1) Most stars emit continuous radiation, which is also called "blackbody radiation". This characteristic spectrum makes it easy to identify a star's temperature.

a) (2 pts) Briefly explain why continuous radiation is called "blackbody radiation".

b) (6 pts) Name and explain the two things that happen to a blackbody radiation curve as the object's temperature rises. Your explanation should focus on why each change occurs.

2) From "Philosophy and the Scientific Method", we learned about the nature of scientific ideas and systems of knowledge.

a) (3 pts) What is the primary difference between a scientific and a non-scientific idea?

b) (5 pts) Describe the major weakness of a belief system that relies on irrefutable ideas.

3) Two interesting problems associated with the theory of nuclear fusion in the Sun's interior are the "solar neutrino problem" and the "faint sun paradox". Describe each of these two problems, then explain the resolution to each.

4) Although all main sequence stars have a large abundance of Hydrogen in their outer layers (usually 90% or more of the star's composition), stars that are much hotter and much cooler than our Sun do not show any evidence of Hydrogen absorption or emission lines in their spectra.

a) (6 pts) Explain both cases: Why are Hydrogen lines typically weak in very hot stars? Why are Hydrogen lines typically weak in very cool stars?

b) (2 pts) Suppose a star has weak Hydrogen lines. Describe briefly how you would determine whether this is due to the star being too hot or too cold.

5) Sirius and Capella have the same spectral line widths; however, Sirius has a much bluer color than Capella. Which of these two stars probably is larger? As part of your answer, explain why the spectral line width of a star has anything at all to do with the size.

6) Below is a side view of the situation in which an observer on the Earth is observing a binary star system (with a stationary central star and a companion star in a circular orbit) that is tilted a bit with respect to the observer's line of sight. There are no eclipses in the light curve, so both stars are always visible, but they are so far away that they appear as a single blob of unresolved light. We only know it is a binary by observing its spectrum over time.

At point A, the companion star is moving across our line of sight. Would you expect the spectral lines for this system to be merged or split here? What about at point B? Explain both answers with a single sentence each.

7) The SETI (Search for Extra Terrestrial Intelligence) project has high hopes of detecting signs of other civilizations someday. So far, however, there is no sign. We try to calculate our chances for success by using the Drake equation, which tells us that the number of civilizations we expect to observe (N) depends on the average lifetime of a civilization (L).

a) (4 pts) Explain why there should be a relationship between N and L.

b) (4 pts) The Fermi paradox is associated with SETI. What is it, and how might it be resolved?

8) Fastballs are a very high energy type of cosmic ray that hits the Earth occasionally. Cosmic rays are an interesting field of study for some Astronomers.

a) (4 pts) Why do cosmic rays seem to come evenly from all directions on the sky even though their original sources are likely concentrated in only a few places?

b) (4 pts) Why do Astronomers think that the source of fastballs is an object that is relatively nearby, within our own galaxy?

9) When the Sun ends the main sequence part of its lifetime, it will change from a yellow main sequence star into a red giant.

a) (4 pts) Explain why, even though the core of the Sun is emitting much more luminosity at this point, the surface temperature of the Sun cools off and the Sun becomes red.

b) (4 pts) We see red, yellow and blue stars in the night sky. Why don't we see any green stars?

10) A kind of star commonly found in star clusters (common to the galactic halo) are "blue stragglers".

a) (5 pts) What are blue stragglers, and what process makes them evolve differently from ordinary stars?

b) (3 pts) Stars and clusters are typically found in the galactic halo, but very little gas and dust is found there. Explain why.

11) One possible form of dark matter is MACHO's, or planet-sized objects distributed throughout the galactic halo.

a) (4 pts) Describe how we detect MACHO's.

b) (4 pts) Describe the experiment in which we used the Hubble Space Telescope to look for very low mass red stars to determine whether they account for some of the dark matter.

12) For distances to other galaxies, we often use the Tully-Fisher (TF) relation, which tends to work very well for edge-on spiral galaxies.

a) (4 pts) Briefly describe how you could use the TF method to determine the distance to an edge-on spiral galaxy (assuming to intervening interstellar material).

b) (4 pts) If the galaxy you are observing is inclined with respect to your line of sight, how will that affect your estimate of its distance? Justify your answer fully.

13) Below is a simple version of the graph used in the Hubble Law distance determination technique. For simplicity, let's assume in this question that there is no comological constant, no accelerating force in the Universe. Let's also assume that gravity in the Universe is negligible and that the Universe today is 12 billion years old.

Now imagine you are an Astronomer observing the Universe 6 billion years ago when it was only 6 billion years old. How would this graph (which represents the 12 billion year old universe described above) look different from its current state? Indicate your answer on the graph below (or, if the graph looked the same back then, just write "no change" on the graph). Next to the graph, explain why you changed (or didn't change) the graph in the way you did.

14) Astronomers believe that during the early Universe, there was an era of nucleosynthesis, when some of the Hydrogen got converted into Helium.

a) (4 pts) Explain why no fusion occured prior to a time of about 1 second, and explain why no fusion occured after about 3 minutes.

b) (4 pts) Explain how and why we can use the abundances of the elements in the Universe as an independent means to estimate the density of the Universe.