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Thread: How do I calculate a star's luminosity/radius?

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    Question How do I calculate a star's luminosity/radius?



    Hello there!
    I'm new to this forum and astrophysics in general. I'm also in early high school, so please be patient if I can't grasp the math at first.

    Anyway, I've been looking for a way to calculate a star's radius without depending on luminosity, and/or a way to calculate luminosity without depending on the radius of the star. I have found this equation(1) : R/Rs = (Ts/T)^2(L/Ls)^1/2. The problem, though, is that although the numbers are quite accurate for stars such as Sirius the calculations start getting off for stars such as Vega or Polaris. Is this equation correct? Or am I lacking some variable I don't know?

    Another problem i've encountered is with (2) L = 4piR^2sigmaT^4. No matter what numbers I plug into it, it never gives the correct luminosity or radius. What am I missing here? I'm pretty sure I did my calculations correct, but the results never match... Also, what are the all units I have to use in it? All numbers in solar units? Or in kilometers and watts? (and by extention, the luminosity I get will be in which unit?)

    There's this equation too (3): L = 4pid^2B. Again, no explanations on which units I should use and/or if it works only for a set of stars, etc.

    I've been searching for 2 days now and all the sites I find are either ambiguous in some way or they use some crazy math i've never encountered (solid angles, spherical coordinates, integral calculus and so on)

    If anyone can give me some insight i'd gladly appreciate it!

    Thanks in advance.

    Ps: Sorry for not properly formating the equations, nor attaching then in an image, nor putting links. The forum doesnt let me ):

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    Default Re: How do I calculate a star's luminosity/radius?

    Quote Originally Posted by Bipo10 View Post
    Hello there!
    I'm new to this forum and astrophysics in general. I'm also in early high school, so please be patient if I can't grasp the math at first.

    Anyway, I've been looking for a way to calculate a star's radius without depending on luminosity, and/or a way to calculate luminosity without depending on the radius of the star. I have found this equation(1) : R/Rs = (Ts/T)^2(L/Ls)^1/2. The problem, though, is that although the numbers are quite accurate for stars such as Sirius the calculations start getting off for stars such as Vega or Polaris. Is this equation correct? Or am I lacking some variable I don't know?
    The equation is correct as it stands.

    Another problem i've encountered is with (2) L = 4piR^2sigmaT^4. No matter what numbers I plug into it, it never gives the correct luminosity or radius. What am I missing here? I'm pretty sure I did my calculations correct, but the results never match... Also, what are the all units I have to use in it? All numbers in solar units? Or in kilometers and watts? (and by extention, the luminosity I get will be in which unit?)
    Units are likely your problem not the equations. The equation (2) requires input in SI units. https://en.wikipedia.org/wiki/Intern...ystem_of_Units

    There's this equation too (3): L = 4pid^2B. Again, no explanations on which units I should use and/or if it works only for a set of stars, etc.
    Where does this come from? Define "B"?
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