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Homework Help: Kepler problem?

  1. Jan 27, 2005 #1
    Our Sun, with mass 2.0 1030 kg, revolves about the center of the Milky Way galaxy, which is 2.2 1020 m away, once every 2.5 108 years. Assuming that each of the stars in the galaxy has a mass equal to that of our Sun, that the stars are distributed uniformly in a sphere about the galactic center, and that our Sun is essentially at the edge of that sphere, estimate roughly the number of stars in the galaxy.

    How can i find this answer if the stars dont fill up the space because they are spheres?
  2. jcsd
  3. Jan 27, 2005 #2


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    Compute the galaxy's mass and divide it to the mass of the sun (i assumed that the average mass of a star is roughly equal to the one of the sun).

  4. Jan 27, 2005 #3


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    It doesn't matter that the stars don't fill space. Treat it as if all mass were concentrated at the center of the galaxy.
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