63) 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 hours.
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.
64) How is the energy that powers radio galaxies, quasars, and other active galactic nuclei
thought to be produced?
A) by nuclear fusion near a central black hole
B) by magnetic fields that trap and accelerate charged particles, which then radiate large amounts
of energy
C) by matter that has been converted to pure energy by interacting with the jets shot out by a
central black hole
D) by gravity, which converts the potential energy of matter falling toward a central black hole
into kinetic energy, which is then converted to thermal energy by collisions among the particles
of matter
E) by matter-antimatter annihilation near a central black hole
65) 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) by multiple X-ray binary systems containing black holes located near the center of the galaxy
E) all of the above
66) Which of the following statements best supports the hypothesis that active galactic nuclei
(AGN) are powered by supermassive black holes?
A) Orbits of gas clouds indicate billions of solar masses in a small volume.
B) AGN are found in the centers of galaxies.
C) AGN have jets and accretion disks.
D) AGN emit radiation at all wavelengths, from the radio to the gamma-rays.
1) All spiral galaxies have both a disk and spheroidal component to their stellar populations.
2) Spiral galaxies have more gas, dust, and young stars than elliptical galaxies.
3) Stars continue to form in the halo of our Galaxy today.
4) A lenticular galaxy is another name for an elongated elliptical galaxy.
5) The largest elliptical galaxies are likely the product of galactic mergers.
6) Elliptical galaxies are more likely to have ongoing star formation than spiral galaxies.
7) Massive-star supernovae and white-dwarf supernovae work equally well as standard candles
for measuring cosmic distances.
8) 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.
9) Distant galaxies are more likely to be irregular in shape than galaxies closer to the Milky Way.
10) A protogalactic cloud with slow star formation is more likely to form a spiral galaxy than an
elliptical galaxy.
11) A protogalactic cloud with very little angular momentum is more likely to form an elliptical
galaxy than a spiral galaxy.
12) Galaxy collisions were more common in the past.
13) Galaxy mergers take hundreds of millions of years to complete.
14) The collision of two spiral galaxies will likely result in a single giant spiral galaxy.
15) Quasars radiate most of their energy as radio emission.
16) The black hole mass at the center of a galaxy is generally larger in galaxies with larger
central bulges.
16.3 Process of Science Questions
1) Supermassive Black Holes: A Scientific Theory? Summarize all of the available evidence that
active galaxies are powered by supermassive black holes. Do your best to assemble enough
evidence that this explanation should be considered a scientific theory in the sense outlined at the
end of Section 3.4 (The Nature of Science) of Chapter 3. Do you think astronomers have met this
challenge?
2) 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?
3) The Redshift Controversy: The early debate on whether or not quasar redshifts were correctly
interpreted by Hubble’s law (to indicate that quasars lay at enormous distances) is an excellent
example of how a scientific debate was (reasonably) quickly settled by appeals to the
observational evidence. Outline all of the observational evidence that quasars are indeed
associated with the nuclei of distant galaxies and, therefore, their redshifts can be correctly
interpreted using Hubble’s law.
4) Building new telescopes: With current telescopes, we can see galaxies as they existed 1 billion
years after the Big Bang. Discuss innovations that could be made with new telescopes in order to
see even further.
16.4 Short Answer Questions
1) Explain how we can estimate that there are about 50-100 billion galaxies in the observable
universe.
2) Summarize the links in the distance chain that allow us to estimate distances to the farthest
reaches of the universe.
3) List at least three qualities that make an astronomical object a good standard candle.
4) Explain why individual galaxies or clusters of galaxies do not expand in size despite the
overall expansion of the universe implied by Hubble’s law.
5) Briefly explain why white-dwarf supernovae are useful for measuring cosmic distances.
6) How does the age of the universe depend on the value of Hubble’s constant, and why?
7) Recall that Hubble’s law is written v = Hd, where v is the recession velocity of a galaxy
located a distance d away from us, and H is Hubble’s constant. Suppose H = 20 km/s/Mly. How
fast would a galaxy located 1375 Mly distant be receding from us? Show all work clearly, and
state your final answer with a complete sentence. Give the speed both in units of kilometers per
second and as a percentage of the speed of light.
v = Hd = (1375 Mly)
= 27,500
= 0.09c
8) Explain why galaxy-galaxy collisions are far more common than star-star collisions.
9) Why does the merger of two spiral galaxies often trigger starbursts?
