edit menu. Using the controls at the right-hand end of the Control Panel, zoom in until Jupiter nearly fills the field of view. In the Control Panel, set the time step to 10 minutes. Use the single-step buttons to observe Jupiter's rotation. Step through enough time to determine the rotation period of the planet. (Hint: You may want to track the motion of an easily recognizable feature, such as the Great Red Spot.) How does your answer compare with the rotation period given in Table 13-1?     e   Jupiter: Lord of the Planets 301   13. How was the Galileo spacecraft used to determine the However, some unknown processes have erased the smallest craters and blanketed the surface with a dark, internal structure of Io and the other Galilean satellites? ` dusty substance. 14. What surface features on Europa provide evidence for Other Satellites and Jupiter's Rings: Jupiter has a total of 16 satellites, including the four Galilean satellites.  Inside Io's orbit are four small satellites and a faint 16. What aspects of Europa lead scientists to speculate that system of dust rings. Like the Galilean satellites, these orbit life may exist there? in the plane of Jupiter's equator. 17. Why is ice an important constituent of Ganymede and Callisto, but not of the Earth's Moon? 4. In what ways did the formation of the Galilean satellites mimic the formation of the planets? In what ways were the two formation processes different? 23. Compare and contrast the surface features of the four r ~ direction. Furthermore, the planes of their orbits all lie and the evolution of these four satellites. within 0.5 of Jupiter's equatorial plane. Explain why these 24, The larger the orbit of a Galilean satellite, the less observations are consistent with the idea that the Galilean geologic activity that satellite has. Explain why. ; - satellites formed from a "Jovian nebula." 25. Explain how the 1:2:4 ratio of the orbital periods of Io, Europa, and Ganymede is related to the geologic activity on these satellites. Magnetic field data seems to suggest that Callisto has a shallow subsurface ocean. Nine small satellites move in much larger orbits that are noticeably inclined to the plane of Jupiter's equator. Five of these orbit in the direction opposite to Jupiter's rotation. REVlEW QUESTIONS 1. Why can't the Galilean satellites be seen with the naked eye? 2. In what ways does the system of Galilean satellites resemble our solar system? In what ways is it different? 3. No spacecraft from Earth has ever landed on any of the Galilean satellites. How, then, can we know anything about the chemical compositions of these satellites? 5. All of the Galilean satellites orbit Jupiter in the same Galilean satellites. Discuss their relative geological activity 6. What is the source of energy that powers Io's volcanoes? How is it related to the orbits of Io and the other Galilean satellites? geologic activity? 7. Io has no impact craters on its surface, while our Moon is covered with craters. What is the explanation for "~: this difference? 9. Despite all the gases released from its interior by volcanic activity, Io does not possess a thick atmosphere. Explain why not. 10. Hov, do lavas on Io differ from typical lavas found on Earth? What does this difference tell us about Io's interior? 11. What is the Io torus? What is its source? 12. What is the origin of the electric current that flows through  Newton's form of Kepler's third law ( ~-4) relates the masses of two objects in orbit around each other to the period and size of the orbit. The small-angle formula is discussed in [~ox 1-1. The best seeing conditions on the Earth give a limiting angular resolution of ~a arcsec. Because the orbits of the Galilean satellites are almost perfect circles, you can easily calculate the orbital speeds of these satellites from the data listed in Table 14-1 or Table 14-2. Data about Jupiter itself are given in i u-~_ i ~- l. The Galilean Satellites of Jupiter 319 15. What is the evidence for an ocean of liquid water beneath Europa's icy surface? What is the evidence that substances other than water are dissolved in this ocean? 18. How do scientists know that the dark terrain on Ganymede is younger than the bright terrain? 19. In what ways is Ganymede like our own Moon? In what ways is it different? What are the reasons for the differences? 20. Why were scientists surprised to learn that Ganymede has a magnetic field? What does this field tell us about Ganymede's history? 21. Why are numerous impact craters found on Ganymede and Callisto but not on Io or Europa? 22. Describe the surprising aspects of Callisto's surface and interior that were revealed by the Galileo spacecraft. Why did these come as a surprise?  26. How would you account for the existence of the satellites of Jupiter other than the Galilean ones? How would you account for the existence of the ring? \ 8. Long before the Voyager flybys, Earth-based astronomers reported that Io appeared brighter than usual for the few ADVANCED (QUESTIONS hours after it emerged from Jupiter's shadow. From what we know about the material ejected from Io's volcanoes, I Problem-solving tips and tools   suggest an explanation for this brief brightening of Io. 320 I CHAPTER 14  27. Using the orbital data in Table 14-1, demonstrate that 38. Suppose you were planning four missions that would land a spacecraft on each of the Galilean satellites. What kinds of questions would you want these missions to answer? What kinds of data would you want your spacecraft to send back? In view of the different " environments on the four satellites, how would the designs of the four spacecraft differ? Be specific about the possible hazards and problems each spacecraft might encounter in 29. Astronomers who observed Io with the Hubble Space landing on the four satellites. If only one of these missions Telescope in 1992 claimed that they could see features as could be funded, which one would you choose? Why? small as 150 km across. When the observations were made, Io was 4.45 AU from the Earth. What was the angular ti~~ERACT, resolution of the Hubble Space Telescope at this time? , yv^ ~ ~ C D - RO M QU~ STl O N S (Subsequent repair missions to the Hubble Space Telescope have further improved its resolution.) 39. Search the World Wide Web for recent information about the orbiting Galileo spacecraft. What new discoveries has it made about Jupiter's satellites and rings? the Galilean satellites obey Kepler's third law. 28. What is the size of the smallest feature you should be able to see on a Galilean satellite through a large telescope under conditions of excellent seeing when Jupiter is near opposition? How does this compare with the best Galileo images, which have resolutions around 25 meters? 30. Using the diameter of Io (3642 km) as a scale, estimate the height to which the plume of Pele rises above the surface of Io in Figure 14-Sa. (You will need to make measurements on this figure using a ruler.) Compare your answer to the value given in the figure caption. 40. Various spacecraft missions have been proposed to explore Europa in greater detail. Search the World Wide Web for information about these. How would these 31. Jupiter, its magnetic field, and the charged particles that missions test for the presence of an ocean beneath are trapped in the magnetosphere all rotate together once Europa's surface? every 10 hours. Io takes 1.77 days to complete one orbit. Using a diagram, explain why particles from Jupiter's 41. The seventeenth satellite of Jupiter, S/1999 J1, was magnetosphere hit Io primarily from behind (that is, on the discovered in 2000 by analyzing observations made in side of Io that trails as it orbits the planet). 1999. Search the World Wide Web for information about how this satellite was discovered. How was it determined that S/1999 J1 is actually in orbit around Jupiter? 32. Assuming material is ejected from Io into Jupiter's magnetosphere at the rate of 1 ton per second (1000 kg/s), how long will it be before Io loses 10% of its mass? How does your answer compare with the age of the solar system? ~ti9E i4.y 42. The Surface of Ganymede. Access and view the video "Jupiter's Moon Ganymede" in Chapter 14 of the Universe web site or 33. How long does it take for Ganymede to enter or leave CD-ROM. Describe the different surface features that you Jupiter's shadow? Assume that the shadow has a sharp edge. see, and explain how each type of feature was formed. _ 34. (a) To an observer floating in Jupiter's Great Red Spot, Io would rise in the east and set in the west, but Metis would rise in the west and set in the east. Explain how this O~SCIZVI N C~ P1Z0.1FCTS is possible. (b) At what distance from the center of Jupiter would a satellite have to orbit so that it would neither rise Observing tips and tools nor set as seen by the observer in (a)? You can easily find the apparent positions of the DISCUSSION ~tl~STiONS 35. If you could replace our Moon with Io, and if Io could maintain its present amount of volcanic activity, what changes would this cause in our nighttime sky? Do you think that Io could in fact remain volcanically active in this case? Why or why not? Galilean satellites for any date and time using the Starry Night software on the CD-ROM that accompanies this textbook. For even more detailed information about satellite positions, consult the "Satellites of Jupiter" section in the Astronomical Almanac for the current year. 36. In the classic science-fiction film 2010: The Year We Make Contact, an alien intelligence causes Jupiter to contract so much that nuclear reactions begin at its center. As a result, Jupiter becomes a star like the Sun. Is this possible in principle? Explain your answer. 37. Speculate on the possibility that Europa, Ganymede, or Callisto might harbor some sort of life. Explain your reasoning. 43. Observe Jupiter through a pair of binoculars. Can you see all four Galilean satellites? Make a drawing of what you observe. If you look again after an hour or two, can you see any changes? 44. Observe Jupiter through a small telescope on three or four consecutive nights. Make a drawing each night showing the positions of the Galilean satellites relative to Jupiter. Record the time and date of each observation. ~Consult the sources listed above in the "Observing tips and ,~pRRY ^"'y 47. Use the Starry Night program to observe tools" to see if you can identify the satellites by name. y~~ the Galilean satellites of Jupiter. First turn `.45. Make arrangements to observe an eclipse, a transit, or ~ off daylight (select Daylight in the Sky menu) .an occultation of one of the Galilean satellites. Consult a and show the entire celestial sphere (select Atlas in the listing of such phenomena in the "Satellites of Jupiter" Go menu). Center on Jupiter by using the Find... section in the Astronomical Almanac for the current year. command in the Edit menu. Using the controls at the Choose the phenomenon you would like to see and right-hand end of the Control Panel, zoom in or out until calculate its scheduled time by converting the universal the field of view is roughly 30 arcminutes (30'). In the time given in the Astronomical Almanac to your local time. Control Panel, set the time step to 1 hour and click on "the Forward" button (a triangle that points to the right). 46. If you are fortunate enough to have access to a large You will see the four Galilean satellites orbiting Jupiter. telescope with a primary mirror 1 meter or more in diameter, (a) Are all four satellites ever on the same side of Jupiter? make arrangements to view Jupiter through that telescope. (b) Observe the satellites passing in front of and behind Describe what you see. Under conditions of excellent seeing Jupiter. (Zoom in as needed.) Explain how your vhen Jupiter is near opposition, the Galilean satellites should observations tell you that all four satellites orbit Jupiter in took like tiny discs rather than starlike pinpoints of light. the same direction.     The Galilean Satellites of Jupiter 1 321 338 CHAPTER 15 KEY W O R DS A ring, p. 323 E ring, p. 328 polymer, p. 334 aerosol, p. 334 Encke gap, p. 327 ring particles, p. 326 B ring, p. 323 F ring, p. 327 ringlets, p. 327 C ring, p. 323 G ring, p. 328 Roche limit, p. 326 Cassini division, p. 323 hydrocarbon, p. 334 shepherd satellite, p. 330 D ring, p. 328 light scattering, p. 328 tidal force, p. 326 KFY 1 D~'t~ S Appearance of Saturn's Rings: Saturn is circled by a system of Titan: The largest Saturnian satellite, Titan, is a terrestrial thin, broad rings lying in the plane of the planet's equator. world with a dense nitrogen atmosphere. A variety of This system is tilted away from the plane of Saturn's orbit, hydrocarbons are produced there by the interaction of which causes the rings to be seen at various angles by an sunlight with methane. These compounds form an aerosol Earth-based observer over the course of a Saturnian year. layer in Titan's atmosphere and possibly cover some of its surface with lakes of ethane. Structure of the Rings: Three major, broad rings can be seen from the Earth. The faint C ring lies nearest Saturn. ~.~ERAC'JG Just outside it is the much brighter B ring, then a dark ~ ~^ Other Satellites: Six moderate-sized moons circle gap called the Cassini division, and then the moderately t~,~ yy Saturn in regular orbits: Mimas, Enceladus, bright A ring. Other, fainter rings were observed by the ~jSE Tethys, Dione, Rhea, and Iapetus. They are Uoyager spacecraft. probably composed largely of ice, but their surface features and histories vary significantly. Saturn also has more than The principal rings of Saturn are composed of numerous twenty much smaller satellites, some of which may be particles of ice and ice-coated rock ranging in size from a captured asteroids. few micrometers to about 10 m. Most of the rings exist inside the Roche limit of Saturn, where disruptive tidal forces are stronger than the gravitational forces attracting the ring particles to each other. Each of the major rings is composed of a great many narrow ringlets. The faint, narrow F ring, which is just outside the A ring, is kept narrow by the gravitational pull of shepherd satellites. Atmosphere and Internal Structure: Saturn's internal structure and atmosphere are similar to those of Jupiter. However, Saturn's core makes up a larger fraction of its volume, and its liquid metallic hydrogen mantle is shallower than that of Jupiter. R~ v~fw (w~ST~o~ls 1. When Saturn is at different points in its orbit, we see different aspects of its rings because the planet has a 27 tilt. If the tilt angle were different, would it be possible to see the upper and lower sides of the rings at all points in Saturn's orbit? If so, what would the tilt angle have to be? Explain your answers. 2. Saturn is the most distant of the planets visible without a telescope. Is there any way we could infer this from naked-eye observations? Explain. (Hint: Think about how Saturn's position on the celestial sphere must change over the course of weeks or months.) 3. Describe the structure of Saturn's rings. What evidence is there that ring particles do not migrate significantly between ringlets? Saturn's atmosphere contains less helium than Jupiter's atmosphere. This lower abundance may be the result of helium raining downward into the planet. 4. If Saturn's rings are not solid, why do they look solid If helium rain does fall, the resulting conversion of when viewed through a telescope? gravitational energy into thermal energy would account for 5, During the planning stages for the Pioneer 11 Saturn's surprisingly strong heat output. mission, when relatively little was known about Saturn's Saturn has belts and zones like those of Jupiter. The rings, it was proposed to have the spacecraft fly cloud layers in Saturn's atmosphere are spread out over a through the Cassini division. Why would this have been greater range of altitude than those of Jupiter. a bad idea?     11. It has been claimed that Saturn would float if one had a large enough bathtub. Using the mass and size of Saturn i given in Table 15-1, confirm that the planet's average ' density is about 690 kg/m3, and comment on this ~omewhat fanciful claim. `13. Both Jupiter and Saturn emit more energy than they , receive from the Sun in the form of sunlight. Compare the ~internal energy sources of the two planets that produce 'rhis emission. t 14. On a warm, humid day, water vapor remains in the atmosphere. But if the temperature drops suddenly, the water vapor forms droplets, clouds appear, and it begins to .v.in. Relate this observation to the explanation why there is :relatively little helium in Saturn's atmosphere compared to xhe atmosphere of Jupiter. ( , 15. Describe Titan's atmosphere. What effect has the Sun's ll ultraviolet radiation had on Titan's atmosphere? 16. Why do scientists suspect that there may be liquid hydrocarbons on Titan?  17. Which of Saturn's moderate-sized satellites show evidence of geologic activity? What might be the energy source for this activity? ~ 18. Explain why debris from Phoebe would be expected to , pile up only on the leading hemisphere of Iapetus. (Hint: How do the orbits of these two satellites compare? How does the orbital motion of debris falling slowly inward toward Saturn compare with the orbital motion of Iapetus?) G. The space shuttle and other manned spacecraft orbit ~ pV~ N CE fO (~11ErSTl O NS the Earth well within the Earth's Roche limit. Explain why these spacecraft are not torn apart by tidal forces. -I Problem-solving tips and tools 7. Although the Voyager spacecraft did not collect any samples of Saturn's ring particles, measurements from these spacecraft allowed scientists to determine the sizes of the particles. Explain how this was done. 8. Why is the term "shepherd satellite" appropriate for the objects so named? Explain how a shepherd satellite operates. 9. Compare the atmospheres of Jupiter and Saturn. Why does Saturn's atmosphere look "washed out" in comparison to that of Jupiter? 10. Compare the interiors of Jupiter and Saturn. What are the similarities? What are the differences? 12. Explain why Saturn is more oblate than Jupiter, even though Saturn rotates more slowly.  19. Saturn's equator is tilted by 27 from the ecliptic, while Jupiter's equator is tilted by only 3. Use these data to explain why we see fewer transits, eclipses, and occultations of Saturn's satellites than of the Galilean satellites. (NASA/1PL) R I ~ U X G The small-angle formula is given in i>ox 1-_ . The Doppler effect is the subject of - , while ;~;; ~ discusses factors affecting the angular resolution of a telescope. Saturn's satellites, whose orbital parameters are given in Table 15-3, obey Newton's form of Kepler's third law (see and v' v -? ' ). For a discussion of escape speed and how planets retain their atmospheres, see Box 7-2. 20. As seen from Earth, the intervals between successive edge-on presentations of Saturn's rings alternate between about 13 years, 9 months, and about 15 years, 9 months. Why do you think these two intervals are not equal? 21. (a) Use Newton's form of Kepler's third law to calculate the orbital periods of particles at the outer edge of the A ring and at the inner edge of the B ring. (b) Saturn's rings orbit in the same direction as Saturn's rotation. If you were floating along with the cloudtops at Saturn's equator, would the outer edge of the A ring and the inner edge of the B ring appear to move in the same or opposite directions? Explain. ~a9E is.~, 22. This Voyager 2 close-up image of Saturn's rings shows a number of dark, straight features called spokes. As these features orbit around Saturn, they tend to retain their shape like the rigid spokes on a rotating bicycle wheel. (The black dots were added by the T~oyager camera system to help scientists calibrate the electronic image.) The spokes rotate at the same rate as Saturn's magnetic field and are thought to be clouds of tiny, electrically charged particles that are kept in orbit by magnetic forces. Explain why the spokes could not maintain their shape if they were kept in orbit by gravitational forces alone. The Spectacular Saturnian System 339   23. It is well known that the Cassini division involves a 2-to-1 resonance with Mimas. Does the location of the Encke gap-133,500 km from Saturn's center-correspond to a resonance with one of the other satellites? If so, which one? 32. Jupiter's satellite lo and Saturn's satellite Enceladus are both geologically active, and both are in 2-to-1 resonances with other satellites. However, the amount of geologic activity of Enceladus is far less than on lo. Discuss some possible reasons for this difference. 24. (a) An astronaut floating above Saturn's cloudtops 33. Imagine that you are in charge of planning the Cassini would see a blue sky, even though Saturn's atmosphere has orbital tour of the Saturnian system. In your opinion, what a very different chemical composition than Earth's. Explain objects in the system should be examined, what data why. (b) The image of Saturn in Figure 15-15 appears should be collected, and what kinds of questions should bluish around the edges of the planet. Explain why this the mission attempt to answer? should be. (Hint: For both (a) and (b), see Box 5-4.) ~,g~ERAC,, 25. Find the escape speed on Titan. What is the limiting molecular weight of gases that could be retained by Titan's W W E R /C W -1ZO M QUESTIONS gravity? (Hint: Use the ideas presented in and assume an average atmospheric temperature of 95 K.) 34. Search the World Wide Web, especially the web sites for NASA's Jet Propulsion Laboratory and the European 26. Many of the gases in the atmosphere of Titan, such as Space Agency, for information about the current status of methane, ethane, and acetylene, are highly flammable. the Cassini mission. When will Cassini arrive at Saturn? Why, then, doesn't Titan's atmosphere catch fire? (Hint: What are the current plans for its tour of Saturn's What gas in our atmosphere is needed to make wood, coal, satellites? What ideas are being considered for the Cassini or gasoline burn?) extended mission, to begin in 2008? 27. Although Jupiter's satellite Ganymede is about the same 35. The two Voyager spacecraft were launched from Earth size and mass as Titan, Ganymede does not have an along a trajectory that took them directly to Jupiter. The appreciable atmosphere. Suggest a reason for this difference. force of Jupiter's gravity then gave the two spacecraft a 28. At infrared wavelengths, the Hubble Space Telescope "kick" that helped push them onward to Saturn. The much can see details on Titan's surface as small as 580 km larger Cassini spacecraft, by contrast, was first launched on (360 mi) across. Determine the angular resolution of the a trajectory that took it past Venus. Search the World Wide Hubble Space Telescope using infrared light. If visible light Web, especially the web sites for NASA's Jet Propulsion is used, is the angular resolution better, worse, or the Laboratory and the European Space Agency, for same? Explain your answer. information about the trajectory that Cassini is taking through the solar system. Explain why this trajectory is so 29. (a) To an observer on Enceladus, what is the time different than that of the Voyagers. interval between successive oppositions of Dione? Explain 36, In 2000 astronomers reported the discovery of your answer. (b) As seen from Enceladus, what is the several new moons of Saturn. Search the World Wide angular diameter of Dione at opposition? How does this Web for information about these. How did astronomers compare to the angular diameter of the Moon as seen from discover them? Have the observations been confirmed? Earth (about lh)? How large are these moons? What sorts of orbits do they appear to follow? DISCUSSION QUCST 1 O N S ,a9E is,, 37. The Rotation Rate of Saturn. Access and view the video "Saturn from the Hubble Space 30. Suppose that Saturn were somehow moved to an orbit Telescope" in Chapter 15 of the Universe web around the Sun with semimajor axis 1 AU, the same as the site or CD-ROM. The total amount of time that actually Earth's. Discuss what long-term effects this would have on elapses in this video is 42.6 hours. Using this information, the planet and its rings. identify and follow an atmospheric feature and determine 31. Comment on the suggestion that Titan may harbor the rotation period of Saturn. How does your answer life-forms. compare with the value given in Table 15-1? ORS^RV1NCa PROJECTS ~ Observing tips-and tools ~e~i"Kls,~ To determine the best time of night to view 3~~,~^' Saturn, consult the current issue of Sky ~' Telescope or Astronomy magazine or their web sites or use the Starry Night software on the CD-ROM that accompanies this textbook. If your goal is to view Saturn's satellites, consult the section entitled "Satellites of Saturn" in the Astronomical Almanac for the current year. This includes a diagram showing the orbits of Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Hyperion. Plan your observing session by looking up the dates and times of the most recent greatest eastern elongations of the various satellites. You will have to convert from universal time (UT), which is the same as Greenwich Mean Time, to your local time zone. Then, using the tick marks along the orbits in the diagram, estimate the positions of the satellites relative to Saturn at the time you will be at the telescope. Another useful resource is the "Celestial Calendar" section of Sky ~ Telescope. During months when Saturn is visible in the night sky, this section of the magazine includes a chart of Saturn's satellites. 