Page 40 Chapter 1 Chapter Review Review and Discussion 1. Briefly describe the geocentric model of the universe. geocentric model The geocentric model of the universe is a model of the solar system which holds that the Earth is at the center of the universe and all other bodies are in orbit around it. The earliest theories of the solar system were goecentric. 2. What was the basic flow? 3. What was the great contribution of Copernicus to our knowledge of the solar system? The Heliocentric model did rectify some small discrepancies and inconsistencies in the Ptolemaic System, but for Copernicus, the primary attraction of heliocentricity was its simplicity, its being more pleasing to the mind. To this day, scientists still are guided by simplicity, symmetry, and beauty in modeling all aspects of the universe. By relegating Earth to a non-central and undistinguished place within the solar system. Retrograde Motion. The Copernican model of the solar system explains both the varying brightnesses of the planets and the phenomenon of retrograde motion, The heliocentric (Sun-centered) model. Copernicus asserted that Earth spins on it axis, and like all other planets, orbits the Sun. This model explains the observed daily and seasonal changes in the heavens, as we have seen, but it also naturally accounts for planetary retrograde motion and brightness variations. The critical realization that Earth is not at the center of the universe is now known as the Copernican revolution. 4. What was the Copernican Revolution? The Copernican Revolution was The Birth of Modern Science. 5. What discoveries of Galileo helped confirm the views of Copernicus? Galileo claimed that Earth orbits the Sun. Galileo alsosaw four small points of light, invisible to the naked eye, orbiting the planet Jupiter, and the four small points of light were 4 moons. 6. Why is Galileo often referred to as the father of experi- mental science? Galileo builr a telescope for himself in 1609 and aimed it at the sky. Using his telescope, Galileo discovered that the Moon had mountains, valleys, and creates-terrain in many ways reminiscent of that of Earth. He found that the Sun had imperfections-dark blemishes now known as sunsports. By noting the changing appearance of these sunspots from day to day, he inferred that the Sun rotates, approximately once per month, around an axis roughly perpendicular to the ecliptic plane. 7. State Kepler's three laws of orbital motion? Kepler's first law has to do with the shapes of the planetary orbits: The orbital paths of the planets are elliptical (not circular), with the Sun at one focus. Kepler's second law addresses the speed at which a planet traverses different parts of its orbit Kepler's second law a line joining the planet to the sun sweeps out equal areas in equal intervals of time. The three shaded areas A, B, and C are equal. Any object traveling along the elliptinal path would take the same amount of time to cover the distance indicated by the three red arrows. Therefore, planets move fast when closer to the Sun. Kepler's third law states that The square of a planet's orbital period is proportional to the cube of its semi-major axis. 8. What is meant by the statement that Kepler's laws are emperical in nature? Kempler's empirical laws of planetary motion. The discovery of a set of simple empirical (that is, based on observation) "laws" that accurately described the motions of the planets. 9. If radio waves cannot be reflectedfrom the Sun, how can radar be used to find the distance from Earth to the Sun? We cannot use radar ranging to measure the distance to the Sun directly because radio signals are absorbed at the solar surface and are not reflected back to Earth. Instead, the planet Venus, whose orbit periodically brings it closest to Earth, is the most common target for this technique. The distance from Earth to Venus at the point of closest approach is approximately 0.3 A.U. Radar signals bounced off Venus at that instant return to Earth in about 300 seconds, indicating that Venus lies 45,000,000 km from Earth. With this information, we can compute the magnitude of the astronomical unit: 0.3 A.U. is 45,000,000 km, so 1 A.U. is 45,000,000 km/0.3, or 150,000,000 km. 10. List the two modifications made by Newton to Ke- pler's laws. Newton's laws of motion and his law of universal gravitation provided a theoretical explanation for Kepler's empirical laws of planetary motion. Just as Kepler modified the Copernican model by introducing ellipses in place of circles, so too did Newton make corrections to Kepler's first and third laws. Because the Sun and a planet feel equal and opposite gravitational forces (by Newton's third law), the Sun must also move (by Newton's first law), due to the planet's gravitational pull. 11. Why would a baseball thrown upwars from the surface of the Moon go higher than one thrown with the same velocity from the surface of Earth? Earth has a much greater effect on thelight baseball than the baseball has onthe much more massive Earth. The Moon, less massive than Earth, has less gravitational influenceon the baseball. This 12. What would happen to Earth if the Sun's gravity were suddenly "turned off"? True or False? ___ 1. Aristotle was first to propoaw that all planets re- volvearound the sun. ___ 2. Prolemy's geocentricmodel accurately prediced the positions of the planets, Moon, and the Sun. ___ 3. The heliocentric model of the universe holds that Earth is at the center of the universe. ___ 4. Copernicus's theories gained widespread scientific acceptance during his own lifetime. ___ 5. Galileo's telescopic observations provided direct ev- idence against the geocentric model of the universe. ___ 6. Kepler's discoveries regarding the orbital motion of the planets were based on his own observations. ___ 7. A circle has an eccentricity of zero. ___ 8. The astronomical unit is a distance equal to the semi-major axis of Earth's orbit around the Sun. ___ 9. The speed of a planet orbiting the Sun is indepen- dent of the planet's position in its orbit. ___ 10. Kepler'slaws hold only for the six planets known in his time. ___ 11. Kepler never determined the true distance between the planets and the Sun, only their relative distances. ___ 12. Using his laws of motion and gravity, Newton was ableto prove Kepler's laws. Planetary Motion The mutual gravitational attraction of the Sun and the planets, as expressedby Newton's law of gravity, is re- sponsible for the observed planetary orbits. As depicted in Figure 1.17, this gravitational force continuously pulls each planet toward the Sun, deflecting its forward motion into a curved orbital path.Because the Sun is much more massivethan any of the planets, it domi- nates the interaction. The Sun controls the planets. Figure 1.17 Sun's Gravity. The Sun's inward pull of gravity on an planet competes with the planet's tendency to continue moving in a straight line. These two effects combine, causing the planet to move smoothly along an intermediate path, which continuously "falls around" the Sun. This unending tug-of-war between the Sun's gravity and the planet's inertia results in a stable orbit. Page 67 Chapter 2 Chapter Review Review and Discussion 1. Define the following wave properties: period, wave- length, amplitude, frequency. The Period. The frequency of a wave is just one divided by the wave's period. wavelength x frequency = velocity. The wave period is the number of seconds needed for the wave to repeat itself at some point in space. The wavelength is the number of meters needed for the wave to repeat itself at a given moment in time. The amplitude is The maximum departure of the wave from the undisturbed state---still air, say, or a flat pond surface---is called its amplitude. 2. What is the relationship between wavelength, wave fre- quency, and wave velocity? Frequency is expressed in units of inverse time (cycles per second), called hertz (Hz). A wave moves a distance equal to one wavelength in one wave period. Te product of wavelength and frequency therefore equals the wave velocity. wavelength x frequency = velocity 3. Compare the gravitational force with the electric force. Unlike gravity, however, which is always attractive, electrical forces can be either attractive or repulsive. Particles having like charges. (both negative or both positive) repel one another, particles having unlike charges attract. Extending outward in all directions from our charged particles is an electric field, which determines the electric force exerted by the particle on other charged particles. The strength of the electric field, like that of the gravitational field, decreases with increasing distance from the source according to an inverse-square law. By means of the electric field, the particle's presence is "felt" by other charged particles, near and far. Charged Particles Particles carrying like electrical charges repel one another, particles with unlike charges attract . A charged particle is surrounded by an electric field, which determines the particle's influence on other charged particles. We represent the field by a series of field lines. If a charged particle begins to vibrate, its electric field changes. The resulting disturbance travels through space as a wave. Electric field lines Stationary charge Field line Distant charge Vibrating charge Wave Distant Charge 4. Describe the way in which light radiation leaves a star, travels through the vacuum of space, and finally is seen by someone on Earth. Electromagnetic radiation Another term for light, electromagnetic radiation transfers energy and information from one place to another, even through the vacuum of empty space. All types of electromagnetic radiation travel through space in the form of waves. Radiation is any way in which energy is transmitted through space from one point to another without the need for any physical connection between those two locations. 5. What do radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays have in common? How do they differ? radio Region of the electromagnetic spectrum corresponding to radiation of the longest wavelengths. infrared Region of the electromagnetic spectrum just outside the visible range, corresponding to light of a slightly longer wavelength than red light. ultraviolet Region of the electromagnetic spectrum, just outside the visible range, corresponding to wavelengths slightly shorter than blue light. Visible light is the particular type of electromagnetic radiation to which the human eye happens to be sensitive. X ray Region of the electromagnetic spectrum corresponding to radiation of high frequency and short wavelengths, far outside the visible spectrum. gamma ray Region of the electromagnetic spectrum, far beyond the visible spectrum, corresponding to radiation of very high frequency and very short wavelength. 6. In what regionsof the electromagnetic spectrum is the atmosphere transparent enough to allow observations from the ground? Where there is no shading at all, our atmosphere is almost totally transparent. Note that there are just a few windows, at well-defined locations in the electromagnetic spectrum, where Earth's atmosphere is transparent. In much of the radio and in the visible portions of the spectrum, the opacity is low, and we can study the universe at those wavelengths from ground level. 7. What is a blackbody? What are the characteristics of the radiation it emits? A blackbody is a curve in figure 2.9 refers to a mathermatical idealization kown as a blackbody--an object that absorbs all radiation falling upon it. In a steady state, a blackbody must reemit the same amount of energy as it absorbs. Figure 2.9 Ideal Blackbody Curve. The blackbody curve represents the distribution of the intensity of the radiation emitted by any object. Intensity is a term often used to specify the amount or strength of radiation at any point in space. Like frequency, and wavelength, intensity is a basic property of radiation. 8. If Earth were completely blanketed with clouds and we couldn't see the sky, could we learn about the realm be- yond the clouds? What forms of radiation might pene- trate the clouds and reach the ground? 9. Describe how its blackbody curvechanges asa red-hot glowing coal cools off. The blackbody curve shifts toward higher frequencies (shorter wavelengths) and greater intensities as an object's temperature increases. The blackbody curve The characteristic way in which the intensity of radiation emitted by a hot object depends on frequency. The frequency at which the emitted intensity is highest is an indication of the temperature of the radiating object. Also referred to as the Planck curve. 10. What is spectroscopy? Why is it so important to as- tronomers? spectroscopy The study of the way in which atoms absorb and emit electromagnetic radiation. Spectroscopy allows astronomers to determine the chemical composition of stars. 11. Describe the basic components of a spectroscope. In its most basic form, this device consists of an opaque barrier with a slit in it (to form a narrow beam of light), a prism (to split the beam into its component colors), and either a detector or a screen (to allow the user to view the resulting spectrum). spectroscope Instrument used to view a light source so that it is split into its component colors. 12. What is continuous spectrum? An absorption spectrum? continuous spectrum Spectrum in which the radiation is distributed over all frequencies, not just a few specific frequency ranges. A prime example is the black-body radiation emitted by a hot, dense body. A light bulb, for instance, emits radiation of all wavelengths (mostly in the visible and near-infrared ranges), with an intensit distribution that is well described by the blackbody curve corresponding to the bulb's temperature. Viewed through a spectroscope, the spectrum of the light from the bulb would show the familiar rainbow running from red to violet without interruption. A absorption spectrum is when a cool gas is placed between a source of continuous radiation (a light bulb) and the detector/screen of a spectroscope, the resulting spectrum is crossed by a series of dark absorption lines. These lines are formed when the cool gas absorbs certain wavelenghts (colors) from the light radiating from the light bulb. The absorption lines appear at precisely the same wavelenghts as the emission lines that would be produced if the gas were heated to high temperatures. 13. What is the normal condition for atoms? What is an ex- cited atom? What are orbitals? atom Building block of matter, composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons. Orbitals are like a hydrogen atom--the Bohr model--pictured its electron orbiting the central proton in a well-defined orbital, like a planet orbiting the Sun. Two electron orbitals of different energies are shown. (a) the ground state and (b) an excited state. excited state State of an atom when one of its electrons is in a higher energy orbital than the ground state. Atoms can become excited by absorbing a photon of a specific energy, or by colliding with a nearby atom. 14. Why do excited atoms absorb and reemit radiation at characteristic frequencies? the absorption of a more energetic (higher-frequency, shorter-wavelength) ultraviolet photon, this one having a wavelength of 102.6 nm (1026 A), causing the atom to jump to the second excited state. From that state, the electron may return to the ground state via either one of two alternate paths. 1. It can proceed directly back to the ground state, in the process emitting an ultraviolet 102.6 nm photon identical to the one that excited the atom in the first place. 2. Alternativel, it can cascade down one orbital at a time, emitting two photons: one having an energy equal to the difference between the second and first excited states, and the other having an energy equal to the difference between the first excited state and the ground state. Because electrons may exist only in orbitals having specific energies, atoms can absorb only specific amounts of energy as their electrons are boosted into excited states. Likewise, atoms can emit only specific amounts of energy as their electrons fall back to lower energy states. 15. Suppose a luminous cloud of gas is discovered emitting an emission spectrum. What can be learned about the cloud from the observation? You can learn that the a luminous cloud of gas solid or liquid, or a sufficiantly dense gas, emits light of all wavelengths and so produces a continuous spectrum of radiation. When using a spectroscope for luminous cloud of gas Can learn to see a slit in the barrier allows a narrow beam of light to pass. The beam passes through a prism and is split into its component colors. A lens then focuses the light into a sharp image that is either projected onto a screen, as shown here, or analyzed as it is passed through a detector. True or False? ___ 1.Light, radio, ultraviolet, and gamma rays are all forms of electromagnetic radiation. ___ 2. Sound is a familiar form of electromagnetic wave. ___ 3. Electromagnetic waves cannot travel through empty space. ___ 4. In a vacuum, electromagnetic waves all travel at the same speed, the speed of light. ___ 5. Visible light makes up the greatest part of the elec- tromagnetic spectrum. ___ 6. Ultraviolet light has the shortest wavelength of any electromagnetic wave. ___ 7. A blackbody emits all its radiation at a single wave- length or frequency. ___ 8. Emission spectra are characterized by narrow, bright lines of different colors. ___ 9. For an emission spectrum produced by a container of hydrogen gas, changing the amount of hydrogen in the container will change the color of the lines in the spectrum. ___ 10. In the previousquestion, changing the gas in the container from hydrogen to helium will change the colors of the lines in the spectrum. ___ 11. An absorption spectrum appears as a continuous spectrum interrupted by a series of dark lines. ___ 12. The wavelengths of the emission lines produced by an element are different from the wavelengths of the absorption lines produced by the same element. ___ 13. The energy of a photon is inversely proportional to the wavelength of the radiation. ___ 14. An electon can have any energy within an atom, so long as that energy is above the ground state energy. ___ 15. An atom can remain in an excited state indefinitely. Page 95 Chapter 3 Chapter Review Review and Discussion 1. List three advantages of reflecting telescopes over re- fracting telescopes. In a Newtonian telescope another type of a reflecting telescope. The light is intercepted by a flat secondary mirror before it reaches the prime focus and deflected 90°, usually to an eyepiece at the side of the instrument. This is a popular design for smaller reflecting telescopes, such as those used by amateur astronomers. Each design of the four telescopes uses a primary mirror at the bottom of the telescope to capture radiation, which is then directed along different paths for analysis. In the reflecting telescopes on the Cassegrain design, light is first reflected by the primary mirror toward the prime focus and reflected back down the tube by a secondary mirror. reflecting telescope A telescope which uses a mirror to gather and focus light from a distant object. refracting telescope A telescope which uses a lens to gather and focus light from a distant object. 2. What are the largest optical telescopes in use today? Currently, the largest optical telescopes are the twin Keck instruments atop Mauna Kea in Hawaii. 3. How does Earth's atmosphere affect what is seen through an optical telescope? The thin air at this high-altitude site in Mauna Kea in Hawaii guarantees less atmospheric absorption of incoming radiation and a clearer view than at sea level, but the air is so thin. 4. What advantages does the Hubble Space Telescope have over ground-based telescopes? List some disadvantages. The Hubble Space Telescope can take much more clearer and sharper images of the Universe. Some disadvantages are the Atmospheric Blurring As we observe a star, atmospheric turbulence produces continuous small changes in the optical properties of the air between the star and our telescope (or eye). The light from the star is refracted slightly, again and again, and the stellar image dances around on the detector (or retina). This is the cause of the well-known twinkling of stars. It occurs for the same basic reason that objects appear to shimmer when viewed across a hot roadway on a summer day. 5. What are the advantages of a CCD over a photo- graphic plate? The first advantage is CCD's are much more efficient than photographic plates, recording as many as 75 percent of the photons striking them, compared with less than five percent for photographic methods. This means that a CCD instrument can image objects 10 to 20 times fainter-or the same object 10 to 20 times faster-than can a photographic plate. The second advantage is CCDs produce a faithful representation of an image in a digital format that can be placed directly on magnetic tape or disk, or even sent across a computer network to an observer's home institution for analysis. 6. Is the resolution of a 2-m telescope on Earth's surface limited more by atmospheric turbulence or by the effects of diffraction? No 7. Why do radio telescopes have to be very large? Radio telescopes are built large in part because cosmic radio sources are extremely faint. 8. What kind of astronomical objects can we best study with radio techniques? Cosmic objects across the sky Can see and study a astronomical object like a Radio Galaxy. Visible image of the radio galaxy Centaurus A, with the radio emission from the region superimposed (in false color, with red indicating greatest radio intensity, blue the least). The resolution of the optical image is about 1", that of the radio map is 12". 9. What is interferometry, and what problem in radioas- tronomy does it address? interferometry Technique in widespread use to dramatically improve the resolution of radio and infrared maps. Several telescopes observe an object simultaneously, and a computer analyzes how the signals interfere with one another to reconstruct a detailed image of the field of view. Radio astronomers can sometimes overcome the problem of poor angular resolution by using a technique known as interferometry. This technique makes it possible to produce radio images of much higher angular resolution than can be achieved with even the best optical telescopes on Earth or in space. 10. Compare the highest resolution attainable with optical telescopes to the highest resolution attainable with radio telescopes (including interferometers). The 5-m Hale telescope should have an angular resolution of around 0.02". In practice, however, it cannot do better than about 1". In fact, apart from instruments using special techniques developed to examine some particularly bright stars, no ground-based optical telescope built before 1990 can resolve astronomical objects to much better than about 1". The 43-m telescope can achieve resolution of about 1' at a wavelength of 1 cm. The best angular resolution obtainable with a single radio telesscope is about 10" (for the largest instruments operating at millimeter wavelengths), at least 10 times coarser than the capabilities of the largest optical mirrors. interferometers a interferometer Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength. The effective diameter of an interferometer is equal to the distance between its outermost telescopes. Interferometr is a technique makes it possible to produce radio images of much higher angular resolution. By means of electronic cables or radio links, the signals received by each antenna in the array making up the interferometer are sent to a central computer that combines and stores the data as the antennas track their target. After extensive computer processing, a high-resolution image of the target results. 11. What special conditions are required to conduct obser- vations in the infrared? The special conditions are by placing telescope instruments above most or all of Earth's atmosphere. Improvements in balloon-, aircraft-, rocket-, and satellite-based telescope technologies have made infrared research a powerful tool for studying the universe. infrared Region of the electromagnetic spectrum just outside the visible range, corresponding to light of a slightly longer wavelength than red light. 12. In what ways do the mirrors in X-ray telescopes differ from those found in optical instruments? X-ray Telescope The arrangement of mirrors in an X-ray telescope allows the rays to be reflected at grazing angles and focused into an image. 13. Why can't gamma-ray astronomy be done from the ground? gamma ray Region of the electromagnetic spectrum, far beyond the visible spectrum, corresponding to radiation of very high frequency and very short wavelength. gamma rays- the types of radiation whose photons have the highest frequencies and the greatest energies. First, it must be captured high above Earth's atmosphere because none of it reaches the ground. Second, its detection requires the use of equipment basically different from that used to capture the relativel low-energy radiation. 14. What are the main advantages of studying objects at many different wavelengths of radiation? The types of astronomical objects that can be observed may differ markedly from one wavelength range to another. Full-spectrum coverage is essential not only to see things more clearly, but even to see some things at all. Because of the transmission characteristics of Earth's atmosphere, astronomers must study most wavelengths other than optical and radio from space. 15. Our eyes can see light with an angular resolution of about 1'--equivalent to about a third of a millimeter at arm's length Suppose our eyes detected only infrared radia- tion, with 1o angular resolution. Would we be able to make our way around on Earth's surface? To read? To sculpt? To create technology? Because of diffraction, the angular resolution of radio telescopes is generally quite poor compared with that of their optical counterparts. True or False? ___ 1. A Newtonian telescope has no secondary mirror. ___ 2. A Cassegrain telescope has a hole in the middle of the primary mirror to allow light reflected from its secondary mirror to reach a focus behind the pri- mary mirror. ___ 3. The term "seeing" is used to describe how faint an object can be detected by a telescope. ___ 4. Because of diffraction, the image made by a telescope becomes sharper as the telescope diameter decreases. ___ 5. The main advantage to using the Hubble Space Tele- scope is the increased amount of "night" time avail- able to it. ___ 6. One of the primary advantages of CCDs over pho- tograph plates is the former's high efficiency in de- tecting light. ___ 7. The Hubble Space Telescope can observe objects in the optical, infrared, and ultraviolet parts of the spectrum. ___ 8. The Keck telescopes contain the largest single mir- rors ever produced. ___ 9. Radio telescopes are large in part to improve their angular resolution, which is poor because of the long wavelengths at which they observe. ___ 10. Radio telescopes are large in part because the sources of radio radiation they observe are very faint. ___ 11. For technological reasons, interferometry is limit- ed to radio astronomy. ___ 12. Infrared astronomy can only be done from space. ___ 13. X-ray astronomy can only be done from space. ___ 14. X-ray and gamma-ray telescopes employ the same basic design as optical instruments. ___ 15. Because gamma rays have very short wavelengths, gamma-ray telescopes can achieve extremely high angular resolution. Page 123 Chapter 4 Chapter Review The organization of the solar system points to forma- tition as a one-time event that occurred 4.6 billion years ago. Review and Discussion 1. Name three differences between terrestrial and jov- ian planets. How the weight of overlying layers squeezes the interiors of the terrestrial planets to different extents (greatest for Earth, least for Mercury) The densities they would have in the absence of any gravitational compression-decreases steadily as earth moves farther from the Sun. jovian planet One of the four giant outer planets of the solar system, resembling Jupiter in physical and chemical composition. Saturn is another jovian planet. Uranus, and Neptune. are all similar to one another chemically and physically (and very different from the terrestrial worlds). The word jovian comes from Jove. The jovian worlds are all much larger than the terrestrials and quite different from them in both composition and structure. Terrestrial planets are close to the Sun closely spaced orbits small masses Jovian planets are far from the Sun widely spaced orbits large masses 2. Why are asteroids and meteoroids important to planetary scientists? They are of crucial importance to our studies, for they are the keys to answering some very fundamental questions about our planetar environment. 3. Do all asteroid orbits lie between Mars and Jupiter? no asteroid One of thousands of very small members of the solar system orbiting the Sun between the orbits of Mars and Jupitar. asteroid belt Region of the solar system, between the orbits of Mars and Jupiter, in which most asteroids are found. 4. What might be the consequences of a 10-km asteroid striking Earth today? Would devastate an area hundreds of kilometers in diameter. Should a sufficiently large asteroid hit our planet, it might even cause the extinction of entire species. for example it is thought to believed that the dinosaurs became extinct by an asteroid. 5. What are comets like when they are far from the Sun? Comets are usually discovered as faint, fuzzy patches of light on the sky while still several astronomical units away from the Sun. What happens when they enter the inner solar system? in the inner solar system, most Kuiper Belt comets move in roughly circular orbits between about 30 and 100 A.U. from the Sun, always remaining outside the orbits of the jovian planets. Occassionally, however, either a chance encounter between two comets or (more likely) the gravitational influence of Neptun "kicks" a Kuiper Belt comet into an eccentric orbit that brings it into the solar system into our view. 6. What are the typical ingredients of a comet nucleus? nucleus Dense, central region of an atom, containing both protons and neutrons, and orbited by one or more electrons. The solid region of ice and dust that composes the central region of the head of a comet. 7. Why can comets approach the Sun from any direction but asteroids generally orbit closeto the ecliptic plane? Comets unlike the orbits of other solar system objects, the orbits of comets are not confined to within a few degrees of the ecliptic plane. Short-period comets do tend to have prograde orbits lying close to the ecliptic, but long-period comets exhibit all inclinations and all orientations, both prograde and retrograde, roughly uniformly distributed in all directions from the Sun. Comets are usually discovered as faint, fuzzy patches of light on the sky while still several astronomical units away from the Sun. Traveling in a highly elliptical orbit with the Sun at one focus, a comet brightens, and develops an extended tail as it nears the Sun. All but one of the known asteroids revolve about the Sun in prograde orbits (that is, in the same sense as Earth and the other planets, and like the planets, most asteroids have orbits fairly close to the ecliptic plane (with inclinations less than 10°-20°). Unlike the almost circular paths of the major planets, however, asteroid orbits are generally noticeably elliptical in shape. 8. Explain the difference between a meteor, a meteoroid, and a meteorite. meteor Bright streak in the sky, often referred to as a "shooting star," resulting from a small piece of interplanetary debris entering Earth's atmosphere and heating air molecules, which emit light as they return to their ground states. meteoroid Chunk of interplanetary debris prior to encountering Earth's atmosphere. meteorite Any part of a meteoroid that survives passage through the atmosphere and lands on the surface of Earth. A meteoroid is a piece of rocky interplanetary debris smaller than 10 m across. 9. What causes a meteor shower? a meteor shower Event during which many meteors can be seen each hour, caused by the yearly passage of the Earth through the debris spread along the orbit of a comet. 10. What do meteorites reveal about the age of the solar system? From cold temperatures how they become frozen, and to hot temperatures how they can become melted. Almost all meteorites are old. Radioactive dating shows most of them to be between 4.4 and 4.6 billion years old-roughly the age of the oldest Moon rocks brought back to Earth by Apollo astronauts. Meteorites, along with asteroids, comets, and some lunar rocks, provide essential clues to the original state of matter in the solar neighborhood and to the birth of our planetary system. 11. Describe the basic features of the nebular theory of solar system formation, and give three examples of how this theory explains some observed features of the present-day solar system. nebular theory One of the earliest models of solar system formation, dating back to Descartes, in which a large cloud of gas began to collapse under its own gravity from the Sun and planets. suppose that planets form out of this spinning disk, we can begin to understand the origin of much of the architecture observed in our planetary system today, such as the near-circularity of the planets' orbits and the fact that they move in prograde orbits in almost the same plane. The idea that planets form from such a disk is called the nebular theory. the collapse and flattening of the solar nebula was essentially correct, a disk of gas would not form clumps of matter that would subsequently evolve into planets. In fact, computer calculations predict just the opposite: any clumps in the gas would tend to disperse.not contract further. According to the condensation theory, the planets formed three stage. Early on, dust grains in the solar nebula formed condensation nuclei around which matter began to accumulate. This vital step greatly hastened the critical process of forming the first small clumps of matter. Once these clumps form, they grew rapidly by sticking to other clumps. 12. What key ingredient in the modern condensation the- ory of solar system formation was missing from the orig- inal nebular theory? interstellar dust in the solar nebula. Astronomers now recognize that the space between the stars is strewn with microscopic dust grains, an accumulation of the ejected matterof many long-dead stars. These dust particles probably formed in the cool atmospheres of old stars, then grew by accumulating more atoms and molecules from the interstellar gas. The end result if that interstellar space is littered with tiny chunks of icy and rocky matter. 13. Why are the jovian planets so much larger than the ter- restrial planets? The four largest protoplanets became large enough to enter a third phase of planetary development, sweeping up large amounts of gas from the solar nebula to form what would ultimately become the jovian planets. The smaller, inner protoplanets never reached that point, and as a result their masses remained relatively low. The outer protoplanets may have grown faster simply because there was more raw material available to them. 14. How did the temperature at various locations in the solar nebula determine planetary composition? At any given location, the only materials to condense out were those able to survive the temperature there. The composition of the material that could condense out at any given distance from the Sun ultimately determined the type of planet that formed there. The cores of the jovian planets were formed under cold conditions out of low-density, icy material. 15. Where did Earth's water come from? Icy fragments-comets-from the outer solar system bombarded the newly born inner planets, supplying them with water after their formation. True or False? ___ 1. Most planets orbit the Sun in nearly the same plane as Earth. ___ 2. The largest planets also have the highest densities. ___ 3. All planets have moons. ___ 4. Asteroids generally move on almost circular orbts. ___ 5. Asteroids were recently formed by the breakup of a planet orbiting in the asteroid belt. ___ 6. The Kulper Belt is another name for the asteroid belt. ___ 7. Some comets orbit as much as 50,000 A.U. from the Sun. ___ 8. Most comets have short periods and orbit close to the ecliptic plane. ___ 9. Comet tails always lie along the comet's orbit. ___ 10. The light emitted from a meteor is due to a mete- oroid burning up in Earth's atmosphere. ___ 11. Comets are the sources of meteor showers. ___ 12. The direction of planetary revolution is the same as thedirection of the Sun's rotation. ___ 13. The solar system is highly differentiated. ___ 14. The terrestrial planets formed out of the original dust that made up the solar nebula. ___ 15. Accretion occurred faster in the inner part of the solar system than in the outer regions. Page 151 Chapter 5 Chapter Review Review and Discussion 1.Explain how the Moon produces tides in Earth's oceans. The tides are a direct result of the gravitational influence of the Moon and the Sun on Earth. When Earth, Moon, and Sun are roughly lined up-at new for full Moon-the gravitational effects reinforce one another, and the highest tides occur. These tides are known as spring tides. When the Earth-Moon line is perpendicular to the Earth-Sun line (at the first and third quarters), the daily tides are smallest. These are termed neap tides. 2. What is a synchronous orbit? How did the Moon's orbit become synchronous? synchronous orbit State of an object when its period of rotation is exactly equal to its average orbital period. The Moon is in a synchronous orbit, and so presents the same face toward Earth at all times. Basically the same process is responsible for the Moon's synchronous orbit. Just as the Moon raises tides on Earth, Earth also produces a tidal bulge in the Moon. Indeed, because Earth is so much more massive than the Moon, the lunar tidal bulge is considerably larger and the synchronization process correspondingly faster. The Moon's orbit became synchronous long ago-the Moon is said to have become tidally locked to Earth. Most of the moons in the solar system are similarly locked by the tidal fields of their parent planets. 3. What is convection? What effect does it have on (a) Earth's atmosphere and (b) Earth's interior? convection Churning motion resulting from the constant upwelling of warm fluid and the concurrent downward flow of cooler material to take its place. Convection Convection occurs whenever cool matter overlies warm matter. The resulting circulation currents are familiar to us as the winds in Earth's atmosphere, caused by the solar-heated ground. Over and over, hot air rises, cools, and falls back to Earth. Eventually, steady circulation patterns are established and maintained provided the source of heat remains intact. The Greenhouse Effect is warming up the ground on the Earth as the temperatures rise, and the Greenhouse Effect when the temperatures rise enough to melt much of the polar ice caps and cause dramatic, perhaps even catastrophic, changes in Earth's climate. 4. In contrast to Earth, the Moon undergoes extremes in temperature. Why? Lacking the moderating influence of an atmosphere, the Moon experiences wide variations in surface temperature. Noontime temperatures can reach 400 K, well above the boiling point of water (373 K). At night (which lasts nearly 14 Earth days) or in the shade, temperatures fall to about 100 K, well below water's freezing point (273 K). 5. Use the concept of escape speed to explain why the Moon has no atmosphere. For all practical purposes, there is no atmosphere on the Moon. All of it escaped long ago. More massive objects have a better chance of retaining their atmospheres because the more massive an object, the greater the speed needed for atoms and molecules to escape. The Moon's escape speed is only 2.4 km/s, compared with 11.2 km/s for Earth. Simply put, the Moon has a lot less pulling power-any atmosphere it might once have had is gone forever. With such high daytime temperatures and no atmosphere to help retain it, any water that ever existed on the Moon's surface has most likely evaporated and escape. Not only is there no lunar hydrosphere, but all the lunar samples were absolutely bone dry. 6. What is the greenhouse effect? Is the greenhouse effect operating in Earth's atmosphere helpful or harmful? What are the consequences of an enhanced greenhouse effect? greenhouse effect The partial trapping of solar radiation by a planetary atmosphere, similar to the trapping of heat in a greenhouse. The greenhouse effect is harmful. When the temperatures rise, and left unchecked, may result in global temperature increases of several kelvins over the next half-century, enough to melt much of the polar ice caps and cause dramatic, perhaps even catastrophic, changes in Earth's climate. 7. The density of water in Earth's hydrosphere and the den- sity of rocks in the crust are both lower than the average density of the planet as a whole. What does this fact tell us about Earth's interior? At some time in the distant past, much of Earth was molten, allowing the higher-density matter to sink to the core, displacing lower-density material toward the surface. differentiation Variation with depth in the density and composition of a body, such as Earth, with low-density material on the surface and higher density material in the core. The high central density suggests to geologists that the inner parts of Earth must be rich in nickel and iron. Density and temperatures both increase with depth. 8. Give two reasons geologists believe that part of Earth's core is liquid. Some paths followed by seismic waves from the site of an earthquake. Some of the waves (called shear, or S-waves, and colored red in the figure) are blocked by Earth's outer coure. The result is the shadow zone shown in Figure 5.8. The explanation for this behavior is that these particular waves cannot pass through liquid. The observation that every earthquake exhibits these shadow zones is the best evidence we have that the outer core of our planet is liquid. There is also evidence that some waves (pressure, or P-waves, shown in green), which can pass through the liquid outer core, are reflected off the surface of a solid inner core, of radius 1300 km. 9. What clue does Earth's differentiation provide to our planet's history? differentiation Variation with depth in the density and composition of a body, such as Earth, with low-density material on the surface and higher density material in the core. Earth, then is not a homogeneous ball of rock. Instead, it has a layered structure, with a low-density rocky crust at the surface, intermediate-density rocky material in the mantle, and a high-density metallic core. This variation in density and composition is known as differentiation. 10. What process is responsible for the surface mountains, oceanic trenches,and other large-scale features on Earth's surface. Plate tectonics The slowly drifting gigantic plates or slabs of Earth's surface, and that these plates are slowly drifting around the surface of our planet. These plate motions, popularly known as continental drift, have created the mountains, oceanic trenches, and many other large-scale features across the face of planet Earth. Earthquakes and volcanic eruptions Erosion by wind and water has obliterated much of the evidence from ancient times.. 11. In what sense were the lunar maria once"seas"? The largest of them (Mare Imbrium, the "Sea of Showers") is about 1100 km in diameter. Today we know that the maria are actually extensive flat plains that resulted from the spread of lava during an earlier, volcanic period of lunar evolution. In a sense, then, the maria are oceans-ancient seas of molten lava, now solidified. 12. What is the primary source of erosion on the Moon? Why is the average rate of lunar erosion so much less than on Earth? Meteoritic impact is the only important source of erosion on the Moon. Over billions of years, collisions with meteoroids, large and small, have scarred, cratered, and sculpted the lunar landscape. The lunar surface has a lack of wind and water on the airless Moon. Despite this barrage from space, the Moon's present-day erosion rate is still very low-about 10,000 times less than on Earth. Lunar Surface The lunar surface is not entirely changeless. Despite the complete lack of wind and water on the airless Moon, the surface has still eroded a little under the constant "rain" of impacting meteoroids, especially micrometeoroids. 13. Name two pieces of evidence indicating that the lunar highlands are older than the maria. Radioactive dating indicates ages of more than four billion years for highland rocks, and from 3.2 to 3.9 billion years for those from the maria. The older highlands are much more heavily cratered than the younger maria. Knowing the ages of the highlands and maria, researchers can estimate the rate of cratering in the past. 14. Give a brief description of Earth's magnetosphere. Why does the Moon have no magnetosphere? Simpy put, the magnetosphere is the region around a planet that is influenced by that planet's magnetic field. It forms a buffer zone between the planet and the high-energy particles of the solar wind. It can also provide important insights into the planet's interior structure. That planetary magnetism requires a rapidly rotating liquid metal core. Because the Moon rotates slowly and because its core is probably neither molten nor particularly rich in metals, the absence of a lunar magnetic field is exactly what we expect. 15. Describe the theory of the Moon's origin currently fa- vored by many astronomers. Today, many astronomers favor a scenario often called the impact theory, which postulates a glancing collision between a large, Mars-sized object and a youthful, molten Earth. Computer simulations of such a catastrophic event show that most of the bits and pieces of splattered Earth could have coalesced into a stable orbit, forming the Moon. True or False? ___1.The average density of Earth is less than the den- sity of water. ___ 2. There is one high tide and one low tide per day at any given coastal location on Earth. ___ 3. Because of tidal forces, Earth's rotation rate is in- creasing. ___ 4. Because of the tides, the Moon is in a synchronous orbit around Earth. ___ 5. Lunar maria are extensive lava-flow regions. ___ 6. Except for the layer of air closest to Earth's surface, the ozone layer is the warmest part of the atmosphere. ___ 7. Earth's atmosphere is composed primarily of oxygen. ___ 8. Water vapor and nitrogen are the primary green- house gases in Earth's atmosphere. ___ 9. The Moon has no detectable atmosphere. ___ 10. Earth's magnetic field is the result of our planet's large permanently magnetized iron core. ___ 11. The Moon has a weak magnetic field. ___ 12. Motion of the crustal plates is driven by convec- tion in Earth's upper mantle. ___ 13. Samples of Earth's core are available from volcanos. ___ 14. Volcanic activity continues today on the surface of the moon. ___ 15. Like Earth, the Moon has a liquid metal core. Page 179 Chapter 6 Chapter Review Review and Discussion 1. In contrast to Earth, Mercury undergoes extremes in temperature. Why? Mercury, the innermost planet, lies close to the Sun. 2. What do Mercury's magnetic field and large average den- sity imply about the planet's interior? Mercury's high average density, for instance, tells us that this planet must have large iron core. 3. How is Mercury's evolutionary history like that of the Moon? How is it different? The planet's rotation axis is almost exactly perpendicular to its orbit plane. Mercury's high average density, for instance, tells us that this planet must have a large iron core, consistent with the condensation theory's account of its formation. However, in many other respects Mercury is similar to Earth's Moon, leading us to use the Moon as a model for understanding Mercury's past. Mercury presents the same face to the Sun not every time around, but every other time. The Sun also influences the tilt of Mercury's spin axis. The similarities of the Mercury and the Moon are striking. Much of the cratered surface on Mercury bears a strong resemblance to the Moon's highlands. 4. Mercury used to be called "the Moon of the Sun." Why do you think it had this name? What piece of scientific evidence proved the name inappropriate? Because of the Sun's influences. The Sun also influences the tilt of Mercury's spin axis. Because of the Sun's tides. Mercury's rotation axis is almost exactly perpendicular to its own orbit plane. The Sun is always directly overhead. 5. Why does Venus appear so bright to the naked eye? Being somewhat farther from the Sun, the next planet, Venus, is visible for a little longer. Venus is the third brightest object in the entire sky. Like all the planets, it shines by reflected sunlight. It is so bright because almost all the sunlight reaching it is reflected from thick clouds that envelop the planet. 6. Venus probably has a molten iron-rich core like Earth. Why doesn't it also have a magnetic field? The lack of any detectable magnetic field, then, is almost surely the result of the planet's extemely slow rotation. The planet was struck by a large body, muck like the one that may have hit Earth and formed the Moon. That impact was sufficient to reduce the planet's spin to almost zerio, leaving Venus rotating in the manner we now observe. 7. How did radio observations of Venus made in the 1950s change our conception of that planet? In the 1950s, however, when radio observations of the planet penetrated the cloud layer and gave the first indication of conditions near the surface, they revealed a temperature exceeding 600 K! Almost overnight, the popular conception of Venus changed from lush tropical jungle to arid, uninhabitable desert. 8. What are the main constituents of Venus's atmosphere? What are clouds in the upper atmosphere made of? Venus's atmosphere is much more massive than our own, and it extends to a much greater height above the planet's surface. The dominant constituent (96.5 percent) of Venus's atmosphere is carbon dioxide. Almost all of the remaining 3.5 percent is nitrogen. The clouds are composed of sulfuric acid droplets. 9. What is the runaway greenhouse effect, and how might it have altered the climate of Venus? runaway greenhouse effect A process in which the heating of a planet leads to an increase in its atmosphere's ability to retain heat and thus to further heating, quickly causing extreme changes in the temperature of the surface and the composition of the atmosphere. 10. Why is Mars red? One important finding of these studies was the high iron content of the planet's surface. Chemical reactions between the iron-rich surface soil and trace amounts of oxygen in the atmosphere are responsible for the iron oxide ("rust") that gives Mars its characteristic red color. 11. What is the evidence that water once flowed on Mars? Is there water on Mars today? Water once flowed on Mars as the evidence is Two types of flow features are seen: runoff channels, and outflow channels. There is no evidence for liquid water anywhere on Mars today. Most likely if there is water on Mars? The water is locked in a layer of permafrost, which is water ice lying just below the planet's surface, with some more water contained in the polar caps. 12. Why were Martian volcanoes able to become so large? The great height of Martian volcanoes is a direct consequence of the planet's low surface gravity. 13. Given that Mars has an atmosphere and its composition is mostly carbon dioxide, why isn't there a significant greenhouse effect to warm its surface? Mar's small size means that any internal heat would have been able to escape more easily than in a larger planet like Earth or Venus. It is too cold on Mars. The magnetic field is to weak. 14. Do you think that sending humans to Mars in the near future is a reasonable goal? Why or why not? It is o.k. to send humans to Mars only if properly well prepared due to it being such a long space flight, and so cold on the surface of Mars. It maybe fun exploring, and walking around on a new planet. 15. Compare and contrast the evolution of the atmospheres of Mars, Venus, and Earth. Include a discussion of the im- portance of volcanoes and water. Earth When Earth formed, any atmosphere it might have had-sometimes called the primary atmosphere-would have consisted of the gases most common in the early solar system: light gases such as hydrogen, helium, methane, ammonia, and water vapor. Mars Presumably, Mars also had first a primary and then a secondary (outgassed) atmosphere early in its history. Around 4 billion years ago, Mars may have had a fairly dense atmosphere, perhaps even with blue skies and rain. Venus Venus's dense atmosphere is made up almost entirely of a prime greenhouse gas, carbon dioxide. Venus is hot because of the greenhouse effect. True or False? ___ 1.Mercury has no detectable atmosphere. ___ 2. Mercury has nighttime low temperatures of 100 K, well below the freezing point of water. ___ 3. Mercury's solar day is longer than its solar year. ___ 4. Some volcanic activity continues today on the sur- face of Mercury. ___ 5. The average surface temperature of Venus is about 250 K. ___ 6. Numerous surface features on Venus can be seen from Earth-based observations made in the ultra- violet part of the spectrum. ___ 7. Lava flows are common on the surface of Venus. ___ 8. There is strong circumstantial evidence that active volcanism continues on Venus. ___ 9. Venus has a magnetic field similar to that of Earth. ___ 10. Mars has the largest volcanoes in the solar system. ___ 11. The northern hemisphere of Mars is much older than the southern hemisphere. ___ 12. There are many indications of plate tectonics on Mars. ___ 13. Valles Marineris is similar in size to Earth's Grand Canyon. ___14. The orange-red color of the surface of Mars is pri- marily due to rust (iron oxide) in its soil. ___ 15. One of the two moons of Mars is larger than Earth's Moon. Page 203 Chapter 7 Chapter Review Review and Discussion 1. How was Uranus discovered? The British astronomer William Herschel discovered the planet Uranus in 1781. 2. Why did astronomers suspect an eighth planet beyond Uranus? Astronomers realized that, although the Sun's gravitational pull dominates the planet's orbital motion, the small deviation meant that some unknown body was exerting a much weaker but still measurable gravitational force on Uranus. There had to be another planet in the solar system influencing Uranus. 3. Why have the jovian planets retained most of their orig- inal atmospheres? The jovian planets are massive enough to have retained even the lightest gas, hydrogen, and very little of their original atmospheres have escaped since the birth of the solar system 4.6 billion years ago. 4. What is differential rotation, and what evidence do we have that it occurs on Jupiter? differential rotation The tendency for a gaseous sphere, such as a jovian planet or the Sun, to rotate at a different rate at the equator than at the poles or for the rotation rate to vary with depth. For a galaxy or other object, a condition where the angular speed varies with location within the object. is normal for fluid bodies such as the jovian planets. The amount of differential rotation on Jupiter is small-the equatorial regions rotate once every 9h 50m, while the higher latitudes take about 6 minutes longer 5. Why does Saturn have a less varied appearance than Jupiter? The colors of Saturn's cloud layers, as well as the planet's overall yellowish coloration, are likely due to the same basic cloud chemistry as on Jupiter. However, because Saturn's clouds are thicker, there are fewer holes and gaps in the top layer, with the result that we rarely glimpse the more colorful levels below. Instead, we see only the topmost layer, which accounts for Saturn's less varied appearance. 6. Compare the thicknesses of Saturn's various layers (clouds, molecular hydrogen, metallic hydrogen, and core) to the equivalent layers in Jupiter. The total thickness of the three cloud layers in Saturn's atmosphere is roughly 250 km-about three times the cloud thickness on Jupiter-and each layer is thicker than its counterpart on Jupiter. The reason for this difference is Saturn's weaker gravity. 7. What is the Great Red Spot? What is known about the source of its energy? Great Red Spot A large, high-pressure, long-lived storm system visible in the atmosphere of Jupiter. The Red Spot is roughly twice the size of the Earth. Astronomers think that the Spot is somehow sustained by Jupiter's large-scale atmospheric motion. The zonal motion north of the Spot is westward, while that to the south is eastward, supporting the idea that the Spot is confined and powered by the zonal flow. Turbulent eddies form and drift away from the spot's edge. The center, however, remains tranquil in appearance, like the eye of a hurricane on Earth. 8. Describe the cause of the colors in the jovian planets' atmospheres. Jupiter's Convection The colored bands in Jupiter's atmosphere are associated with vertical convective motion. Upwelling warm gas results in the lighter-colored zones. Jupiter's Atmosphere The vertical structure of Jupiter's atmosphere contains clouds that are arranged in three main layers, each with quite different colors and chemistry. Scientists believe that complex chemical processes occurring in Jupiter's turbulent atmosphere are responsible for these colors, although the details of the chemistry are not fully understood. The trace elements sulfur and phosphorus may play important roles in influencing the cloud colors-particularly the reds, browns, and yellows. The energy that powers the reactions comes in many forms: the planet's own internal heat, solar ultraviolet radiation, aurorae in the planet's magnetosphere, and lightning discharges within the clouds Saturn's atmosphere. In many respects, it is similar to Jupiter. The colors of Saturn's cloud layers, as well as the planet's overall yellowish coloration, are likely due to the same basic cloud chemistry as on Jupiter. However, because Saturn's clouds are thicker, there are fewer holes and gaps in the top layer, with the result that we rarely glimpse the more colorful levels below. Instead, we see only the topmost layer, which accounts for Saturn's less varied appearance. 9. Compare and contrast the atmospheres of the jovian planets, describing how the differences affect the ap- pearance of each. The blue color of the outer jovian planets is the result of their relatively high percentages of methane. The greater the concentration of methane, the bluer the reflected light. Therefore Uranus, having less methane, looks blue-green, while Neptune, having more methane, looks distinctly blue. Atmospheric Composition of Uranus and Neptune The atmospheres of Uranus and Neptune are very similar in composition to that of Jupiter-the most abundant gas is molecular hydrogen (84 percent), followed by helium (about 14 percent) and methane, which is more abundant on Neptune (about 3 percent) than on Uranus (2 percent). The abundance of hydrogen and helium on these worlds is itself a consequence of the strong jovian gravity. The jovian planets are massive enough to have retained even the lightest gas, hydrogen, and very little of their original stmospheres have escaped since the birth of the solar system 4.6 billion years ago. 10. Explain the theoritical model that accounts for Jupiter's internal heat source. Jupiter would radiate back into space the same amount of energy as it receives from the Sun. Astronomers theorize that the source of Jupiter's excess energy is heat left over from the planet's formating. That heat is slowly leaking out through the planet's heavy atmospheric blanket, resulting in the excess emission we observe. The hdrogen behaves like a liquid metal. Jupiter's magnetic field, this metallic Hydrogen is an excellent conductor of electricity. The energy that powers the reactions comes in many forms: the planet's own internal heat, solar ultraviolet radiation, aurorae in the planet's magnetosphere, and lightning discharges within the clouds. 11. Describe the theoretical model that accounts for Sat- urn's missing helium and the surprising amount of heat the planet radiates. The amount of helium condensation was greatest in the planet's cool outer layers, where the mist turned to rain about two billion years ago. A light shower of liquid helium has been falling though Saturn's interior ever since. This helium precipitation is responsible for depleting the outer layers of their helium content. Saturn's atmospheric helium deficit. As the helium sinks toward the center, the planet's gravitational field compresses it and heats it up. The energy thus released is the source of Saturn's internal heating. In the distant future, the helium rain will stop and Saturn will cool until its outermost laers radiate only as much energy as they receive from the Sun. 12. What is unusual about the rotation of Uranus? Unlike the other major planets, which have their spin axis (very) roughly perpendicular to the ecliptic plane, Uranus's rotation acis list almost within that plane. Uranus is "tipped over" onto its side, for its axial tilt is 98°. As a result, the "north: pole of Uranus, at some point in the planet's orbit, points almost directly toward the Sun. Half a Uranian year later, the "south" pole faces the Sun. 13. What is responsible for Jupiter's enormous magnetic field? The combination of rapid overall rotation and an extensive region of highly conductive fluid in its interior gives Jupiter by far the strongest planetary magnetic field in the solar system. The planet's intrinsic field strength is some 20,000 times that of Earth. Jupiter is surrounded by a vast sea of energetic charged particles (mostly electrons and protons), somewhat similar to Earth's Van Allan belts but very much larger. All four jovian worlds have strong magnetic fields and emit radiation at radio wavelengths. 14. How are the interiors of Uranus and Neptune thought to differ from those of Jupiter and Saturn? We do not expect the hydrogen and helium in the interiors of Uranus and Neptune to be compressed as much as in the two larger jovian worlds, yet the average densities of Uranus and Neptune are actually greater than the desity of Saturn, and similar to that of Jupiter. Neptune does have an internal source of heat-the planet emits 2.7 times more energy than it receives from the Sun. Uranus apparently has no internal energy source, radiating into space the same amount of energy as it receives from the Sun. It never experienced helium precipitation, and its initial supply of heat was lost to space long ago.. 15. How much information do you think we might have gathered about the outer solar system without the Voy- ager and Galileo missions? We would not know very much about the outer solar system without the Voyager, and Galileo missions. True or False? ___ 1. Uranus was discovered by Galileo. ___ 2. After the discovery of Uranus, astronomers started looking for other planets and discovered Neptune less than a century later. ___ 3. There is no evidence to suggest that either Jupiter or Saturn has a rocky core. ___ 4. There is no evidence to suggest that either Uranus or Neptune has a rocky core. ___ 5. Jupiter's solid surface lies just below the cloud lay- ers visible from Earth. ___ 6. The thickness of Jupiter's cloud layer is less than one percent of the planet's radius. ___ 7. Jupiter has only one large storm system. ___ 8. In general, a storm system in Jupiter's atmosphere is much longer-lived than a storm system in Earth's atmosphere. ___ 9. The element helium plays an important role in pro- ducing the colors in jovian atmospheres. ___10. Like that of Jupiter, the rotation of Saturn is rapid and shows differential rotation. ___11. The atmosphere of Saturn contains only half the helium of Jupiter's atmosphere, but overall Saturn probably has a normal helium abundance. ___12. Like the other jovian worlds, Uranus has promi- nent belts and zones in its atmosphere. ___13. Jupiter's magnetic field is much stronger than Earth's. ___14. Both Uranus and Neptune have layers of metallic hydrogen surrounding their central cores. ___15. The fraction of Uranus or Neptune that is core is larg- er than the fraction of Jupiter or Saturn that is core. Page 227 Chapter 8 Chapter Review Review and Discussion 1. How does the density of the Galilean moons vary with increasing distance from Jupiter? Is there a trend to this variation? If so, explain the reason for it. Their densities decrease with increasing distance from Jupiter in a manner very reminiscent of the falloff in density of the terrestrial planets with increasing distance from the Sun. 2. What is unusual about Jupiter's moon Io? It's surface is kept smooth and brightly colored by constant volcanism. Io has active volcanoes! All appear ro be erupting. 3. Why is there speculation that the Galilean moon Eu- ropa might be an abode for life? Europa's icy surface is only lightly cratered. Europa Liquid Water Locked in Ice Europa is covered by an ocean of liquid water whose surface layers are frozen. 4. Why do astronomers think that Ganymede may have ex- perienced a relatively recent episode of internal heating? Ganymede has a weak magnetic field, about one percent that of Earth, suggesting that the moon's core may still be partly molten. If that is so, then Ganymede's heating and differentiation must have happened relatively recently. 5. What property of Saturn's largest moon, Titan, makes it of particular interest to astronomers? Titan's atmosphere. it may contain rain, snow, and fog. 6. What is the predicted fate of Triton? Careful calculations indicate that Triton is doomed to be torn apart by Neptune's tidal gravitational field, probably in no more than about 100 million years. 7. What is unique about Miranda? Give a possible expla- nation for this uniqueness Miranda displays a wide range of surface terrains, including ridges, valles, faults, and many other geological features. To explain why Miranda seems to combine so many types of terrain, some researches have hypothesized that is has been catastrophically disrupted several times, with the pieces falling back together in a chaotic, jumbled way. 8. Seen from Earth, Saturn's rings sometimes appear broad and bright but at other times seem to disappear disappear. Why? As Saturn orbits the Sun, the angle at which the rings are illuminated varies. When the planet's north or south pole is tipped toward the Sun (which neans it is either summer or winter on Saturn), the highly reflective rings are at their brightest. In Saturn's spring and fall, the rings are close to being edge-on, both to the Sun and to observers on Earth, so they seem to disappear. 9. Why do many astronomers think Saturn's rings formed quite recently? The dynamic behavior observed in the rings of Saturn and the other jovian planets implies to many researches that the rings are quite young-perhaps no more than 50 million years old, just one percent of the age of the solar system. If the rings are indeed so young, then either they are replenished from time to time, perhaps by fragments of moons chipped off by meteoritic impact, or they are the result of a relatively recent, possibly catastrophic event in the planet's system-a moon torn apart by tidal forces, or destroyed in an impact with a large comet or even with another moon. 10. What effect does Mimas have on the rings of Saturn? The gravitational influence of Saturn's innermost medium-sized moon, Mimas. Over time, particles orbiting within the Division have been deflected by Mima's gravity into eccentric orbits that cause them to collide with other ring particles, effectively moving them into new orbits at different radii. The net result is that the number of ring particles in the Cassini Division is greatly reduced. 11. Describe the behavior of shepherd satellites? It seems as though the ring's intricate structure, as well as its thinness, arises from the influence of two small moons, known as shepherd satellites, that orbit about 1000 km on either side of it. shepherd satellite Satellite whose gravitational effect on a ring helps preserve the ring's shape. Examples are two satellites of Saturn, Prometheus and Pandora, whose orbits lie on either side of the F ring. 12. How do the rings of Neptune differ from those of Uranus and Saturn? Neptune is surrounded by four dark rings. Three are quite narrow, like the rings of Uranus, and one is quite broad and diffuse, more like Jupiter's ring. The outermost ring is noticeably clumped in places. The clumping is caused by shepherd satellites. 13. How was Pluto discovered? In 1930, the American astronomer Clyde Tombaugh, working with improved equipment and photographic techniques at the Lowell Observatory, finally succeeded in finding the ninth planet Pluto. 14. In what respect is Pluto more like a moon than a jovian or terrestrial planet? Pluto's average density of 2000 kg/m3 is too low for a terrestrial planet but far too high for a jovian one. Instead, the mass, radius, and density of Pluto are just what we expect for one of the icy moons of a jovian planet. Indeed, Pluto is comparable in both mass and radius to Neptune's large moon, Triton. 15. How was the mass of Pluto determined? A 1990 Hubble Space Telescope image that clearly resolves the two bodies. Based on studies of Charon's orbit, astonomers have measured the mass of Pluto to great accuracy. It is just 0.0021 Earth masses (1.3 x 10 22 kg)-0.17 times the mass of Earth’s Moon. Pluto is comparable in both mass and radius to Neptune's. True or False? ___ 1. The densities of the Galilean moons increase with increasing distance from Jupiter. ___ 2. Io has a noticeable lack of impact craters on its surface. ___ 3. The surface of Europa is completely covered by water ice. ___ 4. Most of the medium-sized jovian moons rotate synchronously with their orbits around their par- ent planet. ___ 5. Saturn is unique among the planets in having a ring system. ___ 6. A typical particle in Saturn's rings is more than 100 m across. ___ 7. Although Saturn's ring system is tens of thousands of kilometers wide, it is only a few tens of meters thick. ___ 8. Saturn has small and medium-sized moons, but no large ones. ___ 9. Water ice predominates in Saturn's moons. ___ 10. Titan's atmosphere is denser than Earth's. ___ 11. Titan's surface is obscured by thick cloud layers. ___ 12. Uranus has no large moons. ___ 13. Triton's surface has a marked lack of cratering, in- dicating significant amounts of surface activity. ___ 14. Pluto's moon, Charon, is very small compared with Pluto. ___ 15. Pluto is larger than Earth's Moon. Page 249 Chapter 9 Chapter Review Review and Discussion 1. Name and briefly describe the main regions of the Sun. The main regions of the Sun are the chromosphere The Sun's lower atmosphere, lying just above the visible photosphere. The transition zone The region of rapid temperature increase that separates the Sun's chromosphere from the corona. The corona The tenuous outer atmosphere of the Sun, which lies just above the chromosphere, and at great distances turns into the solar wind. The convection zone Region of the Sun's interior, lying just below the surface, where the material of the Sun is in constant convective motion. This region extends into the solar interior to a depth of about 200,000 km. The radiation zone Region of the Sun's interior where extremely high temperatures guarantee that the gas is completely ionized. Photons are only occasionally diverted by electrons, and travel through this region with relative ease. The core The central region of Earth, surrounded by the mantle. The central region of the Sun. 2. How massive is the Sun, compared with Earth? Having a radius of more than 100 Earth radii, a mass of more than 300,000 Earth masses. 