1) Astronomers believe that the Sun's energy source is nuclear fusion of Hydrogen into Helium in the Sun's core. All main sequence stars are thought to have this same energy source, but some collapsed objects don't ever attain the conditions necessary for fusion in their core and become "brown dwarfs".

a) (4 pts) Explain why high temperature and high density are required in order for nuclear fusion to take place.

b) (4 pts) Because brown dwarfs and very low mass red stars both emit most of their light in the red and infrared portion of the spectrum, they can be difficult to distinguish. Astronomers have found when searching for brown dwarfs that there is a characteristic spectral signature that separates brown dwarfs from very low mass red stars. What is this signature, and what is the origin of this difference? In other words, what causes it?

2) One problem with the theory of nuclear fusion as the Sun's energy source has been dubbed the "solar neutrino problem".

a) (4 pts) Explain what the "solar neutrino problem" is, and briefly state the resolution of this problem.

b) (4 pts) Despite the solar neutrino problem, most Astronomers (correctly) continued to accept that nuclear fusion is the Sun's energy source. Sometimes, though, we cling to theories in spite of clear evidence to the contrary. What is the biggest problem with relying on irrefutable beliefs (beliefs that cannot be falsified)?

3) We study the temperature structure of the Sun's corona by observing emission lines coming from this gas. By looking at spectral line widths and ionization species, it is clear that the coronal gas is hotter further from the surface!

a) (4 pts) Explain what an ionization species is, then explain why ionization species are related to the temperature of a gas.

b) (4 pts) Explain why spectral line widths are related to the temperature of a gas.

4) Two geometric distance determination techniques that we have discussed in class are the parallax method and the maser technique. Diagrams will help you on both answers, but they are not required. Diagrams without explanations will not be given credit.

a) (4 pts) Explain how we use the parallax method to find the distance to a nearby star (what do we measure, and how is it used to solve for the distance).

b) (4 pts) Explain how we use the maser technique to find the distance to a nearby galaxy (what do we measure, and how is it used to solve for the distance).

5) Star Alpha and Beta have the same apparent luminosity. Both have the same red color (and the same peak wavelength in their spectrum). Careful parallax measurements show that star Alpha has a larger parallax angle. Which star would you expect to have broader spectral lines? Justify your answer fully.

6) The interstellar medium often obscures our view of distant stars and galaxies, and we must understand its properties before we can reach any firm conclusions about the properties of these targets.

a) (4 pts) Explain two different methods we can use to determine "X", the interstellar correction factor needed in the inverse square law.

b) (4 pts) Sometimes ISM clouds emit forbidden lines. Explain why forbidden lines only occur in very low density environments.

7) An Astronomer views the light from a binary star system. This system is so far away that the two stars appear as one blob of light. The system is also tilted a little bit with respect to our line of sight, so that the two stars do not eclipse one another during the orbit of the companion. The central star remains stationary with respect to us, but the companion star is moving as shown in the diagram below (a side view of the system).

If we try to calculate the mass of the central star in the binary system shown above by assuming that the orbital velocity of the companion is equal to the maximum measured radial velocity, will our estimate of the central star's mass be too low, too high or correct? Justify your answer.

8) Recent research into the structure and evolution of the Sun have led to an interesting problem called the "Faint Sun Paradox".

a) (4 pts) Explain why the size and absolute luminosity of the Sun very slowly increases during its time on the main sequence (the first half of its lifetime, not near the end when it begins to turn into a red giant).

b) (4 pts) Explain what the "Faint Sun Paradox" is, and explain the resolution to the paradox that makes everything work out.

9) What are blue stragglers, and why do they evolve differently from ordinary stars?

10) The Cepheid Period-Absolute Luminosity relation has been critical to our understanding of the properties of galaxies and the Universe.

a) (4 pts) Briefly explain how to use the P-L relation to find the distance to a nearby galaxy. As part of your answer, briefly state in a single sentence why this distance determination technique is limited to only nearby galaxies.

b) (4 pts) Explain how the Hubble Space Telescope was able to use the P-L relation to find the absolute luminosity of type Ia supernovae, which could then be used as standard candles for more distant galaxies.

11) Two interesting objects we've studied in Astronomy this semester are "fastballs" and quasars.

a) (4 pts) What is a fastball, and how do we know fastballs originate from some place relatively nearby in our galaxy?

b) (4 pts) Explain how we know the quasars have an absolute luminosity hundreds of times that of a typical galaxy.

12) Use a graph of Hubble's Law and the concept of lookback time to help explain how current observations lead us to the conclusion that the expansion of the Universe is accelerating.

13) SETI, the Search for Extra-Terrestrial Intelligence, has so far been unsuccessful despite a fairly significant effort on behalf of the Astronomical community to locate signals from other civilizations.

a) (4 pts) Explain what the Fermi paradox is, and briefly describe one possible resolution of this paradox related to SETI.

b) (4 pts) Some Astronomers feel that our past searches in the radio region of the spectrum tell us we should be looking instead for optical signals. Explain one reason why we think optical searches might be more productive than radio searches.

14) Our study of the dynamics of galaxies have led us to the conclusion that a substantial amount of matter in the Universe is "dark", giving off too little light for us to detect.

a) (5 pts) Describe the evidence in the rotation curve of our galaxy that leads us to believe our galaxy consists mostly of dark matter.

b) (3 pts) One possible dark matter candidate is planet-sized objects called MACHO's. Describe how we detect these objects.