a) (4 pts) Which of these two stars is probably larger in size (A, B, same size)? Justify your answer in the space below.

b) (4 pts) Assume that star A is at rest relative to the Earth. The radial velocity of star B is (toward Earth, away from Earth, zero). Justify your answer.

2) Star Alpha has a smaller parallax angle than Star Beta but the same apparent luminosity. The two stars are known to have the same size (radius) based on their spectral line widths. Which of the two stars will show evidence of higher ionization species in its spectrum? Explain.

3) Name and explain two different pieces of evidence that tell Astronomers the solar corona's temperature increases as one looks further away from the Sun. I am not asking for an explanation of the mechanism behind this heating (equipartition of energy argument) but rather the actual observations that prove the temperature gets hotter further out. Name each piece of evidence and explain why it relates to the temperature.

4) Some stars, despite the fact that they have the same abundance of Hydrogen as the Sun, have much weaker Hydrogen absorption lines. Explain why very cool stars often have weak Hydrogen lines, then explain why very hot stars also have weak Hydrogen lines.

5) Suppose our galaxy had no dark matter. What would the rotation curve for the disk of the galaxy look like? Draw in what you think it would look like on the graph below. The Keplerian curve (which we see in our solar system) is included for reference. If the galaxy¹s rotation curve would exactly match this, just write "no change" on the graph below. After drawing in your curve, explain why you drew it the way you did (or wrote "no change").

6) During star formation, a cold, dense cloud of gas and dust slowly shrinks. Eventually, nuclear fusion ignites in the center and a star is formed. During that shrinking process, but before the onset of nuclear fusion, state in one word (increasing, decreasing, unchanging) and explain (one sentence why for each one) what is happening to the self-gravity, the density, the temperature and the outward-pushing pressure of the cloud.

7) Explain why the Sun appears red at sunset. If we do not correct for this reddening effect when using the inverse square law to determine the distance to the Sun or any other star, we will (underestimate or overestimate?) the distance. Explain why.

8) Suppose you observe the spectrum of a binary star system (the system is so far away that the light from the two stars blends together, it is an "unresolved binary" system). The spectral lines from the system split and merge, split and merge, etc. over time. Assuming we are observing this system edge-on, explain what you would measure and how you would use your measurements in order to determine the central mass of this binary system.

9) This question compares the gravitational force of a black hole to the gravitational force of a normal star like the Sun.

a) Cody feels a certain gravitational force when standing on the surface of the Sun (he's got a really great space suit, so he can survive), about 700,000 km from the center of the Sun. Sarah also feels a gravitational force when in her spaceship, parked 700,000 km away from the center of a one-solar-mass black hole. Who feels a stronger gravitational force, or do both feel the same force? Explain.

b) Suppose Cody flies his spaceship inside the Sun into the radiative zone. He is now about 100,000 km away from the center of the Sun. Sarah also flies her ship closer to the center of the black hole (but still well outside the event horizon) so that she is parked 100,000 km from the center of the black hole. Who feels a stronger gravitational force, or do both feel the same force? Explain.

10) Explain why Helium fusion requires a higher density and temperature compared to Hydrogen fusion, then explain how and why the onset of Helium fusion in low mass stars sometimes leads to a planetary nebula.

11) Explain how we think elliptical galaxies form differently from spiral galaxies. How does this formation process account for the lack of gas and dust in ellipticals? How does it account for the red color of ellipticals (as opposed to the blue color of spirals)?

12) Below is a linear Hubble diagram, which shows what our observations of the Universe would look like if the Universe were expanding uniformly without acceleration or deceleration. Astronomers had hoped to use observations of very distant galaxies on this diagram to determine the overall density of the Universe. Explain how and why they could do this (you don't need to go into the actual observations that showed the Universe is apparently accelerating, just what we expected to see originally).

13) Suppose we measure the abundance of Hydrogen in the spectra of galaxies at a variety of distances from the Earth. Some of the galaxies in our sample are billions of light years away, and some are relatively close. On the graph below, sketch the trend of how you think the observed Hydrogen abundance would vary with distance (would it be constant, increasing or decreasing?), then explain why you drew the graph the way you did.

14) An important aspect of current SETI research is planned NASA missions to Mars and Europa to find out if life has ever existed (or currently exists) in any form on either of these two worlds. If we do discover traces of life (just the simplest microorganisms, nothing intelligent or civilized), explain how and why this will affect our estimate of "N", the number of intelligent communicative civilizations in the galaxy.