38. View Saturn through a small telescope. Make a sketch of what you see. Estimate the angle at which the rings are tilted to your line of sight. Can you see the Cassini division? Can you see any belts or zones in Saturn's clouds? Is there a faint, starlike object near Saturn that might be Titan? What observations could you perform to test whether the starlike object is a Saturnian satellite? 39. If you have access to a moderately large telescope, make arrangements to observe several of Saturn's satellites. At the telescope, you should have no trouble identifying Titan. Tethys, Dione, and Rhea are about one-sixth as bright as Titan and should be the next easiest satellites to find. Can you confidently identify any of the other satellites? ~p~RY Nr~y 40. Use the Starry Night program to observe the `'~~ changing appearance of Saturn. First turn off daylight (select Daylight in the Sky menu) and show the entire celestial sphere (select Atlas in the Go menu). Center on Saturn by using the Find... command in the Edit menu. Using the controls at the right-hand end of the Control Panel, zoom in until Saturn nearly fills the field of view. In the Control Panel, set the time step to 1 year. Use the single-step time control buttons to observe the changing aspect of the rings. During which of the next 30 years will we see the rings edge-on? The Spectacular Saturnian System 341 360 1 CHAPTER 16 KC Y WORDS Great Dark Spot, p. 347 occultation, p. 351 magnetic axis, p. 350 radiation darkening, p. 352 KICY 1DClES Discovery of the Outer Planets: Uranus was discovered by Triton has a young, icy surface indicative of tectonic, chance, while Neptune was discovered at a location activity. The energy for this activity may have been predicted by applying Newtonian mechanics. Pluto was provided by tidal heating that occurred when Triton was discovered after a long search. If another planet exists captured by Neptune's gravity into a retrograde orbit. beyond Pluto, it is either very small or very far away. Triton has a tenuous nitrogen atmosphere. Atmospheres of Uranus and Neptune: Both Uranus and Pluto and Charon: Pluto and its moon, Charon, move Neptune have atmospheres composed primarily of together in a highly elliptical orbit steeply inclined to the hydrogen and helium, with about 2% methane. plane of the ecliptic. They are the only worlds in the solar Methane absorbs red light, giving Uranus and Neptune system not yet visited by spacecraft. their greenish-blue color. Several hundred small, icy worlds have been discovered No white ammonia clouds are seen on Uranus or Neptune. beyond Neptune. Pluto, Charon, and Triton may be part of Presumably the low temperatures have caused almost all the this population. ammonia to precipitate into the interiors of the planets. All of these planets' clouds are composed of methane. Much more cloud activity is seen on Neptune than on Uranus. This is because Uranus lacks a substantial internal heat source. Interiors and Magnetic Fields of Uranus and Neptune: Both Uranus and Neptune may have a rocky core surrounded by a mantle of water and ammonia. Electric currents in these mantles may generate the magnetic fields of the planets. Uranus's magnetic axis is inclined by 59 from its axis of rotation, while Neptune's is inclined by 47. The magnetic and rotational axes of all the other planets are more nearly parallel. The magnetic fields of Uranus and Neptune are also offset from the centers of the planets. Uranus's Unusual Rotation: Uranus's axis of rotation lies nearly in the plane of its orbit, producing greatly exaggerated seasonal changes on the planet. The unusual orientation of Uranus's rotational and magnetic axes may be the result of a collision with a planetlike object early in the history of our solar system. Such a collision could have knocked Uranus on its side. Ring Systems of Uranus and Neptune: Uranus and Neptune are both surrounded by systems of thin, dark rings. The low reflectivity of the ring particles may be due to radiation-darkened methane ice. REVIEW QUESTIONS 1. Why do you suppose that the discovery of Neptune is rated as one of the great triumphs of science, whereas the discoveries of Uranus and Pluto are not? 2. Could astronomers in antiquity have seen Uranus? If so, why was it not recognized as a planet? 3. (a) Draw a figure like Figure 16-2, and indicate on it where Uranus was in 1986 and 1998. Explain your reasoning. (Hint: See Figures 16-1 and 16-2.) (b) In approximately what year will the Sun next be highest in the sky as seen from Uranus's south pole? Explain your reasoning. 4. Why do you suppose the tilt of Uranus's rotation axis was deduced from the orbits of its satellites and not by observing the rotation of the planet itself? 5. Describe the seasons on Uranus. Why are the Uranian seasons different from those on any other planet? 6. A number of storms in the Uranian atmosphere can be seen in Figure 16-3, but none are visible in Figure 16-1. How can you account for the difference? 7. Why are Uranus and Neptune distinctly blue-green in color, while Jupiter or Saturn are not? 8. Why are fewer white clouds seen on Uranus and Satellites of Uranus and Neptune: Uranus has five satellites Neptune than on Jupiter and Saturn? similar to the moderate-sized moons of Saturn, plus at least 12 more small satellites. Neptune has eight satellites, one 9. Discuss some competing explanations of why Uranus of which (Triton) is comparable in size to our Moon or the and Neptune are substantially smaller than Jupiter and Galilean satellites of Jupiter. Saturn.  10. How do the orientations of Uranus's and Neptune's 22. At certain points in its orbit, a stellar occultation by magnetic axes differ from those of other planets? Uranus would not reveal the existence of the rings. What 11. Briefly describe the evidence supporting the idea that Points are those? How often does this circumstance arise? Uranus was struck by a large planetlike object several Explain using a diagram. ` billion years ago. 12. Compare the rings that surround Jupiter, Saturn, Uranus, and Neptune. Briefly discuss their similarities and differences. 23. At what planetary configuration is the gravitational force of Neptune on Uranus at a maximum? For this configuration, calculate the gravitational force exerted by the Sun on Uranus and by Neptune on Uranus. Then calculate the fraction by which the sunward gravitational pull on Uranus is reduced by Neptune at that configuration. Based on your calculations, do you expect that Neptune has a relatively large or relatively small effect on Uranus's orbit? 13. The 1977 occultation that led to the discovery of Uranus's rings was visible from the Indian Ocean. Explain why it could not be seen from other parts of the Earth's night side. 14. As Voyager 2 flew past Uranus, it produced images 24. According to one model for the internal structure of only of the southern hemispheres of the planet's satellites. Uranus, the rocky core and the surrounding shell of water ' Why do you suppose this was? and methane ices together make up 80 / of the planet's mass. This interior region extends from the center of 15. Why do astronomers think that the energy needed to Uranus to about 70% of the planet's radius. (a) Find the resurface parts of Miranda came from tidal heating rather average density of this interior region. (b) How does your than the satellite's own internal heat? answer to (a) compare with the average density of Uranus as a whole? Is this what you would expect? Why? 16. Using the data in Table 16-3, explain why Uranus's satellites Caliban and Sycorax (both discovered in 1997) were probably captured from space rather than having formed at the same time as the planet itself. 17. If you were floating in a balloon in Neptune's upper atmosphere, in what part of the sky would you see Triton rise? Explain your reasoning. 18. Briefly describe the evidence supporting the idea that Triton was captured by Neptune. 19. Why is it reasonable to suppose that Neptune will someday be surrounded by a broad system of rings, perhaps similar to those that surround Saturn? 20. How can astronomers distinguish a faint solar system object like Pluto from background stars within the same field of view? 21. Describe the circumstantial evidence supporting the idea that Pluto is one of thousands of similar icy worlds that once occupied the outer regions of the solar system. ADVANCED QUEST10NS Problem-solving tips and tools See ''; w '~ ! for the small-angle formula. For Question 24, you will need to recall that the volume of a sphere of radius r is 4/3~r3. You can find Newton's formula for the gravitational force between two objects in ~n -. , a discussion of tidal forces in .~tion -* , Wien's law for blackbody radiation in v~-~u, s-~, and a discussion of the transparency of the Earth's atmosphere to various wavelengths of light in Secc<< ~-- _ -7. 25. Show that the ratio of the orbital periods of Neptune and Pluto is very close to 2:3. (This ratio is thought to result from gravitational interactions between Neptune and Pluto. These interactions prevent Neptune and Pluto from ever getting very close to each other.) 26. Suppose you were standing on Pluto. Describe the motions of Charon relative to the Sun, the stars, and your own horizon. Would you ever be able to see a total eclipse of the Sun? (Hint: You will need to calculate the angles subtended by Charon and by the Sun as seen by an observer on Pluto.) In what circumstances would you never see Charon? 27. It is thought that Pluto's tenuous atmosphere may become even thinner as the planet moves toward aphelion (which it will reach in 2113), then regain its present density as it again moves toward perihelion. Why should this be? 28. The brightness of sunlight is inversely proportional to the square of the distance from the Sun. For example, at a distance of 4 AU from the Sun, sunlight is only (1/a)Z ='h6 = 0.0625 as bright as at 1 AU. Compared with the brightness of sunlight on the Earth, what is its brightness (a) on Pluto at perihelion and (b) on Pluto at aphelion? (c) How much brighter is it on Pluto at perihelion compared with aphelion? (Even this brightness is quite low. Noon on Pluto is about as dim as it is on the Earth a half hour after sunset on a moonless night.) 29. If Earth-based telescopes can resolve angles down to 0.25 arcsec, how large could an object be at Pluto's average distance from the Sun and still not present a resolvable disk? 30. The observations of Pluto shown in Figure 16-18 were made using blue and ultraviolet light. What  The Outer Worlds 361 362 I CHAPTER 16 advantages does this have over observations made with orbits do these objects have? How do these orbits compare with that of Pluto? What are the largest and smallest objects of this sort that have so far been found, and how large are they? red or infrared light? 31. Calculate the maximum angular separation between Pluto and Charon as seen from Earth. (Assume that Pluto is at its minimum distance from the Sun and that Pluto is at opposition as seen from Earth.) Compare your answer with the angular separation given in the caption to Figure 16-19. 32. Presumably Pluto and Charon raise tidal bulges on each other. Explain why the average distance between Pluto and Charon is probably constant, rather than increasing like the Earth-Moon distance or decreasing like the Neptune-Triton distance. Include. a diagram like Fiuure 9-1 ~ as part of your answer. ps`^M y6~,~ 41. Separation of Pluto and Charon. Pluto is located about 4.5 billion km from Earth and has a maximum observable separation from Charon of about 0.9 arcseconds. Access the AIMM (Active Integrated Media Module) called "Small-Angle Toolbox" in Chapter 1 of the Universe web site or CD-ROM. Use this AIMM and the above data to determine the distance between Pluto and Charon. How does your answer compare with the value given in the text?  33. Suppose you wanted to search for planets beyond Pluto. Why might it be advantageous to do your Q (3 SE RV 1 N G PROJ ~CTS observations at infrared rather than visible wavelengths? (Use Wien's law to calculate the wavelength range best I Observing tips and tools suited for your search.) Could such observations be done at an observatory on the Earth's surface? Explain. D1_SCtI_SS10N Q,U~STIONS 34. Discuss the evidence presented by the outer planets that suggests that catastrophic impacts of planetlike objects occurred during the early history of our solar system. 35. Some scientists are discussing the possibiliry of placing spacecraft in orbit about Uranus and Neptune. What kinds of data should be collected, and what questions would you like to see answered by these missions?  ~m~-IN"~ l6~ During the period 2001-2005, Uranus will w be at opposition in August, Neptune in late July and early August, and Pluto in June. You can find Uranus and Neptune with binoculars if you know where to look (a good star chart is essential), but Pluto is so dim that it can be a challenge to spot even with a 25-cm (10-inch) telescope. Each year, star charts that enable you to find these planets are printed in the issue of Sky ~' Telescope for the month in which each planet is first visible in the nighttime sky. You can also locate the outer planets using the Starry Nigbt program on the CD-ROM that accompanies this textbook. 36. If Triton had been formed along with Neptune rather than having been captured, would you expect it to be in a prograde or retrograde orbit? Would you expect the satellite 42. Make arrangements to view Uranus through a to show signs of tectonic activity? Explain your answers. telescope. The planet is best seen at or near opposition. Use 37. Would you expect the surfaces of Pluto and Charon to a star chart at the telescope to find the planet. Are you certain that you have found Uranus? Can you see a disk? What is its color? ?~~?: 43. If you have access to a large telescope, make arrangements to view Neptune. Like Uranus, Neptune is best seen at or near opposition and can most easily be , ,,ypE0.l6.~ 38. Miranda. Access and view the video found using a star chart. Can you see a disk? 4~ "Uranus's Moon Miranda" in Chapter 16 of the What is its color? ~I Universe web site or CD-ROM. Discuss some of 44. If you have access to a large telescope (at least 25 cm the challenges that would be involved in launching a in diameter), make arrangements to view Pluto. Using the spacecraft from Earth to land on the surface of Miranda. star chart from Sky ~-' Telescope referred to above, view the part of the sky where Pluto is expected to be seen and make a careful sketch of all of the stars that you see. Repeat this process on a later night. Can you identify the "star" that has moved? be heavily cratered? Explain why or why not. W~~3/CD-RC)M QU~STIONS 39. The discovery image of Charon (Figure 16-18) was made by an astronomer at the U.S. Naval Observatory. Why do you suppose the U.S. Navy carries out work in astronomy? Search the World Wide Web for the answer. N?~y 45. Use the Starry Night program to observe the 40. Search the World Wide Web for a list of small objects y~~ five large satellites of Uranus. First turn off that orbit beyond Neptune. (These are called "trans- daylight (select Daylight in the Sky menu) and Neptunian objects.") Use the list to learn about the current show the entire celestial sphere (select Atlas in the Go status of 1992 QBl and similar objects. What kinds of menu). Center on Uranus by using the Find... command in   the Edit menu. Using the controls at the right-hand end of main window. Zoom out until you can see the orbit of r the Control Panel, zoom in or out until the field of view is Triton, the inner of the two satellites, and click on the , roughly 1 arcminute (1'). Select Planet List in the Window "Forward" button in the Control Panel. Make a diagram menu, and then click on the triangle to the left of the name of the orbit that you see. How does the direction of t Uranus. This will reveal a list of the planet's five large Triton's orbital motion compare to the direction of satellites. Then, in the "Orbit" column on the right-hand Neptune's rotation? Does the orbit appear to lie in the side of the Planet List, click to the right of each satellite's same plane as Neptune's equator? How can you tell? name. As you click, marks will appear in this column and (c) Zoom farther out until you can see all of Nereid's the satellites' orbits will appear in the main window. Set elongated orbit. To follow the motion of this rather dim the time step in the Control Panel to 2 hours. (a) In the satellite, double-click on the name Nereid in the Planet Control Panel, click on the "Forward" button (a triangle List palette, then click on the Centre and Lock button. that points to the right). Describe how the satellites move, Then click on the "Forward" button in the Control and relate your observations to Kepler's third law (see Panel. Can you see Nereid move? Now change the time ). (b) From time to time, Miranda lies step to 10 days and watch Nereid go through several directly between Ariel and Uranus. Using the single-step orbits. Make a diagram of the orbit that you see. How time control buttons, determine how much time elapses does the direction of Nereid's orbital motion compare to between successive instances of this configuration. How the direction of Neptune's rotation? Does the orbit appear does this compare to the orbital periods of Miranda and to lie in the same plane as Nereid's equator? How can Ariel? Relate your observations to the ideas of sidereal you tell? period and synodic period (see Section 4-2).RRY 47 Use the Starry Niht tb ~P 1`1,1,,g programo oserve ypR(tY N,12 y~ 46, Use the Starry Night program to observe Neptune and its two outer satellites. First turn off daylight (select Daylight in the Sky menu) and show the entire celestial sphere (select Atlas in the Go menu). Center on Neptune by using the Find... command in the Edit menu. (a) Using the controls at the right-hand end of the Control Panel, zoom in until you can clearly see surface features on the planet. Set the time step in the Control Panel to 2 hours, then click on the "Forward" button (a triangle that points to the right). In what direction does Neptune appear to rotate? Illustrate your answer with a diagram. (b) Select Planet List in the Window menu and click on the triangle to the left of the name Neptune. This will reveal a list of the planet's two outer satellites. Then, in the "Orbit" column on the right-hand side of the Planet List, click to the right of each satellite's name. As you click, check marks will appear in this column and the satellites' orbits will appear in the ~ Pluto and Charon. First turn off daylight (select Daylight in the Sky menu) and show the entire celestial sphere (select Atlas in the Go menu). Center on Pluto by using the Find... command in the Edit menu. Using the controls at the right-hand end of the Control Panel, zoom in as far as possible. Select Planet List in the Window menu and click on the triangle to the left of the name Pluto. Then, in the "Orbit" column on the right-hand side of the Planet List, click to the right of the name Charon. A mark will appear in this column and Charon's orbit will appear in the main window. In the Control Panel, set the time step to 3 hours. (a) Use the single-step time control buttons to step through enough time to determine the period of Charon's orbit, How does your answer compare to the value given in Section 16-10? (b) What is the apparent shape of Charon's orbit around Pluto? How can you reconcile this with the fact that Charon's actual orbit is nearly a perfect circle?  The Outer Worlds 363   j Analysis of isotopes in certain meteorites suggests that a asteroid. Where might you look to find other members of . = nearby supernova may have triggered the formation of the the same family? ! solar system 4.6 billion years ago. 8. What are the Trojan asteroids, and where are they ~a,~~RACrrG~ Comets: A comet is a chunk of ice with located? ti imbedded rock fragments that generally moves ~F~P ti~ in a highly elliptical orbit about the Sun. 9. What is the difference between a meteoroid, a meteor, and an meteorite? As a comet approaches the Sun, its icy nucleus develops ; a luminous coma, surrounded by a vast hydrogen ~ envelope. An ion tail and a dust tail extend from the comet, pushed away from the Sun by the solar wind and radiation pressure, respectively. 10. Is there anywhere on the Earth where you might find large numbers of stony meteorites that are not significantly weathered? If so, where? If not, why not? 11. Scientists can tell that certain meteorites came from the interior of an asteroid rather than from its outer layers. Explain how this is done. ~+ Fragments of "burned out" comets produce meteoritic swarms. A meteor shower is seen when the Earth passes through a meteoritic swarm. 12. Why are some asteroids differentiated while others are not? Origin of Comets: Comets are thought to originate from two regions, the Kuiper belt and the Oort cloud. 13. Suppose you found a rock you suspect might be a 1 meteorite. Describe some of the things you could do to The Kuiper belt lies in the plane of the ecliptic at determine whether it was a meteorite or a distances between 40 and 500 AU from the Sun. It is "meteorwrong." 3 thought to contain many tens of thousands of comet nuclei. Many Jupiter-family comets probably come from 14. What is the evidence that carbonaceous chondrites are the Kuiper belt, and a number of objects have been essentially unaltered relics of the early solar system? What observed in the Kuiper belt. do they suggest about how the solar system may have formed? The Oort cloud contains billions of comet nuclei in a t~ spherical distribution that extends out to 50,000 AU from the Sun. Intermediate-period and long-period comets are thought to originate in the Oort cloud. As yet no objects in the Oort cloud have been detected directly. 15. With the aid of a drawing, describe the structure of a comet. 16. Why is the phrase "dirty snowball" an appropriate characterization of a comet's nucleus? 17. Why do the ion tail and dust tail of a comet point in R~ V 1 ~1N QLJ~ST ( O N S different directions? 1. How did the first asteroids come to be discovered? How did this discovery differ from what astronomers had expected to find? 2. How do modern astronomers discover new asteroids? 3. Describe the asteroid belt. Does it lie completely within the plane of the ecliptic? What are its inner and outer radii? 4. What are Kirkwood gaps? What causes them? 5. Compare the explanation of the Kirkwood gaps in the asteroid belt to the way in which Saturn's moons help produce divisions in that planet's rings (see Section 1.5-4). 18. Why do comets have prominent tails for only a short time during each orbit? 19. What is the Kuiper belt? How does it compare with , ' the asteroid belt? What are the similarities? What are the differences? 20. What is the Oort cloud? How might it be related to planetesimals left over from the formation of the solar system? 21. Why are comets more likely to break apart at perihelion than at aphelion? 6. What is the evidence that some asteroids are made of a loose conglomeration of smaller pieces? 22. Why do astronomers think that meteorites come from asteroids, while meteor showers are related to comets? 