Astronomy Chapter 15 Study Quizzes – Flashcards
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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 ____________.
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100 billion
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Which of the following is not one of the three major categories of galaxies?
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globular galaxies
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A standard candles is ______________.
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a light source of known luminosity
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What is main sequence fitting?
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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
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What is a Cepheid Variable?
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A type of very luminous star that makes an excellent standard candle
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What two observable properties of a Cepheid variable are directly related to one another?
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the period between its peaks of brightness and its luminosity
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What does Hubble's law tell us?
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The more distant a galaxy, the faster it is moving away from us.
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Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies?
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They are rare events, so we have observed them in only a tiny fraction of all galaxies.
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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?
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the inside of the balloon does not represent any part of our universe
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If we say that a galaxy has a lookback time of 1 billion years, we mean that __________.
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its light traveled
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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 ___________.
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the cosmological horizon
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Cosmological redshift is the result of ____________.
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the expansion of the universe
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Current estimates place the age of the universe at about
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14 billion years
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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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Current understanding holds that a galaxy's type (spiral, irregular, or elliptical) ____________.
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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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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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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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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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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 theory that 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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In a photo like the Hubble Deep Field, we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their life?
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the galaxies that are farthest away
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The most basic difference between elliptical galaxies and spiral galaxies is that ____________.
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elliptical galaxies lack anything resembling the disk of a spiral galaxy
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Hubble's galaxy classification diagram (the "turning fork") ____________.
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relates galaxies according to their shapes, but not according to any evolutionary status.
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Using the technique of main sequence fitting to determine the distance to a star cluster requires that ______________.
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we have telescopes powerful enough to allow us to identify the spectral types of main-sequence stars of many masses in the cluster
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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?
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We can learn the distance to the galaxy
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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?
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By observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation.
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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?
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away from us at 2,000 km/s
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Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
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White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.
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Does Hubble's law work well for galaxies in the Local Group? Why or why not?
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No, because galaxies in the Local Group are gravitationally bound together
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What is the best way to determine a galaxy's redshift?
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Take a spectrum of the galaxy, and measure the difference in wavelength of spectral lines from the wavelengths of these same lines as measured in the labratory.
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Which statement below correctly describes the relationship between expansion rate and age for the universe?
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The faster the rate of expansion, the younger the age of the universe
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What does cosmological redshift do to light?
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stretches its wavelength
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What can't we see past the cosmological horizon?
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Beyond the cosmological horizon, we would be looking back to a time before the universe was born
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We can study how galaxies evolve because ______________.
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we are really smart astronomers.. the farther 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 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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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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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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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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A quasar's spectrum is hugely redshifted. What does this large redshift tell us about the quasar?
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the distance to the quasar
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What evidence tells us that quasars are the centers of distant galaxies?
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Images and spectra show quasars to be embedded at the centers of distant galaxies.
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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 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 theory that 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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According to the theory 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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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.