3. How hot is the solar surface? The solar core? The solar temperature also decreases with increasing radius, but not as rapidly as the density. Computer models indicate a central temperature of about 15 million K. The temperature decreases steadily, reaching the observed value of 5800 K at the photosphere. Surface temperature is 5780 K 4. How do scientists construct models of the Sun? With the Solar Vibration. There are solar vibrations waves on the sun. Standard Solar Model A self-consistent picture of the Sun, developed by incorporating the important physical processes that are believed to be important in determining the Sun's internal structure, into a computer program. The results of the program are then compared with observations of the Sun, and modifications are made to the model. The Standard Solar Model, which enjoys widespread acceptance, is the result of this process. 5. Describe how energy generated at the center of the Sun reaches Earth. All through the convection zone, energy is transported to the surface by a physical motion of the solar gas. 6. Why does the Sun appear to have a sharp edge? The edge appears sharp because the solar photosphere is very thin. The part of the Sun that emits the radiation we see is called the photosphere.Its radius (listed as the radius of the Sun) is about 700,000 km. However, its thickness is probably no more than 500 km-less than 0.1 percent of the radius-which is why we perceive the Sun as having a well-defined, sharp edge. By the outer edge of the radiation zone, 500,000 km from the center, all of the photons produced in the Sun's core have been absorbed. 7. What evidence do we have for solar convection? The visible surface is highly mottled with regions of bright and dark gas known as granules. This granulation of the solar surface is a direct reflection of motion in rhe convection zone. Extending down some 200,000 km below the photosphere is the convection zone, a region where the material of the Sun is in constant convective motion. 8. What is the solar wind? solar wind An outward flow of fast-moving charged particles from the Sun. 9. What is the cause of sunspots, flares, and prominences? convection causes the magnetized gas to upwell toward the surface, twisting and tangling the magnetic field pattern. In some places, the field lines becomes kinked like a knot in a garden hose, causing the field strength to increase Occasionally, the field becomes so strong that it overwhelms the Sun's gravity, and a "tube" of field lines bursts out of the surface and loops through the lower atmosphere, forming a sunspot pair. Sunspots are simply relatively cooler regions of the photospheric gas. Flares are also the result of magnetic instabilities. Magnetic instabilities in the strong fields found in and near sunspot groups may cause the prominences. 10. What is the Maunder minimum? The lengthy period of solar inactivity that extended from 1645 to 1715 is called the Maunder minimum. 11. What fuels the Sun's enormous energy output? Astronomers think that the corona is heated primarily by magnetic activity on the solar surface, particularly prominences and flares, which can inject large amounts of energy into the corona, greatly distorting its shape. Eztensive disturbances often move through the corona above an active site in the photosphere, distributing the energy throughout the coronal gas and often hurling large amount (billions of tons) of matter into space. 12. What is the law of conservation of mass and energy? How is it relevant to nuclear fusion in the Sun? law of conservation of mass and energy A fundamental law of modern physics which states that the sum of mass and energy must always remain constant in any physical process. In fusion reactions, the lost mass in converted into energy, primarily in the form of electromagnetic radiation. 13. What are the ingredients and the end result of the proton- proton chain? Why is energy released in the process? proton-proton chain The chain of fusion reactions, leading from hydrogen to helium, that powers most main-sequence stars. Four hydrogen nuclei (protons) combine to creare one nucleus of the next lightest element, helium-4, creating two new lightweight particles called neutrinos, and releasing energy in the form of gamma-ray radiation. Several alternative reaction sequences exist that produce the same end result. 14. Why are scientists trying so hard to detect solar neutrinos? to see if there is a direct electromagnetic evidence of the core nuclear reactions. learn about conditions in the solar core. They travel clearly out of the Sun, interacting with virtually nothing, and escape into space a few seconds after being created. Neutrinos can pass through the entire Sun without interacting also makes neutrinos difficult to detect on Earth! solar neutrino problem The discrepancy between the theoretically predicted numbers of neutrinos streaming from the Sun as a result of fusion reactions in the core and the numbers actually observed. The observed number of neutrinos is only about half the predicted number. 15. What would we observe on Earth if the Sun's internal energy source suddenly shut off? Would the Sun dark- en instantaneously? If not, how long do you think it might take-minutes, days, years, millions of years- for the Sun's light to begin to fade? Repeat the question for solar neutrinos. The sun would darkened instantly. The solar neutrinos would not be created anymore. supernova Explosive death of a star, caused by the sudden onset of nuclear burning (type I), or an enormously energetic shock wave (type II). One of the most energetic events of the universe, a supernova may temporarily outshine the rest of the galaxy in which it resides. True or False? ___ 1 .The Sun is a fairly typical star. ___ 2. The average density of the Sun is significantly greater than the density of Earth. ___ 3. The Sun's diameter is about 100 times that of Earth. ___ 4. The Sun's mass is comparable to the mass of Jupiter. ___ 5. Convection involves cool gas rising to the solar surface and hot gas sinking into the interior. ___ 6. Absorption lines in the solar spectrum are produced mainly in the corona. ___ 7. There are as many absorption lines in the solar spectrum as there are elements present in the Sun? ___ 8. The faintness of the chromosphere is a result of its low density and temperature. ___ 9. The temperature of the solar corona decreases with increasing radius. ___ 10. Sunspots are regions of intense magnetic fields. ___ 11. Coronal holes are low-density regions of the upper solar atmosphere. ___ 12. Prominences are violent nuclear explosions on the surface of the Sun. ___ 13. Neutrinos are neutrons traveling close to the speed of light. ___ 14. Nuclear fusion releases energy because the total mass of the nuclei involved increases. ___ 15. The nuclear reactions that power the Sun create energy in the form of gamma rays. Page 274 Chapter 10 Chapter Review Review and Discussion 1. How is stellar parallax used to measure the distances to stars? The distances to the nearest stars can be measured using parallax-the apparent shift of an object relative to some distant background as the observer's point of view changes. The parallax is determined by comparing photographs made from the two ends of the baseline. 2. What is a parsec? Compare it to the astronomical unit. parsec The distance at which a star must lie in order that its measured parallax is exactly 1 arc second, equal to 206,000 A.U. 3. Explain how a star's real motion through space trans- lates into motion observable from Earth. proper motion The angular movement of a star across the sky, as seen from Earth, measured in seconds of arc per year. This movement is a result of the star's actual motion through space. 4. Describe some characteristics of red giants and white dwarfs? red giant A giant star whose surface temperature is relatively low, so that it glows with a red color. white dwarf A dwarf star with a surface temperature that is hot, so that the object glows white. 5. What is the difference between absolute and appar- ent magnitude? absolute magnitude The apparent magnitude a star would have if it were placed at a standard distance of 10 parsecs from Earth. apparent magnitude The apparent brightness of a star, expressed using the magnitude scale. 6. How do astronomers measure star temperatures? Astronomers can determine a star's surface temperature by measuring its apparent brightness (radiation intensity) at several frequencies, then matching the observations to the appropriate blackbody curve. 7. Briefly describe how stars are classified according to their spectral characteristics. Researchers classified stars primarily according to their hydrogen-line intensities. They adopted an A, B, C, D,... schemein which A stars, with the strongest hydrogen lines, were thought to have more hydrogen than did B stars, and so on. The classification extended as far as the letter P. 8. What information is needed to plot a star on the Hertzsprung-Russell diagram? H-R Diagram of Prominent Stars A plot of luminosity against surface temperature (or spectral class), known as an H-R diagram, is a useful way to compare stars. Plotted here are the data for some stars mentioned earlier in the text. The Sun, of course, has a luminosity of one solar unit. Its temperature, read off the bottom scale, is 5800 K-a G-type star. Similarly, the B-type star Rigel, at top left, has a temperature of about 15,000 K and a luminosity more than 10,000 times that of the Sun. The M-type star Proxima Centauri, at bottom right, has a temperature of less than 3000 K and a luminosity less than 1/10,000 that of the Sun. 9. What is the main sequence? What basic property of a star determines where it lies on the main sequence? main sequence Well-defined band on the Hertzsprung& Russell diagram, on which most stars are found, running from the top left of the diagram to the bottom right. Stellar Masses More than any other stellar property, mass determines a star's position on the main sequence. Low-mass stars are cool and faint, they lie at the bottom of the main sequence. Very massive stars are hot and bright; they lie at the top of the main sequence. The symbol " M "means "solar mass." 10. How are distances determined using spectroscopic parallax? spectroscopic parallax Method of determining the distance to a star by measuring its temperature and then determining its absolute brightness by comparing with a standard H-R diagram. The absolute and apparent brightnesses of the star give the star's distance from Earth. 11. Which stars are most common in the Milky Way Galaxy? Why don't we see manyof them in H-R diagrams? luminous blue giants are overrepresented in Figure 10.14, low-luminosity red dwarfs are surely uderrepresented. In fact, no dwarfs appear on this diagram. This absence is not surprising, because low-luminosity stars are difficult to observe from Earth. 12. How can stellar masses be determined by observing bi- nary star systems? If the distance to a visual binary is known, then the orbits of each component can be individually tracked, and the masses of the component can be determined. By studying the variation of the light from an eclipsing binary system-called the binary’s light curve-we can derive detailed information not only about the stars’ orbits and masses but also about their radii. As with all other objects, we measure a star’s mass by observing its gravitational influence on some nearby body-another star, perhaps, or a planet. If we know the distance between the two bodies, then we can use Newton’s laws to calculate their masses. 13. If a high-mass star starts off with much more fuel than a low-mass star, why doesn't the high-mass star live longer? Because luminosity increases so rapidly with mass, the most massive stars are by far the shortest lived. For example, according to the mass-luminosity relationship, the lifetime of a 20-solar-mass O-type star is roughly 20/203 = 1/400 that of the Sun, or about 25 million years. We can be sure that all the O- and B-type stars we now observe are quite young-less than a few tens of millions of years old. Their nuclear reactions proceed so rapidly that their fuel is quickly depleted despite their large masses. 14. In general, is it possible to determine the age of an individual star simply by noting its position on an H-R diagram? Yes 15. Visual binaries and eclipsing binaries are relatively rare compared to spectroscopic binaries. Why is this? visual binary A binary star system in which both members are resolvable from Earth. eclipsing binary Rare binary-star system that is aligned in such a way that from Earth we periodically observe one star pass in front of the other, eclipsing the other star. spectroscopic binary A binary-star system which from Earth appears as a single star, but whose spectral lines show back-and-forth Doppler shifts as two stars orbit one another. True or False? ___ 1. One parsec is a little more than 200,000 A.U. ___ 2. There are no other stars within 1 pc of the Sun. ___ 3. Parallax can be used to measure stellar distances out to about 10,000 pc. ___ 4. Most stars have radii between 0.1 and 10 times the radius of the Sun. ___ 5. Star A appears brighter than star B, as seen from Earth. Therefore, star A must be closer to Earth than star B. ___ 6. Star A and star B have the same luminosity, but star B is twice as distant as star A. Therefore, star A appears four times brighter than star B. ___ 7. A star of apparent magnitude five looks brighter than one of apparent magnitude two. ___ 8. Differences among stellar spectra are mainly due to differences in star composition. ___ 9. Stars with very weak hydrogen lines in their spec- tra contain no hydrogen. ___ 10. Red giants are very bright because they are ex- tremely hot. ___ 11. Red dwarfs lie in the lower left part of the H-R di- agram. ___ 12. The brightest stars visible in the night sky are all found in the upper part of the H-R diagram. ___13. In a spectroscopic binary, the orbital motion of the component stars appears as variations in the over- all apparent brightness of the system. ___ 14. It is impossible to have a one-billion-year-old O- or B-type main-sequence star. ___ 15. Astronomers can distinguish between main-sequence and giant stars by purely spectroscopic means. Page 297 Chapter 11 Chapter Review Review and Discussion 1. What is the composition of interstellar gas? Of inter- stellar dust? The gas is made up mainly of individual atoms and small molecules. The dust consists of clumps of atoms and molecules-not unlike the microscopic particles that make up smoke or soot. 2. How is interstellar matter distributed through space? Interstellar matter is distributed very unevenly. In some directions it is largely absent, allowing astronomers to study objects literally billions of parsecs from the Sun. 3. What are some methods that astronomers use to study interstellar dust? From our vantage point on Earth, this assemblage of stellar and interstellar matter stretches all the way across the sky. On a clear night, it is visible to the naked eye as the Milky Way. Milky Way Mosaic The Milky Way photographed almost from horizon to horizon, thus extending over nearly 180°. This band contains high concentrations of stars as well as interstellar gas and dust. The field of view is several times wider than that of Figure 11.1, whose outline is superimposed on this image. (Palomar Observatory/California Institute of Technology) 4. What is an emission nebula? emission nebula A glowing cloud of hot interstellar gas. The gas glows as a result of a nearby young star which is ionizing the gas. Since this gas is mostly hydrogen, the emitted radiation falls predominantly in the red region of the spectrum, because of a dominant hydrogen emission line. 5. Why can't 21 -centimeter radiation be used to probe the interiors of molecular clouds? molecular cloud A cold, dense interstellar cloud which contains a high fraction of molecules. It is widely believed that the relatively high density of dust particles in these clouds plays an important role in the formation and protection of the molecules. Only long-wavelength radio radiation can escape from these dense, dusty parts of interstellar space. Molecular hydrogen (H2) is by far the most common constituent of molecular clouds, but unfortunately this molecule does not emit or absorb radio radiation, so it cannot easily be used as a probe of cloud structure. Nor are 21-cm observations helpful-they are sensitive only to atomic hydrogen, not to the molecular form of the gas. Instead, astronomers use radio observations of other "tracer" molecules, such as carbon monoxide (CO), hydrogen cyanide (HCN), ammonia (NH3), water (H2O), and formaldehyde (H2CO), to study the dark interiors of these dusty regions. 21-Centimeter Radiation In many cases, dark dust clouds absorb so much light from background stars that they cannot easily be studied by the techniques described in Section 11.1. Fortunately, astronomers have an important alternative means of probing their structure that relies on low-energy radio emissions produced by the interstellar gas itself. 6. If our Sun were surrounded by a cloud of gas, would this cloud be an emission nebula? Why or why not? Yes The dark region outlined by the red and green radio contours in Figure 11.12 (not the emission nebula itself, where stars have already formed) probably represents a stage-1 cloud just starting to collapse. Doppler shifts of the observed formaldehyde lines indicate that it is contracting. Less than a light year across, this region has a total mass over 1000 times the mass of the Sun-considerably greater than the mass of M20 itself. Orion Nebula, Up Close (a) The constellation Orion, with the region around its famous emission nebula marked by a rectangle. The Orion Nebula is the middle "star" of Orion’s sword. (b) Enlargement of the framed region of part (a), suggesting how the nebula is partly surrounded by a vast molecular cloud. Various parts of this cloud are probably fragmenting and contracting, with even smaller sites forming protostars. Shown in Figure 11.15(c), they measure about 1010 km, about the diameter of our solar system. Their density is about 1015 particles/m3, much higher than the density of the surrounding cloud. Although the temperatures of these regions cannot be estimated reliably, many researchers regard the regions as objects between stages 2 and 3, on the threshold of becoming protostars. Figures 11.15(d) and (e) are visible-light images of part of the nebula showing other evidence for protostars. 7. Briefly describe the basic chain of events leading to the formation of a star like the Sun. The first stage in the star-formation process is a dense interstellar cloud-the core of a dark dust cloud or perhaps a molecular cloud. These clouds are truly vast, sometimes spanning tens of parsecs (1014 to 1015 km) across. Typical temperatures are about 10 K throughout, with a density of perhaps 109 particles/m3. Stage 1 clouds contain thousands of times the mass of the Sun, mainly in the form of cold atomic and molecular gas. (The dust they contain is important for cooling the cloud as it contracts and also plays a crucial role in planet formation, but it constitutes a negligible fraction of the total mass.) 8. What is an evolution track? evolutionary track A graphical representation of a star's life as a path on the Hertzsprung&151;Russell diagram. 9. Why do stars tend to form in groups? Gravity Once the collapse begins, theory indicates that fragmentation into smaller and smaller clumps of matter naturally follows. As illustrated in Figure 11.14, a typical cloud can break up into tens, hundreds, even thousands, of fragments, each imitating the shrinking behavior of the parent cloud and contracting ever faster. The whole process, from a single quiescent cloud to many collapsing fragments, takes a few million years. Depending on the precise conditions under which fragmentation takes place, an interstellar cloud can produce either a few dozen stars, each much larger than our Sun, or a collection of hundreds of stars comparable to or smaller than our Sun. There is little evidence for stars born in isolation, one star from one cloud. Most stars-perhaps all stars-appear to originate as members of multiple systems. astronomers were aware only of very massive stars forming in clouds far away. IRAS showed that stars are forming much closer to home, and some of these protostars have masses comparable to that of our Sun. Figure 11.19 shows a premier example of a solar-mass protostar-Barnard 5. Its infrared heat signature is that expected of a stage 5 object. 10. What critical event must occur in order for a protostar to become a star? Stages 4 and 5-Protostellar Evolution As the protostar evolves, it shrinks, its density increases, and its temperature rises, both in the core and at the photosphere. Some 100,000 years after the fragment formed, it reaches stage 4, where its center seethes at about 1,000,000 K. The electrons and protons ripped from atoms whiz around at hundreds of kilometers per second, but the temperature is still short of the 107 K needed to ignite the proton-proton nuclear reactions that fuse hydrogen into helium. Still much larger than the Sun, our gassy heap is now about the size of Mercury’s orbit. Its surface temperature has risen to a few thousand kelvins. 11. What are brown dwarfs? brown dwarf Remnant of a fragment of collapsing gas and dust that did not contain enough mass to initiate core nuclear fusion. Such objects are frozen somewhere along their pre-main-sequence contraction phase, continually cooling into compact dark objects. Because of their small sizes and low temperatures they are extremely difficult to detect observationally. 12. Because stars live much longer than we do, how do as- tronomers test the accuracy of theories of star formation? Because all the stars formed at the same time out of the same cloud of interstellar gas, and under the same environmental conditions, clusters are near-ideal "laboratories" for stellar studies-not in the sense that astronomers can perform experiments on them, but because the properties of the stars are very tightly constrained. The only factor distinguishing one star from another in the same cluster is mass, so theoretical models of star formation and evolution can be compared with reality without the complications introduced by broad spreads in age, chemical composition, and place of origin. 13. At what evolutionary stages must astronomers use radio and infrared radiation to study prestellar objects? Why can't they use visible light? The numerical values and evolutionary track just described are valid only for one-solar-mass stars. The temperatures, densities, and radii of prestellar objects of other masses exhibit similar trends, but the details differ, in some cases quite considerably. Perhaps not surprisingly, the most massive fragments formed within interstellar clouds tend to produce the most massive protostars and eventually the most massive stars. Similarly, low-mass fragments give rise to low-mass stars. The opposite is the case for prestellar objects having masses much less than our Sun. A typical M-type star, for example, requires nearly a billion years to form. Table 11.2 lists seven evolutionary stages that an interstellar cloud goes through before becoming a main-sequence star like our Sun. These stages are characterized by different central temperatures, surface temperatures, central densities, and radii of the prestellar object. This portion of the evolutionary track is often called the T Tauri phase, after T Tauri, the first "star" (actually protostar) to be observed in this stage of prestellar development. 14. Explain the usefulness of the Hertzsprung-Russell dia- gram in studying the evolution of stars. Why can't evo- lutionary stages 1 to 3 be plotted on the diagram? Newborn Star on the H-R Diagram The changes in a protostar’s observed properties are shown by the path of decreasing luminosity, from stage 4 to stage 6. At stage 7, the newborn star has arrived on the main sequence. Protostar on the H-R Diagram The red arrow indicates the approximate evolutionary track followed by an interstellar cloud fragment before becoming a stage-4 protostar. (The boldface numbers on this and subsequent H-R plots refer to the prestellar evolutionary stages listed in Table 11.2 and described in the text.) Recall from Chapter 10 that"R" means the radius of the Sun. Stage 1-An Interstellar Cloud The first stage in the star-formation process is a dense interstellar cloud-the core of a dark dust cloud or perhaps a molecular cloud. These clouds are truly vast, sometimes spanning tens of parsecs (1014 to 1015 km) across. Typical temperatures are about 10 K throughout, with a density of perhaps 109 particles/m3. Stage 1 clouds contain thousands of times the mass of the Sun, mainly in the form of cold atomic and molecular gas. (The dust they contain is important for cooling the cloud as it contracts and also plays a crucial role in planet formation, but it constitutes a negligible fraction of the total mass.) (Sec. 4.3) The process of continuing fragmentation is eventually stopped by the increasing density within the shrinking cloud. As fragments continue to contract, they eventually become so dense that radiation cannot get out easily. The trapped radiation causes the temperature to rise, the pressure to increase, and the fragmentation to stop. However, the contraction continues. Stages 2 and 3-A Contracting Cloud Fragment As it enters stage 2, a fragment destined to form a star like the Sun-the end product of the process sketched in Figure 11.14-contains between one and two solar masses of material. Estimated to span a few hundredths of a parsec across, this fuzzy, gaseous blob is still about 100 times the size of our solar system. Its central density is about 1012 particles/m3. The Orion complex harbors several smaller sites of intense radio emission from molecules deep within its core. Shown in Figure 11.15(c), they measure about 1010 km, about the diameter of our solar system. Their density is about 1015 particles/m3, much higher than the density of the surrounding cloud. Although the temperatures of these regions cannot be estimated reliably, many researchers regard the regions as objects between stages 2 and 3, on the threshold of becoming protostars. Figures 11.15(d) and (e) are visible-light images of part of the nebula showing other evidence for protostars. The evolutionary events just described occur over the course of 40 to 50 million years. Although this is a long time by human standards, it is still less than one percent of the Sun’s lifetime on the main sequence. Once an object begins fusing hydrogen and establishes a "gravity-in/pressure-out" equilibrium, it burns steadily for a very long time. The star’s location on the H-R diagram will remain almost unchanged for the next 10 billion years. 15. Compare and contrast the properties of open and glob- ular star clusters. open cluster Loosely bound collection of tens to hundreds of stars, a few parsecs across, generally found in the plane of the Milky Way. a small star cluster called the Pleiades, or Seven Sisters, a well-known naked-eye object in the constellation Taurus, lying about 120 pc from Earth. This type of loose, irregular cluster, found mainly in the plane of the Milky Way, is called an open cluster. Open clusters typically contain from a few hundred to a few tens of thousands of stars and are a few parsecs across. globular cluster Tightly bound, roughly spherical collection of hundreds of thousands, and sometimes millions, of stars, spanning about 50 parsecs. Globular clusters are distributed in the halos around the Milky Way and other galaxies. a very different type of star cluster, called a globular cluster. All globular clusters are roughly spherical (which accounts for their name), are generally found away from the Milky Way plane, and contain hundreds of thousands, and sometimes millions, of stars spread out over about 50 pc. True or False? ___ 1. Interstellar matter is evenly distributed throughout the entire Milky Way Galaxy. ___ 2. In the vicinity of the Sun, there is about as much mass in the form of interstellar matter as in the form of stars. ___ 3. Interstellar gas is deficient in heavy elements because those elements go into making interstellar dust. ___ 4. Because of the obscuration of visible light by in- terstellar dust, we can observe stars only within a few thousand parsecs of Earth. ___ 5. A typical dark dust cloud is many hundreds of par- secs across. ___ 6. Because of their low temperatures, dark dust clouds radiate mainly in the ultraviolet part of the elec- tromagnetic spectrum.. ___ 7. 21-centimeter radiation provides astronomers with information on interstellar molecular hydrogen gas. ___ 8. 21-centimeter radiation can pass unimpeded through the entire Milky Way Galaxy. ___ 9. Emission nebulae display spectra almost identical to those of the stars embedded in them. ___ 10. Given the typical temperatures found in interstel- lar space, a cloud containing as few as 1000 atoms has sufficient gravity for it to begin to collapse. ___ 11. Most stars form as members of clusters. ___ 12. A stage-5 T Tauri (proto)star has a luminosity about 10 times that of the Sun. ___ 13. The rate of evolution speeds up dramatically as a protostar approaches the main sequence. ___ 14. The earliest stages of star formation can be observed using optical telescopes. ___ 15. Globular clusters generally contain between a few hundred and a few thousand stars. Page 324 Chapter 12 Chapter Review Review and Discussion 1. For how long can a star like the Sun keep burning hy- drogen in its core? A star like the Sun, for example, after spending a few tens of millions of years in formation (stages 1-6 in Chapter 11), will reside on or near the main sequence (stage 7) for roughly 10 billion years before evolving into something else. core hydrogen burning The energy burning stage for main sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90% of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by core hydrogen burning. 2. Why is the depletion of hydrogen in the core of a star such an important event? As soon as hydrogen becomes substantially depleted, about 10 billion years after the star arrived on the main sequence, the helium core begins to contract. The shrinkage of the helium core releases gravitational energy, driving up the central temperature and heating the overlying layers, causing the hydrogen there to fuse even more rapidly than before. Figure 12.3 depicts this hydrogen-shell-burning stage, in which hydrogen is burning at a furious rate in a relatively thin layer surrounding the nonburning inner core of helium "ash." The hydrogen shell generates energy faster than did the original main-sequence star’s hydrogen-burning core, and the shell’s energy production continues to increase as the helium core contracts. Strange as it may seem, the star’s response to the disappearance of the nuclear fire at its center is to get brighter! 3. What makes an ordinary star become a red giant? The pressure exerted by this enhanced hydrogen burning causes the star’s nonburning outer layers to increase in radius. Not even gravity can stop them. Even while the core is shrinking and heating up, the overlying layers are expanding and cooling. The star, aged and unbalanced, is on its way to becoming a red giant. The change from normal main-sequence star to elderly red giant takes about 100 million years. 4. How big (in A.U.) will the Sun become when it enters the red giant phase? Relative sizes and colors of a normal G-type star (such as our Sun) during its formative stages, on the main sequence, and while passing through the red giant and white dwarf stages. At maximum swelling, the red giant is approximately 70 times the size of its main-sequence parent; the core of the giant is about 1/15 the main-sequence size and would be barely discernible 5. How long does it take for a star like the Sun to evolve from the main sequence to the top of the red giant branch? The change from normal main-sequence star to elderly red giant takes about 100 million years. 6. What is the helium flash? helium flash An explosive event in the post-main-sequence evolution of a low-mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again. 7. How do stars of low mass die? How do stars of high mass die? The Death of a Low-Mass Star As our red supergiant moves from stage 10 to stage 11, its envelope swells while its core, too cool for further nuclear burning, continues to contract. If the central temperature could become high enough for carbon fusion to occur, still heavier elements could be synthesized, and the newly generated energy might again support the star, restoring for a time the equilibrium between gravity and pressure. For solar-mass stars, however, this does not occur. The temperature never reaches the 600 million K needed for new nuclear reactions to occur. The red supergiant is now very close to the end of its nuclear-burning lifetime. 8. What is a planetary nebula? Why do many planetary nebulae appear as rings? planetary nebula The ejected envelope of a red giant star, spread over a volume roughly the size of our solar system. Planetary Nebulae A planetary nebula is an object with a small, dense core (central blue-white star) surrounded by an extended shell (or shells) of glowing matter. (a) The Helix Nebula appears to the eye as a small star with a halo around it. About 140 pc from Earth and 0.6 pc across, its apparent size in the sky is roughly half that of the full Moon. (b) The appearance of a planetary nebula can be explained once we realize that the shell of glowing gas around the central core is quite thin. There is very little gas along the line of sight between the observer and the central star (path A), so that part of the shell is invisible. Near the edge of the shell, however, there is more gas along the line of sight (paths B and C), so the observer sees a glowing ring. (c) The Cat’s Eye Nebula is an example of a much more complex planetary nebula. It lies about 1000 pc away. It may have been produced by a pair of binary stars (unresolved at the center) that have both shed planetary nebulae. 9. What are white dwarfs? What is their ultimate fate? white dwarf A dwarf star with a surface temperature that is hot, so that the object glows white. The white dwarf continues to cool and dim with time, following the white-yellow-red line near the bottom of Figure 12.10, eventually becoming a black dwarf-a cold, dense, burned-out ember in space. This is stage 14 of Table 12.1, the graveyard of stars. The cooling dwarf does not shrink much as it fades away, however. Even though its heat is leaking away into space, gravity does not compress it further. Its tightly packed electrons will support the star even as its temperature drops (after trillions of years) almost to absolute zero. As the dwarf cools, it remains about the size of Earth. 10. Under what circumstances will a binary star produce a nova? nova A star that suddenly increases in brightness, often by a factor of as much as 10,000, then slowly fades back to its original luminosity. A nova is the result of an explosion on the surface of a white dwarf star, caused by matter falling onto its surface from the atmosphere of a binary companion. 