10) How does a starburst end? What might happen to the galaxy afterwards?
11) Give several observations supporting the idea that the high redshifts of quasars really do
imply great distances.
12) How are the jets produced by radio galaxies similar to those produced by protostars?
13) How are jets thought to be produced by active galaxies?
14) How do quasars produce so much energy in such a small volume?
16.5 Mastering Astronomy Reading Quiz
1) Based on the number of galaxies visible in the Hubble eXtreme Deep Field (Figure 16.1 in
your textbook) and the angular size of the image, the estimated number of galaxies in our
observable universe is about ________.
A) 100 billion
B) 100 million
C) 50,000
D) Infinity
2) Which of the following is not one of the three major categories of galaxies?
A) globular galaxies
B) elliptical galaxies
C) spiral galaxies
D) irregular galaxies
3) A standard candle is ________.
A) a 7-cm-long wax candle
B) a light source of known luminosity
C) another name for a main-sequence star
D) another name for a barred-spiral galaxy
4) What is main-sequence fitting?
A) a method of determining the distance to a star cluster by matching its main sequence on an H-
R diagram with the main sequence of a cluster with a known distance
B) a method for determining the age of a star cluster
C) a way of forcing stars to fit into a standard main sequence, even when they have some
unusual characteristics
D) the way we construct an H-R diagram by plotting the surface temperatures and luminosities
of stars
5) What is a Cepheid variable?
A) a type of luminous star whose variations in luminosity make it a good standard candle
B) a bright source of variable X-ray emission, thought to harbor a supermassive black hole
C) a main-sequence star whose known luminosity makes it a good standard candle
D) a type of galaxy whose variations in luminosity make it a good standard candle
6) What two observable properties of a Cepheid variable are directly related to one another?
A) the period between its peaks of brightness and its luminosity
B) the period between its peaks of brightness and its distance
C) its luminosity and its distance
D) its mass and its distance
7) What does Hubble’s law tell us?
A) The more distant a galaxy, the faster it is moving away from us.
B) The faster a spiral galaxy’s rotation speed, the more luminous it is.
C) The longer the period of a Cepheid variable, the greater its luminosity.
D) For every force, there is an equal and opposite reaction force.
8) Given that white dwarf supernovae are such good standard candles, why don’t we use them to
measure the distance to all galaxies?
A) They are rare events, so we have observed them in only a tiny fraction of all galaxies.
B) We cannot see them beyond a distance of about 100 million light-years.
C) They can occur only in spiral galaxies, not elliptical galaxies.
D) We would, but we don’t have enough telescopes.
9) 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?
A) the center of the universe
B) the entire universe
C) regions of the universe beyond the Milky Way Galaxy
D) The inside of the balloon does not represent any part of our universe.
10) 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
11) 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
________.
A) the cosmological horizon
B) the Big Bang
C) the lookback time
D) the Hubble Deep Field
12) Cosmological redshift is the result of ________.
A) the high speeds at which galaxies move within clusters
B) the expansion of the universe
C) very old, red stars in distant galaxies
D) supermassive black holes
13) Current estimates place the age of the universe at about ________.
A) 10 billion years
B) 10 million years
C) 14 billion years
D) 4 1/2 billion years
14) Telescopes designed to study the earliest stages in galactic lives should be optimized for
observations in ________.
A) infrared light
B) visible light
C) ultraviolet light
D) X-rays
15) 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.
16) 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
17) Collisions between galaxies typically unfold over a period of ________.
A) centuries
B) hundreds of thousands of years
C) millions of years
D) hundreds of millions of years
18) Why are collisions between galaxies more likely than collisions between stars within a
galaxy?
A) Relative to their sizes, galaxies are much closer together than stars.
B) Galaxies are much larger than stars.
C) Galaxies travel through space much faster than stars.
D) Galaxies have higher redshifts than stars.
19) Which of the following features is not a feature of central dominant galaxies?
A) They’re found in clusters of galaxies.
B) They are often spiral galaxies.
C) They often have multiple galactic nuclei near their centers.
D) They are thought to form by the merger of smaller galaxies.
20) Why should galaxy collisions have been more common in the past than they are today?
A) Galaxies were closer together in the past because the universe was smaller.
B) Galaxies were more active in the past and therefore would have collided with each other more
frequently.
C) Galaxies were much bigger in the past since they had not contracted completely.
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.
21) The distinguishing feature of a starburst galaxy is ________.