7. The asteroid 243 Ida, which was viewed by the 23. Why are asteroids, meteorites, and comets all of Galileo spacecraft, is a member of a Hirayama family. special interest to astronomers who want to understand Discuss what this tells us about the history of this the early history of the solar system? _ Vagabonds of the Solar System I 385 386 I CHAPTER 17 1tiDV~NCED Q,U~STIONS I Problem-solving tips and tools We discussed retrograde motion in - ___ ; and described its causes in S_ectvn 4- . You will need to use Kepler's third law, described in m ~- and , in some of the problems below. discusses the concept of escape speed. A spherical object of radius r intercepts an amount of sunlight proportional to its cross-sectional area, equal to 7cr2. The volume of a sphere of radius r is 4/37cr3. from the surface of Dactyl. If you were an astronaut standing on Dactyl's surface, could you throw a baseball straight up so that it would never come down? Professional baseball pitchers can throw at speeds around 40 m/s (140 km/h, or 90 mi/h); your throwing speed is probably a bit less. 29. Imagine that you are an astronaut standing on the surface of a Trojan asteroid. How will you see the phase of Jupiter change with the passage of time? How will you see Jupiter move relative to the distant stars? Explain your answers. 30. Use the percentages of stones, irons, and stony iron 24. When Olbers discovered Pallas in March 1802, the meteorites that fall to Earth to estimate what fraction of asteroid was moving from east to west relative to the stars. their parent asteroids' interior volume consisted of an iron At what time of night was Pallas highest in the sky over core. Assume that the percentages of stones and irons that Olbers's observatory? Explain your reasoning. fall to the Earth indicate the fractions of a parent asteroid's interior volume occupied by rock and iron, respectively. 25. Consider the Kirkwood gap whose orbital period is How valid do you think this assumption is? two-fifths of Jupiter's period. Calculate the distance from the Sun to this gap. Does your answer agree with Figure 17-4? 31. Sun-grazing comets come so close to the Sun that their perihelion distances are essentially zero. Find the orbital periods of Sun-grazing comets whose aphelion distances are (a) 100 AU, (b) 1000 AU, (c) 10,000 AU, and (d) 100,000 AU. Assuming that these comets can survive only a hundred perihelion passages, calculate their lifetimes. (Hint: Remember that the semimajor axis of an orbit is one-half the length of the orbit's long axis.) 26. Suppose that a binary asteroid (two asteroids orbiting each other) is observed in which one member is 16 times brighter than the other. Suppose that both members have the same albedo and that the larger of the two is 120 km in diameter. What is the diameter of the other member? 27. The accompanying image from the Galileo spacecraft shows the asteroid 243 Ida, which has dimensions 56 x 24 x 21 km. Galileo discovered a tiny moon called Dactyl, just 1.6 x 1.4 x 1.2 km in size, which orbits Ida at a distance of about 100 km. (In Greek mythology, the Dactyli were beings who lived on the slopes of Mount Ida.) Describe a scenario that could explain how Ida came to have a moon. Ida   32. Comets are generally brighter a few weeks after passing perihelion than a few weeks before passing perihelion. Explain why might this be. (Hint: Water, including water ice, does an excellent job of retaining heat.) 33. The hydrogen clouds of comets are especially bright at a wavelength of 122 nm. Use I~i~ure 5-22 to explain why. 34. A very crude model of a typical comet nucleus is a cube of ice (density 1000 kg/m3) 10 km on a side. (a) What is the mass of this nucleus? (b) Suppose 1% of the mass of the nucleus evaporates away to form the comet's tail. Suppose further that the tail is 100 million (10g) km long and 1 million (106) km wide. Estimate the average density of the tail (in kg/m3). For comparison, the density of the air you breathe is about 1.2 kg/m3. (c) In 1910 the Earth actually passed through the tail of Comet Halley. At the time there was some concern among the general public that this could have deleterious effects on human health. Was this concern justified? Why or why not? (JPL/NASA) R i p u x ~ DiSCUSSION ~?UESTIONS 35. From the abundance of craters on the Moon and 28. Assume that Ida's tiny moon Dactyl (see Question 27) Mercury, we know that numerous asteroids and has a density of 2500 kg/m3. (a) Calculate the mass of meteoruids struck the inner planets early in the history Dactyl in kilograms. For simplicity, assume that Dactyl is a of our solar system. Is it reasonable to suppose that sphere 1.4 km in diameter. (b) Calculate the escape speed numerous comets also pelted the planets 3.5 to 4.5 billion  ;years ago? Speculate about the effects of such a O(3SERVINC~ PROJECTs .~cometary bombardment, especially with regard to the ievolution of the primordial atmospheres on the terrestrial I Observing tips and tools - planets. ~e~-~"K l?~ Meteors: Informative details concerning upcoming meteor showers appear on the web sites for Sky ~' Telescope and Astronomy magazines. 36. In the 1998 movie Armageddon, an asteroid "the size ~of Texas" is on a collision course with Earth. The asteroid _is first discovered by astronomers just 18 days prior to :mpact. To avert disaster, a team of astronauts blasts the :asteroid into two pieces just 4 hours before impact. Discuss ~he plausibility of this scenario. (Hint: On average, the ,tate of Texas extends for about 750 km from north to south and from east to west. How does this compare with the size of the largest known asteroids?) 37. Suppose astronomers discovered that the near-Earth object 1994 XMl had been disturbed in such a way as to put it on a collision course with Earth. Describe what humanity could do within the framework of present technology to counter such a catastrophe. ~~qERA~T . . wER!cl~-ROM c~t~EST~oNs 38. Search the World Wide Web to find out why some scientists disagree with the idea that a tremendous impact led to the demise of the dinosaurs. (They do not dispute that the impact took place, only what its consequences were.) What are their arguments? From what you learn, what is your opinion? 39. Several scientific research programs are dedicated to the search for near-Earth objects (NEOs), especially those that might someday strike our planet. Search the World Wide Web for information about at least one of these programs. How does this program search for NEOs? How many NEOs has this program discovered? Will any of these pose a threat in the near future? ,ti9EO i?~ 40. Estimating the Speed of a Comet. Access and view the video "Two Comets and an Active Sun" in Chapter 17 of the Universe web site or CD ROM. (a) Why don't the comets reappear after passing the Sun? (b) The white circle shows the size of the Sun, which has diameter 1.39 x 106 km. Using this to set the scale, step through the video and measure how the position of one of the comets changes. Use your measurements and the times displayed in the video to estimate the comet's speed in km/h and km/s. (Assume that the comet moved in the same plane as that shown in the video.) As part of your answer, explain the technique and calculations you used. (c) How does your answer in (b) compare with the orbital speed of Mercury, the innermost and fastest-moving planet (see ' v isl~_ i ~ )? Why is there a difference? (d) If the comet's motion was not in the same plane as that shown in the video, was its actual speed more or less than your estimate in (b)? Explain.  Comets: Because astronomers discover dozens of comets each year, there is usually a comet visible somewhere in the sky. Unfortunately, they are often quite dim, and you will need to have access to a moderately large telescope (at least 35 cm, or 14 in.). You can get up-to-date information from the Minor Planet Center web site. Also, if there is an especially bright comet in the sky, useful information about it might be found at the web sites for Sky ~' Telescope and Astronomy magazines. Asteroids: At opposition, some of the largest asteroids are bright enough to be seen through a modest telescope. Check the Minor Planet Center web site to see if any bright asteroids are near opposition. If so, check the current issue as well as the most recent January issue of Sky ~' Telescope magazine for a star chart showing the asteroid's path among the constellations. You will need such a chart to distinguish the asteroid from background stars. Also, you can locate Ceres, Pallas, Juno, and Vesta using the Starry Night program on the CD-ROM that accompanies this textbook. 41. Make arrangements to view a meteor shower. Table 17-1 lists the dates of major meteor showers. Choose a shower that will occur near the time of a new moon. Set your alarm clock for the early morning hours (1 to 3 A.M.). Get comfortable in a reclining chair or lie on your back so that you can view a large portion of the sky. Record how long you observe, how many meteors you see, and what location in the sky they seem to come from. How well does your observed hourly rate agree with published estimates, such as those in Table 17-1? Is the radiant of the meteor shower apparent from your observations? 42. If a comet is visible with a telescope at your disposal, make arrangements to view it. Can you distinguish the comet from background stars? Can you see its coma? Can you see a tail? 43. Make arrangements to view an asteroid. Observe the asteroid on at least two occasions, separated by a few days. On each night, draw a star chart of the objects in your telescope's field of view. Has the position of one starlike object shifted between observing sessions? Does the position of the moving object agree with the path plotted on published star charts? Do you feel confident that you have in fact seen the asteroid? Vagabonds of the Solar System 387 388 CHAPTER 17 11pRRY NICl y 44. Use the Starry Night program to observe prediction. To check your prediction, zoom out as far as mom several comets. (a) First turn off daylight (select possible, turn the planets and Sun back on (again select Daylight in the Sky menu), show the entire Planets/Sun in the Sky menu) and use the hand cursor to celestial sphere (select Atlas in the Go menu), and turn off scroll the display in the direction of your prediction. the planets and Sun (select Planets/Sun in the Sky menu). Were you correct? (b) Using the Planet List, double-click In the Control Panel, set the date to March 22, 1997. on the name of another comet to center on it, but do not Select Planet List in the Window menu and click on the zoom in. Then move the sky with the hand cursor and triangle to the left of the name Comets. This will reveal a locate the Sun. From your observations, predict the list of comets. Double-click on the name Hale-Bopp to direction of the comet's tail on the sky, and explain how center on that comet. Zoom in until you can see the comet you made your prediction. Center again on the comet and its tail. Predict in what direction the Sun is located and zoom in until you can see its tail. Was your prediction relative to the comet, and explain how you made your correct? 414 1 CHAPTER 18  radiative diffusion, p. 395 solar wind, p. 405 radiative zone, p. 396 spicule, p. 403 solar flare, p. 412 sunspot, p. 406 solar neutrino, p. 399 sunspot cycle, p. 407 solar neutrino problem, p. 399 sunspot maximum, p. 407 sunspot minimum, p. 407 supergranule, p. 402 thermal equilibrium, p. 394 thermonuclear fusion, p. 392 Zeeman effect, p. 408  KEY 1 DChS Hydrogen Burning in the Sun's Core: The Sun's energy is upward from the photosphere into the chromosphere along produced by hydrogen burning, a thermonuclear fusion the boundaries of supergranules. process in which four hydrogen nuclei combine to produce . The outermost layer of the solar atmosphere, the corona, a single helium nucleus. is made of very high-temperature gases at extremely low The energy released in a nuclear reaction corresponds to density. Activity in the corona includes coronal mass a slight reduction of mass according to Einstein's equation ejections and coronal holes. The solar corona blends into E = mc2. the solar wind at great distances from the Sun. Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen burning occurs only at temperatures in excess of about 107 K. In the Sun, fusion occurs only in the dense, hot core. The Active Sun: The Sun's surface features vary in an 11-year cycle. This is related to a 22-year cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity. Models of the Sun's Interior: A theoretical description of a . Sunspots are relatively cool regions produced by local star's interior can be calculated using the laws of physics. concentrations of the Sun's magnetic field. The average The standard model of the Sun suggests that hydrogen number of sunspots increases and decreases in a regular burning takes place in a core extending from the Sun's cycle of approximately 11 years, with reversed magnetic center to about 0.25 solar radius. polarities from one 11-year cycle to the next. Two such The core is surrounded by a radiative zone extending to cycles make up the 22-year solar cycle. about 0.71 solar radius. In this zone, energy travels The magnetic-dynamo model suggests that many features outward through radiative diffusion. of the solar cycle are due to changes in the Sun's magnetic The radiative zone is surrounded by a rather opaque field. These changes are caused by convection and the Sun's convective zone of gas at relatively low temperature and differential rotation. pressure. In this zone, energy travels outward primarily A solar flare is a brief eruption of hot, ionized gases through convection. from a sunspot group. A coronal mass ejection is a much Solar Neutrinos and Helioseismology: Conditions in the larger eruption that involves immense amounts of gas from the corona. solar interior can be inferred from measurements of solar neutrinos and of solar vibrations. Neutrinos emitted in thermonuclear reactions in the Sun's core have been detected, but in smaller numbers than expected. Recent experiments may explain why this is so. Helioseismology is the study of how the Sun vibrates. These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun. The Sun's Atmosphere: The Sun's atmosphere has three main layers: the photosphere, the chromosphere, and the corona. Everything below the solar atmosphere is called the solar interior. The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere. Its spectrum is similar to that of a blackbody at a temperature of 5800 K. Convection in the photosphere produces granules. REVIEW (ZUESTIONS 1. What is hydrogen burning? Why is hydrogen burning fundamentally unlike the burning of a log in a fireplace? 2. Why do thermonuclear reactions occur only in the Sun's core? 3. Describe how the net result of the reactions in Figure 18-2 is the conversion of four protons into a single helium nucleus. What other particles are produced in this process? How many of each particle are produced? 4. Give an everyday example of hydrostatic equilibrium. Give an example of thermal equilibrium. Explain how these equilibrium conditions apply to each example.  Above the photosphere is a layer of less dense but higher- 5. If thermonuclear fusion in the Sun were suddenly to temperature gases called the chromosphere. Spicules extend stop, what would eventually happen to the overall radius of the Sun? Justify your answer using the ideas of hydrostatic equilibrium and thermal equilibrium. The question preceded by an asterisk (*) involves the topic G. Give some everyday examples of conduction, discussed in Box 18-1. convection, and radiative diffusion. Problem-solving tips and tools 7. Describe the Sun's interior. Include references to the main physical processes that occur at various depths within the Sun. 8. Suppose thermonuclear fusion in the Sun stopped abruptly. Would the intensity of sunlight decrease just as abruptly? Why or why not? 9. Explain how studying the oscillations of the Sun's surface can give important, detailed information about physical conditions deep within the Sun. 10. What is a neutrino? Why is it useful to study neutrinos coming from the Sun? What do they tell us that cannot be learned from other avenues of research? 11. Unlike all other types of telescopes, neutrino detectors are placed deep underground. Why? 12. Describe the dangers in attempting to observe the Sun. How have astronomers learned to circumvent these observational problems? 13. Briefly describe the three layers that make up the Sun's atmosphere. In what ways do they differ from each other? 14. What is solar granulation? Describe how convection gives rise to granules. ADVANCED ~l1FST10N5 You may have to review Wien's law and the Stefan-Boltzmann law, which are the subject of Section 5-4. discusses the properties of photons. As we described in , you can simplify calculations by taking ratios, such as the ratio of the flux from a sunspot to the flux from the undisturbed photosphere. When you do this, all the cumbersome constants cancel out. shows the various parts of the electromagnetic spectrum. We introduced the Doppler effect in Section 5-9 and l_3ox ` . For information about the planets, see ' ' ,- l . 24. How much energy would be released if each of the following masses were converted entirely into their equivalent energy: (a) a carbon atom with a mass of 2 x 10-Z6 kg, (b) 1 kilogram, and (c) a planet as massive as the Earth (6 x 1024 kg)? 25. Use the luminosity of the Sun (given in Table 18-1) and the answers to the previous question to calculate how long the Sun must shine in order to release an amount of energy equal to that produced by the complete mass-to-energy conversion of (a) a carbon atom, (b) 1 kilogram, and (c) the Earth.  15. High-resolution spectroscopy of the photosphere reveals that absorption lines are blueshifted in the spectrum of the central, bright regions of granules but are redshifted in the spectrum of the dark boundaries between granules. Explain how these observations show that granulation is due to convection. 27. (a) Estimate how many kilograms of hydrogen the Sun has consumed over the past 4.6 billion years, and estimate the amount of mass that the Sun has lost as a result. Assume that the Sun's luminosity has remained constant during that time. (b) In fact, the Sun's luminosity when it first formed was only about 70% of its present value. With 18. How do astronomers know that the temperature of this in mind, explain whether your answers to part (a) are the corona is so high? an overestimate or an underestimate. 1 19. How do astronomers know when the next sunspot *2g, (a) A positron has the same mass as an electron (see maximum and minimum will occur? Append , ). Calculate the amount of energy released by 20. Why do astronomers say that the solar cycle is really the annihilation of an electron and positron. (b) The 22 years long, even though the number of sunspots varies products of this annihilation are two photons, each of ~; over an 11-year period? equal energy. Calculate the wavelength of each photon, 21. Explain how the magnetic-dynamo model accounts for and confirm from Figure S-7 that this wavelength is the gamma-ray range. he solar cyc l e. t 16. What is the difference between granules and supergranules? 17. What are spicules? Where are they found? How can you observe them? What causes them? 29 22. Explain why the surface of the Sun appears black in Sirius is the brightest star in the night sky. It has a luminosity of 235 L that is it is 235 times as luminous .o,,. as the Sun and burns hydrogen at a rate 23.5 times greater 23. Why should solar flares and coronal mass ejections be a than the Sun. How many kilograms of hydrogen does Sirius convert into helium each second?  Figure 18-28. concern for businesses that use telecommunication satellites? 26. Assuming that the current rate of hydrogen burning in the Sun remains constant, what fraction of the Sun's mass will be converted into helium over the next 5 billion years? How will this affect the overall chemical composition of the Sun? Our Star, the Sun 415  416 1 CHAPTER 18  30. (Refer to the preceding question.) Sirius has 2.3 times why the Sun appears somewhat larger in the ultraviolet the mass of the Sun. Do you expect that the lifetime of image than in the visible-light image. Sirius will be longer, shorter, or the same length as that of the Sun? Explain your reasoning.  31. What would happen if the Sun were not in a state of both hydrostatic and thermal equilibrium? Explain your reasoning. 32. Using the mass and size of the Sun given in Table 18-1, verify that the average density of the Sun is 1410 kg/m3. Compare your answer with the average densities of the Jovian planets. 33. Use the data in Table 18-2 to calculate the average density of material within 0.1 Ro of the center of the Sun. (You will need to use the mass and radius of the Sun as a whole, given in Table 18-1.) Explain why your answer is not the same as the density at 0.1 Ro given in Table 18-2. 34. In a typical solar oscillation, the Sun's surface moves up or down at a maximum speed of 0.1 m/s. An astronomer sets out to measure this speed by detecting the Doppler shift of an absorption line of iron with wavelength 557.6099 nm. What is the maximum wavelength shift that she will observe? 35. The amount of energy required to dislodge the extra electron from a negative hydrogen ion is 1.2 x 10-19 J. 39. Find the wavelength of maximum emission of the (a) The extra electron can be dislodged if the ion absorbs a umbra of a sunspot and the wavelength of maximum photon of sufficiently short wavelength. (Recall from emission of a sunspot's penumbra. In what part of the ' , _ that the higher the energy of a photon, the electromagnetic spectrum do these wavelengths lie? shorter its wavelength.) Find the longest wavelength (in 40. (a) Find the ratio of the energy flux from a patch of a nm) that can accomplish this. (b) In what part of the sunspot's penumbra to the energy flux from an equally electromagnetic spectrum does this wavelength lie? large patch of undisturbed photosphere. Which patch is (c) Would a photon of visible light be able to dislodge the brighter? (b) Find the ratio of the energy flux from a patch extra electron? Explain. (d) Explain why the photosphere, f a sunspot's penumbra to the energy flux from an equally which contains negative hydrogen ions, is quite opaque to o large patch of umbra. Again, which patch is brighter? visible light but is less opaque to light with wavelengths longer than the value you calculated in (a). 41. Suppose that you want to determine the Sun's rotation rate by observing its sunspots. Is it necessary to take the Earth's orbital motion into account? Why or why not? 36. Astronomers often use an Ha filter to view the chromosphere. Explain why this can also be accomplished with filters that are transparent only to the wavelengths of the H and K lines of ionized calcium. 37. Calculate the wavelengths at which the photosphere, chromosphere, and corona emit the most radiation. Explain how the results of your calculations suggest the best way to observe these regions of the solar atmosphere. (Hint: Treat each part of the atmosphere as a perfect blackbody. Assume average temperatures of 50,000 K and 1.5 x 106 K for the chromosphere and corona, respectively.) 38. On November 15, 1999, the planet Mercury passed in front of the Sun as seen from Earth. The TRACE spacecraft made these time-lapse images of this event using ultraviolet light (top) and visible light (bottom). (Mercury moved from left to right in these images. The time between successive views of Mercury is 6 to 9 minutes.) Explain  R I V OX G  (K. Schrijver, Stanford-Lockheed Institute for Space R I 0 U X G Research; TRACE; and NASA) 42. The amount of visible light emitted by the Sun varies only a little over the 11-year sunspot cycle. But the amount of X rays emitted by the Sun can be ten times greater at solar maximum than at solar minimum. Explain why these two types of radiation should be so different in their variability. DISCUSSION QUESTIONS 43. Discuss the extent to which cultures around the world have worshiped the Sun as a deity throughout history. Why do you suppose there has been such widespread veneration? 