11. What occurs in a massive star to cause it to explode? The Death of a High-Mass Star Once the inner core begins to change to iron, our high-mass star is in trouble. Nuclear fusion involving iron does not produce energy, because iron nuclei are so compact that energy cannot be extracted by combining them into heavier elements. In effect, iron plays the role of a fire extinguisher, damping the inferno in the stellar core. With the appearance of substantial quantities of iron, the central fires cease for the last time, and the star’s internal support begins to dwindle. The star’s foundation is destroyed, and its equilibrium is gone forever. Even though the temperature in the iron core has reached several billion kelvins by this stage, the enormous inward gravitational pull of matter ensures catastrophe in the very near future. Gravity overwhelms the pressure of the hot gas, and the star implodes, falling in on itself. Now the core consists entirely of electrons, protons, neutrons, and photons at enormously high densities and it is still shrinking. As the density continues to rise, the protons and electrons are crushed together, combining to form more neutrons and releasing neutrinos. Even though the central density by this time may have reached 1012 kg/m3 or more, most of the neutrinos pass through the core as if it weren’t there. (Sec. 9.5) They escape into space, carrying away still more energy as they go. There is nothing to prevent the collapse from continuing all the way to the point at which the neutrons themselves come into contact with each other, at the incredible density of about 1015 kg/m3. At this density, the neutrons in the shrinking core play a role similar in many ways to that of the electrons in a white dwarf. When far apart, they offer little resistance to compression, but when brought into contact, they produce enormous pressures that strongly oppose further contraction. The collapse finally begins to slow. By the time it is actually halted, however, the core has overshot its point of equilibrium and may reach a density as high as 1018 kg/m3 before beginning to re-expand. Like a fast-moving ball hitting a brick wall, the core becomes compressed, stops, then rebounds-with a vengeance! The events just described do not take long. Only about a second elapses from the start of the collapse to the "bounce" at nuclear densities. Driven by the rebounding core, an enormously energetic shock wave then sweeps outward through the star at high speed, blasting all the overlying layers-including the heavy elements outside the iron inner core-into space. Although the details of how the shock reaches the surface and destroys the star are still uncertain, the end result is not: The star explodes in one of the most energetic events known in the universe (Figure 12.16). This spectacular death rattle of a high-mass star is known as a core-collapse supernova. 12. What are the observational differences between Type I andType II supernovae? Type I and Type II supernovae are actually unrelated to one another. They occur in stars of very different types, under very different circumstances. All high-mass stars become Type II (core-collapse) supernovae, but only a tiny fraction of low-mass stars evolve into white dwarfs that ultimately explode as Type I carbon-detonation) supernovae. However, there are far more low-mass stars than high-mass stars, resulting in the remarkable coincidence that the two types of supernova occur at roughly the same rate. Two Types of Supernovae Type I and Type II supernovae have different causes. These sequences depict the evolutionary history of each type. (a) A Type I supernova usually results when a carbon-rich white dwarf pulls matter onto itself from a nearby red giant or main-sequence companion. (b) A Type II supernova occurs when the core of a high-mass star collapses and then rebounds in a catastrophic explosion. Type I supernova One possible explosive death of a star. A white dwarf in a binary system can accrete enough mass that it cannot support its own weight. The star collapses and temperatures become high enough for carbon fusion to occur. Fusion begins throughout the white dwarf almost simultaneously and an explosion results. Type II supernova One possible explosive death of a star, in which the highly evolved stellar core rapidly implodes and then explodes, destroying the surrounding star. Astronomers divide supernovae into two classes. Type I supernovae contain very little hydrogen, according to their spectra, and have light curves (see Figure 12.17) somewhat similar in shape to those of typical novae-a sharp rise in intensity followed by steady, gradual decline. Type II supernovae are hydrogen-rich, and usually have a characteristic "plateau" in the light curve a few months after the maximum. Observed supernovae are divided roughly evenly between these two categories. 13. How do the mechanisms that cause Type I and Type II supernovae explain their observed differences? What of Type I supernovae? Is there more than one way for a supernova explosion to occur? The answer is yes. To understand the alternative supernova mechanism, we must reconsider the long-term consequences of the accretion-explosion cycle that causes a nova. A nova explosion ejects matter from a white dwarf’s surface, but it does not necessarily expel or burn all the material that has accumulated since the last outburst. In other words, there is a tendency for the dwarf’s mass to increase slowly with each new nova cycle. As its mass grows and the internal pressure required to support its weight rises, the white dwarf can enter into a new period of instability-with disastrous consequences. the characteristics of Type II supernovae are exactly consistent with the core collapse supernovae discussed in the previous section. The Type II light curve is in good agreement with computer simulations of a stellar envelope expanding and cooling as it is blown into space by a shock wave sweeping up from below. In addition, since the expanding material consists mainly of unburned hydrogen and helium, it is not surprising that those elements are strongly represented in the supernova’s spectrum. 14. What evidence do we have that many supernovae have occurred in our Galaxy? We have plenty of evidence that supernovae have occurred in our Galaxy. Occasionally, the supernova explosions are visible from Earth. In other cases, we can detect their glowing remains, or supernova remnants. One of the best-studied supernova remnants is the Crab Nebula (Figure 12.19a). Its brightness has greatly dimmed now, the original explosion in the year A.D. 1054 was so brilliant that it is prominently recorded in the manuscripts of Chinese and Middle Eastern astronomers. For nearly a month, this exploded star reportedly could be seen in broad daylight. Even today, the knots and filaments give a strong indication of past violence. The nebula-the envelope of the high-mass star that exploded to create this Type II supernova-is still expanding into space at several thousand kilometers per second. 15. Why do the cores of massive stars evolve into iron and not heavier elements? However, all this stops at iron. The fact that iron will not fuse to create more massive nuclei is the basic underlying cause of Type II supernovae. How then were even heavier elements, such as copper, lead, gold, and uranium, formed? The answer is that they were created during supernova explosions (both Type I and Type II), as neutrons and protons, produced when some nuclei were ripped apart by the almost unimaginable violence of the blast, were crammed into other nuclei, creating heavy elements that could not have formed by any other means. The heaviest elements, then, were formed after their parent stars had already died, even as the debris from the explosion signaling the stars’ deaths was hurled into interstellar space. True or False? ___ 1. Low-mass stars are conventionally taken to have masses of less than about eight solar masses. ___ 2. All the red dwarf stars that ever formed are still on the main sequence today. ___ 3. Once a star is on the main sequence, gravity is no longer important in determining the star's inter- nal structure. ___ 4. The Sun will get brighter as it begins to run out of fuel in its core. ___ 5. As it evolves away from the main sequence, a star gets smaller. ___ 6. When the Sun becomes a red giant, its core will be smaller than when the Sun was on the main sequence. ___ 7. When helium starts to fuse inside a solar-mass red giant, it does so very slowly at first; the rate of fu- sion increases gradually over many years. ___ 8. A planetary nebula is the disk of matter around a star that will eventually form a planetary system. ___ 9. A nova is a sudden outburst of light coming from an old main-sequence star. ___ 10. High-mass stars can fuse carbon in their cores. ___ 11. It takes less and less time to fuse heavier and heav- ier elements inside a high-mass star. ___ 12. In a core-collapse supernova, the outer part of the core rebounds from the high-density inner part of the core, destroying the entire outer part of the star. ___ 13. The spectrum of a Type I supernova shows the pres- ence of lots of hydrogen. ___ 14. Stellar evolution can account for the existence of all elements except hydrogen and helium. ___ 15. A star cluster 100 million years old still contains many O-type stars. Page 347 Chapter 13 Chapter Review Review and Discussion 1. How does the way in which a neutron star forms deter- mine some of its most basic properties? Recall that, in a Type II supernova, the iron core of a massive star collapses until its neutrons effectively come into contact with one another. At that point, the central portion of the core rebounds, creating a powerful shock wave that races outward through the star, violently expelling matter into space. (Sec. 12.4) The key point here is that the shock wave does not start at the very center of the collapsing core. The innermost part of the core-the region that rebounds-remains intact as the shock wave it produces destroys the rest of the star. After the violence of the supernova has subsided, this ultracompressed ball of neutrons is all that is left. Researchers colloquially call this core remnant1 a neutron star. neutron star A dense ball of neutrons that remains at the core of a star after a supernova explosion has destroyed the rest of the star. Typical neutron stars are about 20 km across, and contain more mass than the Sun. 2. What would happen to a person standing on the surface of a neutron star? The severe pull of a neutron star’s gravity would flatten you much thinner than this piece of paper. 3. Why aren't all neutron stars seen as pulsars? All pulsars are neutron stars, but not all neutron stars are pulsars, for two reasons. First, the two ingredients that make the neutron star pulse-rapid rotation and strong magnetic field-both diminish with time, so the pulses gradually weaken and become less frequent. Theory indicates that, within a few tens of millions of years, the pulses have all but stopped. Second, even a young, bright pulsar is not necessarily visible from Earth. The pulsar beam depicted in Figure 13.3 is relatively narrow-perhaps as little as a few degrees across. Only if the neutron star happens to be oriented in just the right way do we see pulses. When we see the pulses from Earth, we call the object a pulsar. 4. What are X-ray bursters? X-ray burster X-ray source that radiates thousands of times more energy than our Sun, in short bursts that last only a few seconds. A neutron star in a binary system accretes matter onto its surface until temperatures reach the level needed for hydrogen fusion to occur. The result is a sudden period of rapid nuclear burning and release of energy. 5. Why do astronomers think that gamma-ray bursts are very energetic? Some 40 optical or X-ray afterglows of gamma-ray bursts have now been detected, and about a dozen distances are known. All are very large, implying that the bursts must be extremely energetic, since otherwise they wouldn’t be detectable by our equipment. (Sec. 10.2) Each burst apparently generates more energy-and in some cases hundreds of times more energy-than a typical supernova explosion, all in a matter of seconds! 6. What is the favored explanation for the rapid spin rates of millisecond pulsars? The most likely explanation for the high rotation rate of millisecond pulsars is that they have been spun up by drawing in matter from a companion star. As matter spirals down onto the neutron star’s surface in an accretion disk, it provides the "push" needed to make the neutron star spin faster (Figure 13.8). Theoretical calculations indicate that this process can spin the star up to breakup speed in about a hundred million years. 7. Why do you think astronomers were surprised to find a pulsar with a planetary system? However, it is unlikely that any of these planets formed in the same way as our own. Any planetary system orbiting the pulsar’s parent star was almost certainly destroyed in the supernova explosion that created the pulsar. As a result, scientists are still uncertain about how these planets came into being. One possibility involves the binary companion that provided the matter necessary to spin the pulsar up to millisecond speeds. Possibly the pulsar’s intense radiation and strong gravity destroyed the companion, then spread its matter out into a disk (a little like the solar nebula) in whose cool outer regions the planets might have condensed. 8. What does it mean to say that the measured speed of a light beam is independent of the motion of the observer? In 1887, a fundamental experiment carried out by two American physicists, A. A. Michelson and E. W. Morley, demonstrated a further important and unique aspect of light-that the measured speed of a beam of light is independent of the motion of the observer or the source. No matter what our velocity may be relative to the source of the light, we always measure precisely the same value for c-299,792.458 km/s. A moment’s thought leads to the realization that this is a decidedly nonintuitive statement. 9. Use your knowledge of escape speed to explain why black holes are said to be "black." As the core shrinks, the gravitational pull in its vicinity eventually becomes so great that nothing-not even light-can escape. The resultant object therefore emits no light, no other form of radiation, no information whatsoever. Astronomers call this bizarre endpoint of stellar evolution, in which the core of a very-high-mass star collapses in on itself and vanishes forever, a black hole. Despite the fact that general relativity is necessary for a proper description of black-hole properties, we can still usefully discuss some aspects of these strange bodies in more or less Newtonian terms. Let’s reconsider the familiar Newtonian concept of escape speed-the speed needed for one object to escape from the gravitational pull of another-supplemented by two key facts from relativity: 1) nothing can travel faster than the speed of light, and 2) all things, including light, are attracted by gravity. escape speed The speed necessary for an object to escape the gravitational pull of an object. Anything that moves away from the object with more than the escape speed will never return. 10. What is an event horizon? event horizon Imaginary spherical surface surrounding a collapsing star, with radius equal to the Schwarzschild radius, within which no event can be seen, heard, or known about by an outside observer. 11. Why is it so difficult to test the predictions of the theo- ry of general relativity? Describe two tests of the theory? The problem with verifying general relativity is that its effects on Earth and in the solar system-the places where we can most easily perform tests-are very small. Just as special relativity produces major departures from Newtonian mechanics only when speeds approach the speed of light, general relativity predicts large departures from Newtonian gravity only when extremely strong gravitational fields are involved. Here we consider just two “classical” tests of the theory-solar-system experiments that helped ensure acceptance of Einstein’s theory. Bear in mind, however, that there are no known tests of general relativity in the “strong-field” regime-that part of the theory that predicts black holes, for example-so the full theory has never been experimentally tested. At the heart of general relativity is the premise that everything, including light, is affected by gravity because of the curvature of spacetime. Shortly after he published his theory in 1915, Einstein noted that light from a star should be deflected by a measurable amount as it passes the Sun. The closer to the Sun the light comes, the more it is deflected. The maximum deflection should occur for a ray that just grazes the solar surface. Einstein calculated that the deflection angle should be 1.75"-a small but detectable amount. Of course, it is normally impossible to see stars so close to the Sun. During a solar eclipse, however, when the Moon blocks the Sun’s light, the observation does become possible, as illustrated in the accompanying figure. In 1919, a team of observers led by the British astronomer Sir Arthur Eddington succeeded in measuring the deflection of starlight during an eclipse. The results were in excellent agreement with the prediction of general relativity. Another prediction of general relativity is that planetary orbits should deviate slightly from the perfect ellipses of Kepler’s laws. Again, the effect is greatest where gravity is strongest-that is, closest to the Sun. Thus, the largest relativistic effects are found in the orbit of Mercury. Relativity predicts that Mercury’s orbit is not exactly an ellipse. Instead, its orbit should rotate slowly, as shown in the (highly exaggerated) diagram below. The amount of rotation is very small-only 43" per century-but Mercury’s orbit is so well charted that even this tiny effect is measurable. 12. What would happen to someone falling into a black hole? Matter flowing into a black hole is subject to great tidal stress. An unfortunate person falling feetfirst into a solar-mass black hole would find herself stretched enormously in height and squeezed unmercifully laterally. (More Precisely 5-1) She would be torn apart long before she reached the event horizon, for the pull of gravity would be much stronger at her feet (which are closer to the hole) than at her head. The net result of all this stretching and squeezing is numerous and violent collisions among the resulting debris, causing a great deal of frictional heating of any infalling matter. 13. What makes Cygnus X-1 a good black hole candidate? A few close binary systems, however, have peculiarities suggesting that one of their members may be a black hole. Figure 13.14(a) shows the area of the sky in the constellation Cygnus, where the evidence is particularly strong. The rectangle outlines the celestial system of interest, some 2000 pc from Earth. The black-hole candidate is an X-ray source called Cygnus X-1, discovered by the Uhuru satellite in the early 1970s. Its visible companion is a blue B-type supergiant. Assuming that it lies on the main sequence, its mass must be around 25 times the mass of the Sun. Spectroscopic observations indicate that the binary system has an orbital period of 5.6 days. Combining this information with spectroscopic measurements of the visible component’s orbital speed, astronomers estimate the total mass of the binary system to be around 35 solar masses, implying that Cygnus X-1 has a mass of about 10 times that of the Sun. X-ray radiation emitted from the immediate neighborhood of Cygnus X-1 indicates the presence of high-temperature gas, perhaps as hot as several million kelvins. 14. Imagine that you had the ability to travel at will through the Milky Way Galaxy. Explain why you would discov- er many more neutron stars than those known to ob- servers on Earth. Where would you be most likely to find these objects? First, the two ingredients that make the neutron star pulse-rapid rotation and strong magnetic field-both diminish with time, so the pulses gradually weaken and become less frequent. Theory indicates that, within a few tens of millions of years, the pulses have all but stopped. Second, even a young, bright pulsar is not necessarily visible from Earth. The pulsar beam depicted in Figure 13.3 is relatively narrow-perhaps as little as a few degrees across. Only if the neutron star happens to be oriented in just the right way do we see pulses. When we see the pulses from Earth, we call the object a pulsar. Even though most pulsars are probably not visible from our vantage point, current pulsar observations are consistent with the idea that every high-mass star dies in a supernova explosion, leaving a neutron star behind, and that all young neutron stars emit beams of radiation, like the pulsars we actually see. 15. Do you think that planet-sized objects discovered in orbit around a pulsar should be called planets? Why or why not? These remarkable results constitute the first definite evidence of planet-sized bodies outside our solar system. A few other millisecond pulsars have since been found with similar behavior. However, it is unlikely that any of these planets formed in the same way as our own. Any planetary system orbiting the pulsar’s parent star was almost certainly destroyed in the supernova explosion that created the pulsar. As a result, scientists are still uncertain about how these planets came into being. One possibility involves the binary companion that provided the matter necessary to spin the pulsar up to millisecond speeds. Possibly the pulsar’s intense radiation and strong gravity destroyed the companion, then spread its matter out into a disk (a little like the solar nebula) in whose cool outer regions the planets might have condensed. On Saturday July 6, 2002 the Earth reaches its farthest point from the sun during its orbit, but the average global temperature actually goes up this time of year. What gives? Like the other planets in the solar system, the Earth moves in a lopsided orbit around the sun. In July, our planet reaches the most distant point of its annual revolution, called aphelion. In January, it dips to its nearest position, or perihelion. Compared to other planets, the Earth's orbit is only slightly askew. It has an eccentricity of 1.7 percent, while Mars' is more than 9 percent and Mercury's more than 20 percent. Looking at a page-sized map, an observer might mistake the Earth's path around the sun for an exact circle. Scientists know better. "When we're closest the sun, the distance is 147.8 million kilometers [91.8 million miles]. This weekend we will be 152.6 million kilometers [94.8 million miles] away, a 5 million kilometer (3.1 million mile) difference," University of Florida astronomer George Lebo said. The greater the distance, the less intense the rays of the sun. Averaged over the globe, sunlight falling on Earth at aphelion is about 7 percent less intense than it is at perihelion, NASA climate researcher Roy Spencer said. Then why is it so warm now in the Northern Hemisphere? The reason is that the Earth tilts 23.5 degrees on its axis, and because of where it is in its orbit, the northern half is the portion most exposed to the sun during the summer months. The conditions are reversed in the Southern Hemisphere, which basks in the solar rays half a year later, when the South Pole tips more toward the sun. Overall, the entire globe averages higher temperatures when the Earth is farther from the sun. The planet is about 4 degrees F (2.3 degrees C) warmer during aphelion than perihelion, NASA scientists said. The reason? Continents and oceans are distributed unevenly. The north has more land and the south more water. When sunlight strikes the former, it heats up more quickly than the latter, which can absorb more heat before rising in temperature. In other words, Earth is slightly warmer in July because the sun is shining on all the northern continents, which heat up rather easily. In January, the sun focuses its rays on the southern oceans, which have higher heat capacity. Another quirk of physics contributes to even more planetary heat this time of year. During aphelion, planets move in their orbits more slowly than during perihelion. As a consequence, the summer in the north is several days longer than in the south, affording the sun more time to cook the landmasses in the Northern Hemisphere. Astronomy Chapter 14 Review and Discussion 1. What do the globular clusters tell us about our galaxy? Figure 14.2 shows three spiral galaxies thought to resemble our own in overall structure. Figure 14.2(a) is the Andromeda Galaxy, the nearest major galaxy to the Milky Way Galaxy, lying about 900 kpc (nearly three million light-years) away. Andromeda’s apparent elongated shape is a consequence of the angle at which we happen to view it. In fact, this galaxy, like our own, consists of a circular galactic disk of matter that fattens to a galactic bulge at the center. The disk and bulge are embedded in a roughly spherical ball of faint old stars known as the galactic halo. These three basic galactic regions are indicated on the figure 2. How are Cepheid variables used in determining distances? For Cepheids, we make use of a close correlation between average luminosity and pulsation period, discovered in 1908 by Henrietta Leavitt of Harvard University and known simply as the period-luminosity relationship. Cepheids that vary slowly-that is, have long periods-have high luminosities, while short-period Cepheids have low luminosities. Figure 14.6 illustrates the period-luminosity relationship for Cepheids found within a thousand parsecs or so of Earth. Astronomers can plot such a diagram for relatively nearby stars because they can measure their distances, and their luminosities, using stellar or spectroscopic parallax. 3. Roughly how far out into space can we use Cepheids to measure distance? With Cepheids, this method allows astronomers to estimate distances out to about 15 million parsecs, more than enough to take us to the nearest galaxies. 4. What important discoveries were made early in this century using RR Lyrae variables? Early in the twentieth century, the American astronomer Harlow Shapley used observations of RR Lyrae stars to make two very important discoveries about the Galactic globular cluster system. First, he showed that most globular clusters reside at great distances-many thousands of parsecs-from the Sun. Second, by measuring the direction and distance of each cluster, he was able to determine their three-dimensional distribution in space (Figure 14.8). In this way, Shapley demonstrated that the globular clusters map out a truly gigantic, and roughly spherical, volume of space, about 30 kpc across. 5. Why can’t we study the central regions of the Galaxy using optical telescopes? However, as we have seen, optical techniques can cover only a small portion of the dusty Galactic disk. Much of our knowledge of the structure of the disk on larger scales is based on radio observations, particularly of the 21-cm radio emission line produced by atomic hydrogen. (Sec. 11.2) Because long-wavelength radio waves are largely unaffected by interstellar dust, and hydrogen is by far the most abundant element in interstellar space, the 21-cm signals are strong enough that virtually the entire disk can be observed in this way. 6. Of what use is radio astronomy in the study of Galactic structure? Much of our knowledge of the structure of the disk on larger scales is based on radio observations, particularly of the 21-cm radio emission line produced by atomic hydrogen. (Sec. 11.2) Because long-wavelength radio waves are largely unaffected by interstellar dust, and hydrogen is by far the most abundant element in interstellar space, the 21-cm signals are strong enough that virtually the entire disk can be observed in this way. According to radio studies, the center of the gas distribution coincides roughly with the center of the globular cluster system, lying about 8 kpc from the Sun. In fact, this figure is derived most accurately from radio observations of Galactic gas. The densities of both stars and gas in the disk decline quite rapidly beyond about 15 kpc from the Galactic center (although some radio-emitting gas has been observed out to at least 50 kpc). Perpendicular to the Galactic plane, the disk in the vicinity of the Sun is “only” about 300 pc thick, or about 1/100 of the 30 kpc Galactic diameter. Don’t be fooled, though. Even if you could travel at the speed of light, it would take you 1000 years to traverse the thickness of the Galactic disk. The disk may be thin compared with the Galactic diameter, but it is huge by human standards. 7. Contrast the motions of disk and halo stars. This picture of orderly circular orbital motion about the Galactic center applies only to the Galactic disk. The old globular clusters in the halo and the faint, reddish individual stars in both the halo and the bulge do not share the disk’s well-defined rotation. Instead, as shown in Figure 14.11, their orbits are largely random.3 Although they do orbit the Galactic center, they move in all directions, their paths filling an entire three-dimensional volume rather than a nearly two-dimensional disk. At any given distance from the Galactic center, bulge or halo stars move at speeds comparable to the disk’s rotation speed at that radius but in all directions, not just one. Their orbits carry these stars repeatedly through the disk plane and out the other side. 8. Explain why galactic spiral arms are believed to be regions of recent and ongoing star formation. The spiral arms in our Galaxy are made up of much more than just interstellar gas and dust. Studies of the Galactic disk within a kiloparsec or so of the Sun indicate that young stellar and prestellar objects-emission nebulae, O- and B-type stars, and recently formed open clusters-are also distributed in a spiral pattern that closely follows the distribution of interstellar clouds. The obvious conclusion is that the spiral arms are the part of the Galactic disk where star formation takes place. The brightness of the young stellar objects just listed is the main reason that the spiral arms of other galaxies are easily seen from afar (Figure 14.2b). A central problem facing astronomers trying to understand spiral structure is how that structure persists over long periods of time. The basic issue is simple: Differential rotation makes it impossible for any large-scale structure “tied” to the disk material to survive. Figure 14.14 shows how a spiral pattern consisting always of the same group of stars and gas clouds would necessarily “wind up” and disappear within a few hundred million years. How then do the Galaxy’s spiral arms retain their structure over long periods of time in spite of differential rotation? A leading explanation for the existence of spiral arms holds that they are spiral density waves-coiled waves of gas compression that move through the Galactic disk, squeezing clouds of interstellar gas and triggering the process of star formation as they go. 9. Describe the motion of interstellar gas as it passes through a spiral density wave. Over much of the visible portion of the Galactic disk (within about 15 kpc of the center), the spiral wave pattern is predicted to rotate more slowly than the stars and gas. as shown in Figure 14.15, Galactic material catches up with the wave, is temporarily slowed down and compressed as it passes through, then continues on its way. (For a more down-to-earth example of an analogous process, see Interlude 14-1.) As gas enters the arm from behind, the gas is compressed and forms stars. Dust lanes mark the regions of highest-density gas. The most prominent stars-the bright O- and B-type blue giants-live for only a short time, so emission nebulae and young star clusters are found only within the arms, near their birthsites, just ahead of the dust lanes. Their brightness emphasizes the spiral structure. Further downstream, ahead of the spiral arms, w e see mostly older stars and star clusters, which have had enough time since their formation to outdistance the wave and pull away from it. 10. What is self-propagating star formation? self-propagating star formation The theory that the formation and eventual explosion of massive stars can create shock waves that trigger the next round of star formation, possibly accounting for the spiral structure observed in many galaxies. 11. What do the red stars in the Galactic halo tell us about the history of the Milky Way Galaxy? star clusters and star-forming regions. In contrast, the cooler, redder stars-including those found in the old globular clusters-are more uniformly distributed throughout the disk, bulge, and halo. Galactic disks appear bluish because main-sequence O- and B-type blue supergiants are very much brighter than G-, K-, and M-type dwarfs, even though the dwarfs are present in far greater numbers. The explanation for the marked difference in stellar content between disk and halo is that, whereas the gas-rich Galactic disk is the site of ongoing star formation and so contains stars of all ages, all the stars in the Galactic halo are old. The absence of dust and gas in the halo means that no new stars are forming there, and star formation apparently ceased long ago-at least 10 billion years in the past, judging from the ages of the globular clusters. (Sec. 12.6) The gas density is very high in the inner part of the Galactic bulge, making this region the site of vigorous ongoing star formation, and both very old and very young stars mingle there. The bulge’s gas-poor outer regions have properties more similar to those of the halo. 12. What does the rotation curve of our Galaxy tell us about its total mass? Based largely on data obtained from radio observations, Figure 14.17 shows the Galactic rotation curve, which plots the rotation speed of the Galactic disk against distance from the Galactic center. Using this graph, we can repeat our earlier calculation to compute the total mass that lies within any given distance from the Galactic center. We find, for example, that the mass within about 15 kpc from the center-the volume defined by the globular clusters and the known spiral structure-is roughly 2 1011 solar masses, about twice the mass contained within the Sun’s orbit. Does most of the matter in the Galaxy “cut off” at this point, where the luminosity drops off sharply? Surprisingly, the answer is no. 13. What evidence is there for dark matter in the Galaxy? the luminous portion of the Milky Way Galaxy-the region outlined by the globular clusters and by the spiral arms-is merely the “tip of the Galactic iceberg.” Our Galaxy is in reality very much larger. The luminous region is surrounded by an extensive, invisible dark halo, which dwarfs the inner halo of stars and globular clusters and extends well beyond the 15-kpc radius once thought to represent the limit of our Galaxy. But what is this dark halo made of? We do not detect enough stars or interstellar matter to account for the mass that our computations tell us must be there. We are inescapably drawn to the conclusion that most of the mass in our Galaxy exists in the form of invisible dark matter, which we presently simply do not understand. dark matter Term used to describe the mass in galaxies and clusters whose existence we infer from rotation curves and other techniques, but which has not been confirmed by observations at any electromagnetic wavelength. 14. What are some possible explanations for dark matter? Many candidates have been suggested for this dark matter, although none is proven. Among the strongest “stellar” contenders are brown dwarfs, white dwarfs, and faint, low-mass red dwarfs. These objects could in principle exist in great numbers throughout the Galaxy, yet would be exceedingly hard to see. A radically different alternative is that the dark matter is made up of exotic subatomic particles that pervade the entire universe. 15. Why do astronomers believe that a supermassive black hole lies at the center of the Milky Way Galaxy? At the very center of our Galaxy is a remarkable object with the odd-sounding name Sgr A* (pronounced “saj-ay-star”). Its total energy output (at all wavelengths) is estimated to be about 1033 W, more than a million times the luminosity of the Sun. Most of its energy is emitted in the radio and infrared part of the spectrum, although it is also detectable at X- and gamma-ray wavelengths. What could cause all this activity? The leading candidate is a massive black hole at the Galactic center. VLBI observations imply that Sgr A* is small-no more than 10 A.U. across-and infrared spectroscopic observations of stars and gas within a few arc seconds of Sgr A* indicate that the orbital speed increases closer to the center, just as one would expect for matter orbiting a very massive object. Chapter 15 Review and Discussion 1. In what sense are elliptical galaxies "all halo"? Most ellipticals also contain little or no gas and dust and display no evidence of young stars or ongoing star formation. Like the halo of our own Galaxy, ellipticals are made up mostly of old, reddish, low-mass stars. Indeed, having no disk, gas, or dust, elliptical galaxies are, in a sense, "all halo." Again like the halo of our Galaxy, the orbits of stars in ellipticals are disordered, exhibiting little or no overall rotation; objects move in all directions, not in regular, circular paths as in our Galaxy’s disk. 2. Describe the four rungs in the distance-measurement ladder used to determine the distance to a galaxy lying 5 Mpc away. To emphasize their independence, standard candles and the Tully-Fisher relation share the fifth rung of our cosmic distance ladder (Figure 15.11). Just as with the lower rungs, the properties of these new techniques are calibrated using distances measured by more local means. However, in the process, the errors and uncertainties in each step accumulate, so the distances to the farthest objects are the least well known. 3. Describe the contents of the Local Group. How much space does it occupy compared with the volume of the Milky Way Galaxy? All told, some 20 galaxies populate our Galaxy’s neighborhood. Three of them (the Milky Way, Andromeda, and M33) are spirals; the rest are dwarf irregulars and ellipticals. Together, they form a small galaxy cluster called the Local Group (Figure 15.12), a new level of structure in the universe above the scale of our Galaxy. Its diameter is roughly 1 Mpc. The Milky Way Galaxy and Andromeda are by far its largest members. The Local Group is held together by the combined gravity of the galaxies it contains, like stars in a star cluster, but on a millionfold larger scale. 4. How does the Tully-Fisher relation allow astronomers to measure the distances to galaxies? the rotation speed is usually determined by measuring the "smearing," or broadening, it causes in a galaxy’s spectral lines. (Sec. 2.6) The Tully-Fisher relation then tells us the galaxy’s luminosity, and (comparing the known luminosity with the measured apparent magnitude) its distance. The Tully-Fisher relation can be used to measure distances to spiral galaxies out to about 200 Mpc, beyond which the line broadening becomes increasingly difficult to measure accurately. Tully-Fisher relation A relation used to determine the absolute luminosity of a spiral galaxy. The rotational velocity, measured from the broadening of spectral lines, is related to the total mass, and hence the total luminosity. 5. What is the Virgo Cluster? the next large concentration of galaxies we come to is the Virgo Cluster (Figure 15.14). It lies some 17 Mpc from the Milky Way. The Virgo Cluster does not contain a mere 20 galaxies, however. Rather, it houses approximately 2500 galaxies, bound by gravity into a tightly knit group about 3 Mpc across. Figure 15.15 illustrates the approximate locations of Virgo and several other galaxy clusters in our cosmic neighborhood, within about 30 Mpc of the Milky Way. 6. Describe two techniques for measuring the mass of a galaxy. Astronomers can calculate the masses of some spiral galaxies by determining their rotation curves, which plot rotation speed versus distance from the galactic center. The mass within any given radius then follows directly from Newton’s laws. (Sec. 14.5) Rotation curves for a few nearby spirals are shown in Figure 15.18. We can use another statistical technique to derive the combined mass of all the galaxies within a galaxy cluster. As depicted in Figure 15.19(b), each galaxy within a cluster moves relative to all other cluster members, and we can estimate the cluster’s mass simply by asking how massive it must be in order to bind its galaxies gravitationally. Typical cluster masses obtained in this way lie in the range of 1013-1014 solar masses. Notice that this calculation gives us no information whatsoever about the masses of individual galaxies. It tells us only about the total mass of the entire cluster. 7. Why do astronomers believe that galaxy clusters contain more mass than we can see? Astronomers find an even greater discrepancy when they study galaxy clusters. Calculated cluster masses range from 10 to nearly 100 times the mass suggested by the light emitted by individual cluster galaxies. A lot more mass is needed to bind galaxy clusters than we can see. Thus the problem of dark matter exists not just in our own Galaxy, but also in other galaxies and, to an even greater degree, in galaxy clusters too. In that case, we are compelled to accept the fact that upward of 90 percent of the universe is composed of dark matter. As noted in Chapter 14, this matter is not just dark in the visible portion of the spectrum-it is undetected at any electromagnetic wavelength. (Sec. 14.5) The dark matter in clusters apparently cannot simply be the accumulation of dark matter within individual galaxies. Even including the galaxies’ dark halos, we still cannot account for all the dark matter in galaxy clusters. As we look on larger and larger scales, we find that a larger and larger fraction of the matter in the universe is dark. 8. Why do clusters of galaxies emit X-rays? Computer simulations suggest that collisions between spiral galaxies can indeed destroy the spirals’ disks, ejecting much of the gas into intergalactic space (in the process creating the hot intracluster gas noted in Section 15.3) and leaving behind objects that look very much like ellipticals. Observations of interacting galaxies, such as those shown in Figure 15.23, appear to support this scenario. Additional evidence comes from the observed fact that spirals are more common at large distances, which implies that their numbers are decreasing with time, presumably as the result of collisions 9. What evidence do we have that galaxies collide with one another? Theoretical evidence for this picture is provided by computer simulations of the early universe, which clearly show merging taking place. Further strong support comes from observations that indicate that galaxies at large distances from us (meaning that the light we see was emitted long ago) appear distinctly smaller and more irregular than those found nearby. Figure 15.21(b) and 15.22 show some of these images. The vague bluish patches are separate small galaxies, each containing only a few percent of the mass of the Milky Way Galaxy. Their irregular shape is thought to be the result of galaxy mergers; the bluish coloration comes from young stars that formed during the merger process. although there are strong hints that the environment plays an important role. For example, spiral galaxies are relatively rare in the dense central parts of rich galaxy clusters. Is this because they simply tended not to form there, or is it because their disks are easily destroyed by collisions and mergers, which are more common in denser regions? Computer simulations suggest that collisions between spiral galaxies can indeed destroy the spirals’ disks, ejecting much of the gas into intergalactic space (in the process creating the hot intracluster gas noted in Section 15.3) and leaving behind objects that look very much like ellipticals. Observations of interacting galaxies, such as those shown in Figure 15.23, appear to support this scenario. Additional evidence comes from the observed fact that spirals are more common at large distances, which implies that their numbers are decreasing with time, presumably as the result of collisions. 10. Describe the role of collisions in the formation and evolution of galaxies. Most astronomers believe that galaxies grew by repeated merging of smaller objects, as illustrated in Figure 15.21(a). Contrast this with the process of star formation, in which a large cloud fragments into smaller pieces that eventually become stars. 11. Do you think that collisions between galaxies constitute "evolution" in the same sense as the evolution of stars? Close encounters are random events and do not seem to represent any genuine evolutionary sequence linking all spirals to all ellipticals and irregulars. However, it is clear that many galaxies and galaxy clusters have evolved greatly since they first formed long ago. 12. What is Hubble’s law? Figure 15.28(a) shows recessional velocity plotted against distance for the galaxies of Figure 15.27. Figure 15.28(b) is a similar plot for some more galaxies within about one billion parsecs of Earth. Plots like these were first made by Edwin Hubble in the 1920s and now bear his name-Hubble diagrams. The data points generally fall close to a straight line, indicating that the rate at which a galaxy recedes is directly proportional to its distance from us. This rule is called Hubble’s law. Hubble's law Law that relates the observed velocity of recession of a galaxy to its distance from us. The velocity of recession of a galaxy is directly proportional to its distance away. 13. How is Hubble’s law used by astronomers to measure distances to galaxies? Using Hubble’s law, we can derive the distance to a remote object simply by measuring the object’s recessional velocity and dividing by Hubble’s constant. (Notice, however, that the uncertainty in Hubble’s constant translates directly into a similar uncertainty in the distance determined by this method.) Using Hubble’s law in this way tops our inverted pyramid of distance-measurement techniques. 14. What is the most likely range of values for Hubble’s constant? Why is the exact value uncertain? In the 1970s, astronomers obtained a value of around 50 km/s/Mpc, using a chain of standard candles to extend their observations to large distances. However, in the early 1980s, when the Tully-Fisher technique had become fairly well established, other researchers used it to obtain a measurement of H0 that was largely independent of methods relying on standard candles. From observations of galaxies within about 150 Mpc, the latter group deduced a value of H0 = 90 km/s/Mpc, a result inconsistent with the earlier measurements (even allowing for the uncertainties in the methods used). For some reason, the distances obtained using the Tully-Fisher method were only about half those determined using standard candles, and so the measured value of Hubble’s constant nearly doubled using this approach. The most recent measurements of H0 by a number of researchers, using different galaxies and a variety of distance-measurement techniques, give results mostly within the range 50-80 km/s/Mpc. Most astronomers would be quite surprised if the true value of H0 turned out to lie outside this range. However, the width of the quoted range is not the result of measurement uncertainties in any one method-there remain real, and as yet unresolved, inconsistencies between the different techniques currently in use. For now, astronomers must live with this uncertainty. We will adopt H0 = 65 km/s/Mpc as the best current estimate of Hubble’s constant for the remainder of the text. 15. What are voids? What is the distribution of galactic matter on very large (more than 100 Mpc) scales? void Large, relatively empty region of the universe around which superclusters and "walls" of galaxies are organized. The most striking feature of maps such as this is that the distribution of galaxies on very large scales is decidedly nonrandom. The galaxies appear to be arranged in a network of strings, or filaments, surrounding large, relatively empty regions of space known as voids. The biggest voids measure some 100 Mpc across. The most likely explanation for the voids and filamentary structure in Figure 15.30 is that the galaxies and galaxy clusters are spread across the surfaces of vast "bubbles" in space. The voids are the interiors of these gigantic bubbles. The galaxies seem to be distributed like beads on strings only because of the way our slice of the universe cuts through the bubbles. Like suds on soapy water, these bubbles fill the entire universe. The densest clusters and superclusters lie in regions where several bubbles meet. Chapter 16 Review and Discussion 1. Name two basic differences between normal galaxies and active galaxies. a few galaxies differ considerably from the norm. They are far more luminous than even the brightest spiral or elliptical galaxies discussed in the previous chapter. Having luminosities sometimes thousands of times greater than that of the Milky Way, they are known collectively as active galaxies. In addition to their greater overall luminosities, active galaxies differ fundamentally from normal galaxies in the character of the radiation they emit. Most of a normal galaxy’s energy is emitted in or near the visible portion of the electromagnetic spectrum, much like the radiation from stars. Indeed, to a large extent, the light we see from a normal galaxy is just the accumulated light of its many component stars. By contrast, as illustrated schematically in Figure 16.1, the radiation from active galaxies does not peak in the visible. Most active galaxies do emit substantial amounts of visible radiation, but far more energy is emitted at longer wavelengths. Put another way, the radiation from active galaxies is inconsistent with what we would expect if it were the combined radiation of myriad stars. Their radiation is said to be nonstellar. active galaxies The most energetic galaxies, which can emit hundreds or thousands of times more energy per second than the Milky Way. 2. Describe some of the basic properties of Seyfert galaxies. Superficially, Seyferts resemble normal spiral galaxies (Figure 16.2a). Indeed, the stars in a Seyfert’s galactic disk and spiral arms produce about the same amount of visible radiation as the stars in a normal spiral galaxy. However, closer study reveals some peculiarities not found in normal spirals. Nearly all of a Seyfert’s energy is emitted in the form of radio and infrared radiation from a small central region known as the galactic nucleus-the center of the overexposed white patch in Figure 16.2(a), shown in more detail in Figure 16.2(b). A Seyfert nucleus may be quite similar to the center of a normal galaxy (the Milky Way, for example), but with one very important difference: The nucleus of a Seyfert is some 10,000 times brighter than the center of our Galaxy. In fact, the brightest Seyfert nuclei are 10 times more energetic than the entire Milky Way Galaxy. 3. What distinguishes a core-halo radio galaxy from a lobe radio galaxy? One common type of radio galaxy is often called a core-halo radio galaxy. As illustrated in Figure 16.4, the radio energy from such an object comes mostly from a small central nucleus (which radio astronomers refer to as the core) less than 1 pc across, with weaker radio emission coming from an extended halo surrounding the nucleus. The halo typically measures about 50 kpc across, comparable in size to the visible galaxy,1 which is usually elliptical and often quite faint. The radio luminosity from the nucleus can be as great as 1037 W-about the same as the total emission from a Seyfert nucleus, and comparable to the output from the Milky Way Galaxy at all wavelengths. 4. What is the evidence that the radio lobes of some active galaxies consist of material ejected from the galaxy’s nucleus? The radio lobes are roughly symmetrically placed, jutting out from the center of the visible galaxy, roughly perpendicular to the dust lane, suggesting that they consist of material ejected in opposite directions from the galactic nucleus. This conclusion is strengthened by the presence of a pair of smaller secondary lobes (marked in Figure 16.7) closer to the visible galaxy, and by the presence of a roughly 1-kpc-long jet in the galactic center, all aligned with the main lobes. 5. What conditions result in a head-tail radio galaxy? In some systems, the motion of the galaxy through the intracluster gas causes the lobes to be swept back behind the main part of the galaxy, forming a head-tail radio galaxy such as that shown in Figure 16.9. 6. Briefly describe the leading model for the central engine of an active galaxy. the leading model for the central engine of active galaxies is essentially a scaled-up version of the same accretion process, only now the black holes involved are millions or even billions of times more massive than the Sun. As with this model’s smaller-scale counterparts, infalling gas forms an accretion disk and spirals down toward the black hole. It is heated to high temperatures by friction within the disk and emits large amounts of radiation as a result. In this case, however, the origin of the accreted gas is not a binary companion, as in stellar X-ray sources, but entire stars and clouds of interstellar gas that come too close to the hole and are torn apart by its strong gravity. 7. How do astronomers know that the energy-producing region of an active galaxy must be very small? 3. Their energy output can be highly variable, implying that it is emitted from a small central nucleus much less than a parsec across. 4. They often exhibit jets and other signs of explosive activity. 5. Their optical spectra may show broad emission lines, indicating rapid internal motion within the energy-producing region. The principal questions then are: How can such vast quantities of energy arise from these relatively small regions of space? Why is so much of the energy radiated at long wavelengths, in the radio and infrared? And what is the origin of the extended radio-emitting lobes and jets? We first consider how the energy is produced, then turn to the question of how it is actually emitted into intergalactic space. Energy Production To develop a feeling for the enormous emissions of active galaxies, consider for a moment an object having a luminosity of 1038 W. In and of itself, this energy output is not inconceivably large. The brightest giant ellipticals are comparably powerful. Thus, some 1012 stars-a few normal galaxies’ worth of material-could equivalently power a typical active galaxy. However, in an active galaxy this energy production is packed into a region much less than a parsec in diameter. 8. What is synchrotron radiation, and what does it tell us about energy emission from active galaxies? synchrotron radiation Type of nonthermal radiation caused by high-speed charged particles, such as electrons, as they are accelerated in a strong magnetic field. The electromagnetic radiation produced in this way-called synchrotron radiation, after the type of particle accelerator in which it was first observed-is nonthermal in nature: There is no link between the emission and the temperature of the radiating object, so the radiation is not described by a blackbody curve. Instead, its intensity decreases with increasing frequency, as shown in Figure 16.15(b). This is just what is needed to explain the overall spectrum of radiation recorded from active galaxies (compare Figure 16.15b with Figure 16.1). Observations of the radiation received from the jets and radio lobes of active galaxies are completely consistent with this process. Eventually, the jet is slowed and stopped by the intergalactic medium, the flow becomes turbulent, and the magnetic field grows tangled. The result is a gigantic radio lobe emitting virtually all of its energy in the form of synchrotron radiation. even though the radio emission comes from an enormously extended volume of space that dwarfs the visible galaxy, the source of the energy is still the accretion disk-a billion billion times smaller in volume than the radio lobe-lying at the galactic center. The jets serve merely as a conduit to transport energy from the nucleus, where it is generated, into the lobes, where it is finally radiated into space. As we have seen, jet formation seems to be an intermittent process, and many nearby active galaxies (Centaurus A and Cygnus A, for example) appear to have been "caught in the act" of interacting with another galaxy, suggesting that in many, if not all, cases, the fuel supply can be turned on by a companion. Just as tidal forces can trigger star formation in starburst galaxies, they may also divert gas and stars into the galactic nucleus, triggering an outburst that may last for millions or even billions of years. (Sec. 15.4) 9. What evidence do we have that the energy production in active galaxies is intermittent? The small size of the emitting region is a direct consequence of the compact nature of the central black hole. Even a billion-solar-mass black hole has a radius of only 3 109 km, or 10-4 pc-about 20 A.U.-and theory suggests that the part of the accretion disk responsible for most of the emission would be much less than 1 pc across. Instabilities in the accretion disk can cause fluctuations in the energy released, leading to the variability observed in many objects. The broadening of the spectral lines observed in the nuclei of many active galaxies results from the rapid orbital motion of the gas in the black hole’s intense gravity. The jets consist of material (mainly protons and electrons) blasted out into space-and completely out of the visible portion of the galaxy-from the inner regions of the disk. The details of how jets form remain uncertain, but there is a growing consensus among theorists that jets are a common feature of accretion flows, large and small. They are most ikely formed by strong magnetic fields produced within the accretion disk itself, which accelerate charged particles to nearly the speed of light and eject them parallel to the disk’s rotation axis. Figure 16.12 shows a Hubble Space Telescope image of a disk of gas and dust at the core of the radio galaxy NGC 4261 in the Virgo Cluster. Consistent with the theory just described, the disk is perpendicular to the huge jets emanating from the galaxy’s center. Figure 16.13 shows imaging and spectroscopic data from the center of M87 (Figure 16.5), suggesting a rapidly rotating disk of matter orbiting the galaxy’s center, again perpendicular to the jet. Measurements of the gas velocity on opposite sides of the disk indicate that the mass within a few parsecs of the center is approximately 3 109 solar masses-we assume that this is the mass of the central black hole. At M87’s distance, HST’s resolution of 0.05 arc second corresponds to a scale of about 5 pc, so we are still far from seeing the (solar-system-sized) central black hole itself, but the improved "circumstantial" evidence has convinced many astronomers of the basic correctness of the theory. 10. What was it about the spectra of quasars that was so unexpected and surprising? The following year saw a breakthrough when astronomers realized that the strongest unknown lines in 3C 273’s spectrum were simply familiar spectral lines of hydrogen redshifted by a very unfamiliar amount-about 16 percent, corresponding to a recession velocity of 48,000 km/s. Figure 16.17 shows the spectrum of 3C 273. Some prominent emission lines and the extent of their redshift are marked on the diagram. Once the nature of the strange spectral lines was known, astronomers quickly found a similar explanation for the spectrum of 3C 48. Its 37 percent redshift implied that it is receding from Earth at almost one-third the speed of light. These huge speeds mean that neither of these two objects can possibly be members of our Galaxy. Applying Hubble’s law with our adopted value of H0 = 65 km/s/Mpc, we obtain distances of 660 Mpc for 3C 273 and 1340 Mpc for 3C 48. More Precisely 16-1 discusses in more detail how these distances are determined and what they mean. Clearly not stars (because of such enormous redshifts), these objects became known as quasi-stellar radio sources (quasi-stellar means "starlike"), or quasars. Because we now know that not all such highly redshifted, starlike objects are strong radio sources, the term quasi-stellar object (or QSO) is more common today. However, the name quasar persists, and we will continue to use it here. 11. Why do astronomers prefer to speak in terms of redshifts rather than distances to faraway objects? When discussing very distant objects, astronomers usually talk about their redshifts rather than their distances. Indeed, it is very common for researchers to speak of an event occurring "at" a certain redshift-meaning that the light received today from that event is redshifted by the specified amount. Of course, because of Hubble’s law, redshift and distance are equivalent to one another. However, redshift is the preferred quantity because it is a directly observable property of an object, whereas distance is derived from redshift using Hubble’s constant, whose value is not accurately known. (In the next chapter, we will see another reason why astronomers favor the use of redshift in studies of the cosmos.) 12. How do we know that quasars are extremely luminous? Despite their unimpressive opticalappearance, the large distances implied byquasar redshifts mean that these faint "stars" are in fact the brightest known objects in the universe! 3C 273, for example, has a luminosity of about 1040 W. More generally, quasars range in luminosity from around 1038 W-about the same as the brightest radio galaxies-up to nearly 1042 W. A value of 1040 W, comparable to 20 trillion Suns or 1000 Milky Way Galaxies, is fairly typical. Quasars have many of the same general properties as active galaxies. Their radiation is nonthermal, may vary irregularly in brightness over periods of months, weeks, days, or (in some cases) even hours, and some show evidence of jets and extended emission features. Figure 16.19 is an optical photograph of 3C 273. Notice the jet of luminous matter, reminiscent of the jet in M87, extending nearly 3 kpc from the center of the quasar. Often, as shown in Figure 16.20, quasar radio radiation arises from regions lying beyond the bright nucleus, much like the emission from core-halo and lobe radio galaxies. In other cases, the radio emission is confined to the central part of the visible image. Quasars have been observed in all parts of the electromagnetic spectrum, although most emit much of their energy in the infrared. 13. How are the spectra of distant quasars used to probe the space between us and them? In addition to their own strongly redshifted spectra, many quasars also show additional absorption features that are redshifted by substantially less than the lines from the quasar itself. For example, the quasar PHL 938 has an emission-line redshift of 1.955, placing it at a distance of some 3400 Mpc, but it also shows absorption lines having redshifts of just 0.613. These are interpreted as arising from intervening gas that is much closer to us (only about 1700 Mpc away) than the quasar itself. Most probably this gas is part of an otherwise invisible galaxy lying along the line of sight, affording astronomers an important means of probing previously undetected parts of the universe. 14. What evidence do we have that quasars represent an early stage of galactic evolution? Most quasars are very distant, indicating that they were more common in the past than they are today. At the same time, normal galaxies seem to have been less common in the distant past. These two pieces of evidence suggest to many astronomers that, when galaxies first formed, they probably were quasars. This view is strengthened by the fact that the same black hole energy-generation mechanism can account for the luminosity of quasars, active galaxies, and the central regions of normal galaxies. Large black holes do not simply vanish. The presence of supermassive black holes in the centers of many normal galaxies is consistent with the idea that they started off as quasars, then "wound down" to become the relatively quiescent objects we see today. Galaxy Evolution A possible evolutionary sequence for galaxies, beginning with the highly luminous quasars, decreasing in violence through the radio and Seyfert galaxies, and ending with normal spirals and ellipticals. The central black holes that powered the early activity are still there at later times but they run out of fuel as time goes on. We can thus construct the following (speculative) scenario for the evolution of galaxies in the universe. Galaxies began to form some 9-10 billion years ago (corresponding to a redshift of 5, the upper limit on the measured redshifts of known quasars-see Table 16.1). The first massive stars to form in a galaxy may have given rise to many stellar-mass black holes which sank to the galactic center and merged into a supermassive black hole there. Alternatively, the supermassive hole may have formed directly by gravitational collapse of the galaxy’s dense central regions. Whatever their precise origin, large black holes appeared at the centers of many galaxies at a time when there was still plenty of fuel available to power them, resulting in many highly luminous quasars. The brightest quasars-the ones we now see from Earth-were those with the greatest fuel supply. 15. If many galaxies were once quasars, what has happened to the energy sources at their centers? As a young galaxy developed and its central black hole used up its fuel, the luminosity of the nucleus diminished. While still active, this galaxy no longer completely overwhelmed the emission from the surrounding stars. The result was an active galaxy-either radio or Seyfert-still emitting a lot of energy, but now with a definite "stellar" component in its spectrum. The central activity continued to decline. Eventually, only the surrounding galaxy could be seen-a normal galaxy, like the majority of those we now see around us. Today, the black holes lie dormant in galactic nuclei, producing only a relative trickle of radiation. Occasionally, two normal galaxies may interact with one another, causing a flood of new fuel to be directed toward the central black hole of one or both. The engine starts up again for a while, giving rise to the nearby active galaxies we observe. Should this picture be correct-and the confirmation of host galaxies surrounding many quasars lends strong support to this view-then many normal galaxies, including perhaps our own Milky Way Galaxy, were once brilliant quasars. Perhaps some alien astronomer, thousands of megaparsecs away, is at this very moment observing our Galaxy-seeing it as it was billions of years ago-and is commenting on its enormous luminosity, nonstellar spectrum, and high-speed jets, and wondering what exotic physical process could possibly account for its violent activity! Chapter 17 Review and Discussion 1. What evidence do we have that there is no structure in the universe on very large scales? How large is "very large"? Are there even larger structures? Does the hierarchy of clustering of stars into galaxies and galaxies into galaxy clusters, superclusters, and filaments continue forever, on larger and larger scales? Perhaps surprisingly, given the trend we have just described, most astronomers think the answer is no. Figure 17.1 shows a survey of galaxies similar to that presented in Figure 15.30, except that it includes nearly 24,000 galaxies within almost 1000 Mpc of the Milky Way, and so covers a much greater volume of space than the earlier figure. Numerous voids and "Great Wall-like" filaments can be seen but, apart from the general falloff in numbers of galaxies at large distances-basically because more distant galaxies are harder to see-there is no obvious evidence for any structures on scales larger than about 200 Mpc. Careful statistical analysis confirms this impression. 2. What is the cosmological principle? cosmological principle Two assumptions which make up the basis of cosmology, namely that the universe is homogeneous and isotropic on sufficiently large scales. The twin assumptions of homogeneity and isotropy are known as the cosmological principle . 3. What is Olbers’s paradox? How is it resolved? Olber's paradox A thought experiment suggesting that if the universe were homogeneous, infinite, and unchanging, the entire night sky would be as bright as the surface of the Sun. Why is the sky dark at night? Given that the universe is homogeneous and isotropic, then one (or both) of the other two assumptions must be false. Either the universe is finite in extent, or it evolves in time. In fact, the answer involves a little of each, and is intimately tied to the behavior of the universe on the largest scales. 4. Explain how an accurate measure of Hubble’s constant can lead to an estimate of the age of the universe. The Birth of the Universe We have seen that all the galaxies in the universe are rushing away from us as described by Hubble’s law: recession velocity = H0 distance, where we take Hubble’s constant H0 to be 65 km/s/Mpc. (Sec. 15.5) We have used this relationship as a convenient means of determining the distances to galaxies and quasars, but it is much more than that. Assuming that all velocities have remained constant in time, we can ask how long it has taken for any given galaxy to reach its present distance from us. The answer follows from Hubble’s law. The time taken is simply the distance traveled divided by the velocity: For H0 = 65 km/s/Mpc, this time is about 15 billion years. Notice that it is independent of the distance-galaxies twice as far away are moving twice as fast, so the time they took to cross the intervening distance is the same. 5. Why isn’t it correct to say that the expansion of the universe involves galaxies flying outward into empty space? But the Big Bang was not an explosion in an otherwise featureless, empty universe. The only way that we can have Hubble’s law and retain the cosmological principle is to realize that the Big Bang involved the entire universe-not just the matter and radiation within it, but the universe itself. In other words, the galaxies are not flying apart into the rest of the universe. The universe itself is expanding. Like raisins in a loaf of raisin bread that move apart as the bread expands in an oven, the galaxies are just along for the ride. 6. Why are recent observations of distant supernovae so important to cosmology? One such global method is based on observations of Type I (carbon detonation) supernovae. (Sec. 12.5) Recall that these objects are both very bright and have a remarkably narrow spread in luminosities, making them particularly useful as standard candles. (Sec. 15.2) They can be used as probes of the universe because, by measuring their distances (without using Hubble’s law) and their redshifts, we can determine the rate of cosmic expansion in the distant past. 7. Where and when did the Big Bang occur? Hubble’s law therefore implies that, at some time in the past-15 billion years ago, according to the foregoing simple calculation-all the galaxies in the universe lay right on top of one another. In fact, astronomers believe that everything in the universe-matter and radiation alike-was confined to a single point at that instant. Then the point exploded, flying apart at high speeds. The present locations and velocities of the galaxies are a direct consequence of that primordial blast. This gargantuan explosion, involving everything in the cosmos, is known as the Big Bang. It marked the beginning of the universe. The Big Bang provides the resolution of Olbers’s paradox. Whether the universe is actually finite or infinite in extent is irrelevant, at least as far as the appearance of the night sky is concerned. The key point is that we see only a finite part of it-the region lying within roughly 15 billion light-years of us. What lies beyond is unknown-its light has not had time to reach us. To understand why there is no center to the expansion, we must make a great leap in our perception of the universe. If we were to imagine the Big Bang as simply an enormous explosion that spewed matter out into space, ultimately to form the galaxies we see, then the foregoing reasoning would be quite correct. The universe would have a center and an edge, and the cosmological principle would not apply. The Big Bang was not an explosion in an otherwise featureless, empty universe. The only way that we can have Hubble’s law and retain the cosmological principle is to realize that the Big Bang involved the entire universe-not just the matter and radiation within it, but the universe itself. In other words, the galaxies are not flying apart into the rest of the universe. The universe itself is expanding. Like raisins in a loaf of raisin bread that move apart as the bread expands in an oven, the galaxies are just along for the ride. Let’s reconsider some of our earlier statements in this new light. We now recognize that Hubble’s law describes the expansion of the universe itself. Apart from small scale individual random motions, galaxies are not moving with respect to the fabric of space. The component of the galaxies’ motion that makes up the Hubble flow is really an expansion of space itself. The expanding universe remains homogeneous at all times. There is no "empty space" beyond the galaxies into which they rush. At the time of the Big Bang, the galaxies did not reside at a point located at some well-defined place within the universe. The entire universe was a point. The Big Bang happened everywhere at once. 8. How does the cosmological redshift relate to the expansion of the universe? osmological Redshift As the universe expands, photons of radiation are stretched in wavelength, giving rise to the cosmological redshift. 9. What is the cosmic microwave background, and why is it so significant? cosmic microwave background The almost perfectly isotropic radio signal that is the electro-magnetic remnant of the Big Bang. the Milky Way’s emission at microwave wavelengths. In their data, they noticed a bothersome background "hiss"-a little like the background static on an AM radio station. Regardless of where and when they pointed their antenna, the hiss persisted. Never diminishing or intensifying, the weak signal was detectable at any time of the day and on any day of the year, apparently filling all of space. the origin of the mysterious static was nothing less than the fiery creation of the universe itself. The radio hiss detected is now known as the cosmic microwave background. 10. Why do all stars, regardless of their abundance of heavy elements, contain at least 25 percent helium by mass? Astronomers find that, no matter where they look, and no matter how low a star’s abundance of heavy elements may be, there is a minimum amount of helium-a little under 25 percent by mass-in all stars. This is the case even in stars containing very few heavy elements, indicating that the helium already existed when those ancient stars formed. (More Precisely 12-1) The accepted explanation is that this base level of helium is primordial-that is, it was created not by fusion in stars but during the early, hot epochs of the universe, before any stars had formed. The production of elements heavier than hydrogen by nuclear fusion shortly after the Big Bang is called primordial nucleosynthesis. This early burst of fusion did not last long. Conditions in the cooling, thinning universe became less and less conducive to further fusion reactions as time went by. For all practical purposes, primordial nucleosynthesis stopped at helium-4. By about 15 minutes after the Big Bang, the cosmic elemental abundance was set, and helium accounted for approximately one-quarter of the total mass of matter in the universe. The remaining 75 percent of the matter in the universe was hydrogen. It would be almost a billion years before nuclear reactions in stars would change these figures. 11. What is ? How do measurements of the cosmic deuterium abundance provide a lower limit on ? Cosmologists conventionally call the ratio of the actual density of the universe to the critical value the cosmic density parameter and denote it by the symbol ("omega nought"). In terms of this quantity, then, a critical universe has = 1.0. A universe with less than 1.0 has insufficient gravity to halt and reverse the present cosmic expansion. How can we determine the average density of the universe? On the face of it, it would seem simple-just measure the average mass of the galaxies residing within a large parcel of space, calculate the volume of that space, and compute the total mass density. When astronomers do this, they usually find a little less than 10-28 kg/m3 in the form of luminous matter. Largely independent of whether the chosen region contains only a few galaxies or a rich galaxy cluster, the resulting density is about the same, within a factor of two or three. Comparing this density with the critical value, we find that galaxy counts yield a value of of about 0.01. 12. When did the universe become transparent to radiation? Radiation-Matter Decoupling When atoms formed, the universe became virtually transparent to radiation. Thus, observations of the cosmic background radiation allow us to study conditions in the universe around a time at a redshift of 1500, when the temperature dropped below about 4500 K. 13. What is cosmic inflation? How does inflation solve the horizon problem? The flatness problem? Cosmic Inflation During the period of inflation the universe expanded enormously in a very short time. Afterward, it resumed its earlier "normal" expansion rate, except that now the size of the cosmos was about 1050 times bigger than it was before inflation. Inflation provides a natural solution for the horizon and flatness problems. The horizon problem is solved because inflation took regions of the universe that had already had time to communicate with one another-and so had established similar physical properties-and then dragged them far apart, well out of communication range of one another. In effect, the universe expanded much faster than the speed of light during the inflationary epoch, so what was once well within the horizon now lies far beyond it. (Relativity restricts matter and energy to speeds less than the speed of light, but imposes no such limit on the universe as a whole.) Regions A and B in Figure 17.15 have been out of contact with one another since 10-32 s after the Big Bang, but they were in contact before then. Their properties are the same today because they were the same long ago, before inflation separated them. inflation Short period of unchecked cosmic expansion early in the history of the universe. During inflation, the universe swelled in size by a factor of about 1050 horizon problem One of two conceptual problems with the standard Big Bang model, which is that some regions of the universe which have very similar properties are too far apart to have exchanged information in the age of the universe. flatness problem One of two conceptual problems with the Standard Big Bang model, which is that there is no natural way to explain why the density of the universe is so close to the critical density. 14. What is the connection between dark matter and the formation of large-scale structure in the universe? Structure Formation (a) The universe started out as a mixture of (mostly) dark and normal matter. (b) A few thousand years after the Big Bang, the dark matter began to clump. (c) Eventually the dark matter formed large structures (represented here by the two high-density peaks) into which normal matter flowed, ultimately to form the galaxies we see today. The three frames at right represent the densities of dark matter (red) and normal matter (yellow) graphed at left. 15. What did dark-matter models predict that was later found by the COBE satellite? Although dark matter does not interact directly with photons, the background radiation is influenced slightly by the gravity of the growing dark clumps This causes a slight gravitational redshift in the background radiation that varies from place to place depending on the dark-matter density. As a result, dark-matter models predict that there should be tiny "ripples" in the microwave background-temperature variations of only a few parts per million from place to place on the sky. In 1992, after almost two years of careful observation, the COBE team announced that the expected ripples had been detected. The temperature variations are tiny-only 30-40 millionths of a kelvin from place to place in the sky-but they are there. The COBE results are displayed as a temperature map of the microwave sky in Figure 17.20. The ripples seen by COBE, combined with computer simulations such as that shown in Figure 17.19, predict present-day structure that is quite consistent with the superclusters, voids, filaments and Great Walls we actually see around us. Detailed analysis of the ripples also supports the key prediction of inflation theory-that the universe is of exactly critical density and hence spatially flat. For these reasons, most cosmologists now regard the COBE observations as striking confirmation of a central prediction of dark-matter theory. They rank alongside the discovery of the microwave background itself in terms of their importance to the field of cosmology. Chapter 18 Review and Discussion 1. Why is life difficult to define? These rules are not hard and fast, and a good working definition of life is elusive. Stars, for example, react to the gravity of their neighbors, grow by accretion, generate energy, and "reproduce" by triggering the formation of new stars, but no one would suggest that they are alive. A virus is crystalline and inert when isolated from living organisms, but, once inside a living system, it exhibits all the properties of life, seizing control of the cell and using the cell’s own genetic machinery to grow and reproduce. Most researchers believe that the distinction between living and nonliving is more one of structure and complexity than a simple checklist of rules. The general case in favor of extraterrestrial life is summed up in what are sometimes called the assumptions of mediocrity: 1) because life on Earth depends on just a few basic molecules, 2) because the elements that make up these molecules are (to a greater or lesser extent) common to all stars, and 3) if the laws of science we know apply to the entire universe (as we have supposed throughout this book), then-given sufficient time-life must have originated elsewhere in the cosmos. The opposing view maintains that intelligent life on Earth is the product of a series of extremely fortunate accidents-astronomical, geological, chemical, and biological events unlikely to have occurred anywhere else in the universe. The purpose of this chapter is to examine some of the arguments for and against these viewpoints. 2. What is chemical evolution? chemical evolution-the chain of events leading from simple non-biological molecules almost to the point of life itself-has been amply demonstrated. 3. What is the Urey-Miller experiment? What important organic molecules were produced in this experiment? The first experimental demonstration that complex molecules could have evolved naturally from simpler ingredients found on the primitive Earth came in 1953. Scientists Harold Urey and Stanley Miller, using laboratory equipment somewhat similar to that shown in Figure 18.2, took a mixture of the materials thought to be present on Earth long ago-a "primordial soup" of water, methane, carbon dioxide, and ammonia-and energized it by passing an electrical discharge ("lightning") through it. After a few days, they analyzed their mixture and found that it contained many of the same amino acids found today in all living things on Earth. About a decade later, scientists succeeded in constructing nucleotide bases in a similar manner. These experiments have been repeated in many different forms, with more realistic mixtures of gases and a variety of energy sources, but always with the same basic outcomes. Miller-Urey Experiment This chemical apparatus is designed to synthesize complex biochemical molecules by energizing a mixture of simple chemicals. A mixture of gases (ammonia, methane, carbon dioxide, water vapor) is placed in the upper bulb to simulate the primordial Earth atmosphere and then energized by spark-discharge electrodes. After about a week, amino acids and other complex molecules are found in the trap at the bottom, which simulates the primordial oceans into which heavy molecules produced in the overlying atmosphere would have fallen. 4. What are the basic ingredients from which biological molecules formed on Earth? water, methane, carbon dioxide, and ammonia 5. How do we know anything at all about the early episodes of life on Earth? However the basic materials appeared on Earth, we know that life did appear. The fossil record chronicles how life on Earth became widespread and diversified over time. The study of fossil remains shows the initial appearance about 3.5 billion years ago of simple one-celled organisms, such as blue-green algae. These were followed about two billion years ago by more complex one-celled creatures like the amoeba. Multicellular organisms did not appear until about one billion years ago, after which a wide variety of increasingly complex organisms flourished-among them insects, reptiles, mammals, and humans. The fossil record leaves no doubt that biological organisms have changed over time-all scientists accept the reality of biological evolution. As conditions on Earth have shifted and Earth’s surface has evolved, those organisms that could best take advantage of their new surroundings succeeded and thrived-often at the expense of those organisms that could not make the necessary adjustments and consequently became extinct. 6. What is the role of language in cultural evolution? The study of fossil remains shows the initial appearance about 3.5 billion years ago of simple one-celled organisms, such as blue-green algae. These were followed about two billion years ago by more complex one-celled creatures like the amoeba. Multicellular organisms did not appear until about one billion years ago, after which a wide variety of increasingly complex organisms flourished-among them insects, reptiles, mammals, and humans. The fossil record leaves no doubt that biological organisms have changed over time-all scientists accept the reality of biological evolution. As conditions on Earth have shifted and Earth’s surface has evolved, those organisms that could best take advantage of their new surroundings succeeded and thrived-often at the expense of those organisms that could not make the necessary adjustments and consequently became extinct. Perhaps most important of all was the development of language. Experience and ideas, stored in the brain as memories, could be passed down from one generation to the next. A new kind of evolution had begun, namely, cultural evolution-the changes in the ideas and behavior of society. Our more recent ancestors created, within about 10,000 years, the entirety of human civilization. 7. Where else, besides Earth, have organic molecules been found? Instead, they suggest, most of the organic material that combined to form the first living cells was produced in interstellar space, and subsequently arrived on Earth in the form of interplanetary dust and meteors that did not burn up during their descent through the atmosphere. Interstellar molecular clouds are known to contain many complex molecules, and large amounts of organic material were detected on comets Halley and Hale-Bopp when they last visited the inner solar system, so the idea that organic matter is constantly raining down on Earth from space in the form of interplanetary debris is quite plausible. (Sec. 4.2) However, whether this was the primary means by which complex molecules appeared on Earth remains unclear. 8. Where-besides the planet Mars-might we find signs of life in our solar system? However, the possibility of liquid water below Europa’s icy surface has refueled speculation about the development of life there, making this moon of Jupiter a prime candidate for future exploration. (Sec. 8.1) Saturn’s moon Titan, with its atmosphere of methane, ammonia, and nitrogen gases, and possibly with some liquid on its surface, is conceivably another site where life might have arisen, although the results of the 1980 Voyager 1 flyby suggest that Titan’s frigid surface conditions are inhospitable for anything familiar to us. What about the cometary and meteoritic debris that orbits within our solar system? Comets contain many of the basic ingredients for life, but they spend almost all of their time frozen, far from the Sun. A small fraction of the meteorites that survive the plunge to Earth’s surface also contain organic compounds. 9. Do we know whether Mars ever had life at any time during its past? What argues in favor of the position that it may once have harbored life? The planet most likely to harbor life (or to have harbored it in the past) seems to be Mars. This planet is harsh by Earth standards-liquid water is scarce, the atmosphere is thin, and the lack of magnetism and of an ozone layer allows solar high-energy particles and ultraviolet radiation to reach the surface unabated. But the Martian atmosphere was thicker, and the surface warmer and much wetter, in the past. (Sec. 6.8) The Viking landers found no evidence of life, but they landed on the safest Martian terrain, not in the most interesting regions, such as near the moist polar caps. Some scientists think that a different type of biology might be operating, or might have operated, on the Martian surface. They suggest that Martian microbes capable of digesting oxygen-rich compounds in the Martian soil could explain the Viking results. This speculation will be greatly strengthened if recent announcements of fossilized bacteria in meteorites originating on Mars are confirmed. (Interlude 6-1) The consensus among biologists and chemists today is that Mars does not house any life similar to that on Earth, but a solid verdict regarding present or past life on Mars will likely not be reached until we have thoroughly explored our intriguing neighbor. 10. What is generally meant by "life as we know it"? What other forms of life might be possible? Life as We Know It "Life as we know it" is generally taken to mean carbon-based life that originated in a liquid-water environment-in other words, life on Earth. Might such life exist on some other body in our planetary system? Conceivably, some types of biology may be so different from life on Earth that we do not know how to test for them. Some researchers have pointed out that the abundant element silicon has chemical properties somewhat similar to those of carbon and have suggested silicon as a possible alternative to carbon as the basis for living organisms. Ammonia (made of the common elements hydrogen and nitrogen) is sometimes put forward as a possible liquid medium in which life might develop, at least on a planet cold enough for ammonia to exist in the liquid state. Together or separately, these alternatives would surely give rise to organisms having biochemistries radically different from those we know on Earth. Conceivably, we might have difficulty even recognizing these organisms as being alive. 11. How many of the terms in the Drake equation are known with any degree of certainty? Which factor is least well known? Drake Equation Expression that gives an estimate of the probability that intelligence exists elsewhere in the galaxy, based on a number of supposedly necessary conditions for intelligent life to develop. The Drake Equation An early approach to this statistical problem is known as the Drake equation, after Frank Drake, the U.S. astronomer who pioneered this analysis: number of technological, intelligent civilizations now present in the Galaxy= rate of star formation, averaged over the lifetime of the Galaxy fraction of stars having planetary systems average number of habitable planets within those planetary systems fraction of those habitable planets on which life arises fraction of those life-bearing planets on which intelligence evolves fraction of those intelligent- life planets that develop technological society average lifetime of a technologically competent civilization. Several of the terms in this equation are largely a matter of opinion. We do not have nearly enough information to determine-even approximately-all of the terms, so the Drake equation cannot give us a hard-and-fast answer. Its real value is that it subdivides a large and very difficult question into smaller pieces that we can attempt to answer separately. Figure 18.5 illustrates how, as our requirements become more and more stringent, only a small fraction of star systems in the Milky Way Galaxy have the advanced qualities specified by the combination of terms on the right-hand side of the equation. Let’s examine the terms in the equation one by one and make some educated guesses about their values. Bear in mind, though, that if you ask two scientists for their best estimates of any given term, you will likely get two very different answers! 12. What is the relationship between the average lifetime of Galactic civilizations and the possibility of our someday communicating with them? For definiteness, let us assume that the average lifetime of a technological civilization is one million years-only one percent of the reign of the dinosaurs, but 100 times longer than human civilization has survived thus far. Given the size and shape of our Galaxy, we can then estimate the average distance between these civilizations to be some 30 pc, or about 100 light-years. any two-way communication with our neighbors-using signals traveling at or below the speed of light-will take at least 200 years (100 years for the message to reach the planet and another 100 years for the reply to travel back to us). 13. How would Earth appear, at radio wavelengths, to extraterrestrial astronomers? Consider first how "waste" radio wavelengths originating on Earth would look to extraterrestrials. Figure 18.9 shows the pattern of radio signals we broadcast into space. From the viewpoint of a distant observer, the spinning Earth emits a bright flash of radio radiation every few hours as the most technological regions of our planet rise or set. In fact, Earth is now a more intense radio emitter than the Sun. Our radio and television transmissions race out into space, and have been doing so since the invention of these technologies nearly seven decades ago. Another civilization as advanced as ours might have constructed devices capable of detecting this radiation. If any sufficiently advanced (and sufficiently interested) civilization resides within about 65 light-years (20 pc) of Earth, then we have already announced our presence to them. 14. What are the advantages in using radio waves for communication over interstellar distances? A cheaper, and much more practical, alternative is to try to make contact with extraterrestrials using electromagnetic radiation, the fastest known means of transferring information from one place to another. Because light and other short-wavelength radiation are heavily scattered while moving through dusty interstellar space, long-wavelength radio radiation seems to be the best choice. (Sec. 2.3) We do not attempt to broadcast to all nearby candidate stars, however-that would be far too expensive and inefficient. Instead, radio telescopes on Earth listen passively for radio signals emitted by other civilizations. 15. What is the water hole? What advantage does it have over other parts of the radio spectrum? water hole The radio interval between 18 cm and 21 cm, the wavelengths at which hydroxyl (OH) and hydrogen (H) radiate, respectively, in which intelligent civilizations might conceivably send their communication signals. Some basic arguments suggest that civilizations might well communicate at a wavelength near 20 cm. The basic building blocks of the universe, namely, hydrogen atoms, radiate naturally at a wavelength of 21 cm. (Sec. 11.2) Also, one of the simplest molecules, hydroxyl (OH), radiates near 18 cm. Together, these two substances form water (H2O). Arguing that water is likely to be the interaction medium for life anywhere, and that radio radiation travels through the disk of our Galaxy with the least absorption by interstellar gas and dust, some researchers have proposed that the interval between 18 cm and 21 cm is the best wavelength range for civilizations to transmit or listen. Called the water hole, this radio interval might serve as an "oasis" where all advanced Galactic civilizations would gather to conduct their electromagnetic business. This water-hole frequency interval is only a guess, of course, but its use is supported by other arguments. Figure 18.10 shows the water hole’s location in the electromagnetic spectrum and plots the amount of natural emission from our Galaxy and from Earth’s atmosphere. The 18- to 21-cm range lies within the quietest part of the spectrum, where the Galactic "static" from stars and interstellar clouds happens to be minimized. Furthermore, the atmospheres of typical planets are also expected to interfere least at these wavelengths. the water hole seems like a good choice for the frequency of an interstellar beacon, although we cannot be sure of this reasoning until contact is actually achieved. Some radio searches are now in progress, simultaneously sampling millions of frequencies in and around the water hole. So far, however, nothing resembling an extraterrestrial signal has been detected. The space surrounding all of us could be, right now, flooded with radio signals from extraterrestrial civilizations. If only we knew the proper direction and frequency, we might be able to make one of the most startling discoveries of all time. The result would provide whole new opportunities to study the cosmic evolution of energy, matter, and life throughout the universe. Astronomy 105 ASTR 105 Beginning Astronomy Costs Section OAS, Item 3033 Section OAC, Item 3035 6 Credits RM L200 To register for courses, note down the item number of all courses for which you wish to register and then click "Registration" on the left menu. A BCC Online Class - Science with lab credit at BCC. An introductory survey of astronomy, including the history of astronomy, an overview of the solar system, an introduction to the lives of stars, plus an overview of galaxies and the formation and evolution of the universe as a whole. Credit cannot be obtained for both ASTR 101 and 105. Online Class Requirements: Questions? Call (425) 564-2438 To participate in an online course, students must have access to the class via an Internet account, including e-mail and Web access via a server which is Java enabled. Basic computer literacy is essential. Instructor: D. Knight, Syllabus: Textbooks: Books for online classes will be available from the BCC Bookstore at Or call (425) 564-2285. Starting Online Classes: Print out and read the course syllabus. Here's where to find out HOW AND WHEN to start classes. Beginning Astronomy 105 6 credits, No prerequisites Instructor: Dan Knight BCC Office: B249 Text (REQUIRED): Astronomy: A Beginner's Guide to the Universe 3rd Edition (2001), by Chaisson and McMillan, (ISBN# 0-13-087307-1). Included with your text should be a CD-ROM containing the entire book plus a CD-ROM containing SkyChart III, a planetarium program that will be used for labs. Other required tools: computer, internet access, CD-ROM drive, Adobe Acrobat and RealPlayer (free downloads available). Course Description: Astronomy 105 is an introductory survey of astronomy, including the history of astronomy, an overview of the solar system, an introduction to the lives of stars, plus an overview of galaxies and the formation and evolution of the universe as a whole. Learning Outcomes: By the end of the course, the successful student should be able to: 1.Describe the motions of the Earth, Sun, Moon, and planets, and explain how these motions generate observable phenomenon such as the seasons, moon phases and eclipses. 2.Describe where they are in the universe. 3.Relate the history of astronomy including the contributions of several key historical figures. 4.Explain the scientific method. 5.Demonstrate an understanding of light and how the information contained in light can be deciphered by astronomers. 6.Identify the main types of telescopes, their uses and limitations. 7.Show how the nebular theory of the formation of the solar system accounts for the observed properties of the solar system 8.Describe, in comparative terms, the main features of the solar system, including planets, moons, comets and asteroids 9.Summarize the overall properties of the Sun and the processes that occur within it. 10.Describe the life cycle of stars from birth through death. 11.Describe the different types and features of galaxies. 12.Relate how the Big Bang Theory explains the observed features of our universe. 13.Describe the considerations that go into trying to determine the possibility of communicating with other technologically advanced civilizations within the galaxy. Testing and Grading: Your grade will be based on a professional evaluation of your performance in four areas as follows: 10% - Weekly homework assignments. These will consist of all the true false and fill in the blank questions at the end of each chapter, (answers provided on page AK-3 at the back of the book) 20% - Weekly labs. 40% - 3 quizzes. Lowest automatically dropped. The other two count for 20% each. (Each quiz will consist of 50 questions similar in format to the homework questions.) 30% - Comprehensive Final exam (50 questions on the material covered since the last quiz, plus 25 questions from previous material.) Your final grade will be assigned according to the following scale: A 100% - 92% A- 92% - 90% B+ 90% - 88% B 88% - 82% B- 82% - 80% C+ 80% - 76% C 76% - 68% C- 68% - 65% D+ 65% - 60% Tuesday, Sept. 24, Course Begins Assigned: -Reading and Homework: Prologue and Chapter 1 -Lab #1 "Introduction to Sky Chart III" Monday, Sept. 30 Due: -Homework from Prologue and Chapter 1 Assigned: -Reading and Homework: Chapters 2 and 3, this homework will not be collected as there is a quiz due next week Thursday, Oct. 3 Due: -Lab #1 "Introduction to Sky Chart III" Assigned: -Lab #2 "Motions of the Stars and Sun" Available: -Quiz #1 covering Prologue and Chapters 1, 2 and 3 Monday, Oct. 7 Due: -Quiz #1 covering Prologue and Chapters 1, 2 and 3 Assigned: -Reading and Homework: Chapters 4 and 5. Thursday, Oct. 10 Due: -Lab #2 "Motions of the Stars and Sun" Assigned: -Lab#3 "Motions of the Sun Moon and Planets" Friday, Oct. 11, BCC Holiday!! Monday, Oct. 14 Due: -Homework from Chapters 4 and 5 Assigned: -Homework and reading: Chapters 6 and 7 Thursday, Oct. 17 Due: -Lab#3 "Motions of the Sun Moon and Planets" Assigned: -Lab#4 "Lunar Phases and Retrograde Motion" Monday, Oct. 21 Due: -Homework Chapters 6 and 7 Assigned: -Reading and Homework: Chapter 8, but this homework will not be collected since there is a quiz due next week. Thursday, Oct. 24 Due: -Lab#4 "Lunar Phases and Retrograde Motion" Assigned: -Lab#5 "Solar System Scavenger Hunt" Available: -Quiz #2 covering Chapters 4, 5, 6, 7 and 8 Monday, Oct. 28 Due: -Quiz #2 Assigned: -Homework and reading: Chapters 9 and 10 Thursday, Oct. 31 Due: -Lab #5 "Solar System Scavenger Hunt" Assigned: -Lab #6 "The Sun" Monday, Nov. 4 Due: -Homework: Chapters 9 and 10 Assigned: -Reading and Homework: Chapters 11 and 12 Thursday, Nov, 7 Due: -Lab #6 "The Sun" Assigned: -Lab #7 "The Parallax and the H-R Diagram" Monday, Nov. 11, BCC Holiday!! Tuesday, Nov. 12 Due: -Homework: Chapters 11 and 12 Assigned: -Homework and reading: Chapters 13, but the homework from this chapter won't be collected since there is a quiz due next week. Thursday, Nov. 14 Due: -Lab #7 "The Parallax and the H-R Diagram" Assigned: -Lab #8 "Galaxies and the Hubble Law" -Available: -Quiz #3 covering Chapters 9,10,11,12 and 13 Monday, Nov. 18 Due: -Quiz #3 Assigned: -Reading and Homework: Chapters 14 and 15 Thursday, Nov. 21 Due: -Lab #8 "Galaxies and the Hubble Law" Assigned: -Lab #9 "The Search for Extraterrestrial Intelligence" Monday, Nov. 25 Due: -Homework Chapters 14 and 15 Assigned: -Reading and Homework: Chapters 16 and 17 Tuesday, Dec. 3 Due: -Lab #9 "The Search for Extraterrestrial Intelligence" Thursday, Dec. 5 Due: -Homework: Chapters 16 and 17 Available: -Final Exam Monday, Dec. 9 -Final Exam Due Astronomy Chapter 1 Fill in the Blank 1. When the planets Mars, Jupiter, and Saturn appear to move "backwards" (westward) in the sky relative to the stars, this is known as retrograde_ motion. 2. The geocentric model holds that Earth_ is at the center of the solar system. 3. Observation, theory, and testing are the cornerstones of the scientific method_. 4. Galileo discovered Moons_ of Jupiter, the Moons_ of Venus, and the Sun’s rotation from observations of sunspots_. 5. Kepler’s laws were based on telescopic_'s observations. or another scientist's 6. Kepler discovered that the shape of an orbit is an elliptical_, not a circular_ as had previously been believed. 7. According to Kepler’s first law, the Sun lies at the eccentricity_ of a planet’s orbit. 8. Kepler’s third law relates the square of a planet’s_ of the orbital period to the cube_ of the semi-major axis. 9. The modern method of measuring the astronomical unit uses Venus, whose orbit periodically brings it closest to Earth_ measurements of Venus. 10. One astronomical unit is approximately 1 A.U. is 45,000,000 km/0.3, or 150,000,000 km._ km. 11. Newton’s first law states that a moving object will continue to move in a straight line with constant speed unless acted upon by a external force_. 12. Newton’s law of gravity states that the gravitational force between two objects depends on the gravity_ of their masses and inversely on the gravitational force_ of their separation. Chpater 2 Fill in the Blank 1. The wavelength_ of a wave is the distance between any two adjacent wave crests. 2. The hertz (Hz) is a unit used to measure the Frequency_ of a wave. 3. When a charged particle moves, information about this motion is transmitted through space via the particle’s changing Electric_ and magnetic_ fields. 4. The visible spectrum ranges from 400_ nm to 700_ nm in wavelength. 5. Earth’s atmosphere has low opacity for three forms of electromagnetic radiation. They are gamma-ray _, X-ray_, and ultraviolet_. 6. The peak of an object’s emitted radiation occurs at a frequency or wavelength determined by the object’s temperature_. 7. Two identical objects have temperatures of 1000 K and 1200 K. It is observed that one of the objects emits roughly twice as much radiation as the other. Which one is it? electric heater_. 8. Blackbody radiation is an example of a continuous spectra_ spectrum. 9. Fraunhofer discovered absorption lines in the spectrum of solar_. 10. A continuous spectrum can be produced by a luminous solid, a liquid, or a sufficiently dense_ gas. 11. An absorption spectrum is produced when a low-density cool_ gas lies in front of a continuous source. 12. Protons carry a positive_ charge; electrons carry a negative electrical_ charge. 13. When moving to a higher energy level in an atom, an electron red_ a photon of a specific energy. 14. When moving to a lower energy level in an atom, an electron blue_ a photon of a specific energy. 15. The "specific energy" referred to in the preceding two questions is exactly equal to the energy specifc amounts_ between the two energy levels the electron moves. Chapter 3 Fill in the Blank 1. A telescope that uses a lens to focus light is called a refracting_ telescope. 2. A telescope that uses a mirror to focus light is called a reflecting_ telescope. 3. All large modern telescopes are of the reflectors_ type. 4. The light-gathering power of a telescope is determined by the area_ of its mirror or lens. 5. The angular resolution of a telescope is determined by the resolution_ of the telescope and the amount_ of the radiation being observed. 6. The angular resolution of ground-based optical telescopes is more seriously limited by Earth's turbulent atmosphere_ than by diffraction. Atmospheric Blurring. 7. Optical telescopes on Earth can see angular detail down to about 1" or few_ arc second. 8. CCDs produce images in digital_ form that can be easily transmitted, stored, and processed by computers. 9. Active optics and adaptive optics are both being used to improve the resolution_ of ground-based optical telescopes. 10. All radio telescopes are of the curved metal dish _ design. 11. An interferometer_ is two or more telescopes used in tandem to observe the same object, in order to improve angular resolution. 12. Chandra is a telescope designed to make observations in the X-Ray_ part of the spectrum. 13. An object having a temperature of 300 K would be best observed with an infrared_ telescope. 14. Orbiting infrared_ telescopes must be cooled to very low temperatures. 15. Gamma-ray telescopes are unable to form image_ of their fields of view. Chapter 4 Fill in the Blank 1. The two planets with the highest eccentricities and orbital tilts are Mercury_ and Pluto_. 2. Asteroids and meteoroids are often debris rocky_ in composition, in contrast to comets, which are generally icy_ in composition. 3. Most asteroids are found between the orbits of Mars_ and Jupiter_. 4. The largest asteroids are 940_ kilometers in diameter; the smallest are only 200_ meters across. 5. The Trojan asteroids share an orbit with Jupiter_. 6. The nucleus of a comet is typically 70,000 km_ kilometers across, and its tail may be up to 500,000 km_ long. 7. Passage of a comet near the Sun may leave a fragment_ moving in the comet’s orbit. 8. When a meteoroid, asteroid, or comet fragment enters Earth’s atmosphere, you see a brief flash_. 9. The oldest meteorites are between 4.4 and 4.6 billion _ years old. 10. By the time planetesimals had formed, the accretion process was accelerated by the effect of gravity_. 11. In the final stage of accretion, the largest protoplanets were able to attract large quantities of gas_ from the solar nebula. 12. Unlike the planetesimals that formed the terrestrial planets, those that formed the jovian planets were made up of icy_ material. 13. The large number of left-over planetesimals formed beyond 5 A.U. were destined to become terrestrial worlds_. 14. The reason the planetesimals of the asteroid belt did not form a larger object was probably the gravitational influence of Jupiter’s huge gravitational field_. 15. Angular momentum depends on the mass, radius, and rotation rate_ of an object. Chapter 5 Fill in the Blank 1. The radius of the Moon is about 1700 km 0.27 Earth =1_ Earth’s radius. (Give your answer as a simple fraction, not as a decimal.) 2. Of Earth’s crust, mantle, outer core, and inner core, which layer is the thinnest? The crust. 3. Earth is unique among the planets in that it has large quantities of liquid water_ on its surface. 4. The tidal force is due to the tidal bulge_ in the gravitational force from one side of Earth to the other. 5. The maria seas_ on the Moon are dark, flat, roughly circular regions hundreds of kilometers in diameter. 6. Craters on the Moon are primarily caused by "rain" of impacting meteoroids_. 7. The lunar maria’s dark, dense rock originally was part of the lunar surface_ of the Moon. 8. Earth’s atmosphere is 78 percent nitrogen_ and 21 percent oxygen_. 9. The troposphere is where the process called convection_ occurs. 10. Sunlight is absorbed by Earth’s surface and is reemitted in the form of solar ultraviolet_ radiation. 11. An increase in the level of carbon dioxide in Earth’s atmosphere would increase_ our planet’s temperature. 12. When trapped electrons and protons from the magnetosphere eventually collide with the upper atmosphere, they produce an a spectacular light show called an aurora_. 13. Observations of seismic waves imply that Earth’s inner core is solid_ and the outer core is liquid_. 14. For differentiation to have occurred, Earth’s interior must, at some time in the past, have been largely molten_. 15. Plate Tectonics_ is the study of continental drift, and the resulting volcanism, earthquakes, faults, and mountain building. Chapter 6 Fill in the Blank 1. Mercury’s iron core contains a 1800 km_ fraction of the planet’s mass than does Earth’s core. 2. Mercury’s rate of rotation was first measured using Arecibo radio telescope_. 3. Although Mercury’s daytime temperatures are always very hot, it may still be possible for the planet to have sheets of water ice at its poles_. 4. Mercury’s magnetic field_ is about 1/100 that of Earth and was originally thought not to exist at all. 5. Venus’s rotation is unusual because it is retrograde-that is, opposite that of Earth_. 6. The most abundant gas in the atmosphere of Venus is carbon dioxide_. 7. The runaway greenhouse effect on Venus was a result of the planet’s being closer_ to the Sun than is Earth. 8. The surface of Venus has been mapped using the U.S. Magellan spacecraft_. 9. The surface of Venus appears to have been resurfaced by volcanism_ every few hundred million years. 10. The main difficulties in using landers to study Venus’s surface are the planet’s extremely high pressure_ and temperature_. 11. The southern hemisphere of Mars consists of heavily cratered_ highlands. 12. The Tharsis region of Mars is a large equatorial region_. 13. The great height of Martian volcanoes is a direct result of the planet’s low surface gravity_ . 14. The flowing-liquid appearance of the ejecta surrounding Martian impact craters is evidence of a layer of permafrost_ just under the surface. 15. Runoff channels on Mars carried water_ from the southern mountains into the valleys. Chapter 7 Fill in the Blank 1. The masses_ of Jupiter indicates that the planet’s overall composition differs greatly from that of the terrestrial planets. 2. The planet Uranus_ is blue-green and virtually featureless. 3. The planet Neptune_ is dark blue, with white clouds and visible storm systems. 4. The overall bluish coloration of Uranus and Neptune is caused by the gas methane_. 5. The main constituents of Jupiter and Saturn are molecular hydrogen_ and helium_. 6. Jupiter’s clouds consist of a series of light-colored zones_ and darker belts_. 7. Jupiter’s Great Red Spot has similarities to a huricane_ on Earth. 8. Features in the atmospheres of Saturn and Jupiter are mostly hidden by an upper layer of frozen haze_ clouds. 9. Saturn’s cloud layers are thicker than those of Jupiter because of Saturn’s weaker gravity_. 10. The Great Dark Spot was a storm system on Jupiter_. 11. Jupiter’s rapid rotation_ produces a significant equatorial bulge. 12. Uranus’s rotation axis is almost perpendicular_ to the ecliptic plane. 13. Jupiter emits about twice-(125/105)4_ times more radiation than it receives from the Sun. 14. Jupiter’s magnetic field is generated by its rapid rotation and by the element metallic hydrogen _, which becomes metallic in the interior of the planet. 15. Saturn’s excess energy emission is caused by Astronomers theorize that the source of Jupiter’s excess energy is heat left over from the planet’s formation. That heat is slowly leaking out through the planet’s heavy atmospheric blanket, resulting in the excess emission we observe_or liquis metal hydrogen. The explanation for this strange state of affairs also explains of Saturn’s atmospheric helium deficit. At the temperatures and high pressures found in Jupiter’s interior, liquid helium dissolves in liquid hydrogen. In Saturn, where the internal temperature is lower, the helium doesn’t dissolve so easily and tends to form droplets instead. (The basic phenomenon is familiar to cooks who know that it is generally much easier to dissolve ingredients in hot liquids than in cold ones.) Saturn probably started out with a fairly uniform mix of hydrogen and helium, the helium tended to condense out of the surrounding hydrogen, much as water vapor condenses out of Earth’s atmosphere to form a mist. The amount of helium condensation was greatest in the planet’s cool outer layers, where the mist turned to rain about two billion years ago. A light shower of liquid helium has been falling through Saturn’s interior ever since. This helium precipitation is responsible for depleting the outer layers of their helium content. As the helium sinks toward the center, the planet’s gravitational field compresses it and heats it up. The energy released is the source of Saturn’s internal heating. Chapter 8. Fill in the Blank 1. The Galilean moon Ganymede_ is larger than Mercury. 2. In contrast to the inner two Galilean moons, the outer two have compositions that include significant amounts of water ice_. 3. Io is the only moon in the solar system having active volcanoes_. 4. As viewed from Earth, Saturn’s ring system is conventionally divided into three main rings or quite_ broad rings. 5. The Cassini Division lies between the A_ and B_ rings. Cassini Division A relatively empty gap in Saturn's ring system between the A and B rings 6. Saturn’s rings exist because they lie within Saturn’s Roche limit_. 7. Two small moons, known as shepherd_ satellites, are responsible for the unusually complex form of the F ring. 8. The composition of Titan’s atmosphere is 90 percent nitrogen_. 9. There may be large amounts of liquid water_ under the frozen surface of Europa. 10. The Uranian moon that shows the greatest amount of geological activity and disruption over time is Miranda_. 11. Triton’s orbit is unusual because it is retrograde_. 12. Neptune’s moon Triton has a thin atmosphere made of nitrogen_. 13. Nitrogen geysers are found on Triton’s surface_. 14. Overall, Pluto is most similar to which object in the solar system? Triton_ 15. Astronomers measured the radii of Pluto and Charon by observing a fortuitous series of eclipses_ during the late 1980s. Chapter 9 Fill in the Blank 1. The part of the Sun we see is called the photosphere_. 2. The three main regions of the solar atmosphere are the chronosphere_, the transition zone_, and the corona_. 3. Below the solar surface, in order of increasing depth, lie the convection_ zone, the radiation_ zone, and the core_. 4. The granulation_ seen on the surface of the Sun is evidence of convective cells. 5. The Sun appears to have a well-defined edge because the thickness of the Sun_ is only 0.1 percent of the solar radius. 6. Hydrogen_ is the most abundant element in the Sun. 7. Helium_ is the second most abundant element in the Sun. 8. The two most abundant elements in the Sun make up about 99.9_ percent of its composition. 9. The solar wind_ is formed as the hot outer regions of the corona expand into interplanetary space. 10. Sunspots appear dark because they are against an even brighter background (the 5800 K photosphere)._ than the surrounding gas of the photosphere. 11. The sunspot cycle is 11_ years long; the solar cycle is 22 years_ as long. 11-year sunspot cycle is only half of a 22-year solar cycle 12. A Flares_ is a violent explosive event in a solar active region. 13. The Sun’s luminosity is produced in the solar constant_ (give the region). 14. The net result of the proton-proton chain is that hydrogen nuclei_ protons are fused into a nucleus of helium-4_, two neutrinos_ are emitted, and energy is released in the form of gamma-ray radiation_. 15. The solar neutrino problem is the fact that astronomers observe too _ neutrinos coming from the Sun. solar neutrino problem The discrepancy between the theoretically predicted numbers of neutrinos streaming from the Sun as a result of fusion reactions in the core and the numbers actually observed. The observed number of neutrinos is only about half the predicted numbe Chapter 10 Fill in the Blank 1. Parallax measurements of the distances to the stars nearest Earth use _ as a baseline. The distances to the nearest stars can be measured using parallax 2. To determine the transverse velocity of a star, both its proper motion_ and its distance_ must be known. 3. The radius of a star can be indirectly determined if the star’s luminosity_ and temperature_ are known. 4. The smallest stars normally plotted on the H-R diagram are dwarf._. 5. The smallest main-sequence stars are smaller_ (larger/smaller) than the planet Jupiter. 6. Observations of stars through B and V filters are used to determine stellar temperature_. For example, a B (blue) filter rejects all radiation except for a certain range of violet to blue light. Similarly, a V (visual) filter passes only radiation in the green to yellow range (the part of the spectrum to which human eyes happen to be particularly sensitive). 7. The hottest stars show little evidence of hydrogen in their spectra because hydrogen is mostly ionized_ at these temperatures. 8. The coolest stars show little evidence of hydrogen in their spectra because hydrogen is mostly faint and weak_ at these temperatures. 9. The Sun has a spectral type of spectral classes (or spectral types)._. 10. The H-R diagram is a plot of Surface temperature _ on the horizontal scale versus solar luminosity _ on the vertical scale. The vertical luminosity scale, expressed in units of solar luminosity (3.9 1026 W), extends over a large range, from 10-4 to 104. The Sun appears right in the middle of the luminosity range, at a luminosity of one. Surface temperature is plotted on the horizontal axis, although in the unconventional sense of temperature increasing to the left, so that the spectral sequence O, B, A,.. reads left to right. 11. The band of stars extending from the top left of the H-R diagram to its bottom right is known as the main sequence_. from top left (high-temperature, high-luminosity) to bottom right (low-temperature, low-luminosity). In other words, cool stars tend to be faint (less luminous) and hot stars tend to be bright (more luminous). This band of stars spanning the H-R diagram is known as the main sequence. 12. The large, cool stars found at the upper right of the H-R diagram are red dwarf_. red dwarf Small, cool faint star at the lower-right end of the main sequence on the Hertzsprung-Russell diagram. red giant A giant star whose surface temperature is relatively low, so that it glows with a red color. red supergiant An extremely luminous red star. Often found on the asymptotic giant branch of the H-R diagram. 13. The small, hot stars found at the lower left of the H-R diagram are white dwarf_. white dwarf A dwarf star with a surface temperature that is hot, so that the object glows white. 14. binary_-star systems are important for providing measurements of stellar masses. 15. Going from spectral type O to M along the main sequence, stellar masses luminosity_. Chapter 11 Fill in the Blank 1. The interstellar medium is made up of matter in the form of gas_ and dust_. 2. To scatter a beam of radiation most effectively, a particle must be 10-7 m, comparable_ in size to the wavelength of the radiation. 3. 21-centimeter radiation results from a change in the _ of the electron in a _______ atom. Give answer in A.U. proton excited hydrogen atom 21-centimeter radiation Radio radiation emitted when an electron in the ground state of a hydrogen atom flips its spin to become parallel to the spin of the proton in the nucleus. 4. Molecular clouds typically have temperatures of about 20_ K. 5. Emissions from molecular clouds are in the radio portion_ part of the electromagnetic spectrum. 6. The most common constituent of molecular clouds is molecular hydrogen_. 7. A molecular cloud complex may contain as much as _ solar masses of gas. some spanning as much as 50 pc across and containing enough gas to make millions of stars like our Sun. About 1000 such complexes are known in our Galaxy. 8. The temperature of a typical emission nebula is about 8000_ K. 9. Astronomers look for emissions at radio_ wavelengths to identify interstellar clouds in the early stages of collapse. The dark region outlined by the red and green radio contours in Figure 11.12 (not the emission nebula itself, where stars have already formed) probably represents a stage-1 cloud just starting to collapse. Doppler shifts of the observed formaldehyde lines indicate that it is contracting. Less than a light year across, this region has a total mass over 1000 times the mass of the Sun-considerably greater than the mass of M20 itself. Figure 11.14 Cloud Fragmentation As an interstellar cloud contracts, gravitational instabilities cause it to fragment into smaller pieces. The pieces also contract and fragment, eventually forming many tens or hundreds of individual stars. 10. An _ plots a star’s or protostar’s changing location on the H-R diagram as the object evolves. The motion of that point around the diagram as the star evolves is known as the star’s evolutionary track. 11. As the fragments of an interstellar cloud contract, their central densities and temperatures have started to heat up_. 12. Protostars emit a great deal of radiation in the _ part of the electromagnetic spectrum. The changes in a protostar’s observed properties are shown by the path of decreasing luminosity, from stage 4 to stage 6. At stage 7, the newborn star has arrived on the main sequence. 13. When hydrogen is burning stably in the core, the star has reached the zero-age main sequence_. A star is considered to have reached the main sequence when hydrogen burning begins in its core and the star’s properties settle down to stable values. The main-sequence band predicted by theory is usually called the zero-age main sequence 14. It takes a star like the Sun a total of about 40 to 50_ million years to form. The evolutionary events just described occur over the course of 40 to 50 million years. 15. More massive stars evolve more _. The most massive fragments contract into O-type stars in a mere million years, roughly 1/50 the time taken by the Sun. The opposite is the case for prestellar objects having masses much less than our Sun. A typical M-type star, for example, requires nearly a billion years to form. Chapter 12 Fill in the Blank 1. A main-sequence star doesn’t collapse because of the outward pressure_ produced by hot gases in the stellar interior. push 2. The Sun will leave the main sequence in about 10 billion_ years from now. 3. While a star is on the main sequence, hydrogen_ is slowly consumed in the core and pressure_ builds up. 4. A temperature of at least 10 upper 8 K_ is needed to fuse helium. 5. At the end of its main-sequence lifetime, a star’s core starts to weaken_. Without nuclear burning to maintain it, the outward-pushing gas pressure weakens in the helium inner core; however, the inward pull of gravity does not. Once the pressure is relaxed-even a little-structural changes in the star become inevitable. As soon as hydrogen becomes substantially depleted, about 10 billion years after the star arrived on the main sequence, the helium core begins to contract. 6. The various stages of stellar evolution predicted by theory can be tested using observations of stars in Star clusters_. Star clusters provide excellent test sites for the theory of stellar evolution. Every star in a given cluster formed at the same time, from the same interstellar cloud, with virtually the same composition. 7. By the time the envelope of a red supergiant is ejected, the core has shrunk down to a diameter of about.0upper4inupper right corner of the 4_. 8. A typical white dwarf has the following properties: about half a solar mass, fairly white hot_ surface temperature, small size, and original_ luminosity. or solar luminosity. 9. As time goes by, the temperature and the luminosity of a white dwarf both rise_. 10. In a binary consisting of a white dwarf and a main-sequence or giant companion, matter leaving the companion forms an accretion_ disk around the dwarf. 11. A nova explosion is due to hdrogen_ fusion on the surface_ of a white dwarf. 12. When a proton and an electron are forced together, they combine to form a neutrons_ and a releasing neutrinos_. 13. A white dwarf_ supernova occurs when a white dwarf exceeds 1.4 solar masses. carbon-detonation supernova 14. The two types of supernova can be distinguished observationally by their spectra and by their light curves_. 15. As a star cluster ages, the luminosity of the main-sequence turnoff mass_. Chapter 13 Fill in the Blank 1. No neutron star remains after the explosion of a carbon-detonation_ supernova. 2. A typical neutron star is about 20_ km in diameter. 3. Neutron stars may be characterized as having rapid_ rotation rates and strong_ magnetic fields. 4. Pulsars were discovered through observations in the radio radiation _ part of the electromagnetic spectrum. 5. Observed pulsar periods range from few milliseconds_ to about a second_. (Give approximate numbers and units.) 6. The pulse period of pulsar radiation tells us the A small region_ of the neutron star emitting the radiation. lighthouse model The leading explanation for pulsars. A small region of the neutron star, near one of the magnetic poles, emits a steady stream of radiation which sweeps past Earth each time the star rotates. The period of the pulses is the star's rotation period. 7. All millisecond pulsars either are now, or once were, members of binary systems_ star systems. 8. X-ray bursters result when material from a binary companion accretes onto a neutron_ star. 9. Colliding neutron stars would be expected to release a lot of energy in the form of gravitational radiation_. 10. According to the general theory of relativity, space is warped, or curved, by some nearby massive object _. some nearby massive object. The greater the mass, the greater the warping. 11. Photons expend some_ energy as they escape from a gravitational field. 12. The region of extremely high density at the center of a black hole is called a the event horizon_. core 13. Matter accreting onto a stellar-mass black hole emits radiation in the form of X-rays_. the hole’s event horizon, matter falling into the hole emits radiation of its own accord. For a black hole of roughly solar mass, the energy is expected to be emitted in the form of X-rays. 14. Black holes are believed to have been discovered in several binary_ systems. solar system is a system of planets. 15. Scientists infer that the energy-emitting region in Cygnus X-1 must be very small because of the rapid variability_ of the radiation received. Chapter 14 Fill in the Blank 1. One difficulty in studying our Galaxy in its entirety is that the Sun lies within the Galactic disk_. 2. Herschel’s attempt to map the Milky Way by counting stars led to an estimate of the Galaxy’s size that was too flattened_. the Galaxy was a somewhat flattened, roughly disk-shaped collection of stars lying in the plane of the Milky Way Early in the twentieth century, some workers went so far as to estimate the dimensions of this “Galaxy” as about 10 kpc in diameter by 2 kpc thick. 3. The highly flattened, circular part of the Galaxy is called the Galactic disk_. 4. The roughly spherical region of faint old stars in which the rest of the Galaxy is embedded is the Galactic halo_. 5. The variables instabilities_ of Cepheids and RR Lyrae stars are observed to vary in a periodic fashion. Cepheid variable Star whose luminosity varies in a characteristic way, with a rapid rise in brightness followed by a slower decline. The period of a Cepheid variable star is related to its luminosity, so a determination of this period can be used to obtain an estimate of the star's distance. 6. Cepheid pulsation periods range from 1_ to 100_. different Cepheids can have very different pulsation periods, ranging from about 1 to 100 days. 7. Cepheids and RR Lyrae variables lie in a region of the H-R diagram called the instability strip_. They occur in post-main-sequence stars as they evolve through a region of the Hertzsprung-Russell diagram known as the instability strip 8. According to the period-luminosity relationship, the longer the pulsation period of a Cepheid, the high luminosities_ its luminosity. 9. Harlow Shapley used observations of RR Lyrae stars_ to determine distances to the globular clusters. 10. The Sun lies roughly 8_ pc from the Galactic center. 11. The orbital speed of the Sun around the Galactic center is 220_ km/s. 12. The direction of motion of halo objects at any particular location is they move in all directions_. 13. The original cloud of gas from which the Galaxy formed probably had a size and shape similar to those of the present Galactic halo_. 14. Rotational speeds in the outer part of the Galaxy are 50 kpc_ than would be expected on the basis of observed stars and gas, indicating the presence of dark matter_. 15. Spectroscopic observations of gas and stars near the Galactic center indicate that material there is orbiting at extremely fast_ speeds. Chapter 15 Fill in the Blank 1. Galaxies are classified by their type in the Hubble_ classification scheme. 2. Spiral galaxies that have tightly wrapped spiral arms tend to have largest_ central bulges. 3. Galaxies of type Sa galaxies_ have the least amount of gas; type Sc galaxies_ have the most. 4. An E5 galaxy is more circular in appearance_ than an E1 galaxy. 5. The Milky Way Galaxy, the Andromeda Galaxy, and about 20 others form a small cluster known as the Local Group _. Local Group, a new level of structure in the universe above the scale of our Galaxy. Its diameter is roughly 1 Mpc. 6. In the Tully-Fisher relation, a galaxy’s luminosity is found to be related to the measured apparent magnitude its distance_ of its spectral lines. 7. Galaxy clusters are themselves clustered, forming titanic agglomerations of matter known as superclusters_. 8. When galaxies collide, the star formation rate often decreases_. or merge starburst galaxy Galaxy in which a violent event, such as near-collision, has caused a sudden, intense burst of star formation in the recent past. 9. Galaxy mass determinations from rotation curves, line broadening, and binary galaxies all make use of the law of Newton’s laws_. 10. Intergalactic gas in galaxy clusters emits large amounts of energy in the form of X-ray radiation_. 11. More than 90 percent of the mass in galaxy clusters exists in the form of dark matter_. Hot gas. 12. Galaxies form by the process of repeated merging of smaller objects,_ (small objects accumulating into large); they do not form by a large cloud fragments_ (large objects breaking up into small). Contrast this with the process of star formation, in which a large cloud fragments into smaller pieces that eventually become stars. 13. Hubble’s law is a correlation between the redshifts and the recession velocities_ of galaxies. Hubble's law Law that relates the observed velocity of recession of a galaxy to its distance from us. The velocity of recession of a galaxy is directly proportional to its distance away. 14. Hubble’s constant is believed to lie in the range 50_ to 80_ km/s/Mpc. 15. The largest known structures in the universe, such as voids and the Great Wall, have sizes on the order of 100 Mpc across_. the Great Wall, measures at least 70 Mpc (out of the plane of the page) by 200 Mpc (across the page). It is one of the largest known structures in the universe. Chapter 16 Fill in the Blank 1. Active galaxies generally emit most of their radiation at longer_ wavelengths. 2. A Seyfert galaxy looks like a normal spiral, but with a very bright galactic nucleus_. The nucleus of a Seyfert is some 10,000 times brighter than the center of our Galaxy. In fact, the brightest Seyfert nuclei are 10 times more energetic than the entire Milky Way Galaxy. 3. In a core-halo radio galaxy, most of the radio radiation is emitted from the small central region known as the galactic nucleus_. 4. Lobe radio galaxies emit radio radiation from regions that are typically much more than 10 times_ in size than the visible galaxy. The radio lobes of lobe radio galaxies are truly enormous. From end to end, an entire lobe radio galaxy typically is more than 10 times the size of the Milky Way Galaxy, comparable in size to the entire Local Group. 5. Radio lobes are always found aligned with the center_ of the visible galaxy. 6. For all types of active galaxies, the original source of the tremendous energy emitted is the galactic nuclei_. Active Galactic Nucleus The leading theory for the energy source in active galactic nuclei 7. The energy source of an active galaxy is unusual in that a large amount of energy is emitted from a region much less than a parsec across_ in diameter. (Give size and unit.) 8. The mass of the black hole responsible for energy production in the active galaxy M87 is thought to be approximately equal to approximately 3 10 9_ solar masses. 9. The amount of mass that must be consumed by a supermassive black hole to provide the energy for an active galaxy is about 10 stars_ per year_. 10. Quasars are also known as quasi-stellar objects because of their unimpressive_ appearance at visible wavelengths. quasars now known is that their spectra all show large redshifts, ranging from 0.06 (that is, a 6 percent increase in wavelength) 11. Quasar spectra were understood when it was discovered that their radiation is hydrogen_ by an unexpectedly large amount. 12. The distance to a quasar in light-years is not simply equal to the time in years since the quasar emitted the light we see because of the expanding_ of the universe. 13. The fact that a typical quasar would consume an entire galaxy’s worth of mass in 10 billion years suggests that quasar lifetimes are relatively 10 billion years_. old 14. The image of a distant quasar can be split into several images by gravitational lensing, produced by a foreground lensing_ along the line of sight. 15. Quasar host galaxies are hard to see because they are so much fainter_ than the quasar itself. Chapter 17 Fill in the Blank 1. Deep surveys of the universe suggest that the largest structures in space are no larger than about 200_ Mpc across. 2. Homogeneous means "the same everywhere_." 3. Isotropic means "the same in all directions_." 4. Together, the assumptions of cosmic homogeneity and isotropy are known as the cosmological principle_. 5. If the universe had an edge, this would violate the assumption of homogeneity_. 6. If the universe had a center, this would violate the assumption of isotropy_. 7. A value of 65 km/s/Mpc for Hubble’s constant implies an age for the universe of roughly 15_ billion years. 8. Luminous matter makes up at most 95_ percent of the critical density. the resulting density is about the same, within a factor of two or three. Comparing this density with the critical value, we find that galaxy counts yield a value of of about 0.01. currently do not know what the dark matter is, but we do know that it is there. Galaxies may contain as much as 10 times more dark matter than luminous material, and the figure for galaxy clusters is even higher-perhaps as much as 95 percent of the total mass in clusters is invisible. 9. The surface of a sphere is a two-dimensional example of a closed_ universe. 10. The present average temperature of the universe, as measured by the cosmic microwave background, is 3_ K. 11. Comparing the mass density of radiation and matter, we find that, at the present time, matter_ dominates. 12. When the universe was a few minutes old, nuclear fusion produced protons and Neutrons_ As the temperature of the universe fell below about 900 million K, roughly 2 minutes after the Big Bang, conditions became favorable for protons and neutrons to fuse to form deuterium.2 Prior to that time, deuterium nuclei were broken apart by high-energy gamma rays as quickly as they were created. Once the deuterium was at last able to survive the radiation background, other fusion reactions quickly converted it into heavier elements, especially helium-4. In just a few minutes, most of the available neutrons were consumed, leaving a universe whose "normal" matter content was primarily hydrogen and helium. 13. Elements heavier than these were not formed primordially because the density and temperature of the universe became too dense after helium formed. This early burst of fusion did not last long. Conditions in the cooling, thinning universe became less and less conducive to further fusion reactions as time went by. For all practical purposes, primordial nucleosynthesis stopped at helium-4. By about 15 minutes after the Big Bang, the cosmic elemental abundance was set, and helium accounted for approximately one-quarter of the total mass of matter in the universe. The remaining 75 percent of the matter in the universe was hydrogen. It would be almost a billion years before nuclear reactions in stars would change these figures. 14. If the inflation model is correct, then the density of the universe is flat_ (greater than, equal to, less than) the critical density. 15. Theory predicted tiny inhomogeneities in the cosmic microwave background _ of the microwave background; the COBE __ satellite found them. As a result, dark-matter models predict that there should be tiny "ripples" in the microwave background-temperature variations of only a few parts per million from place to place on the sky. Chapter 18 Fill in the Blank 1. Amino acids are the building blocks of Organic molecules _ amino acids Organic molecules which form the basis for building the proteins that direct metabolism in living creatures.complex molecules known as amino acids and nucleotide bases-organic (carbon-based) molecules that are the building blocks of life as we know it. 2. The naturally occurring molecules present on the young Earth included water, carbon dioxide, methane_, and ammonia_. 3. Two sources of energy for chemical reactions on the young Earth were ammonia_ and carbon dioxide_. The surface of the young Earth was a very violent place. Natural radioactivity, lightning, volcanism, solar ultraviolet radiation and meteoritic impacts all provided large amounts of energy that eventually shaped the ammonia, methane, carbon dioxide and water into more complex molecules known as amino acids and nucleotide bases-organic (carbon-based) molecules that are the building blocks of life as we know it. 4. The fossil record clearly shows evidence of life dating back 3.5 billion_ years. 5. Multicellular organisms did not appear on Earth until about one billion_ years ago. 6. The Murchison meteorite was discovered to contain relatively large amounts of amino acids_. 7. The Drake equation estimates the number of star systems _ in the Milky Way Galaxy. 8. Planets in binary-star systems are not considered habitable because the planetary orbits are usually unstable_. 9. The probabilities of the development of life and intelligence on Earth are extremely unlikely_ if random chance is the only evolutionary factor involved. 10. Direct contact between extraterrestrial lifeforms may be impractical because of the large distance_ between civilizations. 11. Radio communication over interstellar distances is practical because the signals travel at the speed of light_. 12. Radio waves can travel throughout the Galaxy because they are not blocked by interstellar dust_. dust interstellar space. light and other short-wavelength radiation are heavily scattered while moving through dusty interstellar space, radiation. 13. Radio waves leaking away from Earth have now traveled a distance of 65_ light-years. 14. Radio wavelengths between 18 cm and 21 cm are referred to as the water hole_. 15. A two-way communication with another civilization at a distance of 100 light-years will require 200_ years. 100 years for the message to reach the planet and another 100 years for the reply to travel back to us. I have enclosed $299.00 for the following Distance Education Course that I would like to enroll into. Apple Math Algebra Problems? How many seeds are in apples? Which apple had more seeds than the other apple? In apple one 3 apple seeds were found in the core of the apple. In apple two 9 apple seeds were found? Apple two had more apple seeds than apple one. 9 - 3 = 6 Apple two had 6 more apple seeds than apple one. 3 + 9 = 12 total apple seeds were found. Mars Exploration Rover-2003 Mission Participation Certificate
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Artist conception of Mars 2003 Lander.

