1) Many scientists speculate on what the motions of our galaxy would be like in the absence of dark matter. 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. Explain your answer fully.

2) Stars can end their lifetimes in a variety of ways. Low mass stars typically end up as planetary nebulae, while high mass stars typically end up as supernovae.

a) (8 pts) Explain what causes planetary nebulae and why only low mass stars end up this way.

b) (6 pts) High mass stars typically end up as supernovae. Explain what causes these incredibly energetic explosions.

3) One of the only indicators Astronomers have for the age of a star is its composition, or metallicity. Color is sometimes an indicator of age, too.

a) (7 pts) Suppose we observe two main sequence stars, one blue and one red. Can we say with confidence which of these two stars has a higher metallicity? If yes, which one has a higher metallicity and why? If no, why canšt we tell?

b) (7 pts) Suppose we observe two clusters of main sequence stars. The light from one cluster is predominantly blue and the other is predominantly red. Can we say with confidence which of these two clusters is younger? If yes, which one is younger and how do we know? If no, why canšt we tell?

4) Molecular clouds are the source of newly formed stars and, some scientists argue, possibly the source of life on Earth.

a) (8 pts) New stars inside molecular clouds often emit large amounts of UV radiation. Explain what role UV radiation plays in the formation of the complex molecules that may be the precursors to life on Earth or other worlds.

b) (6 pts) As a molecular cloud collapses, state and briefly explain what happens to the density, temperature and outward-pushing presure of the cloud.

5) During the Sun's 10-billion-year main sequence lifetime, there will be some internal changes which will likely affect Earth.

a) (8 pts) Name and briefly explain two reasons why the luminosity of the core (and thus the overall star) increases gradually over time during the main sequence.

b) (6 pts) What is the "Faint Sun Paradox", and what is the resolution of this paradox?

6) As the Sun approaches the end of its main sequence lifetime, the overall luminosity will be its highest value yet, but the overall color of the star will change to red.

a) (6 pts) Explain why the Sun will change colors like this.

b) (8 pts) Eventually, the Sun will begin to burn Helium into Carbon in its core. Name and briefly explain two reasons why this process requires a higher temperature and density compared to Hydrogen fusion.

7) The colors of stars can often be surprising, and they can lead to misleading conclusions if not carefully considered.

a) (7 pts) What do we not see green stars in the night sky?

b) (7 pts) What exactly is a blue straggler? How do these stars stand out or differ from the stars around them, and what makes this difference come about?

8) Two hypothetical situations we have dealt with in this part of the course have dealt with lobsters (and extraterrestrials) and black holes.

a) (7 pts) What is the Fermi paradox, and according to Timothy Ferris, what is one possible response to this paradox?

b) (7 pts) Suppose Jack is standing on the surface of a one-solar-mass white dwarf, 5000 miles away from the center. Jill is drifting in space, 5000 miles away from the singularity of a one-solar mass black hole. Which of these two astronauts will feel a stronger gravitational pull (or will they feel the same pull)? Explain your answer.