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{\bf 29:119 Introduction to Astrophysics I\\
     Fall 2008\\
     Second Hour Exam \ldots November 20, 2008}
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Start each question on a new page.  It allows me to make comments and generally keeps me in a better mood.  Write legibly. Explain what ideas you are using and what you are trying to do. There are 10 questions.   Good luck and no whining.
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{\bf Walk with Ursus!!!}
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(1) Consider a binary star with components A and B having masses $m_A$ and $m_B$.  Describe the motion of the stars A and B.  {\bf Point of clarification:} Answer what you can, given the very small amount of information given.\\

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(2) There is a temperature gradient from the center of the Sun to its radius.  Explain why this demonstrates that there must be a flow of energy from the center of the Sun to its surface.  \\

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(3) The Balmer absorption lines of hydrogen are strongest for stars of spectral class A and are weaker for hotter and cooler stars.  Explain what causes this behavior.  \\

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(4) Consider an atom which has three states, the ground state, the first excited state, and a second excited state at a higher excitation energy.  What determines the relative populations of the three levels?    \\

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(5) Write down the radiative transfer equation, at least as well as you remember it (small errors will be tolerated).  Explain what the various terms are describing physically.  Be sure to define the most important terms in the equation.  \\

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(6) What does the Saha equation describe?\\

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(7) The solar disk is slightly darker near the edge than at the center (limb darkening).  Explain why this occurs.  What would be necessary for the Sun to show limb brightening? \\

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(8) What is the name of the process by which helium nuclei undergo fusion to produce carbon.  Does it occur in the Sun?  Why or why not?   \\

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(9) Assuming that we are viewing radiation from a source in thermodynamic equilibrium.  If it is optically thick, what function describes the intensity of the radiation as a function of wavelength which we observe? \\

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(10) In HI clouds, the temperature is about 100K and the density is approximately $10^8$ H atoms/m$^3$.  Approximately what is the mean free path between atomic collisions?  {\bf Information you will need:} $k_B = 1.38 \times 10^{-23}$, $m_H = 1.67 \times 10^{-27}$ kg $a_0 \mbox{ (radius of Bohr atom in ground state) } = 5.29 \times 10^{-11}$ m.  \\
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