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!

[Astrophysics] Star luminosity-radius-temperature problem

  1. Jul 10, 2013 #1
    Hi!

    1. The problem statement, all variables and given/known data

    The light variation of a Cepheid is 2 mag,if its effective temperature at maximum luminosity is
    6000K, while at minimum is 5000K, please estimate the ratio of its maximum and minimum
    radius.

    2. Relevant equations

    $$\Delta m=-2,5lg\frac{L_{min}}{L_{max}}$$
    $$lg\frac{L_{min}}{L_{max}}=2lg\frac{R_{min}}{R_{max}}+4lg\frac{L_{min}}{L_{max}}$$

    3. The attempt at a solution

    Well, actually I should solve the problem with those two formulas above. My question is: why I can not just calculate the $$\frac{L_{min}}{L_{max}}$$ and solve the $$\frac{L_{min}}{L_{max}}=\frac{4\pi R_{min}^2 \cdot T_{min}^4\cdot \sigma}{4\pi R_{max}^2 \cdot T_{max}^4\cdot \sigma}$$ equation? Why I have to calculate with the lgs? As I studied the problem it looked for me that these two methods are equal but I got different results.

    Thanks in advance,


    Andrev
     
  2. jcsd
  3. Jul 10, 2013 #2
    Sorry for spamming the forum with this I solved it on my own:

    Of course I can calculate with the form without lgs too. I missed the calculation at the beginning: I forgot that the magnitude scale is inverse, so $$-0.4\cdot (m_1-m_2)=-0.4 \cdot \Delta m <0$$ It is ok now.


    Andrev
     
    Last edited: Jul 10, 2013
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: [Astrophysics] Star luminosity-radius-temperature problem
  1. Luminosity of a star (Replies: 2)

Loading...