1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Binary star period calculation

  1. May 18, 2014 #1
    The problem is attached, and the official answer is given.
    The step in the answer does not make a lot of sense to me.
    How does the luminosity L converted directly into distance d, and why is the solar distance is used?
    And where does the dm=0.75 come from.
    It's just not making sense to me...

    Please expand and explain the steps in the answer a bit. Thank you.

    Attached Files:

  2. jcsd
  3. May 19, 2014 #2
    m for the binary system is

    m = -2.5log10L + C

    m' for a single star is

    m' = -2.5log10(L/2) + C
    m' = -2.5[log10L - log102] + C
    m' = [-2.5log10L + C] + 2.5log102
    m' = m + 2.5log102 = m + δm

    δm = 2.5log102 = 0.752575... ≈ 0.75
    Last edited: May 19, 2014
  4. May 19, 2014 #3
    The relation between magnitude and distance comes from the inverse square law for the radiation intensity. If you double the distance to a star than its radiation intensity drops to (1/2)2 = 1/4 and so forth so if you know the apparent magnitude you can figure the distance.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted