AST1002 Exam 3 Review

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AST1002 Exam 3 Review
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  1. What is the approximate chemical composition (by mass) with which all stars are born?
    three quarters hydrogen, one quarter helium, no more than 2% heavier elements
  2. The total amount of power (in watts, for example) that a star radiates into space is called its _____________.
    luminosity
  3. According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
    Its apparent brightness will decrease by a factor of 9.
  4. Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
    Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.
  5. If star A is closer to us than star B, then Star A's parallax angle is _________.
    larger than that of Star B
  6. Ten parsecs is about _________.
    32.6 light-years
  7. Star A has an apparent magnitude of 3 and star B has an apparent magnitude of 5. Which star is brighter in our sky?
    Star A
  8. From hottest to coolest, the order of the spectral types of stars is _________.
    OBAFGKM
  9. Our Sun is a star of spectral type _________.
    G
  10. Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?
    The star is a member of a binary star system.
  11. Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?
    spectroscopic binary
  12. The axes on a Hertzsprung-Russell (H-R) diagram represent _________.
    luminosity and surface temperature
  13. What can we infer, at least roughly, from a star's luminosity class?
    its size (radius)
  14. On an H-R diagram, stellar radii _________.
    increase diagonally from the lower left to the upper right
  15. On an H-R diagram, stellar masses _________.
    can be determined for main sequence stars but not for other types of stars
  16. How is the lifetime of a star related to its mass?
    More massive stars live much shorter lives than less massive stars.
  17. Each choice below lists a spectral type and luminosity class for a star. Which one is a red supergiant?
    Spectral type M2, luminosity class I
  18. What is the common trait of all main sequence stars?
    They generate energy through hydrogen fusion in their core.
  19. Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
    Earth would be inside the supergiant.
  20. What is a white dwarf?
    the remains of a star that ran out of fuel for nuclear fusion
  21. Which of the following statements comparing open and globular star clusters is not true?
    Open and globular clusters each typically contain a few hundred stars.
  22. What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?
    It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.
  23. All stars are born with the same basic composition, yet stars can look quite different from one another. Which two factors primarily determine the characteristics of a star?
    its mass and its stage of life
  24. Based on the definition of apparent brightness, which units are appropriate for its measurement?
    watts per square meter
  25. Star A is identical to Star B, except that Star A is twice as far from us as Star B. Therefore:
    Both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A.
  26. A star with a parallax angle of 1/20 arcsecond is _________.
    20 parsecs away
  27. The star Vega has an absolute magnitude of about 4 and an apparent magnitude of about 0. Based on the definitions of absolute and apparent magnitude, we can conclude that ______.
    Vega is nearer than 10 parsecs from Earth.
  28. Which of the following statements about spectral types of stars is not generally true?
    The spectral type of a star can be used to determine its distance.
  29. Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?
    Rigel has a higher surface temperature than Sirius.
  30. To calculate the masses of stars in a binary system, we must measure their _________.
    orbital period and average orbital distance
  31. Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours. The most likely explanation is that _________.
    the star is a member of an eclipsing binary star system
  32. You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star's _________.
    mass
  33. The approximate main-sequence lifetime of a star of spectral type O is _________.
    3 million years
  34. How did astronomers discover the relationship between spectral type and mass for main sequence stars?
    By measuring the masses and spectral types of main-sequence stars in binary systems.
  35. The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?
    The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.
  36. The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is most likely to be located in the halo of our galaxy?
    The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.
  37. The interstellar clouds called molecular clouds are _________.
    the cool clouds in which stars form
  38. What is a protostar?
    a star that is still in the process of forming
  39. Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.
    thermal pressure
  40. Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
    infrared
  41. What kind of gas cloud is most likely to give birth to stars?
    a cold, dense gas cloud
  42. Which of the following phenomena is not commonly associated with the star formation process?
    intense ultraviolet radiation coming from a protostar
  43. When does a protostar become a main-sequence star?
    when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space
  44. Approximately what core temperature is required before hydrogen fusion can begin in a star?
    10 million K
  45. Which star spends the longest time in the protostellar phase of life?
    a 1 solar mass star
  46. What is the approximate range of masses that newborn main sequence stars can have?
    0.1 to 150 solar masses
  47. The vast majority of stars in a newly formed star cluster are _________.
    less massive than the Sun
  48. Which of the following statements about brown dwarfs is not true?
    Brown dwarfs eventually collapse to become white dwarfs.
  49. What can we learn about a star from a life track on an H-R diagram?
    the surface temperature and luminosity the star will have at each stage of its life
  50. Which of the following lists the stages of life for a low-mass star in the correct order?
    protostar, main-sequence star, red giant, planetary nebula, white dwarf
  51. What happens when a main-sequence star exhausts its core hydrogen fuel supply?
    The core shrinks while the rest of the star expands.
  52. The main source of energy for a star as it grows in size to become a red giant is _________.
    hydrogen fusion in a shell surrounding the central core
  53. The overall helium fusion reaction is:
    Three helium nuclei fuse to form one carbon nucleus.
  54. What is a helium flash?
    The sudden onset of helium fusion in the core of a low-mass star
  55. What is a planetary nebula?
    Gas ejected from a low-mass star in the final stage of its life
  56. The ultimate fate of our Sun is to _________.
    become a white dwarf that will slowly cool with time
  57. What is the CNO cycle?
    a set of steps by which four hydrogen nuclei fuse into one helium nucleus
  58. In order to predict whether a star will eventually fuse oxygen into a heavier element, you mainly want to know what fact about the star?
    its mass
  59. Why is iron significant to understanding how a supernova occurs?
    Iron cannot release energy either by fission or fusion.
  60. After a supernova explosion, the remains of the stellar core _________.
    may be either a neutron star or a black hole
  61. Why is Supernova 1987A particularly important to astronomers?
    It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.
  62. Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
    The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.
  63. Where does gold (the element) come from?
    it is produced during the supernova explosions of high-mass stars
  64. Which two processes can generate energy to help a star maintain its internal thermal pressure?
    nuclear fusion and gravitational contraction
  65. Our Sun is considered to be a _________.
    low-mass star
  66. Which process is required to allow a gravitationally-collapsing gas cloud to continue to collapse?
    The cloud must radiate much of its thermal energy.
  67. Angular momentum plays an important role in star formation. Which of the following characteristics of a protostellar system is probably not strongly affected by the star's angular momentum?
    the onset of core hydrogen fusion
  68. Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes?
    A main-sequence star is hotter and dimmer than it was as a protostar.
  69. Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?
    A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.
  70. We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?
    Brown dwarfs would outnumber all ordinary stars.
  71. Where would a brown dwarf be located on an H-R diagram?
    below and to the right of the lowest part of the main sequence
  72. Which of the following types of data provide evidence that helps us understand the life tracks of low-mass stars?
    H-R diagrams of globular clusters
  73. During which stage is the star's energy supplied by primarily by gravitational contraction?
    ii
  74. During which stage does the star have an inert (non-burning) helium core?
    iv
  75. Which stage lasts the longest?
    iii
  76. During which stage does the star have an inert (non-burning) carbon core surrounded by shells of helium and hydrogen burning?
    viii
  77. What will happen to the star after stage viii?
    Its outer layers will be ejected as a planetary nebula and its core will become a white dwarf.
  78. Why is a 1 solar-mass red giant more luminous than a 1 solar-mass main sequence star?
    Fusion reactions are producing energy at a greater rate in the red giant.
  79. Carbon fusion occur in high-mass stars but not in low-mass stars because _________.
    the cores of low-mass stars never get hot enough for carbon fusion
  80. Which of the following statements about various stages of core nuclear burning (hydrogen, helium, carbon, and so on) in a high-mass star is not true?
    • Each successive stage lasts for approximately the same amount of time.
    • Which event marks the beginning of a supernova?
    • The sudden collapse of an iron core into a compact ball of neutrons
  81. Suppose that the star Betelgeuse (the upper left shoulder of Orion) were to supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?
    Betelgeuse would remain a dot of light, but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime.
  82. Observations show that elements with atomic mass numbers divisible by 4 (such as oxygen-16, neon-20, and magnesium-24) tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case?
    At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions.
  83. A spinning neutron star has been observed at the center of a _________.
    supernova remnant
  84. Which is more common: a star blows up as a supernova, or a star forms a planetary nebula/white dwarf system?
    Planetary nebula formation is more common.
  85. You discover a binary star system in which one star is a 15 MSun main-sequence star and the other is a 10 MSun giant. How do we think that a star system such as this might have come to exist?
    The giant must once have been the more massive star, but is now less massive because it transferred some of its mass to its companion.
