1) One of the most famous spectral line fingerprints ever observed by Astronomers in interstellar clouds is that of "Nebulium", an element some scientists thought only existed in interstellar space, since its fingerprint was never observed on Earth. It turns out "Nebulium" is an example of a forbidden line of ionized Oxygen. Explain what property of an atom, electron and/or energy level causes a line to be considered "forbidden", and explain why we never see "forbidden" lines in laboratory spectra on Earth.

2) This question is about the interstellar medium, its effects on our observations of stars and perhaps life on Earth.

a) (4 pts) If we fail to take reddening into account when looking through the interstellar medium (ISM) at a distant star, our estimate of the distance will be (overestimated or underestimated?). Choose one, and justify your answer.

b) (4 pts) In some ISM clouds, nearby stars shine Ultraviolet (UV) light onto the clouds, and this can lead to very complex organic molecules, some of which may have made their way to Earth long ago to help start up the first complex chemical reactions that led to life, according to some theories. Explain how UV light contributes to the formation of such complex organic molecules in interstellar clouds.

3) Below is a view of a binary star system seen almost edge-on from an observer on the Earth. You may assume that the central star is motionless with respect to the Earth. The direction of motion of the companion star is indicated. In position A, the companion star is *not* eclipsing the central star, but it is moving transverse to the line of sight of the observer. When the companion star is in position A, are the spectral line fingerprints from the two stars in this system merged or split? Explain. As part of your answer, explain why spectral lines sometimes merge and sometimes split in binary systems.

4) Explain why the innermost part of the Sun transports energy outward in a radiative fashion instead of some other way. As part of your answer, explain why energy transport changes to a different method (convection) in the Sunıs outer layers.

5) Nuclear fusion is the source of the Sun's energy, and we hope to someday harnass this energy source in a practical way here on the Earth.

a) (4 pts) Explain why high temperatures and high densities are needed in order for nuclear fusion to work in the core of the Sun.

b) (4 pts) Some scientists have claimed to achieve fusion in lab experiements at room temperatures. Name and very briefly explain two reasons why these experiemental claims are not generally accepted by the scientific community.

6) Note the location of star A on the H-R diagram given below.

a) (4 pts) Is star A larger than, smaller than or the same size as the Sun? Justify your answer (show work).

b) On the H-R diagram above, plot the location of a star with one-half the surface temperature of the Sun and 10 times the size (radius) of the Sun. The position doesnıt have to be exact, just the right general vicinity. Show work above for possible partial credit.

7) Name and briefly explain two separate pieces of evidence that confirm gas in the Sunıs corona gets hotter as one looks further away from the surface of the Sun. You do not need to explain why the temperature gets hotter like this, just explain the evidence proving it.

8) For the purposes of this problem, assume that the Universe is 10 billion years old. Also assume that the Universe is undergoing no acceleration or deceleration, that it is expanding at a constant velocity and always has been. The graph below shows the Hubble relation between radial velocity and distance for galaxies as it appears today. On the graph, sketch how this relationship looked when the Universe is 5 billion years old. If the graph will remain unchanged, just write "no change" on the graph. Explain why you changed (or didnıt change) the graph in the way you did.

9) Describe what the Helium-3 instability is, and explain why it may have a long-term impact on the Earthıs climate.

10) Suppose there were no dark matter in our galaxy. How would our rotation curve look with respect to the Keplerian curve? Would it be identical to the curve or different somehow? In either case, justify your answer. For your reference, the Keplerian rotation curve has been reproduced below.

11) The Tully-Fisher relationship shows that the absolute luminosity of a galaxy is directly proportional to its rotation speed. Explain how you could use this relationship to determine the distance to some edge-on spiral galaxy? Also, explain why this method of distance determination doesnıt work well for very high redshift galaxies.

12) Two possible candidates for the dark matter in our galaxy are very low mass stars and brown dwarfs.

a) (4 pts) Briefly describe how astronomers attempted to determine whether very low mass stars make up the dark matter. What was the result of these observations?

b) (4 pts) Astronomers also attempted to find brown dwarfs, but we now know that their initial searches found far fewer brown dwarfs than exist in reality. Explain how initial searches for brown dwarfs were biased to find too few.

13) While observing an eclipsing binary system, an Astronomer takes a series of spectra, showing the spectral fingerprints of each star as the companion star's lines shift back and forth around the stationary central star's lines. The Astronomer measures the amount of this shift and also how long it takes for the shifting pattern to repeat.

a) (5 pts) Explain how this data could be used to determine the mass of the central star in the system.

b) (3 pts) Similar techniques have been used to guess that there is a black hole at the center of our galaxy. The region around this area has been observed using radio telescopes, but we are unable to observe it with optical telescopes. Explain briefly why we can see radio wavelengths of light from this region but not visible wavelengths of light.

14) Two important components of the Big Bang theory are Hubble's Law (the correlation between radial velocity and distance for galaxies) and the Microwave Background Radiation (MBR).

a) (4 pts) Explain why the MBR is seen as more convincing evidence for the Big Bang, compared to Hubble's Law.

b) (4 pts) Most alternative theories to the Big Bang are not considered seriously by the scientific community, usually because they lack a certain quality. What is the major weakness of non-scientific theories that make them unsuitable as a basis for a system of knowledge? Explain.