Mars Exploration Rover-2003 Mission

Participation Certificate

Presented to

Robert Michael Alberg

On November 12, 2002

Thank you for joining us on this mission of exploration and discovery. A compact disc bearing your name will be included in one of the next Mars Exploration Rover-2003 missions that will explore the planet's surface in search of geologic evidence of water in Mars' past.

Together, we will journey into space to discover and understand the many wonders of our universe.

Dr. Edward J. Weiler
Associate Administrator
Office of Space Science

Certificate No. 3140044

Image revised June 19, 2002

Astronaut Bio: K. Bowersox 3/02
National Aeronautics and Space Administration

Lyndon B. Johnson Space Center
Houston, Texas 77058

Biographical Data

NAME: Kenneth D. Bowersox (Captain, USN)
NASA Astronaut

PERSONAL DATA: Born November 14, 1956, in Portsmouth, Virginia, but c onsiders Bedford, Indiana, to be his hometown.

EDUCATION: Graduated from Bedford High School, Bedford, Indiana, in 1974; received a bachelor of science degree in aerospace engineering from the United States Naval Academy in 1978, and a master of science degree in mechanical engineering from Columbia University in 1979.

The Space Shuttle Clickable Map: Crew Cabin
The Space Shuttle

Crew Cabin

The two-to eight person crew occupies a two-level cabin at the forward end of the orbiter. They operate the vehicle from the upper level, the flight deck. The flight controls for the mission commander and pilot are at the front. A station at the rear, overlooking the payload bay through two windows, containing the controls a mission specialist astronaut uses to operate the Remote Manipulator System arm which handles some items in the payload. Mission operations displays and controls are on the right side of the cabin, and payload controls on the left. The latter are operated by payload specialists, who are usually not career astronauts. The living, eating and sleeping area for off-duty crew members, called the middeck, is located below the flight deck. It contains pre-packaged food, a toilet, bunks, and other amenities.

The daily routine for crew members aboard flights will vary according to crew assignments but each member will follow a detailed schedule each day. Time is allotted for each person for sleep, personal hygiene, work, meal preparation, and eating as well as routine Orbiter subsystem housekeeping. Housekeeping duties include cleaning the waste compartment, dumping excess water, replacing the carbon dioxide scrubbing canisters, purging the fuel cells, giving daily status reports to the ground controllers, and aligning the inertial measurement unit (the device that directs the vehicle attitude in space). A 24-hour time period is normally divided into an 8-hour sleep period and a 16-hour awake period for each crew member.

Spacelab: Science in Orbit

Periodically the Shuttle is scheduled to carry a complete scientific laboratory called "Spacelab" into Earth orbit. Two complete Spacelabs (plus instrument-carrying platforms exposed to space, called "pallets") have been built by the European Space Agency (ESA), which paid for the development expense and manufacturing costs of the first one. NASA purchased the second unit. A Spacelab is similar to a small but well-equipped laboratory on Earth, but has been designed for zero-gravity operation. It provides a shirt-sleeve environment where up to four people, who eat and sleep in the orbiter, can perform scientific tests utilizing the high vacuum and microgravity of orbital space, and make observations above the abscurring atmosphere.

Spacelab payload specialists are men and women of many nations, experts in their fields, who must be in reasonably good health. They are required to have only a few weeks of spaceflight training, may have spent years preparing to perform their experiments in orbit.

Currently, most of the experiments on a given Spacelab mission are devoted to a single broad field, such as medicine, manufacturing, astronomy, space physics or pharmaceuticals. Some previous Spacelab flights combine experiments from several fields. One mission utilized an all-pallet configuration, where all the instruments were exposed to space and operated from inside the orbiter.

Astronauts on 
Astronauts adjust to the microgravity environment aboard the Space Shuttle, in this photo taken from the mid-deck.

Spaceflight is no longer limited to intensively trained, physically perfect astronauts. Experienced scientists and technicians can fly in support of their payloads. Crew members experience a designed maximum gravity load of 3g during launch, and less than 1.5g during re-entry. These accelerations are about one-third the levels experienced on previous manned flights. Many other features of the Space Shuttle, such as a standard sea-level atmosphere, make spaceflight more comfortable for the astronauts.

The Johnson Space Center in Houston, Texas maintains a site of Astronaut Biographies. Pineapple Drink (B)

MEAL Split Grilled Pork Chop (T) Macaroni Cheese (R) Creamed Spinach (R) Applesauce sans-serif" SIZE="-2">Tea (B) MEAL 3 Pike Perch in Baltica Sauce (T) Chicken w/ Rice (T) Borodinsky Bread (IM) Visit Cracker (NF) Prunes w/ Nuts (IM) Green Tea w/o Sugar (B) Vitamins MEAL 4 Sweet Almonds (NF)Kuraga (IM) Apple-Black Curr Jce (R) computer, internet access, CD-ROM drive, Adobe Acrobat and RealPlayer. (Free downloads available.) You will be required to listen to lectures via audio streaming using RealPlayer. (Almost any computer built within the last five years should be capable.) Course Description Astronomy 105 is an introductory survey of astronomy, including the history of astronomy, an overview of the solar system, an introduction to the lives of stars, plus an overview of galaxies and the formation and evolution of the universe as a whole. Learning Outcomes By the end of the course, the successful student should be able to: Describe the motions of the Earth, Sun, Moon, and planets, and explain how these motions generate observable phenomenon such as the seasons, moon phases and eclipses. Describe where they are in the universe. Relate the history of astronomy including the contributions of several key historical figures. Explain the scientific method. Demonstrate an understanding of light and how the information contained in light can be deciphered by astronomers. Identify the main types of telescopes, their uses and limitations. Show how the nebular theory of the formation of the solar system accounts for the observed properties of the solar system Describe, in comparative terms, the main features of the solar system, including planets, moons, comets and asteroids Summarize the overall properties of the Sun and the processes that occur within it. Describe the life cycle of stars from birth through death. Describe the different types and features of galaxies. Relate how the Big Bang Theory explains the observed features of our universe. Describe the considerations that go into trying to determine the possibility of communicating with other technologically advanced civilizations within the galaxy. Testing and Grading Your grade will be based on a professional evaluation of your performance in four areas as follows: