1) On the graph below, the continuous spectrum of the Sun has been plotted. Sketch the spectrum of a star (3 pts) that is somewhat hotter than the Sun, then answer the following in the space below:

a) (8 pts) Explain why the hotter object has a higher intensity of light at all wavelengths and why the peak wavelength of the hotter object is shorter than for the Sun.

b) (3 pts) If both of these objects were viewed through a filter that only allowed red light through, which would appear brighter? No need to explain this answer.

2) Even though all normal stars are composed mostly of Hydrogen atoms, some of them do not show very strong Hydrogen absorption lines in their spectra. We know the Hydrogen is there, but often its fingerprint is very weak or even nonexistant. Explain why very cool stars are unlikely to contain many Hydrogen absorption lines, then explain why very hot stars are unlikely to contain many Hydrogen absorption lines.

3) Two stars, A and B, have the same apparent luminosity and the same temperature (which we determine from their color, among other things). The spectral absorption lines in the spectrum of star A are much wider than those of star B. Which of these two stars probably has a larger parallax angle, if both are close enough to be measured using Earth's orbit as a baseline? Explain your answer.

4) Suppose the temperature structure of the photosphere is this: The temperature at the base of the photosphere is about 5000K while the temperature as one moves closer to the surface approaches 6000K. If an observer were to observe the color and brightness of the limb of the Sun and the center of the Sun's visible disk, how would these things compare? Would the center be brighter or dimmer? Redder or bluer? Same? As part of your answer, explain or draw a simple diagram showing the geometry of the different lines of sight into the Sun's atmosphere.

5) The process that generates energy in the core of the Sun is believed to be nuclear fusion, the combining together of Hydrogen nuclei to form Helium. The mass lost during this process is converted into energy.

a) (8 pts) Explain why nuclear fusion is thought to require both high temperatures (and thus, high nuclear speeds) and high densities in order to occur. It is not sufficient to simply say that high temperature leads to high speed, for example. You need to explain why high speeds are necessary. Similarly, *why* is high density necessary?

b) (6 pts) An alternative form of energy recently in the news here on Earth is "cold fusion", a process that supposedly uses fairly simple lab apparatus to achive fusion of atoms. Explain how cold fusion supposedly works at such low temperatures.

6) In the Sun's corona, we know that the temperature of the gas is higher the further away you look from the surface of the Sun while the density of gas is lower the further away you look from the surface.

a) (7 pts) Use the principle of equipartition of energy to help explain why this is true.

b) (7 pts) Would you be more likely to find lower ionization species (e.g. singly-ionized atoms) or higher ionization species (e.g. triply-ionized atoms) as you look further away from the surface of the Sun? Explain. As part of your answer, explain why ionization species are related to the temperature of the gas.

7) Irrefutable beliefs can be very compelling. The fact that these beliefs can't be disproven can be comforting, providing us with a basis of unchanging truths about the way the world works. However, this isn't the whole story.

a) (7 pts) Explain the major weakness of irrefutable beliefs when used as a basis for a system of knowledge.

b) (7 pts) Refutable beliefs can be (and usually are) disproven through scientific observations that serve as tests of the theories' predictions. Why, then, are refutable beliefs thought to be a better basis for knowledge among scientists?

8) Based on information in the graph below, determine the size (relative to the size of the Sun) of stars A and B. Show your work. You may answer either with a numerical ratio or simply like ³Star A is larger than the Sun² or ³Star B is the same size as the Sun², etc. Just be sure your answer is unambiguous and clearly marked (circled).