Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Determining the individual magnitudes of two stars

  1. Jun 2, 2009 #1
    Firstly please if you reply don't post the solution, rather the method you would use to get it as I'm trying to learn how to answer this kind of question for an exam.

    the problem (quoting my question paper):

    the stars S1 and S2 are seen along the same line of sight from Earth. Srat S1 is at a distance of 10pc, the star S2 is at a distance of 100pc. Their apparent flux ratio is known to be (f1/f2)=2.
    The combined apparent brightness of both stars seen from Earth is zero magnitudes, i.e m1+2=0.0mag. Determine the individual apparent magnitudes m1 and m2 of the two stars, if they could be resolved.
  2. jcsd
  3. Jun 17, 2009 #2


    User Avatar
    Homework Helper

    I'm not familiar with the term "apparent flux ratio", but I'm guessing it means "apparent brightness ratio". If so, the fact that S1 is twice as bright as S2 means that 3 times S2's intensity equals mag. 0. Do you know how to convert "3 times" into a magnitude difference?

    Using the same logic, 2/3 of the combined brightness equals S1's brightness. Convert 2/3 (or 3/2; it doesn't matter) into a magnitude difference; the question should be easy from there.
  4. Jun 17, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper

    You need to know the relation between brightness (or flux) and magnitude (see http://en.wikipedia.org/wiki/Absolute_magnitude)
    And you need to know how the brightness of an object decreases with distance.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook