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Homework Help: Luminosity physics question

  1. Jan 31, 2007 #1
    1. The problem statement, all variables and given/known data
    Show that the Hubble profile of surface brightness

    [tex]I(r) = I_0 \left(1+\frac{r}{R}\right)^{-2}[/tex]

    leads to an infinite total luminosity. r is the distance from the center and R is a constant.

    2. The attempt at a solution
    For large r this is related as

    [tex]I(r) \propto r^{-2}[/tex]

    but how is the surface brightness related to the total luminosity?
  2. jcsd
  3. Jan 31, 2007 #2


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    Science Advisor
    Homework Helper

    Can't you find it in the textbook ? Even though the problem seems to be purely mathematical (basically a computation which gives infinity as its result), you have problems in the conceptual domain. Well, the/my advice is to look again in the book.
  4. Jan 31, 2007 #3
    I have, all there is about this is one half page of text.
  5. Feb 1, 2007 #4
    This is still a problem... Haven't anyone dealt with this before?
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