Physics 5
Introduction to Astronomy

H. E. Smith                                                                                                       Spring 2007

Quiz 8 Answer Key

Physics 5 – Spring Quarter 2007

Prof. Smith

 

 

1. How does gravitational lensing tell us about the mass of a galaxy cluster?

A. Newton's universal law of gravitation predicts how mass can distort light, so we can apply Newton's law to determine the mass of the cluster.

B. The lensing allows us to determine the orbital speeds of galaxies in the cluster, so that we can determine the mass of the cluster from the orbital velocity law.

C. Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.

D. The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.

 

2. Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did NOT contain dark matter. How would their rotation curves be different?

A. The orbital speeds would fall off sharply with increasing distance from the galactic center.

B. The rotation curve would be a straight, upward sloping diagonal line, like the rotation curve of a merry-go-round.

C. The orbital speeds would rise upward with increasing distance from the galactic center, rather than remaining approximately constant.

D. The rotation curve would look the same with or without the presence of dark matter.

 

3. 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?

A. All quasars have large redshifts.

B. 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.

C. We now have images and spectra that clearly show quasars to be embedded at the centers of distant galaxies and within distant galaxy clusters.

D. No one could think of a way to explain quasar speeds if they are nearby, so we concluded they must be far way.

 

4. It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?

A. Stars in elliptical galaxies are dimmer, making them harder to study.

B. We cannot observe spectral lines for elliptical galaxies.

C. Elliptical galaxies contain much less dark matter than spiral galaxies, so it's much more difficult to measure.

D. Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.

 

 

5. Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?    

A. The temperature of the gas tells us the gas density, so we can use the density to determine the cluster's mass.

B. Temperature is always directly related to mass, which is why massive objects are always hotter than less massive objects.

C. The question is nonsense --- gas temperature cannot possibly tell us anything about mass.

D. The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.

 

6. 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. Supernovae occur so frequently that their effects combine to drive a galactic wind that blows material into intergalactic space.

C. The observed features of the starburst are thought to be caused by the presence of a supermassive black hole in the galaxy's center.

D. Its rate of star formation is many times higher than the rate of star formation in the Milky Way.

 

7. Briefly explain how we think quasars produce so much energy.

 

The energy is produced in the accretion disk around a supermassive black hole.