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{\bf Introduction to Astrophysics II, 29:120\\
     Winter, 2009\\
Second Homework Set...January 29, 2009. Due February 5, 2009}\\
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Problems (1) - (4) use the interstellar extinction curve given in the online illustrations and diagrams under the title ``Measurements of the ISM''\\

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{\bf (1)} Consider light with a wavelength of 500 nm. For what size of particles is there a maximum interaction with light of this wavelength? \\

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{\bf (2)} Assume that all of the particles responsible for extinction at 500 nm
have the size calculated above.  (a) Using the interstellar extinction curve online, calculate the number density of such grains in the interstellar medium. (b) Assuming that the grains (i.e. interstellar dust particles) have a density of 1000 kg/m$^3$, what is the mass density of the interstellar medium {\em due only to grains}?  \\

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{\bf (3)} A spectral class A0V star is 2 kiloparsecs distant in the galactic plane.  How much is its light attenuated at a wavelength of 330 nm relative to the case if there were no interstellar extinction? \\

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{\bf (4)} Astronomers like magnitudes.  What is the apparent magnitude of the star in problem \# 4, including the effect of interstellar extinction? Assume that the visual magnitude of the star is defined at about 500 nm. \\

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{\bf (5)} Assume that a spectral class O5V star radiates like a blackbody.  Calculate its Lyman luminosity in photons/sec ($N$ in equation 12.32).  You will have to use {\em Mathematica}  to come up with an answer.  Be sure to make it clear what you are doing.  {\bf Hint:} You will have to do some searching, in the book or elsewhere, for some of the data that you need. 
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{\bf (6)} Assume that the star in problem \# 5 is placed in a cloud of hydrogen gas with a number density of $10^7 \mbox{ m}^{-3}$.  What is the radius of the HII region which is produced?\\

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{\bf (7)} Problem 12.4 from textbook \\

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{\bf (8)} Problem 12.5 from textbook \\

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{\bf (9)} Problem 12.7 from textbook \\


 
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