Astronomy Ch. 16 – Flashcards
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Based on the number of galaxies visible in the Hubble Deep Field (Figure 16.1 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 candle is _________.
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a light source of known luminosity
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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 through space for 1 billion years to reach us
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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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Collisions between galaxies typically unfold over a period of _________.
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hundreds of millions of years
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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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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 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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In a photo like the Hubble Deep Field (Figure 16.1 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?
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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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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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Assume that Hubble's constant is 22 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.)
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away from us at 2,200 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 those same lines as measured in the laboratory.
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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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Why 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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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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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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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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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 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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According to the model in which 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.
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Elliptical galaxy:
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-stars form rapidly as protogalactic cloud shrinks -protogalactic cloud has high density -galaxy collision strips away gas -protogalactic cloud rotates very slowly
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Spiral galaxy:
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-most protogalactic gas settles into a disk -protogalactic cloud has higher angular momentum
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Suppose a collision strips gas out of a spiral galaxy. Why would this tend to change the spiral galaxy into an elliptical galaxy?
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A galaxy cannot have a disk if it does not have gas.
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High density tends to lead to more rapid star formation in a protogalactic cloud. Why does this rapid star formation tend to lead to an elliptical galaxy, rather than a spiral galaxy?
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Rapid star formation means that there may not be enough gas left to make a disk.
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High angular momentum leads to faster rotation. Why does faster rotation tend to lead to a spiral galaxy, rather than an elliptical galaxy?
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Faster rotation leads to collisions among gas particles that cause the gas to settle into a spinning disk, rather than a more spread out cloud.
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Radar, the first link in the cosmic distance chain, is used to establish the baseline distance necessary for the second link, parallax. What baseline distance must we know before we can measure parallax?
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the Earth-Sun distance
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Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?
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Cepheids
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Hubble's law expresses a relationship between __________.
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the distance of a galaxy and the speed at which it is moving away from us
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We can always determine the recession velocity of a galaxy (at least in principle) from its redshift. But before we can use Hubble's law, we must first calibrate it by __________.
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measuring the distances to many distant galaxies with a standard candle technique
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Our sun belongs to the _____ of the Milky Way.
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disk component
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The globular cluster M13 belongs to the _______ of the Milky Way.
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spheroidal component
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An _______ contains hot, ionized gas but very little cool gas or dust.
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elliptical galaxy
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The type of galaxy known as an ________ was more common in the universe 10 billion years ago.
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irregular galaxy
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Scientists investigating _______ study hot the distribution of galaxies changes with time.
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cosmology
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__________ have the highest rates of star formation.
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Starburst galaxies
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The largest individual galaxies in the universe are known as _________.
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central dominant galaxies
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The collective activity of many supernova events in a relatively small volume of a galaxy can create _________.
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galactic winds
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The most luminous objects in the universe are _____.
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quasars
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The energy for all active galactic nuclei is thought to come from in-fall of matter into _________.
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supermassive black holes
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________ are often characterized by sources of immense energy located hundreds of thousands of light-years away on either side of their centers.
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radio galaxies
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Which is not present in all galaxies?
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young stars
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Which processes contribute to final galaxy?
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everything but supernova explosions (initial density and rotation/collisions)
