1) The gas and dust in a galaxy determines the galaxy's history, type and color. The two major types of galaxies, Spirals and Ellipticals, have very different properties when you look at the gas and dust in them.

a) (7 pts) Although stars in the Milky Way orbit around the center of the galaxy both in the halo and in the disk, virtually all of the gas and dust in our galaxy is located in the disk. Explain why the gas/dust in the disk instead of in the halo.

b) (7 pts) Explain the process by which elliptical galaxies probably lost all of their gas, which explain why they have so little gas and dust compared to spirals.

2) Another distance determination technique involves the use of masers, interstellar clouds that emit a certain wavelength of light very strongly. By tracking the motion of these masers in an edge-on galaxy, we can estimate the distance to the galaxy.

a) (10 pts) Explain how we estimate distances using masers. What quantities do we measure, and how do we use these to calculate the radial distance to the galaxy? A simple diagram may help, but by itself, it is not a sufficient answer.

b) (4 pts) Like many other techniques, the maser method is limited to galaxies within a certain distance to the Earth. Briefly explain why.

3) Two very mysterious, highly energetic objects in the Universe are quasars and gamma-ray bursters, both thought to lie at very large distances from the Earth.

a) (8 pts) Describe what we think is the energy source for quasars. As part of your answer, state why Astronomers donąt think quasars have the same sort of energy source as galaxies.

b) (6 pts) Explain how we know that gamma-ray bursters are extremely far away from the Earth.

4) In the past five years or so, many Astronomers have come to accept the observations that show the Universe apparently contains some sort of "Cosmological Constant" (CC), or repulsive force that pushes galaxies apart.

a) (6 pts) Explain where the CC first originated in theory. Why did some Astronomers long ago originally think there was a CC in the Universe?

b) (8 pts) Describe the evidence today that there is some sort of CC in the Universe.

5) The Standard Candle technique is a very common method of distance determination. Suppose we find a globular cluster in a distant galaxy and assume it has the same absolute luminosity as a well-known cluster in our own galaxy.

a) (6 pts) Explain what we could then do to estimate the distance to the galaxy using this information.

b) (8 pts) This technique is unreliable because the absolute luminosity of globular clusters varies so much. Explain how and why, by carefully selecting a certain cluster in the target galaxy, you could more accurately estimate the distance.

6) The early composition of our Universe, about 90% Hydrogen and 10% Helium, was determined during the era of nucleosynthesis, which lasted from about a time of 3 seconds until a time of about 3 minutes old.

a) (8 pts) Explain why no heavy elements were created in the Universe prior to a time of about 1 second. Also, explain why nucleosynthesis ended after 3 minutes.

b) (6 pts) Explain why the abudance of Helium in the Universe today is related to the overall density of the Universe.

7) For this particular problem, you may assume that there is no acceleration or deceleration of the expansion of the Universe due to gravity or any kind of cosmological constant (just trying to keep it simple). Today, when the Universe has an age of about 12 billion years, we can draw a Hubble relation as below. Suppose Astronomers had been alive 6 billion years ago and drawn a Hubble diagram based on the data they observed at that time. Would it look different? If so, show on the diagram how it would change. If it wouldnąt look different, just write "no change" on the diagram. In either case, justify your answer in 2-3 sentences.

(Helpful hint: Think about the diagram of the car race...how does it look after 2 hours? After 1 hour?)

8) Two factors in the Drake equation are f(L), the fraction of earth-like planets on which life will develop, and L, the lifetime of an average civilization once it begins trying to communicate across interstellar distances.

a) (7 pts) Explain how and why current research on Jupiterąs moon Europa and the planet Mars will affect our estimate of f(L).

b) (7 pts) Why is L relevant in the Drake equation?