44. In the movie Star Trek IV The Voyage Home, the starship Enterprise flies on a trajectory that passes close to the Sun's surface. What features should a real spaceship have to survive such a flight? Why?  ,,;9'o 241 49. Determining the Lifetime of a Solar Granule. Access and view the video "Granules on the Sun's Surface" in Chapter 18 of the Universe Web site or CD-ROM. Your task is to determine the approximate lifetime of a solar granule on the photosphere. Select an area, then slowly and rhythmically repeat "start, stop, start, stop" until  you can consistently predict the appearance and disappearance of granules. While keeping your rhythm, move to a different area of the video and continue monitoring the appearance and disappearance of granules. When you are confident you have the timing right, move your eyes (or use a partner) to the clock shown in the video. Determine the length of time between the appearance and disappearance of the granules and record your answer.   45. Discuss some of the difficulties in correlating solar Control Panel to zoom in until the sunspots are clearly activity with changes in the Earth's climate. visible. Set the time step to 1 day. Using the single-step time control buttons, step through enough time to determine the rotation rate of the Sun. Which part of the actual Sun's surface rotates at the rate shown in Starry Night? (The program does not show the Sun's differential rotation.) 46. Describe some of the advantages and disadvantages of observing the Sun (a) from space and (b) from the Earth's South Pole. What kinds of phenomena and issues might solar astronomers want to explore from these locations? yaqERq~, "'WES/CD-ROM QUESTIONS 47. Search the World Wide Web for the latest information from the neutrino detectors at the Super-Kamiokande Observatory and the Sudbury Neutrino Observatory. What are the most recent results from these detectors? What is the current thinking about the solar neutrino problem? What is the status of a new detector called Borexino? 48. Search the World Wide Web for information about features in the solar atmosphere called sigmoids. What are they? What causes them? How might they provide a way to predict coronal mass ejections? OSSERVINQ PROJECTS 50. Use a telescope with a solar filter to observe the surface of the Sun. Do you see any sunspots? Sketch their appearance. Can you distinguish between the umbrae and penumbrae of the sunspots? Can you see limb darkening? Can you see any granulation? 51. If you have access to an Ha filter attached to a telescope especially designed for viewing the Sun safely, use this instrument to examine the solar surface. How does the appearance of the Sun differ from that in white light? What do sunspots look like in Ha? Can you see any prominences? Can you see any filaments? Are the filaments in the Ha image near any sunspots seen in white light? (Note that the amount of activity that you see will be much greater at some times during the solar cycle than at others.) pRRY NjG~, 52. Use the Starry Night program to measure the ~ Sun's rotation. Set the time in the Control Panel to 12:00:00 P.M., center on the Sun (select Find... in the Edit menu), and use the controls in the Observing tips and tools At the risk of repeating ourselves, we remind you to never look directly at the Sun, because it can easily cause permanent blindness. You can view the Sun safely without a telescope just by using two pieces of white cardboard. First, use a pin to poke a small hole in one piece of cardboard; this will be your "lens," and the other piece of cardboard will be your "viewing screen." Hold the "lens" piece of cardboard so that it is face-on to the Sun and sunlight can pass through the hole. With your other hand, hold the "viewing screen" so that the sunlight from the "lens" falls on it. Adjust the distance between the two pieces of cardboard so that you see a sharp image of the Sun on the "viewing screen." This image is perfectly safe to view. It is actually possible to see sunspots with this low-tech apparatus. For a better view, use a telescope with a solar filter that fits on the front of the telescope. A standard solar filter is a piece of glass coated with a thin layer of metal to give it a mirrorlike appearance. This coating reflects almost all the sunlight that falls on it, so that only a tiny, safe amount of sunlight enters the telescope. An H filter, which looks like a red piece of glass, keeps the light at a safe level by admitting only a very narrow range of wavelengths. (Filters that fit on the back of the telescope are not recommended. The telescope focuses concentrated sunlight on such a filter, heating it and making it susceptible to cracking-and if the filter cracks when you are looking through it, your eye will be ruined instantly and permanently.) To use a telescope with a solar filter, first aim the telescope away from the Sun, then put on the filter. Keep the lens cap on the telescope's secondary wide-angle "finder scope" (if it has one), because the heat of sunlight can fry the finder scope's optics. Next, aim the telescope toward the Sun, using the telescope's shadow to judge when you are pointed in the right direction. You can then safely look through the telescope's eyepiece. When you are done, make sure you point the telescope away from the Sun before removing the filter and storing the telescope. Note that the amount of solar activity that you can see (sunspots, filaments, flares, prominences, and so on) will depend on where the Sun is in its 11-year sunspot cycle. Our Star, the Sun 417 r 448 1 CHAPTER 19 The Population of Stars: Stars of relatively low luminosity Mass-Luminosity Relation for Main-Sequence Stars: The are more common than more luminous stars. Our own Sun mass-luminosity relation expresses a direct correlation is a rather average star of intermediate luminosity. between mass and luminosity for main-sequence stars. The greater the mass of a main-sequence star, the greater The Magnitude Scale: The apparent magnitude scale is an its luminosity. alternative way to measure a star's apparent brightness. Spectroscopic Observations of Binary Stars: Some binaries can be detected and analyzed, even though the system may be so distant or the two stars so close together that the two star images cannot be resolved. The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 parsecs. A version of the inverse-square law relates a star's absolute magnitude, apparent magnitude, and distance. Photometry and Color Ratios: Photometry measures the apparent brightness of a star. The color ratios of a star are the ratios of brightness values obtained through different standard filters, such as the U, B, and V filters. These ratios are a measure of the star's surface temperature. A spectrum binary appears to be a single star but has a spectrum with the absorption lines for two distinctly different spectral types. A spectroscopic binary has spectral lines that shift back and forth in wavelength. This is caused by the Doppler effect, as the orbits of the stars carry them first toward then away from the Earth. Spectral Types: Stars are classified into spectral types (subdivisions of the spectral classes O, B, A, F, G, K, and M), based on the major patterns of spectral lines in their spectra. The spectral class and type of a star is directly related to its surface temperature: O stars are the hottest and M stars are the coolest. An eclipsing binary is a system whose orbits are viewed nearly edge-on from the Earth, so that one star periodically eclipses the other. Detailed information about the stars in an eclipsing binary can be obtained from a study of the binary's radial velocity curve and its light curve. Most brown dwarfs are in even cooler spectral classes called L and T. Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusion. REVIEW QllE .ST10N.S Hertzsprung-Russell Diagram: The Hertzsprung-Russell (H-R) diagram is a graph plotting the absolute magnitudes of stars against their spectral types-or, equivalently, their luminosities against surface temperatures. The positions on the H-R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperature. 1. Describe how the parallax method of finding a star's distance is similar to binocular (two-eye) vision in humans.  2. Why are measurements of stellar parallax difficult to make? What are the advantages of making these measurements from orbit? 3. What is the inverse-square law? Use it to explain why an ordinary lightbulb can appear brighter than a star, even - , though the lightbulb emits far less light energy per second. On the H-R diagram, giant and supergiant stars lie above the main sequence, while white dwarfs are below the main sequence. 4. Briefly describe how you would determine the luminosity of a nearby star. Of what value is knowing the luminosity of various stars? By carefully examining a star's spectral lines, astronomers 5, Why is the magnitude scale called a "backward" scale? can determine whether that star is a main-sequence star, What is the difference between apparent magnitude and giant, supergiant, or white dwarf. Using the H-R diagram absolute magnitude? and the inverse-square law, the star's luminosity and distance can be found without measuring its stellar parallax. 6. The star Zubenelgenubi (from the Arabic for - "scorpion's southern claw") has apparent magnitude Binary Stars: Binary stars, in which two stars are held in +2.75, while the star Sulafat (Arabic for "tortoise") has orbit around each other by their mutual gravitational apparent magnitude +3.25. Which star appears brighter? attraction, are surprisingly common. Those that can be From this information alone, what can you conclude about resolved into two distinct star images by an Earth-based the luminosities of these stars? Explain. telescope are called visual binaries. 7. Would it be possible for a star to appear bright when viewed through a U filter or a V filter, but dim when viewed through a B filter? Explain. Each of the two stars in a binary system moves in an elliptical orbit about the center of mass of the system. Binary stars are important because they allow g, Explain why the color ratios of a star are related to the astronomers to determine the masses of the two stars in a star's surface temperature. binary system. The masses can be computed from measurements of the orbital period and orbital dimensions 9. Which gives a more accurate measure of a star's surface of the system. ' temperature, its color ratios or its spectral lines? Explain.   10. A fellow student expresses the opinion that since the Sun's spectrum has only weak absorption lines of hydrogen, this element cannot be a major constituent of the Sun. How would you enlighten this person? 11. What are the most prominent absorption lines you would expect to find in the spectrum of a star with a surface temperature of (a) 35,000 K, (b) 2800 K, and (c) 5800 K (like the Sun)? Briefly describe why these stars have such different spectra even though they have essentially the same chemical composition. 12. If a red star and a blue star both have the same radius and both are the same distance from the Earth, which one looks brighter in the night sky? Explain why. 13. Sketch a Hertzsprung-Russell diagram. Indicate the regions on your diagram occupied by (a) main-sequence stars, (b) red giants, (c) supergiants, (d) white dwarfs, and (e) the Sun. 14. Most of the bright stars in the night sky (see Appendix S) 24. Find the average distance from the Sun to Pluto in are giants and supergiants. How can this be, if giants and parsecs. Compared to Pluto, how many times farther away supergiants make up only 1 % of the population of stars? from the Sun is Proxima Centauri? 15. Explain why the dashed lines in Figure 19-15 slope down and to the right. 16. Some giant and supergiant stars are of the same spectral type (G2) as the Sun. What aspects of the spectrum of a G2 star would you concentrate on to determine the star's luminosity class? Explain what you would look for. ADVANCCD QUESTIONS Questions preceded by an asterisk (*) involve topics discussed in the Boxes. I Problem-solving tips and tools Look carefully at the worked examples in Boxes 19-1, 19-2, 19-3, and 19-4 before attempting these exercises. For data on the solar system, see api~ - . , c, 1 an,! -' at the back of this book. Remember that a telescope's light-gathering power is proportional to the area of its objective or primary mirror. The volume of a sphere of radius R is 4/s7cR3. Make use of the H-R diagrams in this chapter to answer questions involving spectroscopic parallax. As Box 19-3 shows, some of the problems concerning magnitudes may require facility with logarithms. 25. Suppose that a dim star were located 2 million AU from the Sun. Find (a) the distance to the star in parsecs and (b) the parallax angle of the star. Would this angle be measurable with present-day techniques? 26. Kapteyn's star, named after the Dutch astronomer who found it, has a parallax angle of 0.255 arcsec. How far away is the star?  17. Suppose that you want to determine the temperature, diameter, and luminosity of an isolated star (not a member of a binary system). Which of these physical quantities require you to know the distance to the star? Explain. 18. What is the mass-luminosity relation? Does it apply to stars of all kinds? 19. Use Figure 19-22 to (a) estimate the mass of a main-sequence star that is 1000 times as luminous as the Sun, and (b) estimate the luminosity of a main-sequence star whose mass is one-fifth that of the Sun. Explain your answers. 20. How do white dwarfs differ from brown dwarfs? Which are more massive? Which are larger in radius? Which are denser? 'E30. In the spectrum of a particular star, the Balmer line Ha has a wavelength of 656.41 nm. The laboratory value for the wavelength of Ha is 656.28 nm. (a) Find the star's radial velocity. (b) Is this star approaching us or moving away? Explain. (c) Find the wavelength at which you 22. Give two reasons why a visual binary star is unlikely to would expect to find Hp in the spectrum of this star, given that the laboratory wavelength of Hp is 486.13 nm. (d) Do your answers depend on the distance from the Sun to this star? Why or why not? 21. Sketch the radial velocity curves of a binary consisting of two identical stars moving in circular orbits that are (a) perpendicular to and (b) parallel to our line of sight. also be a spectroscopic binary star. 23. Sketch the light curve of an eclipsing binary consisting of two identical stars in highly elongated orbits oriented so that (a) their major axes are pointed toward the Earth and 'F31. Derive the equation given in Box 19-1 relating proper (b) their major axes are perpendicular to our line of sight. motion and tangential velocity. (Hint: See Box 1-1.) *27. Kapteyn's star (see the previous question) has a proper motion of 8.67 arcsec per year and a radial velocity of +246 km/s. (a) What is the star's tangential velocity? (b) What is the star's actual speed relative to the Sun? (c) Is Kapteyn's star moving toward the Sun or away from the Sun? Explain. *28. How far away is a star that has a proper motion of 0.08"/year and a tangential velocity of 40 km/s? For a star at this distance, what would its tangential velocity have to be in order for it to exhibit the same proper motion as Barnard's star (Box 19-1)? *29. The space velocity of a certain star is 120 km/s and its radial velocity is 48 km/s. Find the star's tangential velocity.  The Nature of the Stars 449 450 1 CHAPTER 19 32. How much dimmer does the Sun appear from Neptune with the naked eye, what is its absolute magnitude? Is such than from Earth? (Hint: The average distance between a a star more or less luminous than the Sun? Explain. planet and the Sun equals the semimajor axis of the 43. (a) On a copy of Figure 19-8, sketch the intensity curve planet's orbit.) for a blackbody at a temperature of 3000 K. Note that this 33. Stars A and B are both equally bright as seen from Earth, figure shows a smaller wavelength range than Figure 19-7a. but A is 60 pc away while B is 15 pc away. Which star has (b) Repeat part (a) for a blackbody at 12,000 K (see Figure the greater luminosity? How many times greater is it? 19-7c). (c) Use your sketches from parts (a) and (b) to 34. Stars C and D both have the same luminosity, but C is explain why the color ratios bv/bB and bB/bU are less than 36 pc from Earth while D is 12 pc from Earth. Which star 1 for very hot stars but greater than 1 for very cool stars. appears brighter as seen from Earth? How many times '144. Astronomers usually express a star's color using brighter is it? apparent magnitudes. The star's apparent magnitude as 35. Suppose two stars have the same apparent brightness, viewed through a B filter is called mB, and its apparent but one star is 12 times farther away than the other. What magnitude as viewed through a V filter is mv. The is the ratio of their luminosities? Which one is more difference mB - mv is called the B-V color index ("B luminous, the closer star or the farther star? minus V"). Is the B-V color index positive or negative for very hot stars? What about very cool stars? Explain 36. The solar constant, equal to 1370 W/m2, is the amount your answers. of light energy from the Sun that falls on 1 square meter of the Earth's surface in 1 second (see Section 19-2). What *45. (See Advanced Question 44.) The B-V color index is would the distance between the Earth and the Sun have to related to the color ratio bv/bB by the equation be in order for the solar constant to be 1 watt per square ,~B\ meter (1 W/m2)? MB - mv = 2.5 log I` ~fl 37. The star Procyon in Canis Minor (the Small Dog) is a prominent star in the winter sky, with an apparent brightness 1.3 x 10-11 that of the Sun. It is also one of the nearest stars, being only 3.50 parsecs from Earth. What is the luminosity of Procyon? Express your answer as a multiple of the Sun's luminosity. (a) Explain why this equation is correct. (b) Use the data in Table 19-1 to calculate the B-V color indices for Bellatrix, the Sun, and Betelgeuse. From your results, describe a simple rule that relates the value of the B-V color index to a star's color. 38. The star HIP 92403 (also called Ross 154) is only 2.97 parsecs from Earth but can be seen only with a telescope, because it is 60 times dimmer than the dimmest star visible to the unaided eye. How close to us would this star have to be in order for it to be visible without a telescope? Give your answer in parsecs and in AU. Compare with the semimajor axis of Pluto's orbit around the Sun. *39. The star HIP 72509 has an apparent magnitude of +12.1 and a parallax angle of 0.222 arcsecond. (a) Determine its absolute magnitude. (b) Find the approximate ratio of the luminosity of HIP 72509 to the Sun's luminosity. *40. Suppose you can just barely see a twelfth-magnitude star through an amateur's 6-inch telescope. What is the magnitude of the dimmest star you could see through a 60-inch telescope? *41. A certain type of variable star is known to have an average absolute magnitude of 0.0. Such stars are observed in a particular star cluster to have an average apparent magnitude of +14.0. What is the distance to that star cluster? 46. The bright star Rigel in the constellation Orion has a surface temperature about 1.6 times that of the Sun. Its luminosity is about 64,000 Lo. What is Rigel's radius compared to the radius of the Sun? 47. Suppose a star experiences an outburst in which its surface temperature doubles but its average density (its mass divided by its volume) decreases by a factor of 8. The mass of the star stays the same. By what factors do the star's radius and luminosity change? 48. The Sun experiences solar flares (see `--, . ' ' ). The amount of energy radiated by even the strongest solar flare is not enough to have an appreciable effect on the Sun's luminosity. But when a flare of the same size occurs on a main-sequence star of spectral class M, the star's brightness can increase by as much as a factor of 2. Why should there be an appreciable increase in brightness for a main-sequence M star but not for the Sun? 49. (See Figure 19-12.) What temperature and spectral classification would you give to a star with equal line strengths of hydrogen (H) and neutral helium (He I)? Explain. *42. (a) Find the absolute magnitudes of the brightest and 50. The bright star Zubeneschmali ((3 Librae) is of spectral dimmest of the labeled stars in Figure 19-66. Assume that type B8 and has a luminosity of 130 Lo. What is the star's all of these stars are 110 pc from Earth. (b) If a star in the approximate surface temperature? How does its radius Pleiades cluster is just bright enough to be seen from Earth compare to that of the Sun?  ; 51. Castor (a Geminorum) is an A1 V star with an apparent brightness of 4.4 x 10-12 that of the Sun. Determine the approximate distance from the Earth to Castor (in parsecs).  ; 52. A brown dwarf called CoD-337795 B has a ^ luminosity of 0.0025Lo. It has a relatively high surface temperature of 2550 K, which suggests that it is very young and has not yet had time to cool down by emitting radiation. (a) What is this brown dwarf's spectral class? (b) Find the radius of CoD-337795 B. Express your answer in terms of the Sun's radius and in kilometers. How does this compare to the radius of Jupiter? Is the name "dwarf" justified? '11 53. The visual binary 70 Ophiuchi (see Figure 19-20) has a period of 87.7 years. The parallax of 70 Ophiuchi is 0.2 arcsec, and the apparent length of the semimajor axis as seen through a telescope is 4.5 arcsec. (a) What is the distance to 70 Ophiuchi in parsecs? (b) What is the actual length of the semimajor axis in AU? (c) What is the sum of the masses of the two stars? Give your answer in , solar masses. 54. An astronomer observing a binary star finds that one of the stars orbits the other once every 5 years at a distance of 10 AU. (a) Find the sum of the masses of the two stars. (b) If the mass ratio of the system (Ml/M2) is 4.0, find the individual masses of the stars. Give your answers in terms of the mass of the Sun.  DISCUSSION QUfSTIONS *55. As seen from the starship Enterprise in the Star Trek television series and movies, stars appear to move across the sky due to the starship's motion. How fast would the Enterprise have to move in order for a star 1 pc away to appear to move 1 per second? (Hint: The speed of the star as seen from the Enterprise is the same as the speed of the Enterprise relative to the star.) How does this compare with the speed of light? Do you think the stars appear to move as seen from an orbiting space shuttle, which moves at about 8 km/s?  56. From its orbit around the Earth, the Hipparcos satellite could measure stellar parallax angles with acceptable accuracy only if the angles were larger than about 0.002 arcsec. Discuss the advantages or disadvantages of making parallax measurements from a satellite in a large solar orbit, say at the distance of Jupiter from the Sun. If this satellite can also measure parallax s r angles of 0.002 arcsec, what is the distance of the most remote stars that can be accurately determined? How much bigger a volume of space would be covered compared to the Earth-based observations? How many more stars would you expect to be contained in that volume?  57. It is desirable to be able to measure the radial velocity of stars (using the Doppler effect) to an accuracy of 1 km/s  or better. One complication is that radial velocities refer to the motion of the star relative to the Sun, while the observations are made using a telescope on the Earth. Is it important to take into account the motion of the Earth around the Sun? Is it important to take into account the Earth's rotational motion? To answer this question, you will have to calculate the Earth's orbital speed and the speed of a point on the Earth's equator (the part of the Earth's surface that moves at the greatest speed because of the planet's rotation). If one or both of these effects are of importance, how do you suppose astronomers compensate for them? A-Ra cp, WET3/Cl1-ROM QUESTIONS 58. Search the World Wide Web for recent discoveries about brown dwarfs. Are all brown dwarfs found orbiting normal stars, as in Figure 7-22, or are they also found in isolation? The Sun experiences flares (see _rv,n_ I ~ !