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Homework Help: [Astrophysics] Star luminosity-radius-temperature problem

  1. Jul 10, 2013 #1

    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

    2. Relevant equations

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

    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,

  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.

    Last edited: Jul 10, 2013
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