Question 1

Why is 21-cm radiation a good tracer of spiral arm structure?

Accepted Answer

A)  Emitted by neutral hydrogen, it traces the location of high concentrations of gas. 
B)  Emitted by O and B stars, it traces the location of newly formed stars. 
C)  Emitted by ionized hydrogen, it traces the location of H II regions. 
D)  Emitted by molecular hydrogen, it traces the location of clouds in the process of collapsing into stars. 
E)  Emitted by red supergiants, it traces the presence of massive stars that are experiencing high rates of mass loss. 
A)  Emitted by neutral hydrogen, it traces the location of high concentrations of gas. 
B)  Emitted by O and B stars, it traces the location of newly formed stars. 
C)  Emitted by ionized hydrogen, it traces the location of H II regions. 
D)  Emitted by molecular hydrogen, it traces the location of clouds in the process of collapsing into stars. 
E)  Emitted by red supergiants, it traces the presence of massive stars that are experiencing high rates of mass loss. A

Question 2

The 3-D shape of an elliptical galaxy depends on the average velocity of its stars along each of the three axes.

Accepted Answer

A) True 
 B)False  True

Question 3

How do we know that the stellar disks in spiral galaxies are flat? How do we know that elliptical galaxies are not flat?

Accepted Answer

Individual galaxies are oriented at a variety of angles to our line of sight. Sometimes we see that spirals look round on the sky; this means we are looking at the disk face-on. Sometimes spirals look quite thin, which means we are seeing these edge-on. The fraction of galaxies that look thin are what we expect to see if all spiral galaxies have thin disks. Elliptical galaxies, on the other hand, never look thin, so they must be roundish (elongated but not flat) in three dimensions.

Question 4

If you measure the spectrum emitted by a cloud of gas that orbits a nearby spiral galaxy and the observed wavelength of a hydrogen emission line is 658.4 nm while the rest wavelength of that line is 656.3 nm, what is the velocity of this gas cloud along your line of sight (i.e., the radial velocity)? Note that the speed of light is 3 *105 km/s.

Accepted Answer

A)  960 km/s 
B)  1,200 km/s 
C)  2,300 km/s 
D)  3,800 km/s 
E)  4,400 km/s 
A)  960 km/s 
B)  1,200 km/s 
C)  2,300 km/s 
D)  3,800 km/s 
E)  4,400 km/s

Question 5

Which of the following is NOT a difference between spiral and elliptical galaxies?

Accepted Answer

A)  Spiral galaxies still have ongoing star formation whereas elliptical galaxies do not. 
B)  Spiral galaxies are sometimes very thin in appearance whereas elliptical galaxies are always round or elliptical. 
C)  Spiral galaxy disks contain lots of cold gas and dust whereas elliptical galaxies do not. 
D)  Spiral galaxies tend to have bluish areas whereas elliptical galaxies tend to be all reddish. 
E)  Spiral galaxies have dark matter halos whereas elliptical galaxies do not. 
A)  Spiral galaxies still have ongoing star formation whereas elliptical galaxies do not. 
B)  Spiral galaxies are sometimes very thin in appearance whereas elliptical galaxies are always round or elliptical. 
C)  Spiral galaxy disks contain lots of cold gas and dust whereas elliptical galaxies do not. 
D)  Spiral galaxies tend to have bluish areas whereas elliptical galaxies tend to be all reddish. 
E)  Spiral galaxies have dark matter halos whereas elliptical galaxies do not.

Question 6

In some radio galaxies, we only see one side of the jet because:

Accepted Answer

A)  the black hole only produces a jet in one direction 
B)  of relativistic beaming 
C)  the other side is inside the black hole's event horizon 
D)  of dust obscuration 
E)  of dark matter that blocks the light from one side of the jet 
A)  the black hole only produces a jet in one direction 
B)  of relativistic beaming 
C)  the other side is inside the black hole's event horizon 
D)  of dust obscuration 
E)  of dark matter that blocks the light from one side of the jet

Question 7

Which of these statements about quasars is FALSE?

Accepted Answer

A)  Quasars are extremely luminous. 
B)  Quasars are rare in our local vicinity. 
C)  Quasars have high brightnesses. 
D)  Quasars are extremely massive. 
E)  Quasars are located at the centers of some active galaxies. 
A)  Quasars are extremely luminous. 
B)  Quasars are rare in our local vicinity. 
C)  Quasars have high brightnesses. 
D)  Quasars are extremely massive. 
E)  Quasars are located at the centers of some active galaxies.

Question 8

Star formation in spiral galaxies is enhanced in the spiral arms because more hot gas is concentrated there.

Accepted Answer

A) True 
 B)False

Question 9

What happens to active galaxies when their AGNs run out of fuel?

Accepted Answer

A)  The black holes powering them collapse even further. 
B)  They simply become normal galaxies. 
C)  They slowly dissipate into clouds of gas and dust. 
D)  They eventually pull in material from farther out, starting the AGN process all over again. 
E)  They explode as gamma-ray bursts. 
A)  The black holes powering them collapse even further. 
B)  They simply become normal galaxies. 
C)  They slowly dissipate into clouds of gas and dust. 
D)  They eventually pull in material from farther out, starting the AGN process all over again. 
E)  They explode as gamma-ray bursts.

Question 10

The density of ordinary luminous matter can exceed the density of dark matter in which parts of galaxies?