  86. A white dwarf is _________.
    what most stars become when they die
  87. A typical white dwarf is _________.
    as massive as the Sun but only about as large in size as Earth
  88. If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh _________.
    as much as a truck
  89. The maximum mass of a white dwarf is _________.
    about 1.4 times the mass of our Sun
  90. What is an accretion disk?
    a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
  91. According to our modern understanding, what is a nova?
    an explosion on the surface of a white dwarf in a close binary system
  92. Suppose that a white dwarf is gaining mass through accretion in a binary system. What happens if the mass someday reaches the 1.4 solar mass limit?
    The white dwarf will explode completely as a white dwarf supernova.
  93. A neutron star is _________.
    the remains of a star that died in a massive star supernova (if no black hole was created)
  94. A typical neutron star is more massive than our Sun and about the size (radius) of _________.
    a small asteroid (10 km in diameter)
  95. If you had something the size of a sugar cube that was made of neutron star matter, it would weigh _________.
    about as much as a large mountain
  96. Pulsars are thought to be _________.
    rapidly rotating neutron stars
  97. How is an X-ray burst (in an X-ray binary system) similar to a nova?
    Both involve explosions on the surface of stellar corpse.
  98. What is the basic definition of a black hole?
    an object with gravity so strong that not even light can escape
  99. Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly _________.
    3 solar masses
  100. What do we mean by the event horizon of a black hole?
    It is the point beyond which neither light nor anything else can escape.
  101. Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). What would happen to Earth in its orbit?
    Nothing - Earth's orbit would remain the same.
  102. What do we mean by the singularity of a black hole?
    It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
  103. What makes us think that the star system Cygnus X-1 contains a black hole?
    It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.
  104. The Schwarzschild radius of a black hole depends on ________.
    only the mass of the black hole
  105. Based on current evidence, which of the following statements about gamma ray bursts is true?
    All those that we have detected occurred in distant galaxies.
  106. Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true?
    Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars.
  107. Which of the following statements about degeneracy pressure is not true?
    Degeneracy pressure can arise only from interactions among electrons.
  108. The more massive a white dwarf, the _________.
    smaller its radius
  109. Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit?
    Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4-solar-mass limit.
  110. The white dwarf that remains when our Sun dies will be mostly made of ______.
    carbon
  111. Which statement about accretion disks is not true?
    The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.
  112. According to present understanding, a nova is caused by _________.
    hydrogen fusion on the surface of a white dwarf
  113. Which of the following is not true about differences between novae and supernovae?
    Supernovae eject gas into space but novae do not.
  114. Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?
    No, because it is not orbited by another star.
  115. Which of the following best describes what would happen if a 1.5-solar-mass neutron star, with a diameter of a few kilometers, were suddenly (for unexplained reasons) to appear in your home town?
    • The entire Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star.
    • Each Voyager spacecraft carries a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why did the scientists choose pulsars rather than some other type of star?
    • Pulsars are easy to identify by their almost perfectly steady periods of pulsation.
  116. Which statement about pulsars is not thought to be true?
    Pulsars can form only in close binary systems.
  117. How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?
    The accretion disk around a neutron star is much hotter and emits higher-energy radiation.
  118. Which statement concerning black hole masses and Schwarzschild radii is not true?
    In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.
  119. Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?
    Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.
  120. Which of statement below about black holes is not true?
    A spaceship passing near a 10 solar mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10 solar mass main-sequence star.
  121. When we see X rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would rule out the possibility of a black hole?
    intense X-ray bursts
  122. Which of the following observatories is most likely to discover a black hole in a binary system?
    • the Chandra X-Ray Observatory
    • Which of the following statements about gamma ray bursts is not true?
    • The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect.
  123. Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion disk. Sometime within the next million years or so, the civilization's planet is likely to be doomed because ________.
    the red giant will probably supernova within the next million years
  124. Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) One foolhardy day, a daring individual in their space force (let's call him Major Tom) decides to become the first of his species to cross the event horizon of the black hole. To add to the drama, he decides to go in wearing only a thin space suit, which offers no shielding against radiation, no cushioning against any forces, and so on. Which of the following is most likely to kill him first (or at least to start the process of killing him first)?
