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Homework Help: Astrophysics - Apparent Magnitude of stars in a close binary system

  1. Jan 26, 2009 #1
    1. The problem statement, all variables and given/known data

    2 solar type stars are in a close binary system. Each indivual star has an apparent brightness of m = 10 mag.

    Determine the apparent magnitudes of the combined system of the 2 stars assuming they cannot be resolved as individual objects. Determine the minimum brightness for this eclipsing binary when 100%, 75%, 50% and 25% of the stellar survace are covered during the eclipse.

    2. Relevant equations

    m1 - m2 = -2.5 log(f1/f2)

    3. The attempt at a solution

    m1 - m2 = -2.5 log (f1/f2)
    10-10 = -2.5log(f1/f2)
    f1/f2 =1

    m1+2 - m2 = -2.5 log (f1+f2 \ f2) f1 = f2
    m1+2 - m2 = -2.5 log (2f1 \ f1)
    m1+2 - m2 = -2.5 log 2
    m1+2 = -2.5log2 + 5 mag
    m1+2 = 4.25 mag

    Then I'm not sure how to continue from here - any help would be most appreciated!
  2. jcsd
  3. Jan 26, 2009 #2


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    Science Advisor
    Homework Helper

    One magnitude represents a change in brightness of a factor of around 2.5
    so if you double the brightness (ie two stars) the apparent mag of the combination is only going to change by less than 1mag
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