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Star and a Sphere of Dust

  1. Feb 2, 2009 #1
    1. The problem statement, all variables and given/known data

    A star surrounded by a sphere of dust with a radius R. The star emits a short bright flash of light, it is absorbed and re-emitted by the dust. Someone much further away than R sees the light, first at a point on the shell and then in an expanding ring.

    A)Show that the rate dr/dt of the ring expanding is given in an appropriate first order approximation by: dr/dt=(speed of light)(cot(theta))

    3. The attempt at a solution

    I have really know idea how to start this one. I could really use a hint to get started.
  2. jcsd
  3. Feb 3, 2009 #2


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    Homework Helper

    Welcome to PF!

    Hi stellaman! Welcome to PF! :smile:

    Essentially, this is a sphere which lights up all at the same time.

    You have to calculate the time t(θ) when the light from angle θ arrives on Earth.

    Then convert θ to the "sideways" distance x from the axis, and get dx/dt. :wink:
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