()),as do other normal stars; is there any evidence that brown dwarfs also experience flares? If so, is there anything unusual about these flares? p~l 59. Distances to Stars Using Parallax. Access the Active Integrated Media Module "Using Parallax to Determine Distance" in Chapter 19 of the Universe web site or CD-ROM. Use this to determine the distance in parsecs and in light-years to each of the following stars: (a) Betelgeuse (parallax p = 0.00763"); (b) Vega (p = 0.129"); (c) Antares (p = 0.00540"); (d) Sirius (p = 0.379"). ORSERVINQ PROJECTS Observing tips and tools Even through a telescope, the colors of stars are sometimes subtle and difficult to see. To give your eye the best chance of seeing color, use the "averted vision" trick: When looking through the telescope eyepiece, direct your vision a little to one side of the star you are most interested in. This places the light from that star on a more sensitive part of your eye's retina. 60. The table at the top of page 452 lists five well-known red stars. It includes their right ascension and declination (celestial coordinates described in ), apparent magnitudes, and color ratios. As their apparent magnitudes indicate, all these stars are somewhat variable. Observe at least two of these stars both by eye and through a small telescope. Is the reddish color of the stars readily apparent, especially in contrast to neighboring stars? (The Jesuit priest and astronomer Angelo Secchi named Y Canum The Nature of the Stars 451 Chapter 21 452 J CHAPTER 19 Star Apparent Right ascension Declination magnitude bv/bB Betelgeuse Sh 55.2m +7 24' 0.4-1.3 5.5 Y Canum Venaticorum 1245.1 +4526 5.5-6.0 10.4 Antares 1629.4 -2626 0.9-1.8 5.4 g Cephei 2143.5 +5847 3.6-5.1 8.7 TX Piscium 2346.4 +329 5.3-5.8 11.0 Note: The right ascensions and declinations are given for epoch 2000. Venaticorum "La Superba," and p Cephei is often called Time (the same as Greenwich Mean Time), so you will have William Herschel's "Garnet Star.") to convert the time to that of your own time zone. Algol is 61. The table of double stars at the bottom of this page normally the second brightest star in the constellation of includes vivid examples of contrasting star colors. The Prseus~Because of its position on the celestial sphere (R.A. _ table lists the angular separation between the stars of each 3h 08.2 , Decl. =+40 57 ), Algol is readily visible from double. Observe at least four of these double stars northern latitudes during the fall and winter months. through a telescope. Use the spectral types listed t0 pRRY Nz- 63, Use the Starry Night program to investigate estimate the difference in surface temperature of the stars the brightest stars. (a) In the Control Panel, click in each pair you observe. Does the double with the on the "Home" button and set the date to greatest difference in temperature seem to present the today's date and the time to midnight (12:00:00 A.M.). greatest color contrast? From what you see through the Display the outlines of the constellations (select telescope and on what you know about the H-R diagram, Boundaries, Labels, and Astronomical in the Constellations explain why all the cool stars (spectral types K and M) menu). Scroll around the sky and identify at least five of listed are probably giants or supergiants. the brighter stars by moving the cursor over them. 62. Observe the eclipsing binary Algol ((3 Persei), using (Brighter stars are shown as larger dots.) In which nearby stars to judge its brightness during the course of an constellation does each of the stars you selected lie? Which eclipse. Algol has an orbital period of 2.87 days, and, with of the stars you selected are listed in ' ? Of these, the onset of primary eclipse, its apparent magnitude drops which is the most luminous? Which is the most distant? from 2.1 to 3.4. It remains this faint for about 2 hours. The (b) In the Control Panel, set the date to six months from entire eclipse, from start to finish, takes about 10 hours. today, and again set the time to 12:00:00 A.M. Which of Consult the "Celestial Calendar" section of the current issue the stars that you selected in part (a) are visible? Which of Sky & Telescope for the predicted dates and times of the are not? Explain why the passage of six months should minima of Algol. Note that the schedule is given in Universal make a difference. Right Apparent Angular Spectral Star ascension Declination magnitudes separation types 55 Piscium Oh 39.9m +21 26' 5.4 and 8.7 6.5" KO and F3 ,y Andromedae 203.9 +42 20 2.3 and 4.8 , 9.8" K3 and AO 32 Eridani 3 54.3 - 257 4.8 and 6.1 6.8" GS and A2 t Cancri 846.7 +2846 4.2 and 6.6 30.5" GS and AS y Leonis 1020.0 +1951 2.2 and 3.5 4.4" KO and G7 24 Comae Berenices 1235.1 +18 23 5.2 and 6.7 20.3" KO and A3 v Bootis 1445.0 +2704 2.5 and 4.9 2.8" KO and AO a Herculis 1714.6 +1423 3.5 and 5.4 4.7" MS and GS 59 Serpentis 1827.2 + 012 5.3 and 7.6 3.8" GO and A6 (3 Cygni 1930.7 +2758 3.1 and 5.1 34.3" K3 and B8 8 Cephei 2229.2 +5825 4* and 7.5 20.4" FS and AO `The brighter star in the S Cephei binary system is a variable star of approximately the fourth magnitude. Note: The right ascensions and declinations are given for epoch 2000. 474 I CHAPTER 20 R~V1EW QUESTIONS 1. If an interstellar medium fills the space between the stars, how is that we are able to see the stars at all? 2. Summarize the evidence that interstellar space contains (a) gas and (b) dust. 19. What are giant molecular clouds? What role do these clouds play in the birth of stars? 20. Giant molecular clouds are among the largest objects in our Galaxy. Why, then, were they discovered only relatively recently? 21. Consider the following stages in the evolution of a 3. What are H II regions? Near what kinds of stars are young star cluster: (i) H II region, (ii) dark nebula, they found? (iii) formation of O and B stars, (iv) giant molecular cloud. 4. What are stationary absorption lines? In what sort of Put these stages in the correct chronological order and spectra are they seen? How do they give evidence for the discuss how they are related. existence of the interstellar medium? 22. Briefly describe four mechanisms that compress the 5. Why is the daytime sky blue? Why are distant interstellar medium and trigger star formation. mountains purple? Why is the Sun red when seen near the horizon at sunrise or sunset? In what ways are your answers analogous to the explanations for the bluish color of 1~ D V~ N CE D ~l ~F SYl O N S reflection nebulae and the process of interstellar reddening? Problem-solving tips and tools 6. Why are low temperatures necessary in order for protostars to form inside dark nebulae? `~ You may find it helpful to review [~ox 19-4, which 7. Compare and contrast Barnard objects and Bok describes the relationship among a star's luminosity, globules. How many Sun-sized stars could you make out of radius, and surface temperature. Orbital periods are a Barnard object? Out of a Bok globule? described by Kepler's third law, which we discussed in ' mcs 4-2 and 4-4. Remember that the Stefan- 8. The interior of a dark nebula is billions of times less Boltzmann law (' ) relates the temperature of a dense than the air that you breathe. How, then, are dark blackbody to its energy flux. Remember, too, that the nebulae able to block out starlight? volume of a sphere of radius R is 4/3~R3. 9. Describe the energy source that causes a protostar to shine. How does this source differ from the energy source inside a main-sequence star? 10. What is an evolutionary track? How can evolutionary tracks help us interpret the H-R diagram? 11. What happens inside a protostar to slow and eventually halt its gravitational contraction? 12. Why are the evolutionary tracks of high-mass stars different from those of low-mass stars? For which kind of star is the evolution more rapid? Why? 13. In what ways is the internal structure of a 1-Mo main-sequence star different from that of a 5-Mo main-sequence star? From that of a 0.5-Mo main-sequence star? What features are common to all these stars? 14. What sets the limits on the maximum and minimum masses of a main-sequence star? 15. What are Herbig-Haro objects? Why are they often found in pairs? 16. Why do disks form around contracting protostars? What is the connection between disks and bipolar outflows? 17. Young open clusters like those shown in Figures 20-17 and 20-18 are found only in the plane of the Galaxy. Explain why this should be.  18. Why are observations at (a) infrared wavelengths and (b) millimeter wavelengths so much more useful in exploring interstellar clouds than observations at visible wavelengths? (Anglo-Australian Observatory) R I ~ U X G 23. The visible-light photograph below shows the Trifid Nebula in the constellation Sagittarius. Label the following features on this photograph: (a) reflection nebulae (and the star or stars whose light is being reflected); (b) dark nebulae; (c) H II regions; (d) regions where star formation may be occurring. Explain how you identified each feature.    infrared image from the Infrared Space Observatory, while a the lower picture (shown to the same scale) was made with visible light. Explain why the dark streaks in the visible-light image appear bright in the infrared image. 24. If you looked at the spectrum of a reflection nebula, would you see absorption lines, emission lines, or no lines? Explain your answer. As part of your explanation, describe how the spectrum demonstrates that the light was reflected , from nearby stars. 29. A newly formed protostar and a red giant are both 25. In the direction of a particular star cluster, interstellar located in the same region on the H-R diagram. Explain extinction allows only 15% of a star's light to pass through how you could distinguish between these two. each kiloparsec (1000 pc) of the interstellar medium. If the star cluster is 3.0 kiloparsecs away, what percentage of its 30. At one stage during its birth, the protosun had a photons survive the trip to the Earth? luminosity of 1000 Lo and a surface temperature of about 1000 K. At this time, what was its radius? Express your 26. Find the density (in atoms per cubic centimeter) of a answer in three ways: as a multiple of the Sun's present- Bok globule having a radius of 1 light-year and a mass day radius, in kilometers, and in astronomical units. of 100 Mo. How does your result compare with the density of a typical H II region, between 80 and 600 atoms per cm3? 31. (a) Determine the radius of the circumstellar accretion (Assume that the globule is made purely of hydrogen atoms.) disk in Figure 20-14. (You will need to measure this image with a ruler. Note the scale bar in this figure.) Give your answer in astronomical units and in kilometers. (b) Assume that the young star at the center of this disk has a mass of 1 Mo. What is the orbital period (in years) of a particle at the outer edge of the disk? (c) Using your ruler again, determine the length of the jet that extends to the right of the circumstellar disk in Figure 20-14. At a speed of 200 km/s, how long does it take gas to traverse the entire visible length of the jet? 27. The infrared-bright object at the center of Figure 20-106 is called the Becklin-Neugebauer object after its discoverers. Like the other bright spots in that image, it is a newly formed star. Assuming that all these stars began the process of formation of the same time, what can you conclude about the mass of the Becklin-Neugebauer object compared with those of the other stars? Does your conclusion depend on whether or not the stars have reached the main sequence? Explain your reasoning.   32. The concentration or abundance of ethyl alcohol in a typical molecular cloud is about 1 molecule per 10g cubic meters. What volume of such a cloud would contain < enough alcohol to make a martini (about 10 grams of alcohol)? A molecule of ethyl alcohol has 46 times the mass of a hydrogen atom (that is, ethyl alcohol has a molecular weight of 46). 28. The two false-color images below show a portion of the Trifid Nebula (see Question 23). The upper view is an  D1SCUSSION QUESTIONS 34. Some science-fiction movies show stars suddenly becoming dramatically brighter when they are "born" (that is, when thermonuclear fusion reactions begin in their cores). Discuss whether this is a reasonable depiction. 35. Suppose that the electrons in hydrogen atoms were not as strongly attracted to the nuclei of those atoms, so that these atoms were easier to ionize. What consequences might this have for the internal structure of main-sequence stars? Explain your reasoning.  36. What do you think would happen if our solar system (ESA/ISO, ISOCAM, and J. Cernicharo et al.; R I ~ U X G were to pass through a giant molecular cloud? Do you IAC, Observatorio del Teide, Tenerife) think the Earth has ever passed through such clouds? R ~ V U X G 33. (a) Find the angular diameter of the Cygnus Loop, shown in Figure 20-22. (b) Assuming that the supernova shock wave that spawned the Cygnus Loop has always traveled outward at the same speed, determine that speed in meters per second and kilometers per hour. (c) How long would an Earth astronomer have to wait to see the bright regions of the Cygnus Loop move outward by an angle of 1 arcminute? The Birth of Stars 475 476 I CHAPTER 20  37. Many of the molecules found in giant molecular clouds 42. Use a telescope to observe at least two of the following are orga~zic molecules (that is, they contain carbon). H II regions. In each case, can you guess which stars are Speculate about the possibility of life-forms and biological probably responsible for the ionizing radiation that causes processes occurring in giant molecular clouds. In what the nebula to glow? Can you see any obscuration or ways might the conditions existing in giant molecular silhouetted features that suggest the presence of interstellar clouds favor or hinder biological evolution? dust? Draw a picture of what you see through the telescope 38. Speculate on why a shock wave from a supernova and compare it with a photograph of the object. Take note seems to produce relatively few high-mass O and B stars, of which portions of the nebula were not visible through compared to the lower-mass A, F, G, and K stars. your telescope.  ~'(ERA~T ti~ tiVER/CD-ROM QUESTIONS 39. In recent years astronomers have been able to learn about the character of the interstellar medium in the vicinity of the Sun. Search the World Wide Web for information about aspects of the nearby interstellar medium, including features called the Local Interstellar Cloud and the Local Bubble. How do astronomers study the nearby interstellar medium? What makes these studies difficult? Is the interstellar medium relatively uniform in our neighborhood, or is it clumpy? If the latter, is our solar system in a relatively thin or thick part of the interstellar medium? How is our solar system moving through the interstellar medium? 40. Search the World Wide Web for recent discoveries about how brown dwarfs form. Do they tend to form in the same locations as "real" stars? Do they form in relatively small or relatively large numbers compared to "real" stars? What techniques are used to make these discoveries? Nebula Right ascension Declination M42 (Orion) Sh 35.4m M43 5 35.6 M20 (Trifid) 18 02.6 M8 (Lagoon) 18 03.8 M17 (Omega) 18 20.8 -5 2T -5 16 -23 02 -24 23 -16 11 Note: The right ascensions and declinations are given for epoch 2000. 43. On an exceptionally clear, moonless night, use a telescope to observe at least one of the following dark nebulae. These nebulae are very difficult to find, because they are recognizable only by the absence of stars in an otherwise starry part of the sky Are you confident that you actually saw the dark nebula? Does the pattern of background stars suggest a particular shape to the nebula? ti~P~' ~ ~,0 41. Measuring a Stellar Jet. Access the animation Nebula Right ascension Declination a ` W "A Stellar Jet in the Trifid Nebula" in Chapter 20 of the Universe web site/CD-ROM. (a) The Trifid Barnard 72 17h 23.Sm -23 38' Nebula as a whole has an angular diameter of 28 arcmin. By (The Snake) stepping through the animation, estimate the angular size of garnard 86 18 02.7 -27 50 the stellar jet shown at the end of the animation. (b) The Trifid Nebula is about 2800 pc (9000 ly) from Earth. Barnard 133 19 06.1 -6 50 Estimate the length of the jet in light-years. (c) If gas in the jet Barnard 142 and 143 19 40.7 -10 57 travels at 200 km/s, how long does it take to traverse the length of the jet? Give your answer in years. (Hint: There are Note: The right ascensions and declinations are given for epoch 2000: 3.16 x 10~ seconds in a year.) ORSCRVINC~ PROJCCTS Observing tips and tools After looking at the beautiful color photographs of nebulae in this chapter, you may find the view through a telescope a bit disappointing at first, but fear not. You can see a great deal with even a small telescope. To get the best view of a dim nebula using a telescope, use the same "averted vision" trick we described in O ,~;., ~;~ r;, -,~1 ~ta}~~ -, c h.:~; ___ ~_O: If you direct your vision a little to one side of the object that you are looking at, the light from that object will go onto a more sensitive part of the retina. 44. A few fine objects cover such large regions of the sky that they are best seen with binoculars. If you have access to a high-quality pair of binoculars, observe the North American Nebula in Cygnus and the Pipe Nebula in Ophiuchus. Both nebulae are quite faint, so you should attempt to observe them only on an exceptionally dark, ' clear, moonless night. The North America Nebula is a ; cloud of glowing hydrogen gas located about 3 east of Deneb, the brightest star in Cygnus. While searching for the North America Nebula, you may glimpse another diffuse H II region, the Pelican Nebula, located about 2 southeast of Deneb. The Pipe Nebula is a 7-long, meandering, dark nebula to the south and to the east of the star 9 (theta) Ophiuchi, which is in a section of Ophiuchus that extends southward between the   constellations of Scorpius and Sagittarius. Located about 12 east of the bright red star Antares, you can locate A Ophiuchi using the Starry Night software on the CD-ROM that accompanies this book.   1RRY NI~.y 45. Use the Starry Night program to investigate - a star-forming region. First turn off daylight (select Daylight in the Sky menu) and show the ntire celestial sphere (select Atlas in the Go menu). Center e field of view on M20, the Trifid Nebula, shown in the    figure that accompanies Question 23 (select Find... in the Edit menu). Using the controls at the right-hand end of the Control Panel, zoom out to the maximum field of view. Then, using the time controls in the Control Panel, step through the months of the year. (a) In which month is M20 highest in the sky at noon? Explain how you determined this. (b) In which month is M20 highest in the sky at midnight, so that it is best placed for observing with a telescope? Explain how you determined this.   14  The Birth of Stars 477  496 CHAPTER 21  Pulsating Variable Stars: When a star's evolutionary track carries it through a region in the H-R diagram called the instability strip, the star becomes unstable and begins to pulsate. Cepheid variables are high-mass pulsating variables. There is a direct relationship between their periods of pulsation and their luminosities. RR Lyrae variables are low-mass, metal-poor pulsating variables with short periods. Long-period variable stars also pulsate but in a fashion that is less well understood.  As a cluster ages, the main sequence is "eaten away" 11. There is a good deal of evidence that our universe is no from the upper left as stars of progressively smaller mass more than about 15 billion years old (see Chapter 28). .,f~r evolve into red giants. Explain why no main-sequence stars of spectral class M have yet evolved into red-giant stars. Relatively young Population I stars are metal rich; ancient Population II stars are metal poor. The metals (heavy elements) in Population I stars were manufactured by thermonuclear reactions in an earlier generation of Population II stars, then ejected into space and incorporated into a later stellar generation. 12. (a) The main-sequence stars Sirius (spectral type A1), Vega (AO), Spica (B1), Fomalhaut (A3), and Regulus (B7) are among the 20 brightest       !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~stars in the sky. Explain how you can tell that all these stars are younger than the Sun. (b) The third-brightest star in the sky, although it can be seen only south of 29 north latitude, is a (alpha) Centauri A. It is a main-sequence star of spectral type G2, the same as the Sun. Can you tell from this whether a Centauri A is younger than the Sun, the same age, or older? Explain your reasoning.  13. Using the same horizontal and vertical scales as in Figure 21-6a, make points on an H-R diagram for each of the stars listed in Table 21-1. Label each point with the star's mass and its main-sequence lifetime. Which of these stars will remain on the main sequence after 109 years? After 1011 years? , 14. How do astronomers know that globular clusters are . made of old stars? Close Binary Systems: Mass transfer in a close binary system occurs when one star in a close binary overflows its Roche lobe. Gas flowing from one star to the other passes across the inner Lagrangian point. This mass transfer can affect the evolutionary history of the stars that make up the binary system. RF'~!1^W Qt~fSTl(~NS 15. Red giant stars appear more pronounced in composites of infrared images and visible-light images, like those in Figure 21-36 and Figure 21-8. Explain why. 16. The horizontal-branch stars in Figure 21-8 appear blue (a) Explain why this is consistent with the color-magnitude diagram shown in Figure 21-9. (b) All horizontal-branch stars were once red giants. Explain what happened to these stars to change their color. 1. Why do high-mass main-sequence stars have shorter 1~ What is the difference between Population I and lifetimes than those of lower mass? Population II stars? In what sense can the stars of one population be regarded as the "children" of the 2. On what grounds are astronomers able to say that the Sun other population? has about 5 x 109 years remaining in its main-sequence stage? 18. The star whose spectrum is shown in Figure 21-11a has a lower percentage of heavy elements than the Sun, whose spectrum is shown in Figure 21-116. Hence, the star in Figure 21-11a has a higher percentage of hydrogen. Why, then, isn't the Hs absorption line of hydrogen noticeably darker for the star in Figure 21-11a? 3. What will happen inside the Sun 5 billion years from now, when it begins to mature into a red giant? 4. Explain how it is possible for the core of a red giant to contract at the same time that its outer layers expand. 5. How is a degenerate gas different from ordinary gases? 6. What is the helium flash? Why does it happen in some stars but not in others? 19. Why do astronomers attribute the observed Doppler shifts of a Cepheid variable to pulsation, rather than to some other cause, such as orbital motion? 7. Explain why the majority of the stars in the sky are main-sequence stars. 8. What does it mean when an astronomer says that a star "moves" from one place to another on an H-R diagram? 9. On an H-R diagram, main-sequence stars do not lie along a single narrow line but are spread out over a band (see Figure 21-6b). On the basis of how stars evolve during their main-sequence lifetime, explain why this should be so. 10. Explain how and why the turnoff point on the H-R diagram of a cluster is related to the cluster's age. 20. Why do Cepheid stars pulsate? Why are these stars important to astronomers who study galaxies beyond the Milky Way? 21. Would you expect the color of a Cepheid variable star ,~ (see Figure 21-14) to change during the star's oscillation period? If not, why not? If so, describe why the color should change, and describe the color changes you would expect to see during an oscillation period. , 22. What is a Roche lobe? What is the inner Lagrangian e ~ point? Why are Roche lobes important in close binary y star systems?  24. _\tassive main-sequence stars turn into red giants `-,before less massive stars. Why, then, is the more massive ~ ' '0'*~star in Algol a main-sequence star and the less massive star a red giant? `5. Tl ~ie radius of the Sun has increased over the past several billion years. Over the same time period, the size of the N on's orbit around the Earth has also increased. ' few bi alion years ago, were annular eclipses of the Sun see ' t =) more or less common than they are today? Exl~ lain. t  23. What is the difference between a detached binary, a semi- the Sun's luminosity remains nearly constant during the detached binary, a contact binary, and an overcontact binary? entire 101 years.  ~.OW.NCED Qt)FSTIONS Questions preceded by an asterisk (*) involve topics discussed in the BoAce in this cbapter or in Chapter 19. Problem-solving tips and tools Recall from ' that 6 x 1011 kg of hydrogen is converted into helium each second at the Sun's center. Recall also that you must use absolute (Kelvin) temperatures when using the Stefan- ~ Boltzmann law. You may find it helpful to review the ; discussion of apparent magnitude, absolute ~ magnitude, and luminosity in ~ a.x l . Section 19-4 ~ discusses the connection between the surface ;temperatures and colors of stars. Newton's form of tiKepler's third law (see . ~~) describes the )rbits of stars in binary systems. *30. Calculate the main-sequence lifetimes of (a) a 9-Mo star and (b) a 0.25-Mo star. Compare these lifetimes with that of the Sun. *31. The earliest fossil records indicate that life appeared on the Earth about a billion years after the formation of the solar system. What is the most mass that a star could have in order that its lifetime on the main sequence is long enough to permit life to form on one or more of its planets? Assume that the evolutionary processes would be similar to those that occurred on the Earth. 32. As a red giant, the Sun's luminosity will be about 2000 times greater than it is now, so the amount of solar energy falling on the Earth will increase to 2000 times its present-day value. Hence, to maintain thermal equilibrium, each square meter of the Earth's surface will have to radiate 2000 times as much energy into space as it does now. Use the Stefan-Boltzmann law to determine what the Earth's surface temperature will be under these conditions. (Hint: The present-day Earth has an average surface : temperature of 9C.) 33. When the Sun becomes a red giant, its luminosity will be about 2000 times greater than it is today. Assuming ` that this luminosity is caused only by the burning of the Sun's remaining hydrogen, calculate how long our star will be a red giant. (In fact, only a fraction of the remaining hydrogen will be consumed, so your answer will be an overestimate.) 34. What observations would you make of a star to determine whether its primary source of energy is hydrogen or helium burning? 35. The brightness of a certain Cepheid variable star increases and decreases with a period of 10 days. (a) What must this star's luminosity be if its spectrum has strong absorption lines of hydrogen and helium, but no strong absorption lines of heavy elements? (b) Repeat part (a) for the case in which the star's spectrum also has strong absorption lines of heavy elements.  26. The Sunas increased in radius by 6% over the past 4.6 billion years. 'izs present-day radius is 696,000 km. What was its radius 4.6 billion years ago? (Hint: The answer is not 654,000 km.) . -27. ( ',:ulate the escape speed from (a) the surface of the prc>cnt-Liay Sun and (b) from the surface of the Sun when it becomes a red giant, with essentially the same mass as today but with a radius that is 100 times larger. (c) Explain how your results show that a red-giant star can lose mass more easily than a main-sequence star. 36. The star X Arietis is an RR Lyrae variable. Its apparent brightness varies between 2.0 x 10-15 and 4.9 x 10-15 that *28. Calculate the average speed of a hydrogen atom . , of the Sun with a period of 0.65 day. Interstellar extinction (mass 1.67 x 10-27 kg) (a) in the atmosphere of the dims the star by 37% . Approximately how far away is present-day Sun, with temperature 5800 K, aud (b) in the the star? atmosphere of a 1-M,., red giant, with temperature 3500 K. 37. The apparent brightness of S Cephei (a Type I Cepheid ov-'(c) Compare your results with the escape speeds that you ; variable) varies with a period of 5.4 days. Its average calculated in Question 27. Use this comparison to discuss: apparent brightness is 5.1 x 10-13 that of the Sun. how well the present-day Sun and a 1-Mo red giant can % Approximately how far away is S Cephei? (Ignore retain hydrogen in their atmospheres. interstellar extinction.) 29. What mass of hydrogen will the Sun convert into 38. Suppose the detached star in (3 Lyrae (Figure 21-176) helium during its entire main-sequence lifetime of 101 did not have an accretion disk. Would the deeper dips in ., years? What fraction does this represent of the total mass the light curve be deeper, shallower, or about the same? -h-wm of hydrogen that was originally in the Sun? Assume that What about the shallower dip? Explain your answers. Stellar Evolution: After the Main Sequence 1 497  498 I CHAPTER 21 39. The two stars that make up the overcontact binary W Ursae Majoris (Figure 21-17c) have estimated masses of 0.99 Mo and 0.62 Mo. (a) Find the average separation between the two stars. Give your answer in kilometers. (b) The radii of the two stars are estimated to be 1.14 Ro and 0.83 Ro. Show that these values and your result in part (a) are consistent with the statement that this is an overcontact binary. ~PRRY tiI~-y 40. Consult recent issues of Sky ~' Telescope ~ and Astronomy to find out when Mira will next reach maximum brightness. Look up the star's location in the sky using the Starry Night program on the CD-ROM that comes with this book. (Use the Find... command in the Edit menu to search for Omicron Ceti.) Why is it unlikely that you will be able to observe Mira at maximum brightness? D1SClISSION QU~STIONS 41. The half-life of the gBe nucleus is 2.6 x 10-16 second, which is the average time that elapses before this unstable nucleus decays into two alpha particles. How would the universe be different if instead the 8Be half-life were zero? How would the universe be different if the 8Be nucleus were stable? 42. If the universe has a finite age (as a broad array of evidence indicates to be the case), what observational consequences does this have for H-R diagrams of star clusters? Could we use these consequences to establish constraints on the possible age of the universe? Explain. : :. : :viW. r r , ,.,. ~0~-1"K ~a,~ 43. Suppose that an oxygen nucleus were fused "' with a helium nucleus. What element would be formed? Look up the relative abundance of this element in, for example, the Handbook of Chemistry and Physics or on the World Wide Web. Comment on whether such a process is likely. 44. Although Polaris, the North Star, is a Cepheid variable, it pulsates in a somewhat different way than other Cepheids. Search the World Wide Web for information about this star's pulsations and how they have been measured by astronomers at the U.S. Naval Observatory How does Polaris pulsate? How does this differ from other Cepheids? Almach K3 II 2h 03.9'" +42 20" (y Andromedae) l~ Aldebaran (a Tauri) KS III 4 35.9 +16 31 Betelgeuse M2 I +~ 5 55.2 +07 24 (a Orionis) Arcturus (a Bootis) K2 III 14 15.7 +19 11 Antares (a Scorpii) M1 I 16 29.5 -26 26 Eltanin (yDraconis) KS III,;~ 17 56.7 +~I'~4.5 Enif (e Pegasi) K2 I 21 44.2 +09 ~2 ~~P'f ION~1 45, Observing Stellar Evolution. Step through < ~ the animation "The Hertzsprung-Russell Diagram and Stellar Evolution" in Chapter 21 of the Universe web site or CD-ROM. Use this animation to answer the following questions. (a) How does a 1-Mo star move on the H-R diagram during its first 4.6 billion , (4600 million) years of existence? Compare this with the ~t 47. Several of the open clusters referred to in Figure 21-10 discussion in Section 21-1 of how the Sun has evolved over can be seen with a good pair of binoculars. Observe as ` ` the past 4.6 billion years. (b) What is the zero-age spectral many of these clusters as you can, using both a telescope  Note: The right ascensions and declinations are given for eN~cb 2000. class of a 2-Mo star? At what age does such a star evolve into a red giant of spectral class K? (c) What is the .r" approximate zero-age luminosity of a 1.3-Mo star? What is its approximate luminosity when it becomes a red giant? - (d) Suppose a star cluster has no main-sequence stars of spectral classes O or B. What is the approximate age of the cluster? (e) Approximately how long do the most ma~aive ^ stars of spectral class B live before leaving the main sequence? What about the most massive stars of spectral ~,~ class F? . .I'~ OBSERVINC~ PR0.3CCTS Observing tips and tools  ~0~-1"K~a~ An excellent resource for learning how to W observe variable stars is the web site of th American Association of Variable Star Observers. A wealth of data about specific variable stars can be found on the Sky ~ Telescope web site and in the three volumes of Burnharrt's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System (Dover, 1978). This book also provides useful information about star cluste~ .s as does the web site of Students for the Exploratic ,,~ and Development of Space. 46. Observe several of the red giants and supergiant; listed below with the naked eye and through a telescope. ~Note that y Andromedae, a Tauri, and E Pegasi are all tr.~ultiple-star systems. The spectral type and luminosity cla;s refer to the brightest star in these systems.) You can loca_~e these stars in the sky using the Starry Night prograr._; on the CD-ROM that comes with this book. Is r'~e reddish color of these stars apparent when they are eompared with neighboring stars? ` . Star Spectral type R.A. Decl.     and binoculars. (Some are actually so large that they will not " center of each cluster? Do you notice any differences in the fit in the field of view of many telescopes.) You can locate overall distribution of stars between clusters? these clusters in the sky using the Starry Night program on the CD-ROM that comes with this book. Note that in the Star cluster Constellation R.A. Decl. following table, the M prefix refers to the Messier Catalog, M3 (NGC 5272) Canes Venatici 13h 42.2m +28 23' NGC refers to the New General Catalogue, and Mel refers to the Melotte Catalog. In making your own observations, MS (NGC 5904) Serpens 15 18.6 + 2 OS note the overall distribution of stars in each cluster. Which M4 (NGC 6121) Scorpius ~+~ 16 23.6 -26 32 clusters are seen better through binoculars than through a M13 (NGC 6205) Hercules 16 41.7 +36 28 telescope? Which clusters can you see with the naked eye?  M12 (NGC 6218) Ophiuchus 16 47.2 - 1 57 Star cluster Constellation R.A. Decl. 1VI28 (NGC 6626) Sagittarius 18 24.5 -24 52 h Persei Perseus 2h 19.Om +57 09' M22 (NGC 6656) Sagittarius 18 36.4 -23 54 (NGC 869) M55 (NGC 6809) Sagittarius 19 40.0 -30 58 x Persei Perseus 2 22.4 +57 07 M15 (NGC 7078) Pegasus 21 30.0 +12 10 , (NGC 884) ~ Pleiades (M45) Taurus 3 47.0 +24 07 Hyades (Mel 25) Taurus 4 27 +16 00 Praesepe (M44) Cancer 8 40.1 +19 59 ~P~RY ,Nyy 49. Use the Starry Night program to view the Note: Tbe right ascensions and declinations are given for epoch 2000.  Coma (Mel 111) Coma Berenices 12 25 +26 00 y ~ sky as it appears at midnight (12:00:00 n.~t.) on today's date from your location. (Use the time Wild Duck (M11) Scutum 18 51.1 -06 16 controls in the Control Panel and the Viewing Locations... ' command in the Go Settings menu.) Locate the giant Note: The right ascensions and declinations are given for epoch 2000. star Aldebaran, the open cluster M44, and the globular ; cluster M12. (Use the Find... command in the Edit menu.) + 48. There are many beautiful globular clusters scattered Which of these are visible from your location tonight at ; around the sky that can be easily seen with a small midnight? For each object that is visible, in which direction ' telescope. Several of the brightest and nearest gbbulars are of the compass would you have to look to see it (that is, listed below. You can locate these clusters in the sky using what is its azimuth), and how far above the horizon would y the Starry Night program on the CD-ROM that comes you have to look (that is, what is its altitude)? (Hint: ; with this book. Observe as many of these globular clusters Double-click on each object to bring up an information as you can. Can you distinguish individual stars toward the window.)  R c  Stellar Evolution: After the Main Sequence 499  ~ble 22 ~ ~ , v - v Stage Core temperature (K) Core density (kg/m3) rogen burning 4 x 10~ 5 x 103 ~m burning 2 x 108 7 x 10s ~on burning 6 x 10g 2 x 10g 1 burning 1.2 x 109 4 x 109 gen burning 1.5 x 109 1010 m burning 2.7 x 109 3 x 1010 ~ collapse 5.4 x 109 3 x 1012 ~ bounce 2.3 x 101 4 x 101s osive (supernova) about 109 varies  y elements and isotopes that are not produced directly in The entire core supply of silicon in a 25-Mo star is used up in m reactions. only one day! Table 22-1 summarizes the evolutionary stages ach stage of thermonuclear reactions in a high-mass star in the life of such a 25-Mo star. s to trigger the succeeding stage. In each stage, when the Each stage of core burning in a high-mass star generates a xhausts a given variety of nuclear fuel in its core, gravi- new shell of material around the core. After several such stages, 1 contraction takes the core to ever-higher densities and the internal structure of a truly massive star-say, 25 Mo or .ratures, thereby igniting the "ash" of the previous greater-resembles that of an onion, as Figure 22-13 shows. stage-and possibly the outlying shell of unburned fuel Because thermonuclear reactions can take place simultane- . ously in several shells, energy is released at such a rapid rate e increasing density and temperature of the core make that the star's outer layers expand tremendously. The result is uccessive thermonuclear reaction more rapid than the a supergiant sta~ whose luminosity and radius are much at preceded it. As an example, Stanford Woosley at the larger than those of a giant (see tif;r~~~ 1 ~~ -). Fsity of California, Santa Cruz, and Thomas Weaver at Several of the brightest stars in the sky are supergiants, ~ce Livermore National Laboratory have made detailed including Betelgeuse and Rigel in the constellation Orion and ions for a star with a zero-age mass of 25 Mo. They Antares in the constellation Scorpius. ( shows the at carbon burning in such a star lasts for 600 years, neon locations of these stars on an H-R diagram.) They appear g for 1 year, and o~cygen burning for only 6 months. The bright not because they are particularly close, but because ~'i briefest, stage of nuclear reactions is silicon burning. they are extraordinarily luminous. Duration of stage 7 x 106 years 7 x 10s years 600 years 1 year 6 months 1 day ~/a second milliseconds 10 seconds  ;rHelium-1>urning shell -Carbon-burning , shelt, '  `5ilicon-buring shelt f igure 22-13 The Structure of an Old High-Mass Star Near the end of its life, a high-mass star becomes a supergiant whose overall size can be as large as Jupiter's orbit around the Sun. The star's energy comes from six concentric burning shells, all contained within a volume roughly the same size as the Earth. Thermonuclear reactions cannot occur within the iron core, because fusion reactions that involve iron absorb energy rather than releasing energy. Stellar Evolution: The Deaths of Stars I 509 520 1 CHAPTER 22  KEY WORDS asymptotic giant branch, p. 502 mass-radius relation, p. 506 supergiant, p. 509 asymptotic giant branch star neon burning, p. 508 supernova (plural supernovae), (AGB star), p. 502 neutron capture, p. 508 p. 511 carbon burning, p. 508 nuclear density, p. 510 supernova remnant, p. 518 carbon star, p. 503 oxygen burning, p. 508 Cerenkov radiation, p. 514 photodisintegration, p. 510 Chandrasekhar limit, p. 507 planetary nebula, p. 504 core helium burning, p. 501 progenitor star, p. 512 dredge-up, p. 503 red-giant branch, p. 501 helium shell flash, p. 504 shell helium burning, p. 502 horizontal branch, p. 502 silicon burning, p. 508 thermal pulses, p. 504 Type la supernova, p. 516 Type Ib supernova, p. 516 Type Ic supernova, p. 516 Type II supernova, p. 515 white dwarf, p. 506 KCY 1 DCAS ~~(+Arpr its matter is ejected into space at high speeds. The w`' ~^ Late Evolution of Low-Mass Stars: A low-mass luminosity of the star increases suddenly by a factor of Z star becomes a red giant when shell hydrogen around 108 during this explosion, producing a supernova. '~cISE 2 burning begins, a horizontal-branch star when More than 99% of the energy from such a supernova is core helium burning begins, and an asymptotic giant branch emitted in the form of neutrinos from the collapsing core. (AGB) star when the helium in the core is exhausted and shell helium burning begins. The matter ejected from the supernova, moving at supersonic speeds through interstellar gases and dust, As a low-mass star ages, convection occurs over a larger glows as a nebula called a supernova remnant. portion of its volume. This takes heavy elements formed in the star's interior and distributes them throughout the star. Other Types of Supernovae: An accreting white dwarf in a Planetary Nebulae and White Dwarfs: Helium shell flashes in close binary system can also become a supernova when - an old, low-mass star produce thermal pulses during which carbon burning ignites explosively throughout such a degenerate star. more than half the star's mass may be ejected into space. This exposes the hot carbon-oxygen core of the star. Type la supernovae are those produced by accreting white dwarfs in close binaries. Type II supernovae are created by Ultraviolet radiation from the exposed core ionizes and the deaths of massive stars, as are supernovae of Type Ib excites the ejected gases, producing a planetary nebula. and Type Ic; these latter types occur when the star has lost No further nuclear reactions take place within the a substantial part of its outer layers before exploding. exposed core. Instead, it becomes a degenerate, dense sphere about the size of the Earth and is called a white from our view by interstellar dust and gases. dwarf. It glows from thermal radiation; as the sphere cools, it becomes dimmer. Most supernovae occurring in our Galaxy are hidden RC-VIEW QUESTIONS 1. What is the horizontal branch? Where is it located on an H-R diagram? How do stars on the horizontal branch differ from red giants or main-sequence stars? Late Evolution of High-Mass Stars: Unlike a low-mass star, a high-mass star undergoes an extended sequence of thermonuclear reactions in its core and shells. These include carbon burning, neon burning, oxygen burning, and silicon burning. 2. What is the asymptotic giant branch? Where is it In the last stages of its life, a high-mass star has an iron- located on an H-R diagram? How do asymptotic giant rich core surrounded by concentric shells hosting the branch stars differ from red giants or main-sequence stars? various thermonuclear reactions. The sequence of thermonuclear reactions stops here, because the formation 3. What is the connection between dredge-ups in old stars of elements heavier than iron requires an input of energy and life on Earth? rather than causing energy to be released. 4. How is a planetary nebula formed? The Deaths of Massive Stars: A high-mass star dies in a 5. What is a white dwarf? Does it produce light in the violent cataclysm in which its core collapses and most of same way as a star like the Sun?   6. How does the radius of a white dwarf depend on its 20. The globular cluster M15 depicted in Figure 22-6a mass? How is this different from other types of stars? contains 30,000 old stars, but only one of these stars is 7. What is the significance of the Chandrasekhar limit? presently in the planetary nebula stage of its evolution. Explain why planetary nebulae are not more prevalent in M15. 8. On an H-R diagram, sketch the evolutionary track that the Sun will follow from when it leaves the main sequence to when it becomes a white dwarf. Approximately how much mass will the Sun have when it becomes a white dwarf? Where will the rest of the mass go? 21. Explain why infrared light was used to detect the faint wisps in planetary nebula NGC 7027 (Figure 22-6c). ' 9. Why do you suppose that all the white dwarfs known ~o astronomers are relatively close to the Sun? ,...10. What prevents thermonuclear reactions from occurring a't the center of a white dwarf? If no thermonuclear reactions a Te occurring in its core, why doesn't the star collapse? s 1A. Why does the mass of a star play such an important rcole in determining the star's evolution?  12. Why is the temperature in a star's core so important in determining which nuclear reactions can occur there? 13. What is the difference between a red giant and a red supergiant? 14. What is nuclear density? Why is it significant when a star's core reaches this density? 15. Why is SN 1987A so interesting to astronomers? In what ways was it not a typical supernova? 16. What are the differences among Type la, Type Ib, Type Ic, and Type II supernovae? Which type is most unlike the other three, and why? 17. How can a supernova continue to shine for many years ,after it explodes? 18. Why have radio searches for supernova remnants been more fruitful than optical searches? 19. Is our own Sun likely to become a supernova? Why or why not? NDVANGCD QUESTIONS Questions preceded by an asterisk (*) involve topics ;discussed in the Boxes in ihapter or Chapter 19. Problem-solving tips and tools You may find it useful to review , which discusses stellar radii and their relationship to temperature and luminosity. ),i-4--_,explains the i formula for gravitational force, and Box i-L explains the concept of escape speed. tirar;f)ns_S-2-pnd_s-4 ~ describe some key properties of light, especially ~ blackbody radiation. We discussed the relationship among luminosity, apparent brightness, and distance ~ in The relationship among absolute :` magnitude, apparent magnitude, and distance was the topic of hwx I,) -.h.  22. The central star in a newly formed planetary nebula has a luminosity of 1000 Lo and a surface temperature of 100,000 K. What is the star's radius? Give your answer as a multiple of the Sun's radius. 23. You want to determine the age of a planetary nebula. What observations should you make, and how would you use the resulting data? 24. The Ring Nebula in the constellation Lyra has an angular size of 1.4 x 1.0 arcmin and is expanding at the rate of about 20 km/s. Approximately how long ago did the central star shed its outer layers? Assume that the nebula is 2,700 ly from Earth.  25. (a) Calculate the wavelength of maximum emission of the white dwarf Sirius B. In what part of the electromagnetic spectrum does this wavelength lie? (b) In a visible-light photograph such as Figure 22-8, Sirius B appears much fainter than its primary star. But in an image made with an X-ray telescope, Sirius B is the brighter star. Explain the difference. 26. (a) Find the average density of a 1-Mo white dwarf having the same diameter as the Earth. (b) What speed is required to eject gas from the white dwarf's surface? (This is also the speed with which interstellar gas falling from a great distance would strike the star's surface.) 27. (a) What kinds of stars would you monitor if you wished to observe a supernova explosion from its very beginning? (b) Examine , which list the nearest and brightest stars, respectively. Which, if any, of these stars are possible supernova candidates? Explain. *28. Consider a high-mass star just prior to a supernova explosion, with a core of diameter 20 km and density 4 x 1011 kg/m3. (a) Calculate the mass of the core. Give your answer in kilograms and in solar masses. (b) Calculate the force of gravity on a 1-kg object at the surface of the core. How many times larger is this than the gravitational force on such an object at the surface of the Earth, about 10 newtons? (c) Calculate the escape speed from the surface of the star's core. Give your answer in m/s and as a fraction of the speed of light. What does this tell you about how powerful a supernova explosion must be in order to blow material away from the star's core? 29. The shock wave that traveled through the progenitor star of SN 1987A took 3 hours to reach the star's surface. (a) Given the size of a blue supergiant star (see Section 22-7), estimate the speed with which the shock wave traveled through the star's outer layers. (The core of the progenitor star was very small, so you may consider the shock wave    522 1 CHAPTER 22 to have started at the very center of the star.) Give your you decide if this object is a planetary nebula? Could your answer in meters per second. (b) Compare your answer with object be something else? Explain. the speed of sound waves in our atmosphere, about 340 m/s, 37, Suppose the convective zone in AGB stars did not and with the speed of light. (c) A shock wave traveling reach all the way down into their carbon-rich cores. through a gas is a special case of a sound wave. In general, How might this have affected the origin and evolution of sound waves travel faster through denser, less easily life on Earth? compressed materials. Thus, sound travels faster through water (about 1500 m/s) than through our atmosphere, and faster still through steel (about 5900 m/s). Use this idea to compare the gases within the progenitor star of SN 1987A with the gases in our atmosphere in terms of their average density and how easily they are compressed. 