A) a rate of star formation that may be 100 or more times greater than that in the Milky Way
B) the presence of an unusually large number of binary star systems containing X-ray bursters
C) a very large luminosity compared to the total luminosity of the Milky Way
D) strong radio emission from “lobes” of material well outside the visible boundaries of the
galaxy
22) The unusually bright centers found in some galaxies are called ________.
A) active galactic nuclei
B) halos
C) supermassive black holes
D) starbursts
23) 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
24) Which of the following phenomena is probably not related to the presence of a supermassive
black hole?
A) quasars
B) the radio emission from radio galaxies
C) the huge jets seen emerging from the centers of some galaxies
D) the presence of globular clusters in the halos of galaxies
25) The mass of a supermassive black hole thought to power a typical bright active galactic
nucleus is roughly ________.
A) 3 solar masses
B) 10 solar masses
C) 1 trillion solar masses
D) 1 billion solar masses
26) Quasars are rare and most have very large redshifts. What can you conclude from this
statement?
A) Quasars are more common in the young (early) universe than they are today.
B) Dormant quasars may exist in nearby galaxies.
C) Most quasars are at very large distances.
D) all of the above
16.6 Mastering Astronomy Concept Quiz
1) In a photo like the Hubble eXtreme 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?
A) the galaxies that are farthest away
B) the galaxies that have the most hot, young O and B stars
C) the galaxies that are the bluest in color
D) the galaxies that are nearest to us
2) The most basic difference between elliptical galaxies and spiral galaxies is that ________.
A) elliptical galaxies lack anything resembling the disk of a spiral galaxy
B) elliptical galaxies lack anything resembling the halo of a spiral galaxy
C) elliptical galaxies have a spheroidal component (of stars distributed spherically about the
galactic center), and spiral galaxies do not
D) elliptical galaxies are very old and spiral galaxies are very young
3) Hubble’s galaxy classification diagram (the “tuning fork”) ________.
A) shows how galaxies evolve from one form to another
B) explains active galactic nuclei
C) suggests the existence of black holes
D) relates galaxies according to their shapes, but not according to any evolutionary status
4) Using the technique of main-sequence fitting to determine the distance to a star cluster
requires that ________.
A) we are able 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
5) 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?
A) We can learn the distance to the galaxy.
B) It will allow us to calculate the rotation rate of the galaxy.
C) It will allow us to determine the mass of the galaxy.
D) Under the rules of the International Astronomical Union, we will be entitled to naming rights
for the galaxy.
6) 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?
A) By observing individual Cepheid variable stars in Andromeda and applying the period-
luminosity relation
B) He was able to measure the parallax of the Andromeda Galaxy.
C) He found that the universe is expanding, and therefore concluded that Andromeda must lie
outside our own galaxy.
D) He was the first person ever to look through a telescope at the object we now call the
Andromeda Galaxy.
7) 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? (Assume the motion
is due only to Hubble’s law.)
A) away from us at 2,000 km/s
B) away from us at 200 km/s
C) toward us at 2,000 km/s
D) away from us at 20,000 km/s
8) Why are white dwarf supernovae more useful than massive star supernovae for measuring
cosmic distances?
A) White dwarf supernovae all have roughly the same true peak luminosity, while massive
supernovae come in a wide range of peak luminosities.
B) We can see only white dwarf supernovae in distant galaxies, not massive star supernovae.
C) White dwarf supernovae are much more common than massive star supernovae.
D) White dwarf supernovae follow a period-luminosity relation, while massive supernovae do
not.
9) Does Hubble’s law work well for galaxies in the Local Group? Why or why not?
A) No, because galaxies in the Local Group are gravitationally bound together.
B) No, because Hubble did not know the Local Group existed when he discovered his law.
C) No, because we do not know the precise value of Hubble’s constant.
D) Yes, it works so well that we have never detected any measurable deviations from its
predictions.
10) What is the best way to determine a galaxy’s redshift?
A) Find the galaxy’s apparent distance, and look up the redshift based on Hubble’s law.
B) Find the color of the galaxy, and estimate its distance based on how red the galaxy is.
C) 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.
D) Measure the magnitude of the galaxy, estimate its distance, and calculate its redshift using
Hubble’s law.
11) Which statement below correctly describes the relationship between expansion rate and age
for the universe?
A) The faster the rate of expansion, the younger the age of the universe.
B) The faster the rate of expansion, the older the age of the universe.
