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Homework Help: Density of matter problem

  1. Nov 16, 2008 #1
    The sun orbits the galaxy at 220 km/s at a distance of 8500 parsecs from the centre of the galaxy. If the density of matter in the galaxy drops off proportional to 1/r, what would be the Keplerian velocity of a star orbiting at a radius of 1000 parsecs? What would be the Keplerian velocity of that star if the density of matter in the galaxy drops proportional to 1/r2 ? What is the density of matter in the vicinity of the sun in this 1/r2 case?
  2. jcsd
  3. Nov 17, 2008 #2


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    Re: Astrophysics

    show attempt to solution if you want to get some help, read and follow the forum rules.
  4. Nov 18, 2008 #3
    Re: Astrophysics

    I have solved it using v^2=(GM/R)^1/2 and M=(4/3*3.14*R^3)1/R
    If density is proportional to 1/r, the keplerian velocity at 1000 parsecs is 75km/s
    And if the density is 1/r^2, then the speed is constant at all radiuses.
    I Think this does make sense?
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