Physics 5
Introduction to Astronomy

H. E. Smith                                                                                                       Spring 2007

Quiz 6 Answer Key

Physics 5 – Spring Quarter 2007

Prof. Smith

 

1. Which statement concerning black holes masses and Schwarzschild radii is NOT true?

A. For black holes produced in massive star supernovae, Schwarzschild radii are typically a few to a few tens of kilometers.

B. The more massive the black hole, the larger the Schwarzschild radius.

C. The maximum mass for a black hole corresponds to one with a Schwarzschild radius of 100 km.

D. Even an object as small as you could become a black hole if there were some way to compress you to a size smaller than your Schwarzschild radius. 

 

2. Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?

A. Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.

B. The clock will fall toward the black hole at a steady rate, so that you'll see it plunge through the event horizon within just a few minutes.

C. Time on the clock will run faster as it approaches the black hole, and light from the clock will be increasingly blueshifted.

D. The clock will fall faster and faster, reaching the speed of light as it crosses the event horizon.

 

3. How does the diameter of the disk of Milky Way Galaxy compare to its thickness?

A. The diameter is about 100 times greater than the thickness.

B. The diameter and thickness are roughly equal.

C. The diameter is about 100,000 times greater than the thickness.

D. The diameter is about 10 times greater than the thickness.

 

4. All the following types of objects are found almost exclusively in the disk (rather than halo) of the Milky Way EXCEPT ______.

A. high-mass, red supergiant stars

B. globular clusters

C. young stars

D. x-ray binaries

 

5. What is the best evidence for an extremely massive black hole in the center of the Milky Way?

A. We can see stars vanishing in the center of the Galaxy as they are sucked into the black hole and vanish forever from our sight.

B. The center of our galaxy hosts a pulsar which is spinning so fast that it could only be a black hole.

C. Huge amounts of X-rays are pouring out of the center of the galaxy, an incredibly high luminosity which could only be sustained by an accretion disk surrounding a super massive black hole.

D. The 3-D motions of stars in the center of the Galaxy and Newton's version of Kepler's 3rd law indicate that the central object has a mass of 3-4 million solar masses in a region no bigger than our Solar System.

 

6. How does the interstellar medium affect our view of most of the galaxy?

A. It absorbs all wavelengths of light.

B. It produces so much visible light that it is opaque and blocks our view of anything beyond it.

C. It has no effect on visible-light observations, but prevents us from studying the galactic center with radio waves or X-rays.

D. It prevents us from seeing most of the galactic disk with visible, ultraviolet, and some infrared light.

 

7. Briefly explain why stars that formed early in the history of the galaxy contain a smaller proportion of heavy elements than stars that formed more recently.

 

The star-gas-star cycle gradually enriches the interstellar medium with heavy elements. Therefore, stars that formed early in the history of the galaxy were formed before much enrichment from supernova events could take place. Stars that formed more recently where formed from material that had been enriched by the many previous generations of stars.