Mastering Astronomy Chapter 21 – Flashcards
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Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______.
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infrared light
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Which of the following is an important starting assumption in models of galaxy formation?
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Some regions in the universe start out denser than others.
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According to observations, what type of galaxy was much more common when the universe was 2 billion years old than it is today?
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Irregular galaxies.
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Collisions between galaxies typically unfold over a period of ______.
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hundreds of millions of years
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Why are collisions between galaxies more likely than collisions between stars within a galaxy?
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Relative to their sizes, galaxies are closer together than stars.
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Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ______.
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may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies
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Which of the following features is not a feature of central dominant galaxies?
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They are often spiral galaxies.
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The distinguishing feature of a starburst galaxy is _________.
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a rate of star formation that may be 100 or more times greater than that in the Milky Way
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The unusually bright centers found in some galaxies are called ______.
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active galactic nuclei
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According to current understanding, what is a quasar?
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an active galactic nucleus that is particularly bright
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Which of the following phenomena is probably not related to the presence of a supermassive black hole?
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the presence of globular clusters in the halos of galaxies.
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The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is roughly ______.
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1 billion solar masses
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According to the model in which active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______.
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light emitted by hot gas in an accretion disk that swirls around the black hole
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According to the model in which that active galactic nuclei are powered by supermassive black holes, the energy released as light comes from _____.
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gravitational potential energy released by matter that is falling toward the black hole
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Intergalactic hydrogen clouds are easiest to study by looking at ______.
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absorption lines in quasar spectra
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Hubble Space Telescope observations have shown that when the mass of the central black hole is very large, then ________.
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the mass of the bulge of the host galaxy is also very large
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The best evidence for the existence of supermassive black holes is:
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Very high orbital velocities in a very compact region.
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We can study how galaxies evolve because ______.
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The farther away we look, the further back in time we see.
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Which of the following statements is not an assumption used in models of galaxy formation?
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Gas contracted to form the disks of galaxies before any stars were born.
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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 ______.
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a spiral galaxy
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Two ways in which the starting conditions in a protogalactic cloud might cause it to become an elliptical (rather than spiral) galaxy are if the cloud begins with either:
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relatively little angular momentum or relatively high density
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Which of the following phenomena are not thought to be results of collisions or other interactions between galaxies?
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The fact that spiral galaxies have both disk and halo components.
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If the Andromeda Galaxy collided with the Milky Way, what would most likely happen to Earth?
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Nothing
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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?
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The fact that more distant galaxies have larger redshifts.
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Observations indicated that over billions of years, galaxies in general tend to change from _________.
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smaller and bluer to larger and redder
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Which characteristic is not generally true of a starburst galaxy?
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The observed features of the starburst are thought to be caused by the presence of a supermassive black hole in the galaxy's center.
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Why should galaxy collisions have been more common in the past than they are today?
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Galaxies were closer together in the past because the universe was smaller.
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A quasar's spectrum is hugely redshifted. What does this large redshift tells us about the quasar?
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the distance to the quasar
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Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?
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Active galactic nuclei exist tend to become less active as they age.
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Suppose we observe a source of X rays that varies substantially in brightness over a period of a few days. What can we conclude?
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The X-ray source is no more than a few light-days in diameter.
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All of the following observations are real. Which one does not support the model in which active galactic nuclei are powered by accretion disks around massive black holes?
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The most luminous active galactic nuclei have huge redshifts.
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Central black holes can be very efficient for converting the mass-energy of infalling matter to thermal energy in the accretion disk. Roughly what percentage of the mass-energy can be converted to other forms of energy as matter falls into a black hole?
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10-40%
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The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that:
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Galaxy formation and supermassive black hole formation must be related somehow.
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Quasar spectra often show many absorption lines that all appear to be due to the same electron transition (such as level 1 to level 2 in hydrogen) but that fall at different wavelengths in the spectrum. Why do we think this is the case?
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We are seeing absorption lines from clouds of gas that lie between us and the quasar, and therefore each cloud has a different redshift.
