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 bright active galactic nucleus is typically ______.
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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 large, then ________.
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the mass of the bulge of the host galaxy is also large
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The best evidence for the existence of supermassive black holes is ___________________.
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high orbital velocities in a 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 as a result of 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 efficient for converting the mass-energy of in-falling 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 as a result of 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.
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The three cubes represent three stages in a computer simulation of the formation of protogalactic clouds; they represent a region about 500 million light-years across. Which of the following statements about this model is true?
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Although the gas looks uniform in the first frame, the model assumes there were small regions with slightly higher densities than other regions.
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This diagram shows several stages in a computer simulation of a collision between two galaxies. What happens?
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Two spiral galaxies merge to become an elliptical galaxy surrounded by debris.
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This large fuzzy blob in the center of this image is the central dominant galaxy of the cluster that surrounds it. The arrow points to several large clumps of stars near the center of the galaxy. What do those clumps probably represent?
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Each clump was probably once the center of an individual galaxy
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This photo shows the central region of a cluster of galaxies. Based on what you have learned from your text, which of the following statements about this photo is not true?
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The oldest stars in these galaxies are much older than the oldest stars in our own Milky Way.
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Notice the long "tail" of stars and gas extending out (to the right) from the main galaxy in this photo. According to what you have learned from your text, the most likely explanation for this long tail is _______________.
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it contains gas pulled out of the galaxy during a collision with another galaxy
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The photo shows a galaxy known as Arp 220, and the inset shows this galaxy's spectrum. What can you infer?
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Most of the light from this galaxy is infrared, indicating active star formation.
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This visible-light photo shows a starburst galaxy. What is going on with the reddish gas that seems to be emerging on both sides of it?
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It is gas being blown out in a galactic wind.
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This Hubble Space Telescope photo shows the bright active galactic nucleus in the center of galaxy M87. What is the long bluish streak coming out of it?
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A "jet" of material being shot out at high speed.
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The bright object in this photo is a quasar (named 3C 273). According to current understanding, about how big is the source of the bright light?
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about the size of our solar system
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This image combines a visible-light view of the galaxy Hercules A with an image showing radio wave emission (the reddish colors) in its vicinity. The likely explanation for the two huge clumps of radio emission far to either side of the visible galaxy is __________.
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the radio emission comes from gas shot out into space by jets powered by a supermassive black hole in the galactic center.