    X rays from the accretion disk
  125. Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) Through a bizarre (and scientifically unexplainable) fluctuation in the space-time continuum, a copy of a book from that civilization arrives on your desk; it is entitled Iguoonos: How We Evolved. In the first chapter, you learn that these beings evolved from organisms that lived 5 billion years ago. Which of the following statements should you expect to find as you continue to read this book?
    They evolved on a different planet in a different star system, and moved to their current location.
  126. How does the diameter of the disk of Milky Way Galaxy compare to its thickness?
    The diameter is about 100 times as great as the thickness.
  127. What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?
    the galaxy's bulge
  128. The Sun's location in the Milky Way Galaxy is _________.
    in the galactic disk, roughly halfway between the center and the outer edge of the disk
  129. What do we mean by the interstellar medium?
    the gas and dust that lies in between the stars in the Milky Way galaxy
  130. What are the Magellanic Clouds?
    two small galaxies that orbit the Milky Way Galaxy
  131. How do disk stars orbit the center of the galaxy?
    They all orbit in roughly the same plane and in the same direction.
  132. How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?
    by applying Newton's version of Kepler's third law (or the equivalent orbital velocity law) to the Sun's orbit around the center of the Galaxy
  133. Fill in the blank: Elements heavier than hydrogen and helium constitute about _________ of the mass of the interstellar medium.
    2%
  134. What do we mean by the star-gas-star cycle?
    It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium.
  135. What are cosmic rays?
    subatomic particles that travel close the speed of light
  136. The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with _________.
    radio telescopes observing at a wavelength of 21 centimeters
  137. Which of the following molecules is the most abundant in molecular clouds?
    H2
  138. Interstellar dust consists mostly of _________.
    microscopic particles of carbon and silicon
  139. Which of the following models best explains why our galaxy has spiral arms?
    The spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy.
  140. What do we mean by a protogalactic cloud?
    a cloud of hydrogen and helium that contracts to become a galaxy
  141. Most stars in the Milky Way's halo are _________.
    very old
  142. What is an ionization nebula?
    a colorful cloud of gas that glows because it is heated by light from nearby hot stars
  143. What do halo stars do differently from disk stars?
    They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.
  144. Where does most star formation occur in the Milky Way Galaxy?
    in the spiral arms
  145. Based on observations, which of the following statements about stars in the Milky Way is generally true?
    The older the star, the lower its abundance of heavy elements.
  146. What kind of object do we think lies in the center of the Milky Way Galaxy?
    a 3 to 4 million solar mass black hole
  147. If we could see our own galaxy from 2 million light-years away, it would appear _________.
    as a flattened disk with a central bulge and spiral arms
  148. How does the interstellar medium affect our view of most of the galaxy?
    It prevents us from seeing most of the galactic disk with visible and ultraviolet light.
  149. About what percentage of the mass of a molecular cloud is in the form of dust?
    1%
  150. How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?
    It should have a higher fraction of elements heavier than hydrogen and helium.
  151. Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?
    The 10% figure is too high because there are not enough heavy elements to make that much dust.
  152. The most common form of gas in the disk of the Milky Way galaxy is _________.
    atomic hydrogen gas
  153. How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?
    The total amount of gas will be much less than it is today.
  154. Over time, the star-gas-star cycle leads the gas in the Milky Way to _________.
    have a greater abundance of heavy elements
  155. Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?
    the Chandra X-ray Observatory
  156. If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?
    Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.
  157. All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________.
    globular clusters
  158. Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found _________.
    only in or near star-forming clouds
  159. Which of the following statements comparing halo stars to our Sun is not true?
    Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.
  160. Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?
    It is probably a halo star that is currently passing through the disk.
  161. What evidence suggests that most of the mass of the Milky Way is in the form of dark matter?
    The orbital speeds of stars far from the galactic center are surprisingly high.
  162. Spiral arms appear bright because ________.
    they contain more hot young stars than other parts of the disk
  163. How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
    The stars that formed first could orbit the center of the galaxy in any direction at any inclination.
  164. If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?
    Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.
  165. What is the best evidence for an extremely massive black hole in the center of the Milky Way?
    The orbits of stars in the center of the galaxy indicate that the presence of 3 to 4 million solar mass object in a region no larger than our Solar System.
  166. Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?
    It is by far the brightest source of visible light lying in the direction of the galactic center.
  167. Based on the number of galaxies visible in the Hubble Deep Field (Figure 15.1 in your textbook), the estimated number of galaxies in our observable universe is about ______.