C) Age is independent of the expansion rate.
12) What does cosmological redshift do to light?
A) stretches its wavelength
B) makes it brighter
C) makes it slow down
D) makes all light infrared
13) Why can’t we see past the cosmological horizon?
A) Beyond the cosmological horizon, we would be looking back to a time before the universe
was born.
B) We do not have big enough telescopes.
C) The cosmological horizon is infinitely far away, and we can’t see to infinity.
D) Every galaxy in the entire universe (not just the observable universe) exists within the
cosmological horizon, so there’s nothing to see beyond it.
14) We can study how galaxies evolve because ________.
A) we can watch as they interact in real time
B) the farther away we look, the further back in time we see
C) galaxies are transparent to visible light
D) we are really smart astronomers
15) 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.
16) 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 ________.
A) an irregular galaxy
B) an elliptical galaxy
C) a spiral galaxy
D) a torn and incoherent galaxy
17) Which of the following phenomena are not thought to be results of collisions or other
interactions between galaxies?
A) the presence of very large, central dominant galaxies in clusters of galaxies
B) the fact that elliptical galaxies are more common in clusters of galaxies than outside clusters
C) starburst galaxies
D) the fact that spiral galaxies have both disk and halo components
18) Interactions among galaxies are thought to influence a galaxy’s type in some cases. Which of
the following does not support the idea that interactions can shape galaxies?
A) the fact that more distant galaxies have larger redshifts
B) computer modeling of collisions between galaxies
C) the fact that galaxies with distorted appearances are more common at great distances than
nearby
D) the presence of features, such as “tails” extending out of galaxies, bridges between galaxies,
and rings of stars around galaxies
19) Which characteristic is not generally true of a starburst galaxy?
A) The observed features that cause us to classify it as a “starburst” must be only temporary
phenomena in the galaxy’s history.
B) Its rate of star formation is many times higher than the rate of star formation in the Milky
Way.
C) Supernovae occur so frequently that their effects combine to drive a galactic wind that blows
material into intergalactic space.
D) The observed features of the starburst are thought to be caused by the presence of a
supermassive black hole in the galaxy’s center.
20) A quasar’s spectrum is highly redshifted. What does this large redshift tell us about the
quasar?
A) the composition of the quasar
B) the distance to the quasar
C) the size of the quasar’s central, supermassive black hole
D) the type of host galaxy in which the quasar resides
21) What evidence tells us that quasars are the centers of distant galaxies?
A) Images and spectra show quasars to be embedded at the centers of distant galaxies.
B) All quasars have large redshifts.
C) Quasars are extremely luminous.
D) Rapid variations in quasar luminosity tell us that they must be quite small in size.
22) Most active galactic nuclei are found at large distances from us, with relatively few nearby.
What does this imply?
A) Active galactic nuclei tend to become less active as they age.
B) Supermassive black holes existed only when the universe was young, and no longer exist
today.
C) Active galactic nuclei can form only at large distances from the Milky Way.
D) The jets seen in many active galactic nuclei must cause them to move far away from us.
23) Suppose we observe a source of X-rays that varies substantially in brightness over a period
of a few days. What can we conclude?
A) The X-ray source is no more than a few light-days in diameter.
B) The X-ray source is a quasar.
C) The X-ray source contains a black hole with an accretion disk.
D) The X-ray source must have a strong, rapidly varying magnetic field.
24) 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?
A) The most luminous active galactic nuclei have huge redshifts.
B) X-ray emission from active galactic nuclei can vary significantly in times as short as a few
hours.
C) The total luminosity of an active galactic nucleus can be as high as about 10 billion times that
of the Sun.
D) Spectra of active galactic nuclei show that clouds of gas are orbiting a central object at very
high speed.
25) According to the theory that active galactic nuclei are powered by supermassive black holes,
the energy released as light comes from ________.
A) gravitational potential energy released by matter that is falling toward the black hole
B) nuclear fusion in the accretion disk surrounding the black hole
C) matter-antimatter annihilation occurring just outside the event horizon of the black hole
D) jets emerging from the accretion disk
26) The observed relationship between the masses of central black holes and the bulge masses of
galaxies implies that ________.
A) galaxy formation and supermassive black hole formation must be related somehow
B) the black hole will eventually suck in the rest of the galaxy
C) the biggest galaxies have the most massive black holes
D) quasars were more common 10 billion years ago than they are today