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Which of the following is an important starting assumption in models of galaxy formation?


A) Some regions in the universe start out denser than others.
B) Galaxies form first, then black holes.
C) All galaxies start out as spiral galaxies.
D) Black holes form first, seeding the formation of galaxies.

E) A) and B)
F) C) and D)

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Briefly explain white-dwarf supernovae are useful for measuring cosmic distances.

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They all come from explosions of white d...

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Recall that Hubble's law is written v = Hâ‚€d, where v is the recession velocity of a galaxy located a distance d away from us, and Hâ‚€ is Hubble's constant. Suppose Hâ‚€ = 20 km/s/Mly. How fast would a galaxy 1000 Mly distant be receding from us?


A) 20 km/s
B) 20 Mly/s
C) 20,000 km/s
D) 50 km/s
E) 0.20 times the speed of light

F) C) and D)
G) B) and D)

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Where are the X-rays produced that are emitted by quasars and other active galactic nuclei?


A) in the hot gas in an accretion disk around a central black hole
B) in an ionization nebulae of interstellar gas surrounding the accretion disk
C) in dust grains in molecular clouds that encircle the active galactic nucleus
D) in fast-moving jets of electrons that are ejected from the active galactic nucleus
E) all of the above

F) B) and D)
G) A) and D)

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How do observations of galaxies at different distances help us learn about galaxy evolution?


A) Observations of different distances show galaxies of different ages and therefore different stages of evolution.
B) Observations of the most distant galaxies will allow us to observe the birth of galaxies.
C) We can observe the evolution of a single galaxy over time.
D) We can observe galaxies at different distances merge, helping us learn how mergers affect evolution.
E) We can see what our galaxy used to look like over time, helping us theorize about the physical processes that led to its current appearance.

F) All of the above
G) A) and E)

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Explain how we can estimate that there are about 50-100 billion galaxies in the observable universe.

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While it's impossible to count so many g...

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Which of the following statements is not an assumption used in models of galaxy formation?


A) The universe started out filled almost uniformly with hydrogen and helium.
B) Some regions in the universe were slightly more dense than others.
C) The universe is expanding.
D) Gas contracted to form the disks of galaxies before any stars were born.

E) A) and C)
F) A) and D)

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According to current understanding, what is a quasar?


A) An active galactic nucleus that is particularly bright
B) A very large galaxy thought to be formed by the merger of several smaller galaxies, typically found in the center of a galaxy cluster
C) Any object with an extremely large redshift
D) A galaxy with an unusually high rate of star formation

E) A) and D)
F) B) and D)

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How did Edwin Hubble measure the distance to the Andromeda Galaxy?


A) He measured its parallax.
B) He applied main-sequence fitting to star clusters in Andromeda.
C) He applied the period-luminosity relation to Cepheid variables in Andromeda.
D) He measured its redshift and applied Hubble's Law.
E) He detected white dwarf supernovae in Andromeda.

F) B) and E)
G) B) and D)

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The larger the value of Hubble's constant, the more rapid the expansion of the universe and the smaller (and hence younger) our estimate for the age of the universe.

A) True
B) False

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Which of the following is not a piece of evidence supporting the conclusion that active galactic nuclei are powered by accretion disks around massive black holes?


A) Observed radiation from the galactic center can vary significantly in brightness in times as short as a few days.
B) Infrared observations show that many stars are forming near the centers of active galaxies.
C) Radio observations sometimes show long jets of material that can extend millions of light-years out from the galactic center.
D) The total amount of radiation coming from the galactic center can, in some cases, exceed 100 times the total luminosity of the Milky Way Galaxy.
E) Spectral lines from the galactic center indicate that clouds of gas are orbiting a central object at very high speed.

F) A) and B)
G) A) and C)

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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 ________.


A) 100 billion
B) 100 million
C) 50,000
D) Infinity

E) A) and B)
F) A) and C)

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Why should galaxy collisions have been more common in the past than they are today?


A) Galaxies were more active in the past and therefore would have collided with each other more frequently.
B) Galaxies were much bigger in the past because they had not contracted completely.
C) Galaxies were closer together in the past because the universe was smaller.
D) Galaxies attracted each other more strongly in the past because they were more massive; they had not yet turned most of their mass into stars and light.
E) Galaxy collisions shouldn't have been more common in the past than they are now.

F) A) and B)
G) A) and C)

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If we say that a galaxy has a lookback time of 1 billion years, we mean that ________.


A) its light traveled through space for 1 billion years to reach us
B) it is now 1 billion light-years away
C) it was 1 billion light-years away when the light left the galaxy
D) it is 400 million years old

E) C) and D)
F) A) and B)

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Which of the following is true about irregular galaxies?


A) They are composed solely of old stars.
B) They have significant spheroidal component.
C) They were more common when the universe was younger.
D) They have reddish colors.
E) They have well defined spiral arms.

F) A) and C)
G) A) and D)

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What makes white-dwarf supernovae very good standard candles for distance measurements?


A) They are very bright, so they can be used to determine the distances to galaxies billions of light-years away.
B) They should all have approximately the same luminosity.
C) They occur so frequently that we can use them to measure the distances to virtually all galaxies.
D) We have had several occur close to us in the Milky Way, so we have been able to determine their luminosities very accurately.
E) both A and B

F) B) and E)
G) B) and C)

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Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately


A) 100 million.
B) 1 billion.
C) 10 billion.
D) 100 billion.
E) 1 trillion.

F) B) and D)
G) All of the above

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Using the technique of main-sequence fitting to determine the distance to a star cluster requires that ________.


A) we have telescopes powerful enough to allow us to identify the spectral types of main-sequence stars of many masses in the cluster
B) the cluster be near enough for us to measure the parallax of its stars
C) we use ultraviolet and X-ray telescopes
D) we have a well-calibrated period-luminosity relation for Cepheid variable stars

E) A) and B)
F) None of the above

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Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ________.


A) may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies
B) is always determined by the angular momentum of the protogalactic cloud that formed it
C) is determined by whether the galaxy is located in a cluster where collisions are likely or outside a cluster where collisions are less likely
D) may either be a result of the mass of the protogalactic cloud that formed it or the result of the heavy element abundance in that cloud

E) None of the above
F) C) and D)

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Does It Matter What the Universe Expands Into? A natural question of anyone who meets Hubble's Law for the first time is to ask what exactly the universe is expanding into. This is often further motivated by the analogy of an inflating balloon universe, where two-dimensional galaxies ride along on the surface of the balloon. Imagine, that you live in one of these balloon-borne galaxies. Your universe is the surface of the balloon. You can move along the surface, but you can never exist above or below it. Now one of your two-dimensional friends asks what your (balloon) space is expanding into. Do you need to answer this question, or can the expansion you experience be completely described without reference to an unseen, third-dimension? Imagine someone argues that question "what is the universe expanding into?" does not even need an answer! Would you agree or disagree? Explain. If you disagree, would there be any way to gather scientific evidence to answer this question?

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