Physics 2083 - Introductory Astronomy - Spring 1997
Exam #2

Answer any 8 of the following 10 questions. Each is worth 12 points.

1) The Interstellar Medium (ISM) can be thought of as a very thin inter-cloud gas surrounding small pockets of very dense clouds. We know that a condition of Pressure Equilibrium exists in the ISM.

a) (3 pts) Is the inter-cloud gas hotter or colder than the pockets of dense clouds?

b) (9 pts) Explain your answer to (a) by using Pressure Equilibrium.

2) Draw a graph of a blackbody spectrum (intensity on the y-axis, wavelength on the x-axis) for a typical star like the Sun and a dust cloud (both on the same graph). Use this graph to help explain why it is easier to observe a dust cloud in the infrared. Label the parts of the graph where we are seeing visible and infrared (IR) light.

3) The line of sight between Earth and Star A contains no material whatsoever. The line of sight between Earth and Star B contains a dust cloud. Both star A and star B have the same intrinsic brightness and same intrinsic color.

a) (3 pts) If we ignore the effect of the dust cloud and try to determine the distance to star B, will we think B is further away or closer than it actually is?

b) (9 pts) Explain your reasoning for part (a).

4) Suppose I plot the properties of the star cluster M200 as below. In the diagram, each point marks an individual star within the cluster. The position of the Sun is marked on the diagram, but the Sun is not part of the cluster. The Sun has a main sequence lifetime of 10 billion years.

a) (3 pts) Would you expect a typical star in M200 to have a higher or lower metallicity than a star with an age of 10 billion years (or is there insufficient information to decide)?

b) (9 pts) Explain your answer to (a).

5) At the beginning of its main sequence lifetime, the Sun was actually a bit smaller than it is now. Explain why.

6) The mass of a black hole depends only upon the velocity of gas that is orbiting the black hole (measurable with Doppler shifts) and the distance of the gas from the center of the black hole. Describe how you could determine the actual distance (as opposed to the angular distance) between the gas and black hole. A diagram may prove helpful.

7) If you compare the properties of a main sequence Sun-like star to the properties of a more evolved giant star (with mass equal to the Sun's mass), you find some interesting similarities and differences.

a) (2 pts) Would you expect the giant star core to be bigger, smaller or the same size as the Sun's core (both stars have equal mass)? If there is insufficient information to determine the answer, say so.

b) (10 pts) Explain your answer to part (a).

8) Suppose we were awarded time on a radio telescope to study a strange, spinning cloud of cold, neutral Hydrogen gas. We decide to observe its 21-cm emission.

a) (3 pts) On one side of the cloud, we measure the wavelength of neutral hydrogen emission to be 20.95 cm. Is this part of the cloud moving toward us, away from us, or simply stationary? If there is insufficient information to determine the answer, then please say so.

b) (9 pts) Explain your answer to (a).

9) Suppose star A in a binary system evolves more quickly than star B, which is a star about the mass of the Sun. Describe how the evolution of star A affects the mass, luminosity and lifetime of star B.

10) Imagine you are observing a young cluster of very hot, very bright main sequence stars. Around two of the stars, you notice regions of ionized gas. The extent of the ionized region around star A is twice as large as the region around star B.

a) (3 pts) Which of the two stars is more massive (or are the masses equal)? If you cannot determine the answer from the information given, say so.

b) (9 pts) Explain your reasoning to part (a).