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Homework Help: Relative population density of stars and galaxies

  1. Feb 19, 2013 #1
    1. The problem statement, all variables and given/known data

    The number of stars around the Sun, within a distance of 17 ly, is 75. The number of galaxies in the local group, within a distance of 4.0 x106 ly from the Sun, is 26.

    Calculate the average popluation density, per ly3, of stars and galaxies.

    2. Relevant equations

    3. The attempt at a solution
    Stars: 75/173
    Galaxies: 26/(4.0 x106)3

    But the answers I obtain from my attempt is incorrect :s
    Help please!
  2. jcsd
  3. Feb 19, 2013 #2
    What is the shape of the volume element?
  4. Feb 19, 2013 #3
    The question assumes that the Sun is a sphere. So I suppose the volume element is sphere.
  5. Feb 19, 2013 #4
    What's the volume of the sphere?
  6. Feb 19, 2013 #5
    I think my brain went dead, because I got it after saying that we assume the Sun as the volume of a sphere.

    So basically ly is the radius and we divide 75 by the volume of the star and galaxy to get the two answers :smile:

    Am I right?! Please approve! :approve:
  7. Feb 19, 2013 #6
    Pedantry: it is not the volume of the star, and not the volume of the galaxy. It is the volumes of two balls of the specified radii.

  8. Feb 19, 2013 #7


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    Staff Emeritus
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    Gold Member

    Hah, I would say not pedantry at all! It's kind of a crucial detail that you're dividing the number of objects by the volume of the region. I hope the OP gets this now.
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