38. In the classic 1960s science-fiction comic book The Atom, a physicist discovers a basketball-sized meteorite (about 10 cm in radius) that is actually a fragment of a white dwarf star. With some difficulty, he manages to hand-carry the meteorite back to his laboratory. Estimate the mass of such a fragment, and discuss the plausibility of this scenario.  30. The neutrinos from SN 1987A arrived 3 hours before the visible light. While they were en route to the Earth, what was the distance between the neutrinos and the first photons from SN 1987A? Assume that neutrinos are massless and thus travel at the speed of light. Give your answers in kilometers and in AU. *31. Compared to SN 1987A (Figure 22-15), the supernova SN 1993J (see Figure 22-19) had a maximum apparent brightness only 9.1 x 10' as great. Using the distances from Earth to each of these supernovae, determine the ratio of the maximum luminosity of SN 1993J to that of SN 1987A. Which of the two supernovae had the greater maximum luminosity? 39. SN 1987A did not agree with the theoretical picture outlined in Section 22-6. Does this mean that the theory was wrong? Discuss. 40. The major final product of silicon burning is 56Fe, an '' isotope of iron with 26 protons and 30 neutrons. This is: also the most common isotope of iron found on Earth. Discuss what this tells you about the origin of the solar system.  1NF(3/CD-ROM ~1JCSTlONS ~'32. Suppose that the brightness of a star becoming a supernova increases by 20 magnitudes. Show that this 41. It has been claimed that the Dogon tribe in western corresponds to an increase of 108 in luminosity. Africa has known for thousands of years that Sirius is a binary star. Search the World Wide Web for information *33. Suppose that the red-supergiant star Betelgeuse, which about these claims. What is the basis of these claims? Why lies some 1400 light-years from the Earth, becomes a Type are scientists skeptical, and how do they refute these claims? II supernova. (a) At the height of the outburst, how bright would it appear in the sky? Give your answer as a fraction 42. Search the World Wide Web for recent information of the brightness of the Sun (bo). (b) How would it about SN 1993J. Has the shape of the supernova's light compare with the brightness of Venus (about 10-9 bo)? curve been adequately explained? Has the supernova *34. In July 1997, a supernova named SN 1997cw produced any surprises? exploded in the galaxy NGC 105 in the constellation Cetus 43. Search the World Wide Web for information about (the Whale). It reached an apparent magnitude of +16.5 at SN 19941, a supernova that occurred in the galaxy MS1 maximum brilliance, and its spectrum showed an (NGC 5194). Why was this supernova unusual? Was it absorption line of ionized silicon. Use this information to bright enough to have been seen by amateur astronomers? find the distance to NGC 105. (Hint: Inspect the light curves in Figure 22-21 to find the absolute magnitudes of =ti?o~44. Convection Inside a Giant Star. Access and typical supernovae at peak brightness.) Q ^' view the animation "Convection Inside a Giant Star" in Chapter 22 of the Universe web site or 35. Figure 22-22 shows a portion of the Veil Nebula in CD-ROM. Describe the motion of material in the interior Cygnus. Use the information given in the caption to find of the star. In what ways is it similar to convection within the average speed at which material has been moving away the present-day Sun (see =)? In what ways is it from the site of the supernova explosion over the past different? Is a dredge-up taking place in this animation? 15,000 years. Express your answer in km/s and as a How can you tell? fraction of the speed of light. ,M...` _ilG'_ h~p'f ION~~ h~p'f IOM~~ 45. Types of Supernovae. Access an a W a ~ r view the animations "In the Heart c DISCUSSION QUFSTI ONS a Type II Supernova" and "A Type Supernova" in Chapter 22 of the Universe web site or 36. Suppose that you discover a small, glowing disk of CD-ROM. Describe how these two types are fundamental light while searching the sky with a telescope. How would different in their origin.   ~oKsFRVirvG pRO~EeTs Observing tips and tools While planetary nebulae are rather bright objects, their brightness is spread over a relatively large angular size, which can make seeing them a challenge for the beginning observer. For example, the Helix Nebula (see Figure 22-6) has the largest angular size of any planetary nebula but is also one of the most difficult to see. To improve your view, make your observations on a dark, moonless night fro~n a location well shielded from city lights. Another useful trick, mentioned in ~~ pvr, I o ;m~1 ~~~, is to use "averted vision." Once you have the nebula centered in the telescope, you will get a brighter and clearer image if you look at the nebula out of the corner of : your eye. The so-called Blinking Planetary in Cygnus affords an excellent demonstration of this effect; the nebula seems to disappear when you look straight at it, but it reappears as soon as you look toward the side of your field of view. Another useful tip is to view the nebula through a green filter (a #58, or O III, filter available from telescope supply houses). Green light is emitted by excited, doubly ionized oxygen atoms, which are common in planetary nebulae but not in most other celestial objects. Using such a filter can make a planetary nebula stand out more distinctly against the sky. As a side benefit, it also helps to block out stray light from street lamps. The same tips also apply to observing supernova remnants.  ~~~ DEEpf,~ 46, Although they represent a fleeting stage at >~Z the end of a star's life, planetary nebulae are found all across the sky. Some of the brightest a, ite listed in Chapter 22 of the Universe web site and ~:D-ROM. Observe as many of these planetary nebulae as y, ~u can on a clear, moonless night using the largest t, :lescope at your disposal. Note and compare the various .hapes of the different nebulae. In how many cases can you .ee the central star? The central star in the Eskimo Nebula i, supposed to be the "nose" of an Eskimo wearing a parka ~ ee the cover of this book). Can you see this pattern?  47. Northern hemisphere observers with modest telescopes can see two supernova remnants, one in the winter sky and the other in the summer sky. Both are quite faint, however, so you should schedule your observations for a moonless night. The winter sky contains the Crab Nebula, which is discussed in detail in Chapter 23. The coordinates are R.A. = Sh 34.Sm and Decl. =+22 00', which places the object near the star marking the eastern horn of Taurus (the Bull). Whereas the entire Crab Nebula easily fits in the field of view of an eyepiece, the Veil or Cirrus Nebula in the summer sky is so vast that you can see only a small fraction of it at a time. The easiest way to find the Veil Nebula is to aim the telescope at the star 52 Cygni (R.A. = 20h 45.7"' and Decl. =+30 43'), which lies on one of the brightest portions of the nebula. If you then move the telescope slightly north or south until 52 Cygni is just out of the field of view, you should see faint wisps of glowing gas. (52 Cygni is the bright star toward the lower right corner of ~ mur~l~-ZZ, which shows the entire Cygnus Loop.) 48. Use a telescope to observe the remarkable triple star 40 Eridani, whose coordinates are R.A. = 4h 15.3m and Decl. =-7 39'. The primary, a 4.4-magnitude yellowish star like the Sun, has a 9.6-magnitude white dwarf companion, the most easily seen white dwarf in the sky. On a clear, dark night with a moderately large telescope, you should also see that the white dwarf has an 11th-magnitude companion, which completes this most interesting trio. ~QRRY NI~.y 49. The red supergiant Betelgeuse in the y~_,~~ constellation Orion will someday explode as a supernova. Use the Starry Night program to investigate how the supernova might appear if this was to happen tonight. Click on the "Now" button in the Control Panel to make the program display the sky as it is right now. Then use the Find... command in the Edit menu to center the field of view on Betelgeuse. (If Betelgeuse is below the horizon, allow the program to reset the time to when it rises.) Then determine the times when Betelgeuse rises and sets on today's date at your location. (You can use the controls in the Control Panel to move forward or backward in time as needed.) If Betelgeuse became a supernova today, would it be visible in the daytime? How would it appear at night? Do you think it would cast shadows? Are Betelgeuse and the Moon both in the night sky tonight? If they are, what kinds of shadows might they both cast if Betelgeuse became a supernova?     Stellar Evolution: The Deaths of Stars ! 523 538 I CHAPTER 23   KEY iD^~S REViEW ~IlEST10NS ~a,~ERacrr`~ Neutron Stars: A neutron star is a dense stellar 1. What are neutron stars? What led scientists to propose ` ' m corpse consisting primarily of closely packed their existence? ~~~ISE 2~ degenerate neutrons. 2. What prevents a neutron star from collapsing under its A neutron star typically has a diameter of about 30 km, a own gravity? mass less than 3 Mo, a magnetic field 1012 times stronger than that of the Sun, and a rotation period of roughly 3. Why do astronomers think that pulsars are rapidly i 1 second. rotating neutron stars? A neutron star consists of a superfluid, superconducting core surrounded by a superfluid mantle and a thin, brittle crust. r 4. During the weeks immediately following the discovery ! of the first pulsar, one suggested explanation was that the pulses might be signals from an extraterrestrial civilization. , Why did astronomers soon discard this idea? Intense beams of radiation emanate from regions near the north and south magnetic poles of a neutron star. These beams are produced by streams of charged particles 5. Why do neutron stars rotate so much more rapidly moving in the star's intense magnetic field. than ordinary stars? Why do they have such strong magnetic fields? Pulsars: A pulsar is a source of periodic pulses of radio radiation. These pulses are produced as beams of radio 6. Do all supernova remnants contain pulsars? Are all ~ waves from a neutron star's magnetic poles sweep past pulsars found within supernova remnants? For each the Earth. question, explain why or why not. The pulse rate of many pulsars is slowing steadily. This reflects the gradual slowing of the neutron star's rotation as it radiates energy into space. Sudden speedups of the pulse rate, called glitches, may be caused by interactions between the neutron star's crust and its superfluid interior. Neutron Stars in Close Binary Systems: If a neutron star is in a close binary system with an ordinary star, tidal forces will draw gas from the ordinary star onto the neutron star. The transfer of material onto the neutron star can make it rotate extremely rapidly, giving rise to a millisecond pulsar. 7. Is our Sun likely to end up as a neutron star? Why or why not? / 8. Compare the internal structure of a white dWarf to that of a neutron star. What are the similarities; What are the differences? 9. What is the difference between supercotiductiviry and superfluidity?  10. How does a glitch affect a pulsar's period? What could be the cause of glitches? 11. Astronomers have deduced that the Vela pulsar is about 11,000 years old. How do you suppose they did this? 12. Why do astronomers think that millisecond pulsars are very old? Magnetic forces can funnel the gas onto the neutron star's magnetic poles, producing hot spots. These hot spots then radiate intense beams of X rays. As the neutron star rotates, the X-ray beams appear to flash on and off. Such a system is called a pulsating X-ray variable. 14. What is the connection between pulsating X-ray Novae and Bursters: Material from an ordinary star in a sources and millisecond pulsars? close binary can fall onto the surface of the companion white dwarf or neutron star to produce a surface layer in 15. What is the difference between a nova and a Type Ia which thermonuclear reactions can explosively ignite. supernova?  13. Describe a pulsating X-raj source like Hercules Centaurus X-3. What produzes the pulsation? Explosive hydrogen burning may occur in the surface 16. What are the similarities between novae and X-rav layer of a companion white dwarf, producing the sudden bursters? What are the differences? increase in luminosity that we call a nova. The peak luminosity of a nova is only 10~ of th,at observed in a 17. What are the similarities between pulsating X-ray supernova. sources and X-ray bursters? What are the differences? Explosive helium burning may occur in the surface layer 18. Why is the maximum mass of neutron stars not of a companion neutron star. This produces a sudden known as accurately as the Chandrasekhar limit for white increase in X-ray radiation, which we call a burster. dwarfs?   .~ ~~Dti<~NC~D QUFSTIONS Problem-solving tips and tools You will need the formulas for the Doppler effect introduced in Sectiam 5-9 and Box s-~ (see Fi~ure ~-'-a). The volume of a sphere of radius r is ~4/s~tr3. The small-angle formula is given in Box 1-1. Sc, ~ __ describes how to relate the temperature of a blackbody to its wavelength of maximum emission. Secn<,y 19-( gives the formula relating a star's luminosity, surface temperature, and radius (see Box 19-G for worked examples). ~cction 3-5 gives the angular diameter of the Moon.  Earth's motion. Explain why the size of the correction is greatest for pulsars located near the ecliptic. 26. A neutron has a mass of about 1.7 x 10-2~ kg and a radius of about 10-15 m. (a) Compare the density of matter in a neutron with the average density of a neutron star. (b) If the neutron star's density is more than that of a neutron, the neutrons within the star are overlapping; if it is less, the neutrons are not overlapping. Which of these seems to be the case? What do you think is happening at the center of the neutron star, where densities are higher than average? 27. X-ray pulsars are speeding up but ordinary (radio) pulsars are slowing down. Propose an explanation for this difference. i 19. Using a diagram like Figure 23-3, explain why the ! number of pulsars that we observe in nearby space is i probably quite a bit less than the number of rotating, ~ magnetized neutron stars in nearby space. ~ 20. There are many more main-sequence stars of low mass 1 (less than 8 Mo) than of high mass (8 Mo or more). Use ; this fact to explain why white dwarf stars are far more ' common than neutron stars. '21. The distance to the Crab Nebula is about 2000 parsecs. ~,In what year did the star actually explode? Explain your ~nswer. 2~.. How do we know that the Crab pulsar is really Y em.bedded in the Crab Nebula and not simply located at a ~ diff~,rent distance along the same line of sight? `t 23. T.)le apparent expansion rate of the Crab Nebula is 0.23 arcsec per year. The apparent size of the Crab Nebula is aboux 4 by 6 arcmin. If the expansion rate remains constant, calculate how long it will be until the long axis of the Crab Nebula has the same apparent size as the full moon. 24. Emission lines in the spectrum of the Crab Nebula exhibit a Doppler shift, which indicates an expansion velocity of about 1200 km/s. (a) From this and the data ; given in the preceding question, calculate the distance to the Crab Nebula and estimate the date on which it exploded. (b) Do your answers agree with figures quoted in this chapter? If not, can you point to assumptions you made in your computations that led to the discrepancies? Or do you think your calculations suggest additional physical effects are at work in the Crab Nebula, over and above a constant rate of expansion? 28. If the model for Hercules X-1 discussed in the text is correct, at what orientation of the binary system do we see its maximum optical brightness? Explain your answer. 29. In an X-ray burster, the surface of a neutron star 10 km in radius is heated to a temperature of 3 x 10~ K. (a) Determine the wavelength of maximum emission of the heated surface (which you may treat as a blackbody). In what part of the electromagnetic spectrum does this lie? (See .) (b) Find the luminosity of the heated neutron star. Give your answer in watts and in terms of the luminosity of the Sun, given in Table 18-1. How does this compare with the peak luminosity of a nova? Of a TypeIa supernova? 30. The nearest neutron star, called RX J185635-3754, is just 60 pc (200 ly) from Earth. It is thought to be the relic of a star that underwent a supernova explosion about 1 million years ago. The explosion ejected the neutron star at high speed, so it is now moving through nearly empty space. (a) RX J185635-3754 is not a pulsar, that is, it does not emit pulses of radiation. Suggest why this might be so. (b) The neutron star has a surface temperature of 600,000 K. Find the wavelength at which it emits most strongly, and explain why the neutron star appears as a steady, nonpulsating object in an X-ray telescope. (c) RX J185635-3754 has a total luminosity at all wavelengths of about 0.046Lo. Calculate its radius, and explain why astronomers conclude that it is a neutron star. DISCUSSION QUCSTIONS 31. How might astronomers be able to detect the presence of an accretion disk in a close binary system? `- 25. To determine accurately the period of a pulsar, 32. When neutrons are very close to one another, they repel astronomers must take into account the Earth's orbital one another through the strong nuclear force. If this motion about the Sun. (a) Explain why. (b) Knowing that repulsion were made even stronger, what effect might this the Earth's orbital velocity is 30 km/s, calculate the have on the maximum mass of a neutron star? Explain maximum correction to a pulsar's period because of the your answer.  Neutron Stars 539 540 1 CHAPTER 23 R.A. = Sh 34.5' and Decl. = 22 00', which is near the star marking the eastern horn of Taurus (the Bull). Be sure to schedule your observations for a moonless night. The large the telescope you use, the better, because the Crab Nebula is quite dim. $ERA`'', WEB/CD-ROM Q,UfSTIONS %0 33. Search the World Wide Web for information about the latest observations of the stellar remnant at the center of SN 1987A. Has a pulsar been detected? If so, how fast is it spinning? Has the supernova's debris thinned out enough to give a clear view of the neutron star? ~O37. Consult the World Wide Web to see if any ~ novae have been sighted recently. If by good 34. Search the World Wide Web for information about a ~ fortune one has been sighted, what is its class of neutron stars called magnetars. How do they differ apparent magnitude? Is it within reach of a telescope at from ordinary neutron stars? What is their connection to your disposal? If so, arrange to observe it. Draw what you objects called soft gamma-ray repeaters? see through the eyepiece, noting the object's brightness in comparison with other stars in the field of view. If possible, ,ti9E 2~.~ 35. Monitoring the Crab Pulsar. Access the video observe the same object a few weeks or months later to see 4P "The Crab Pulsar" in Chapter 23 of the Universe how its brightness has changed. web site or CD-ROM. View the video and use it to answer the following questions. For each part, explain yP~RY Njcy~ 38. Use the Starry Night program to observe the how you determined your answer. (a) How many rotations sky in July 1054, when the supernova that does the neutron star complete during the duration of the spawned the Crab Nebula was visible from the video? (b) Is the neutron star visible at the beginning of the American Southwest. Select Viewing Location... in the video? If not, explain why not. (c) How does the peak Go menu, then set the latitude to 36 N and the longitude brightness of the Crab pulsar compare to the steady to 109 W Set the time to 5:00 A.M. on July 5, 1054. Use brightness of the nearby star? (d) What total amount of Find... in the Edit menu to find and center on the Crab time is depicted in the video? Nebula. Zoom in or out until you can see both the position of the nebula and the Moon. You may find it helpful to turn off daylight (select Daylight in the Sky menu). What is the phase of the Moon? Investigate how the relative positions of the Moon and the Crab Nebula change when you set the date to July 4, 1054, or July 6, 1054. On which date do the relative positions of the Moon and the Crab Nebula give the best match to the pictograph in Figure 23-1? ORSCRViNC4 PROJCCTS  36. If you did not take the opportunity to observe the Crab Nebula as part of the exercises in Chapter 22, do so now. The Crab Nebula is visible in the night sky from October through March. Its epoch 2000 coordinates are   manifest themselves as a gravitational force. These distortions of space and time are most noticeable in the vicinity of large masses or compact objects. The general theory of relativity is our most accurate description of gravitation. It predicts a number of phenomena, including the bending of light by gravity and the gravitational redshift, whose existence has been confirmed by observation and experiment. The general theory of relativity also predicts the existence of gravitational waves, which are ripples in the overall geometry of space and time produced by moving masses. Gravitational waves have been detected indirectly, and specialized antennas are under construction to make direct measurement of the gravitational waves from cosmic cataclysms. p, Black Holes: If a stellar corpse has a mass greater than about 3 Mo, gravitational compression will overwhelm any and all forms of internal pressure. The stellar corpse will collapse to such a high density that its escape speed exceeds the speed of light. Observing Black Holes: Black holes have been detected using indirect methods. Some binary star systems contain a black hole. In such a svstem, gases captured from the companion star by the black hole emit detectable X rays.  2. Sally flies past Martin in her spaceship at nearly the speed of light. According to Martin, Sally's clock runs slow. According to Sally, does Martin's clock run slow, fast, or at the normal rate? Explain. 3. Why does the speed of light represent an ultimate speed limit? 4. Why is Einstein's general theory of relativiry a better description of gravity than Newton's universal law of gravitation? 5. Describe two different predictions of the general theory of relativity and how these predictions were tested experimentally. Do the results of the experiments agree with the theory? 6. In what circumstances are degenerate electron pressure and degenerate neutron pressure incapable of preventing the complete gravitational collapse of a dead star? 7. Should we worry about the Earth's being pulled into a black hole? Why or why not? 8. All the stellar-mass black hole candidates mentioned in the text are members of very short-period binary systems. Explain how this makes it possible to detect the presence of the black hole. w     Many galaxies have supermassive black holes at their centers. These are detected by observing the motions of material around the black hole. 9. Astronomers cannot actually see the black hole candidates in close binary systems. How, then, do they know that these candidates are not white dwarfs or ,~ERncT1 neutron stars? - ~ ti~~s,~ ;,~^ Properties of Black Holes: The entire mass of ~ a black hole is concentr d in an infinitely ;: ~a de~se singularity. 10. How do astronomers locate supermassive black holes in galaxies? ` The singularity is surrourided by a surface called the 11. In what way is a black hole blacker than black ink or a E= event horizon, where the escape speed equals the speed of black piece of paper? light. Nothing-not even light~an escape from inside the ,~, event horizon. 