1) During the Helium fusion phase of a star's lifetime, the star expands enormously in size. Name and explain two reasons why this occurs.

2) The equation of escape velocity tells you how hard it is to escape from an object. The higher the escape velocity, the harder it is to escape the gravitational pull.

a) (4 pts) Star X is twice as large (size) as the Sun, but it has the same mass. Escaping from the surface of star X would be (harder, easier, about the same) as escaping from the surface of the Sun. Show work.

b) (4 pts) Suppose ship A is 500,000 miles from the center of a one-solar-mass black hole. Star B is 500,000 miles away from the center of star X, which has a radius of about 800,000 miles. Which ship will have a harder time moving outward from its present position due to the force of gravity? Explain.

c) (6 pts) Explain how we use the equation of escape velocity to help define the boundary of a black hole.

3) Prior to its collapse into a star, an interstellar cloud of gas and dust has a relatively high density (compared to its surroundings) and low temperature.

a) (6 pts) The gas surrounding this cloud is in pressure equilibrium with the cloud and has a much lower density. The temperature of the surrounding gas will likely be (hotter, cooler, about the same) as the temperature of the cloud. Explain your answer.

b) (8 pts) Explain (don't just state) what happens to the temperature and density of this interstellar cloud as it collapses to form a star (but before fusion begins in its core).

4) Below you will find the period-luminosity relationship for Cepheid variable stars as well as the light curves of two Cepheids, A and B. Assume for the purposes of this problem that stars A and B have the same apparent luminosity:

Assuming no interstellar extinction along the line of sight to either star A or B, which of these two stars is further away from the Earth? Explain your answer.

5) Although the disk of the Milky Way galaxy generally has a blue color, the stars within the disk have a variety of ages.

a) (7 pts) Explain why the light from the disk of the Milky Way as a whole is blue.

b) (7 pts) The vast majority of stars in the halo of our galaxy are red. Explain why this is true.

6) Suppose star X is at the main sequence turnoff point in the H-R diagram for the Alpha Cluster of stars. Based on spectroscopic observations, Astronomers have deduced that star X is 90% as massive as the Sun, which has a main sequence lifetime of 10 billion years.

a) (7 pts) What is the main sequence lifetime of star X relative to the Sun (longer, shorter, about the same)? Explain why the lifetime of a star is related to the star's mass.

b) (7 pts) Explain how we know that the current age of star X is equal to its main sequence lifetime.

7) Suppose we know the angular size of a planetary nebula, the distance of this nebula from the Earth and the velocity of the expanding gas at the edge of the nebula (via Doppler shift measurements). Explain how we can use these three pieces of information to deduce the age of the nebula (the time since the nebula first exploded).

8) Below is the spectrum showing the absorption pattern of Nitrogen gas in the spectrum of a star with an intervening interstellar cloud along the line of sight to the star (the cloud also contains some Nitrogen)

a) (2 pts) Which set of lines represents the absorption of the star (wider lines or narrower lines)?

b) (8 pts) Explain your answer to part (a). Just saying something like "because interstellar cloud lines are narrower than stellar lines" is circular...you must explain why line widths are wider or narrower for interstellar clouds).

c) (4 pts) Assuming the star is stationary with respect to the Earth, how is the gas cloud moving with respect to the Earth (toward, away, not moving radially)? No explanation needed here.