Chapter 21
Conceptual Quiz
Part A
We can study how galaxies evolve because ______.
Hint A.1
Section 21.1
ANSWER:

The farther away we look, the further back in time we see. Galaxies are transparent to optical light. We can watch as they interact in real time. We are really smart astronomers.
Part B
Which of the following phenomena are NOT thought to be results of collisions or other interactions between galaxies?
Hint B.1
Study Sections 21.2
ANSWER:

The fact that elliptical galaxies are more common in clusters of galaxies than outside clusters. The presence of very large, central dominant galaxies in clusters of galaxies. Starbursts. The fact that spiral galaxies have both disk and halo components.
Part C
Which situation is mostly likely to form a spiral galaxy?
Hint C.1
Study Section 21.2
ANSWER:

The collapse of a dense gas cloud. The direct, face on, collision of two other spiral galaxies. The collision and merger of two elliptical galaxies. The collapse of a diffuse, rotating gas cloud.
Part D
If the Andromeda Galaxy collided with the Milky Way, what would most likely happen to the Earth?
Hint D.1
Study Section 21.2
ANSWER:

The Sun and all its planets would be crushed by collisions with stars and planets from the Andromeda Galaxy. Nothing. The Sun will turn into a red giant and explode. The Sun would not be disturbed, but the solar system would be ripped apart.
Part E
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 ______.
Hint E.1
Study Section 21.2
ANSWER:

an irregular galaxy. an elliptical galaxy. a torn and incoherent galaxy. a spiral galaxy.
Part F
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:
Hint F.1
Study Section 21.2
ANSWER:

relatively little angular momentum or relatively high density. relatively high mass and a relatively low abundance of heavy elements. relatively low mass and a relatively high abundance of heavy elements. relatively high angular momentum or relatively low density.
Part G
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?
Hint G.1
Study Section 21.2
ANSWER:

Computer modeling of collisions between galaxies. The fact that more distant galaxies have larger redshifts. The fact that galaxies with distorted appearances are more common at great distances than nearby. The presence of features such as "tails" extending out of galaxies, bridges between galaxies, and rings of stars around galaxies.
Part H
Which of the following does NOT support the theory that active galactic nuclei are powered by accretion disks around massive black holes?
Hint H.1
Study Section 21.3
ANSWER:

Infrared observations show that many stars are forming near the centers of galaxies with active galactic nuclei. Observed radiation from the galactic center varies significantly in brightness in times as short as a few days. The total amount of radiation coming from the galactic center is, in some cases, comparable to the amount of radiation put out by 10 billion or more ordinary stars. Spectral lines from the galactic center indicate that clouds of gas are orbiting a central object at very high speed.
Part I
Which characteristic is NOT generally true of a starburst galaxy?
Hint I.1
Study Section 21.2
ANSWER:

The observed features that cause us to classify it as a "starburst" must be only temporary phenomena in the galaxy's history. Supernovae occur so frequently that their effects combine to drive a galactic wind that blows material into intergalactic space. The observed features of the starburst are thought to be caused by the presence of a supermassive black hole in the galaxy's center. Its rate of star formation is many times higher than the rate of star formation in the Milky Way.
Part J
A quasar's spectrum is hugely redshifted. What do most astronomers think this large redshift tells us about the quasar?
Hint J.1
Study Section 21.3
ANSWER:

The distance to the quasar. The composition of the quasar. The size of the quasar's central, supermassive black hole. The type of host galaxy in which the quasar resides.
Part K
A few decades ago, there was great controversy among astronomers over the question of quasar distances, with some arguing that quasars are much nearer than application of Hubble's law would seem to imply. Why do nearly all astronomers now agree that quasars really are quite far away?
Hint K.1
Study Section 21.3
ANSWER:

All quasars have large redshifts. Now that we can explain bright quasar emission with power due to the presence of supermassive black holes, there is no reason to doubt that quasars are far away. We now have images and spectra that clearly show quasars to be embedded at the centers of distant galaxies and within distant galaxy clusters. No one could think of a way to explain quasar speeds if they are nearby, so we concluded they must be far way.
Part L
Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?
Hint L.1
Study Section 21.3
ANSWER:

Supermassive black holes existed only when the universe was young, and no longer exist today. Active galactic nuclei exist tend to become less active as they age. Active galactic nuclei can form only at large distances from the Milky Way. The jets seen in many active galactic nuclei must cause them to move far away from us.
Part M
Suppose that we observe a source of X-rays that varies substantially in brightness over a period of a few days. What can we conclude?
Hint M.1
Study Section 21.3
ANSWER:

The X-ray source contains a black hole with an accretion disk. The X-ray source is no more than a few light-days in diameter. The X-ray source must have a strong, rapidly varying magnetic field. The X-ray source is a quasar.
Part N
Which of the following statements is NOT an assumption for galaxy formation scenarios?
Hint N.1
Section 21.1
ANSWER:

The universe started out filled almost uniformly with hydrogen and helium. Gas contracted to form the disks of galaxies before any stars were born. The universe is expanding. Some regions in the universe were a tiny bit more dense than the rest.
Part O
Central black holes can be very efficient for converting the mass-energy of infalling 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?
Hint O.1
Section 21.3
ANSWER:

10-40% 100% 0.7% 1%-4%
Part P
Quasar spectra often show many absorption lines that all appear to be due to 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?
Hint P.1
Study Section 21.3
ANSWER:

We are seeing absorption lines from clouds of gas that lie between us and the quasar, and therefore each cloud has a different redshift. Quasars are rotating rapidly, and this rotation produces spectral lines with a wide variety of Doppler shifts. The lines fall at different wavelengths because they are produced by different chemical elements. No one knows. It remains perhaps the greatest mystery about quasars.
Part Q
The relationship between the masses of central black hole and the bulge masses of galaxies implies that:
Hint Q.1
Study Section 21.3, especially Figure 21.23.
ANSWER:

The biggest galaxies have the most luminous quasars. Quasars were more common 10 billion years ago than they are today. Galaxy formation and supermassive black hole formation must be related somehow. The black hole will eventually eat the galaxy.
Part R
If an intergalactic gas cloud is between you and a quasar, the wavelength of its hydrogen line, compared to the same hydrogen line in the quasar, is _______.
Hint R.1
Study Section 21.3
ANSWER:

At the same wavelength as the quasar's line wavelength. At longer wavelengths compared to the quasar's line wavelength. At shorter wavelengths compared to the quasar's line wavelength. An emission line.