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Astrophysics - darkened sun

  1. Dec 14, 2009 #1
    1. The problem statement, all variables and given/known data
    Explain, as quantitatively as you can, what would be observed from Earth if the Sun's surface instantaneously darkened and hence explain why the brightness of a body cannot significantly alter on time scales significantly less than the light travel time across it.

    2. Relevant equations

    Hubble's constant = H
    velocity of galaxy = v
    distance from earth = d
    d = v / H

    redshift = z
    speed of light = c
    z = v / c

    3. The attempt at a solution

    The first thing that came to my mind is that it will take ~ 8 minutes for the photons to reach the Earth, meaning that any observation from the Earth would be 8 min after the event. The Sun is still receding from the Earth. However, we will not be able to calculate the redshift.

    Am I nowhere near what they are looking for in the answer?

    thanks x
  2. jcsd
  3. Dec 14, 2009 #2
    Think about the fact that the Sun is a sphere (almost!), and that the centre of the visible sphere is nearer to you than the edge.
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