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Homework Help: Which star has the greater luminosity

  1. Jan 2, 2010 #1
    1. The problem statement, all variables and given/known data

    Question 1 - If two stars A of luminosity 9000 and B of luminosity 90 (both relative to the Sun) appear equally bright from the Earth, how much further away is A from B?

    Question 2 - Star A is twice as bright as Star B, but B is twice as far away as A. Which star has the greater luminosity, and by what ratio?

    Question 3 - If two stars, A and B, are the same distance away, but B is 20 times more luminous than A, how much brighter will B appear than A?

    I could use some assistance as to how I should set these out in accordance with

    brightness (proportional to) Luminosity / R^2

    2. Relevant equations

    See above

    3. The attempt at a solution

    Question 1 - Worked out to - by moving Star A 10 times further away we have diminished the apparent brightness of Star A by a factor 10 x10 = 100 so that it now matches the brightness of Star B.

    Question 2 - Worked out to - Luminosity of A
    Luminosity of B = (½)2 x 2 = ½

    This means that Star A really only has half the luminosity of Star B, but appears brighter because it is closer.
    Ratio 1:2

    Question 3 - Worked out to 1/20th .

    Just need assistance to set these out correctly - and also make sure that they are correct.
  2. jcsd
  3. Jan 3, 2010 #2
    Re: Luminosity

    Any thoughts?
  4. Jan 3, 2010 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: Luminosity

    Looks good so far.

    So are you saying that B appears to be 20 times dimmer than A? I will suggest rethinking this one. Hint: distance is not a factor here, since it is the same for both stars.
  5. Jan 3, 2010 #4
    Re: Luminosity

    Zebra, you seem to be really close on the last one. When distance is constant, the apparent brightness is just proportional to the luminosity.
  6. Jan 4, 2010 #5
    Re: Luminosity

    Many thanks for the replies.

    Yes, I think I understand what you are saying about question 3.

    Again, many thanks for the replies.

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