Accepted Answer

A)  everywhere inside galaxies 
B)  only in the outer regions of galaxies 
C)  only in star clusters 
D)  near the centers of galaxies 
E)  only around an AGN 
A)  everywhere inside galaxies 
B)  only in the outer regions of galaxies 
C)  only in star clusters 
D)  near the centers of galaxies 
E)  only around an AGN

Question 11

The dark matter in galaxies is __________ the luminous material.

Accepted Answer

A)  distributed in roughly the same way as 
B)  much more extended in radius than 
C)  much more centrally concentrated than 
D)  much less uniform in density than 
E)  much clumpier than 
A)  distributed in roughly the same way as 
B)  much more extended in radius than 
C)  much more centrally concentrated than 
D)  much less uniform in density than 
E)  much clumpier than

Question 12

If a Seyfert galaxy's nucleus varies in brightness on the timescale of 10 hours, then approximately what is the size of the emitting region?

Accepted Answer

A)  20 AU 
B)  30 AU 
C)  50 AU 
D)  70 AU 
E)  120 AU 
A)  20 AU 
B)  30 AU 
C)  50 AU 
D)  70 AU 
E)  120 AU

Question 13

Why are a higher percentage of distant galaxies classified as AGNs compared to the percentage of galaxies around us locally?

Accepted Answer

More distant galaxies are seen at an ear

Question 14

What makes up the majority of the mass of an individual spiral galaxy?

Accepted Answer

A)  a central supermassive black hole 
B)  dark matter 
C)  massive O- and B-type stars 
D)  cold molecular gas clouds 
E)  low-mass G-, K-, and M-type stars 
A)  a central supermassive black hole 
B)  dark matter 
C)  massive O- and B-type stars 
D)  cold molecular gas clouds 
E)  low-mass G-, K-, and M-type stars

Question 15

How does the appearance of an AGN depend on the viewing angle to the nucleus?

Accepted Answer

An AGN of any type (e.g., Seyferts or qu

Question 16

What determines the three-dimensional shapes of galaxies?

Accepted Answer

The velocities of st

Question 17

If a galaxy had no dark matter, how would the velocity (v) of a star that orbited far outside the visible extent of the galaxy depend on its distance (r) from the center of the galaxy?

Accepted Answer

A)  "$\nu$ $\infty$ r  - 2" 
B)  "$\nu$ $\infty$ r  - 1" 
C) "$\nu$ $\infty$ " 
D)  "$\nu$ $\infty$ r" 
E)  "$\nu$ $\infty$ r2" 
A)  "$\nu$ $\infty$ r  - 2" 
B)  "$\nu$ $\infty$ r  - 1" 
C) "$\nu$ $\infty$ " 
D)  "$\nu$ $\infty$ r" 
E)  "$\nu$ $\infty$ r2"

Question 18

Any asymmetry in the gravity field of a spiral galaxy can create spiral arms.

Accepted Answer

A) True 
 B)False

Question 19

Astronomers have evidence that all giant galaxies probably contain supermassive black holes.

Accepted Answer

A) True 
 B)False

Question 20

Although elliptical and irregular galaxies have been observed in both dwarf and giant varieties, we find only giant spiral galaxies.

Accepted Answer

A) True 
 B)False

Exam 20: Galaxies

Why is 21-cm radiation a good tracer of spiral arm structure?

The 3-D shape of an elliptical galaxy depends on the average velocity of its stars along each of the three axes.

How do we know that the stellar disks in spiral galaxies are flat? How do we know that elliptical galaxies are not flat?

Which of the following is NOT a difference between spiral and elliptical galaxies?

In some radio galaxies, we only see one side of the jet because:

Which of these statements about quasars is FALSE?

Star formation in spiral galaxies is enhanced in the spiral arms because more hot gas is concentrated there.

What happens to active galaxies when their AGNs run out of fuel?

The density of ordinary luminous matter can exceed the density of dark matter in which parts of galaxies?

The dark matter in galaxies is __________ the luminous material.

If a Seyfert galaxy's nucleus varies in brightness on the timescale of 10 hours, then approximately what is the size of the emitting region?

Why are a higher percentage of distant galaxies classified as AGNs compared to the percentage of galaxies around us locally?

What makes up the majority of the mass of an individual spiral galaxy?

How does the appearance of an AGN depend on the viewing angle to the nucleus?

What determines the three-dimensional shapes of galaxies?

If a galaxy had no dark matter, how would the velocity (v) of a star that orbited far outside the visible extent of the galaxy depend on its distance (r) from the center of the galaxy?

Any asymmetry in the gravity field of a spiral galaxy can create spiral arms.

Astronomers have evidence that all giant galaxies probably contain supermassive black holes.

Although elliptical and irregular galaxies have been observed in both dwarf and giant varieties, we find only giant spiral galaxies.

Why Learn Astronomy

Patterns in the Skymotions of Earth

Motion of Astronomical Bodies

Gravity and Orbits

Light

The Tools of the Astronomer

The Birth and Evolution of Planetary Systems

The Terrestrial Planets and Earths Moon

Atmospheres of the Terrestrial Planets

Worlds of Gas and Liquid the Giant Planets

Planetary Adornmentsmoons and Rings

Dwarf Planets and Small Solar System Bodies

Taking the Measure of Stars

Our Starthe Sun

Star Formation and the Interstellar Medium

Evolution of Low-Mass Stars

Evolution of High-Mass Stars

Relativity and Black Holes

The Expanding Universe

The Milky Waya Normal Spiral Galaxy

Modern Cosmology

Large-Scale Structure in the Universe

Life

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