12. If the Sun suddenly became a black hole, how would the Earth's orbit be affected? Explain. A black hole has onlv three physical properties: mass, electric charge, and angular momentum. 13. According to the general theory of relativity, why can't  some sort of yet-undiscovered degenerate pressure prevent the matter inside a black hole from collapsing all the way down to a singularity? ~ A rotating black hole (one with angular momentum) has an ergoregion around the outside of the event horizon. In the ergoregion, space and time themselves are dragged along wirh the rotation of the black hole. " 14. What is the law of cosmic censorship? Black holes can evaporate, but in most cases at an extremely slow rate. 15. What is the no-hair theorem? 16. What kind of black hole is surrounded by an ergoregion? What happens inside the ergoregion? R~v>IEw (ZUESTIONS 17. As seen by a distant observer, how long does it take an 1. In Einstein's special theory of relativity, two different object dropped from a great distance to fall through the observers moving at different speeds will measure event horizon of a black hole? Explain. the same value of the speed of light. Will these same observers measure the same value of, say, the speed of 18. If even light cannot escape from a black hole, how is it an airplane? Explain. possible for black holes to evaporate? Black Holes 559  560 I CHAPTER 24 1kDVL~NC~D QUESTIC)NS Questions preceded by an asterisk (*) involve topics discussed in the Boxes. Problem-solving tips and tools Remember that the time to travel a certain distance is equal to the distance divided by the speed, and that the density of an object is its mass divided by its volume. The volume of a sphere of radius r is 4/s~tr3. Section 4-7 describes Newton's law of universal gravitation. ('~uax ~}-~1 shows how to use Newton's formulation of Kepler's third law, which explicitly includes masses; when using this formula, note that the period P must be expressed in seconds, the semimajor axis a in meters, and the masses in kilograms. For another version of this formula, in which period is in years, semimajor axis in AU, and masses in solar masses, see Section 19-9. *19. A clock on board a moving starship shows a total elapsed time of 2 minutes, while an identical clock on the Earth shows a total elapsed time of 10 minutes. What is the speed of the starship relative to the Earth? "-20. How fast should a meter stick be moving in order to appear to be a "centimeter stick"? are very small, so they will collide when the distance between them is equal to zero.) 25. The orbital period of the binary system containing A0620-00 is 0.32 day, and Doppler shift measurements reveal that the radial velocity of the X-ray source peaks at 457 km/s (about 1 million miles per hour). (a) Assuming that the orbit of the X-ray source is a circle, find the radius of its orbit in kilometers. (This is actually an estimate of the semimajor axis of the orbit.) (b) By using Newton's form of Kepler's third law, prove that the mass of the X-ray source must be at least 3.1 times the mass of the Sun. (Hint: Assume that the mass of the KSV~ible star-about 0.5 Mo from the mass-luminosity relationship-is negligible compared to that of the invisible companion.) 26. Find the orbital period of a star moving in a circular orbit of radius 500 AU around the supermassive black hole in M87. *27. Find the Schwarzschild radius for an object having a mass equal to that of the planet Jupiter. *28. What is the Schwarzschild radius of a black hole whose mass is that of (a) the Earth, (b) the Sun, (c) the supermassive black hole in M87? In each case, also calculate what the density would be if the matter were spread uniformly throughout the volume of the event horizon. *21. An astronaut flies from the Earth to a distant star at 80% of the speed of light. As measured by the astronaut, the one-way trip takes 15 years. (a) How long does the trip take as measured by an observer on the Earth? (b) What is the distance from the Earth to the star (in light-years) as measured by an Earth observer? As measured by the astronaut? 22. In the binary system of two neutron stars discovered by Hulse and Taylor (Section 24-2), one of the neutron stars is a pulsar. The distance between the two stars varies between 1.1 and 4.8 times the radius of the Sun. The time interval between pulses from the pulsar is not constant: It is greatest when the two stars are closest to each other and least when the two stars are farthest apart. Explain why this is consistent with the gravitational slowing of time (Figure 24-7a). *29. To what density must the matter of a dead 10-M . star be compressed in order for the star to disappear inside its event horizon? How does this compare with the density at the center of a neutron star, about 3 x 101g kg/m3? ~30. Prove that the density of matter needed to produce a black hole is inversely proportional to the square of the mass of the hole. If you wanted to make a black hole from matter compressed to the density of water (1000 kg/m3), how much mass would you ~ed? - DISCUSSION QUESTIONS  31. The speed of light is the same for all observers, regardless of their motion. Discuss why this requires us to abandon the Newtonian view of space and time. 32. Describe the kinds of observations you might make in order to locate and identify black holes. 23. Find the total mass of the neutron star binary system discovered by Hulse and Taylor (Section 24-2), for which the orbital period is 7.75 hours and the average distance between the neutron stars is 2.8 solar radii. Is your result reasonable for a pair of neutron stars? Explain. 33. Speculate on the effects you might encounter on a trip to the center of a black hole (assuming that you could survive the journey). 24. Estimate how long it will be until the two neutron stars that make up the binary system discovered by Hulse and 'y`'~ER cr' Taylor collide with each other. Assume that the distance ' w ~ (3 ~ C [~ _ /z C ) M ~ll ~ ST l O 1~I 5 between the two stars will continue to decrease at its present rate of 3 mm every 7.75 hours, and use the data 34. Search the World Wide Web for information about a given in Question 23. (You can assume that the two stars stellar-mass black hole candidate named V4641 Sgr. In  what ways does it resemble other black hole candidates such as Cygnus X-1 and V404 Cygni? In what ways is it different and more dramatic? How do astronomers explain why V4641 Sgr is different?  35. Search the World Wide Web for information about the "mid-mass" black hole candidate in M82. Is this still hought to be a black hole? What new evidence has been used to either support or oppose the idea that this object is a black hole? *1 , ~~ptION~y 36, The Equivalence Principle. Access the animation "The Equivalence Principle" in Chapter 24 of the Universe web site or CD-ROM. View the animation and answer the following questions. (a) Describe what is happening as viewed from the frame of reference of the elevator. What causes the apple to fall to the floor of each elevator? (b) Describe what is happening as viewed from the frame of reference of the stars. What causes the apple to fall to the floor of each elevator? (c) Think of another experiment you could perform with the apple. Describe what would happen during this experiment as seen by Newton (in the left-hand box) and by Einstein (in the right-hand box).       ORSERVINC, PROJECT 37. You cannot see a black hole with a telescope. Nevertheless, you might want to observe the visible companion of Cygnus X-1. The epoch 2000 coordinates of this ninth-magnitude star are R.A. = 19h 58.41 and Decl. _ +35 12', which is quite near the bright star tl (eta) Cygni. Compare what you see with the photograph in Figure 24-10. ,~pRRY, Njcy 38. Use the Starry Night program to plan ' observations of Cygnus X-1. First turn off daylight (select Daylight in the Sky menu). Center the field of view on Cygnus X-1 (select Find... in the Edit menu). If Cygnus X-1 is below the horizon, allow the program to reset the time to when it rises. Using the controls at the right-hand end of the Control Panel, zoom out to the maximum field of view. (a) Using the time controls in the Control Panel, step through time and determine when Cygnus X-1 rises and sets on today's date from your location. (b) At approximately what time on today's date is Cygnus X-1 highest in the sky? Is tonight a good night for observing this star with a visible-light telescope? Would it be better placed in the sky for observation six months from now? Explain how you determined this.   Black Holes 561        _ KEY 1 D~1~S ,~F,R A Cp1. w`' ~^ The Shape and Size of the Galaxy: Our Galaxy According to the theory of self-propagating star f m has a disk about ~ 0 kpc (160,000 ly) in formation, spiral arms are caused by the birth of stars over ~~CISE 2y diameter and about 600 pc (2000 ly) thick, with an extended region in a galaxy. Differential rotation of the a high concentration of interstellar dust and gas in the disk. galaxy stretches the star-forming region into an elongated The galactic center is surrounded by a large distribution arch of stars and nebulae. f stars called the central bulge. This bulge is not perfectly ~`` The Galactic Nucleus: The innermost part of the Galaxy, symmetrical, but may have a bar or peanut shape. or galactic nucleus, has been studied through its radio and. infrared emissions (which are able to pass through interstellar dust). The disk of the Galaxy is surrounded by a spherical distribution of globular clusters ar~d old stars, called the galactic halo. A strong radio source called Sagittarius A'~ is located at the galactic center. This marks the position of a supermassive black hole with a mass of about 3 x 106 Mo. There are about 200 billion (2 x 10~1) stars in the Galaxy's disk, central bulge, and halo. The Sun's Location in the Galaxy: Our Sun lies within the galactic disk, some 8000 pc (26,000 ly) from the center of the Galaxy. Interstellar dust obscures our view at visible wavelengths along lines of sight that lie in the plane of the galactic disk. , As a result, the Sun's location in the Galaxy was unknown 'for many years. This dilemma was resolved by observing p~; rts of the Galaxy outside the disk. R~V1~W Q,U~STIONS 1. How did interstellar extinction mislead astronomers into believing that we are at the center of our Galaxy? 2. How did observations of globular clusters help astronomers determine our location in the Galaxy? 3. Why are infrared telescopes useful for exploring the structure of the Galaxy? Why is it important to make observations at both near-infrared and far-infrared wavelengths? '`ffiF Sun orbits around the center of the Galaxy at a speed of about 790,000 km/h. It takes about 220 million years to complete one orbit. 4. The galactic halo is dominated by Population II stars, The Rotation of the Galaxy and Dark Matter: From whereas the galactic disk contains predominantly studies of tha rotaticm of the Galaxy, astronomers estimate Population I stars. In which of these parts of the Galaxy that the total mass of the Galaxy is about 1012 Mo. Only has star formation taken place recently? Explain. about 10% of this mass is in the form of visible stars, gas, 5, Most interstellar hydrogen atoms emit only radio waves and dust. The remaining 90% is in some-nonvisible form, at a wavelength of 21 cm, but some hydrogen clouds called dark matter, that extends beyond the edge of the emit profuse amounts of visible light (see, for example, luminous material in the Galaxy.  i~, . _ ). What causes this difference? Our Galaxv's dark matter may be a combination of ~1ACHOs Idim, star-sized objects), massive neutrinos, and WIMPs (relatively massive subatomic particles). The Galaxv's Spiral Structure: OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms. Spiral arms can be traced from the positions of clouds of atomic hydrogen. These can be detected throughout the galactic disk by the 21-cm radio waves emitted by the spin-flip transition in hydrogen. These emissions easily penetrate the intervening interstellar dust. Theories of Spiral Structure: There are two leading theories of spiral structure in galaxies. 6. How do astronomers determine the distances to H I (neutral hydrogen) clouds? 7. In a spiral galaxy, are stars in general concentrated in the spiral arms? Why are spiral arms so prominent in photographs of spiral galaxies? 8. What kinds of objects (other than H I clouds) do astronomers observe to map out the Galaxy's spiral structure? What is special about these objects? Which of these can be observed at great distances? 9. Why don't astronomers detect 21-cm radiation from the hydrogen in giant molecular clouds? 10. How do astronomers determine how fast the Sun moves in its orbit around the Galaxy?  According to the density-wave theory, spiral arms are created by density waves that sweep around the Galaxy. 11. In what way are the orbits of stars in the galactic disk The gravitational field of this spiral pattern compresses the different from the orbits of planets in our solar system? interstellar clouds through which it passes, thereby 12. How do astronomers conclude that vast quantities of triggering the formation of the OB associations and H II dark matter surround our Galaxy? How is this dark matter regions that,,~ir~te the spiral arms. distributed in space? Our Galaxy 583   21. Explain why globular clusters spend most of their time ~32. Show that the form of Kepler's third law stated in in the galactic halo, even though their eccentric orbit~ Box 25-2, PZ = 4ttZa3/G(M + Mo), is equivalent to them close to the galactic center. . M = rv2/G, provided the orbit is a circle. [Hint: The mass    .~ ~,u. 584 I CHAPTER 25 14. What proposals have been made to explain the nature of dark matter? What experiments or observations have been made to investigate these proposals? What are the results of this research? 16. Do density waves form a stationary pattern in a galaxy? If not, do they move more rapidly, le~,rapidly, or at the same speed as stars in the disk? Problem-solving tips and tools Several of the following questions make extensive use of Newton's form of Kepler's third law, and you might find it helpful to review R~Y 4-4. Another useful version of Kepler's third law is given in Section 19-9. We discussed the relationship between the energy and wavelength of a photon in . According to the Pythagorean theorem, an isosceles right triangle has a hypotenuse that is longer than its sides by a factor of , 2 = 1.414. The formula for the Schwarzschild radius of a black hole is given in tiox Z~ . You will find the small-angle formula in Rqx 1-1 useful. It is also helpful to remember that an object 1 AU across viewed at a distance of 1 parsec has an angular size of 1 arcsecond. Remember, too, that the volume of a cylinder is equal to its height multiplied by the area of its base, and that the area of a circle of radius r is rtr2. You can find other geometrical formulae in 20. Discuss how the Milky Way would appear to us if the Sun were relocated to (a) the edge of the Galaxy; (b) the galactic halo; (c) the galactic bulge. 13. What is the difference between dark matter and 22. The disk of the Galaxy is about ~0 kpc in diameter and dark nebulae? 600 pc thick. (a) Find the volume of the disk in cubic parsecs. (b) Find the volume (in cubic parsecs) of a sphere 300 pc in radius centered on the Sun. (c) If supernovae occur randomly throughout the volume of the Galaxy, what is the probability that a given superno~ a will occur within 300 pc of the Sun? If there are about three 15. What is the winding dilemma? What does it tell us ~ supernovae each century in our Galaxy, how often, on about the nature of spiral arms? average, should we expect to see one within 300 pc of the Sun? 23. A typical hydrogen atom in interstellar space undergoes a spin-flip transition only once every 10~ years. How, then, 17. In our Galaxy, why are stars of spectral classes O and is it at all possible to detect the 21-cm radio emission from B found in or near the spiral arms? Is the same true for interstellar hydrogen? stars of other spectral classes? Explain why or why not. 24. Calculate the energy of the photon emitted when a 18. Compare the kinds of spiral arms produced by density hydrogen atom undergoes a spin-flip transition. How manv waves with those produced by self- propagating star such photons would it take to equal the energy of a single formation. By examining Figure 25-15, cite evidence that Ha photon of wavelength 656.3 nm? both processes probably occur in our Galaxy. 25. Suppose you were to use a radio telescope to measure 26. Calculate approximately how many times our solar system has orbited the center of our Galaxy since the Sun and planets were formed some 5 x 109 years ago. 27. Sketch the rotation curve you would obtain if the Galaxy were rotating like a rigid body. 28. A gas cloud located in the spiral arm of a distant galaxy is observed to have an orbital velocity of 400 km/s. If the cloud is 20,000 pc from the center of the galaxy and is moving in a circular orbit, find (a) the orbital period of the cloud and (b) the mass .q~ the galaxy contained within the cloud's orbit. 29. The mass of our Galaxy interior to the Sun's orbit is calculated from the radius of the Sun's orbit and its orbital speed. By how much would this estimate be in error if the calculated distance to the galactic center were off by 10%? By how much would tl:is estimate be in error if the calculated orbital velocity were off by 10%? 30. Speculate on the reasons for the rapid rise in the Galaxy's rotation curve (see Figure 25-17) at distances close to the galactic center. 31. According to the Galaxy's rotation curve in Figure 2~-17, a star 16 kpc from the galactic center has an orbital speed of about 270 km/s. Calculate the mass within that star's orbit. 19. Describe the evidence for a supermassive black hole at the Doppler shift of 21-cm radiation in the plane of the the center of our Galaxy. Galaxy. (a) If you observe 21-cm radiation from clouds of atomic hydrogen at an angle of 45 from the galactic center~ you will see the highest Doppler shift from a cloud that is,-' 1~ D V~ N CF D ~IIf=ST 1 O N S as far from the galactic center as it is from the Sun. Expl ain Questions preceded by an asterisk ('~) involve topics this statement using a diagram. (b) Find the distance:; ~rn discussed in the Boxes. ~ the Sun to the particular cloud mentioned in (a).           of the Sun (Mo) is much less than the mass of the Galaxy orbit's semimajor axis as seen from Earth, which is 8000 pc from the center of the Galaxy. Explain why 33. The accompanying image shows the spiral galaxy M74, extremely high-resolution infrared images are required to a located about 55 million light-years from the Earth in the observe the motions of these stars. constellation Pisces (the Fish). It is actually a superposition 36. Consider a star that orbits around Sagittarius A* in a of two false-color images: The red portion is an optical circular orbit of radius 1000 AU. (a) If the star's orbital image taken at visible wavelengths, while the blue portion speed is 1500 km/s, what is its orbital period? Give your is an ultraviolet image made by NASA's Ultraviolet answer in years. (b) Determine the sum of the masses of Imaging Telescope, which was carried into orbit by the Sagittarius A* and the star. Give your answer in solar , space shuttle Columbia during the Astro-1 mission in masses. (Your answer is an estimate of the mass of 1990. Compare the visible and ultraviolet images and, Sagittarius A", because the mass of a single star is + from what you know about stellar evolution and spiral negligibly small by comparison.) ~ structure, explain the differences you see. inside the Sun's orbit (M).] DISCUSSION Q,tICSTIONS 37. From what you know about stellar evolution, the interstellar medium, and the density-wave theory, explain the appearance and structure of the spiral arms of grand-design spiral galaxies. 38. What observations would you make to determine the nature of the dark matter in our Galaxy's halo? 39. Describe how the appearance of the night sky might change if dark matter were visible to our eyes.  40. Clouds of positrons have been discovered recently near the center of the Galaxy. These are particles with the same properties as electrons except that their charge is positive rather than negative (see Boa_a_u-l ). Search the World Wide Web for information about this discovery. How were the positrons discovered? What is thought to be the origin of these particles? What happens when these positrons encounter ordinary electrons?  ~ti9E 2SZ 41. Fast-Moving Stars at the Galactic Center. Access and view the video "Fast-Moving Stars at the Galactic Center" in Chapter 25 of the R 10 MX G Universe web site or CD-ROM. Explain how you can tell which of the stars in the video are actually close to *3X1. (a) Calculate the Schwarzschild radius of a supermassive Sagittarius A" and which just happen to lie along our line black hole of mass 3 x 106 M o, the estimated mass of the of sight to the center of the Galaxy. ~ black hole at the galactic center. Give your answer in both kilorrteters and in astronomical units. (b) What is the angular diameter of such a black hole as seen at a distance of 8 kpc, OBSE /2V1 N Q P120JFCT5 t, the distance from the Earth to the galactic center? Give your ~pRRY, Nz~y 42. Use the Starry Night program to observe the f answer in arcseconds. Observing an object with such a small angular size will be a challenge indeed! Milky Way. First turn off daylight (select angu ;~. Daylight in the Sky menu) and show the entire celestial sphere (select Atlas in the Go menu). To make the Milky Way visible, make sure that the selection Milky Way in the Sky menu has a check mark next to it. If you find the Milky Way difficult to see, select Sky Settings... in the Sky menu and click on Milky Way Colour. Choose a color that will stand out against the dark night sky, then click on (As.tronomical Society of the Pacific) 35. The orbital periods of the stars SO-1 and SO-2 shown in Figure 25-25 are estimated to be 63 and 17 years, respectively. (a) Assuming that the supermassive black hole in Sagittarius A* has a mass of 3 x 106 Mo, determine the semimajor axes of the orbits of these two stars. Give your answers in AU. (b) Calculate the angular size of each  Our Galaxy 585   OK. (a) Using the "hand" cursor, look at different parts of Guide menu if you can't see this.) Finally, move your field the Milky Way. Can you identify the direction toward the of view until you can see where the Milky Way crosses the galactic nucleus? In this direction the Milky Way appears celestial equator. (a) Measure the angle between the Milky broadest. Check your identification by centering the field Way and the celestial equator either on the screen or on a of view on the constellation Sagittarius (use the Find... printout of the screen. (Some estimation is needed here.) command in the Edit menu). (b) Click on the "Home" How well aligned with each other are the plane of the button in the Control Panel to turn the horizon back on, Milky Way and the plane of the Earth's equator, then set the local time to midnight (12:00:00 A.M.). Use the (b) A third plane of interest is the plane of the ecliptic. The controls in the Time palette to step through the months of ecliptic is shown as a green line (select The Ecliptic in the the year. In which month is the galactic nucleus highest in Guide menu if you can't see this.) Adjust your field of view the sky at midnight, so that it is most easily seen? so that the ecliptic appears as a straight line rather than as a curve (which means you are viewing in a direction that ypRRY Nr~y 43. Use the Starry Night program to determine lies in the ecliptic plane) and so that you can see where the ~ how the plane of the Galaxy is oriented on the ecliptic crosses the Milky Way. Measure the angle between ~ celestial sphere. As in the preceding Observing the Milky Way and the ecliptic either on the screen or on a Project, turn off daylight, show the entire celestial sphere, printout of the screen. (Again, some estimation will be and make the Milky Way visible. Next, adjust your field of needed.) 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