1) In 1987, a supernova went off inside the nearby galaxy known as the Large Magellanic Cloud. Theorists believe that there should be a remnant pulsar left behind, but it has not yet been directly detected by observers.

a) (7 pts) Briefly describe what a pulsar is and explain why we might not be able to detect it from the Earth.

b) (7 pts) Describe what causes this most common type of supernovae, the type II (which occur in massive stars only, not white dwarfs in binary systems).

2) Recent observations tell astronomers that stars, gas and dust moving around a common point in the center of our galaxies help "prove" the existence of a supermassive black hole there.

a) (7 pts) Explain how we "prove" the existence of black holes.

b) (7 pts) Suppose there is a big black hole at the center of the galaxy, and a star is orbiting around it. Will the star eventually be sucked in? Why or why not?

3) Regions in the disk of our galaxy tend to have a blue color and a high average metallicity for stars relative to other parts of the galaxy. Explain each of these two properties (7 pts each).

4) Many scientists speculate on what the motions of our galaxy would be like in the absence of dark matter.

a) (4 pts) On the graph below (which shows the true Keplerian motion, as for our solar system), assume our galaxy has no dark matter. Graph the average rotation velocity of our Galaxy vs the distance from the center. If it is the same as the Keplerian motion for our solar system, simply write "no change" on the graph.

b) (10 pts) How would your graph change if dark matter were present? Explain why.

5) One possible candidate for dark matter is a collection of planet-sized masses distributed throughout the galaxy, called MACHOs. These MACHOs emit no light of their own, but we do have a way to detect their presence.

a) (10 pts) With the help of a simple diagram, explain how we can detect a MACHO. In addition, explain why a similar technique cannot be used to find solitary black holes.

b) (4 pts) There are many stars in the night sky that vary in brightness. What two unique properties do MACHO lens light curves have compared to other kinds of stellar variations? You can just name the two, no need for explanation here.

6) Explain how we can determine the distance to a galaxy simply by knowing the rotation speed of a galaxy (via Doppler shifts of emission lines in ISM clouds) and by looking at two high-resolution photographs of the same galaxy taken 30 years apart (which show a given cloud's position slightly changing by some small angle A in the two photographs). A simple diagram may greatly help your answer, but it is not required. A diagram alone without explanation is *not* sufficient.

7) One problem with the standard candle technique for distance determination is finding bright objects with predictable absolute luminosities.

a) (7 pts) Explain why certain planetary nebulae (PN) in galaxies have predictable absolute luminosities.

b) (7 pts) Explain what causes a type Ia supernovae and why it has such a predictable brightness. Briefly mention why these are harder to observe.

8) One popular theory for the different origins of elliptical galxies and spiral galaxies has to do with galaxy mergers.

a) (8 pts) What are two major differences between spirals (S) and ellipticals (E) and how does the merger hypothesis explain each difference?

b) (6 pts) The fact that most E's are found near the center of galaxy clusters is seen as evidence in favor of the merger hypothesis. Explain why.