    100 billion
  168. Which of the following is not one of the three major categories of galaxies?
    globular galaxies
  169. A standard candle is _________.
    a light source of known luminosity
  170. What is main-sequence fitting?
    a method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram
  171. What is a Cepheid variable?
    A type of very luminous star that makes an excellent standard candle
  172. What two observable properties of a Cepheid variable are directly related to one another?
    the period between its peaks of brightness and its luminosity
  173. What does Hubble's law tell us?
    The more distant a galaxy, the faster it is moving away from us.
  174. Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies?
    They are rare events, so we have observed them in only a tiny fraction of all galaxies.
  175. When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent?
    The inside of the balloon does not represent any part of our universe.
  176. If we say that a galaxy has a lookback time of 1 billion years, we mean that _________.
    its light traveled through space for 1 billion years to reach us
  177. Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called _________.
    the cosmological horizon
  178. Cosmological redshift is the result of ______.
    the expansion of the universe
  179. Current estimates place the age of the universe at about _________.
    14 billion years
  180. Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______.
    infrared light
  181. Which of the following is an important starting assumption in models of galaxy formation?
    Some regions in the universe start out denser than others.
  182. Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ______.
    may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies
  183. Collisions between galaxies typically unfold over a period of _________.
    hundreds of millions of years
  184. Why are collisions between galaxies more likely than collisions between stars within a galaxy?
    Relative to their sizes, galaxies are closer together than stars.
  185. Which of the following features is not a feature of central dominant galaxies?
    They are often spiral galaxies.
  186. Why should galaxy collisions have been more common in the past than they are today?
    Galaxies were closer together in the past because the universe was smaller.
  187. The distinguishing feature of a starburst galaxy is _________.
    a rate of star formation that may be 100 or more times greater than that in the Milky Way
  188. The unusually bright centers found in some galaxies are called _________.
    active galactic nuclei
  189. According to current understanding, what is a quasar?
    an active galactic nucleus that is particularly bright
  190. Which of the following phenomena is probably not related to the presence of a supermassive black hole?
    the presence of globular clusters in the halos of galaxies
  191. The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is roughly ______.
    1 billion solar masses
  192. According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______.
    light emitted by hot gas in an accretion disk that swirls around the black hole
  193. In a photo like the Hubble Deep Field (Figure 15.1 in your textbook), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?
    the galaxies that are farthest away
  194. The most basic difference between elliptical galaxies and spiral galaxies is that _________.
    elliptical galaxies lack anything resembling the disk of a spiral galaxy
  195. Hubble's galaxy classification diagram (the "tuning fork") ______.
    relates galaxies according to their shapes, but not according to any evolutionary status
  196. Using the technique of main-sequence fitting to determine the distance to a star cluster requires that _____.
    we have telescopes powerful enough to allow us to identify the spectral types of main-sequence stars of many masses in the cluster
  197. Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation?
    We can learn the distance to the galaxy.
  198. In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thus putting to rest the idea that it might have been a cloud within our own galaxy. How was he able to prove this?
    By observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation
  199. Suppose that Hubble's constant were 20 kilometers per second per million light-years. How fast would we expect a galaxy 100 million light-years away to be moving? (Assume the motion is due only to Hubble's law.)
    away from us at 2,000 km/s
  200. Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
    White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.
  201. Does Hubble's law work well for galaxies in the Local Group? Why or why not?
    No, because galaxies in the Local Group are gravitationally bound together.
  202. What is the best way to determine a galaxy's redshift?
    Take a spectrum of the galaxy, and measure the difference in wavelength of spectral lines from the wavelengths of those same lines as measured in the laboratory.
  203. Which statement below correctly describes the relationship between expansion rate and age for the universe?
    The faster the rate of expansion, the younger the age of the universe.
  204. What does cosmological redshift do to light?
    stretches its wavelength
  205. Why can't we see past the cosmological horizon?
    Beyond the cosmological horizon, we would be looking back to a time before the universe was born.
  206. We can study how galaxies evolve because _________.
    the farther away we look, the further back in time we see
  207. Which of the following statements is not an assumption used in models of galaxy formation?
    Gas contracted to form the disks of galaxies before any stars were born.
  208. One possible explanation for a galaxy's type invokes the angular momentum of the protogalactic cloud from which it formed. Suppose a galaxy forms from a protogalactic cloud with a lot of angular momentum. Assuming its type has not changed due to other interactions, we'd expect this galaxy to be _________.
    a spiral galaxy
  209. Which of the following phenomena are not thought to be results of collisions or other interactions between galaxies?
    The fact that spiral galaxies have both disk and halo components
  210. Interactions among galaxies also are thought to influence a galaxy's type in at least some cases. Which of the following does not support the idea that interactions can shape galaxies?
    The fact that more distant galaxies have larger redshifts.
  211. Which characteristic is not generally true of a starburst galaxy?
    The observed features of the starburst are thought to be caused by the presence of a supermassive black hole in the galaxy's center.
  212. A quasar's spectrum is hugely redshifted. What does this large redshift tells us about the quasar?
    the distance to the quasar
  213. What evidence tells us that quasars are the centers of distant galaxies?
    Images and spectra show quasars to be embedded at the centers of distant galaxies.
  214. Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?
    Active galactic nuclei tend to become less active as they age.
  215. Suppose we observe a source of X rays that varies substantially in brightness over a period of a few days. What can we conclude?
    The X-ray source is no more than a few light-days in diameter.
  216. All of the following observations are real. Which one does not support the theory that active galactic nuclei are powered by accretion disks around massive black holes?
    The most luminous active galactic nuclei have huge redshifts.
  217. According to the theory that active galactic nuclei are powered by supermassive black holes, the energy released as light comes from _____.
    gravitational potential energy released by matter that is falling toward the black hole
  218. The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that:
    Galaxy formation and supermassive black hole formation must be related somehow.
  219. Which of the following best summarizes what we mean by dark matter?
    matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
  220. Which of the following best summarizes what we mean by dark energy?
    It is a name given to whatever is causing the expansion of the universe to accelerate with time.
  221. The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that _________.
    dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way
  222. What is a rotation curve?
    a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
  223. What is the primary way in which we determine the mass distribution of a spiral galaxy?
    We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.
  224. What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?
    Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
  225. Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?
    Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.
  226. Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?
    measuring the temperatures of stars in the halos of the galaxies
  227. When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
    It bends or distorts the light coming from galaxies located behind it.
  228. Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
    Dark matter is the dominant form of mass in both clusters and in individual galaxies.
  229. What is the distinguishing characteristic of what we call ordinary matter (such as the matter that makes up stars, planets, and people)?
    It consists of atoms or ions with nuclei made from protons and neutrons.
  230. Some dark matter may be ordinary matter that orbits in the halo of the galaxy. Which of the following would not be considered a type of ordinary dark matter?
    globular clusters in the halo of the galaxy
  231. What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
    They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
  232. Which of the following best sums up current scientific thinking about the nature of dark matter?
    Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.
  233. When we speak of the large-scale structure of the universe, we mean _________.
    the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe
  234. The critical density of the universe is the _______.
    average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist
  235. What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?
    Observations of white dwarf supernovae
  236. Which of the following best sums up current scientific thinking about the nature of dark energy?
    Dark energy probably exists, but we have little (if any) idea what it is.
  237. Why do we call dark matter "dark"?
    It emits no radiation that we have been able to detect.
  238. Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?
    There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
  239. Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?
    The orbital speeds would fall off sharply with increasing distance from the galactic center.
  240. The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?
    Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.
  241. It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?
    Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
  242. How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
    We detect this gas with X-ray telescopes.
  243. Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
    The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.
  244. How does gravitational lensing tell us about the mass of a galaxy cluster?
    Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.
  245. What evidence tells us that at least some dark matter consists of objects made of ordinary matter that orbit in the halo of the galaxy?
    We have detected gravitational lensing of distant objects that appears to be caused by unseen objects in the halo of our galaxy
  246. If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
    They travel at speeds close to the speed of light.
  247. Is space expanding within clusters of galaxies?
    No, because their gravity is strong enough to hold them together even while the universe as a whole expands.
  248. Which of the following statements about large-scale structure is probably not true?
    Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.
  249. Based on current evidence, a supercluster is most likely to have formed in regions of space where _________.
    the density of dark matter was slightly higher than average when the universe was very young
  250. Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?
    The actual density, even with dark matter included, is less than about a third of the critical density.
  251. Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
    We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
  252. Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be the case, which of the following would have to be true (based on current understanding)?
    Dark energy does not exist and there is much more dark matter than we are aware of to date.

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