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Homework Help: Calculating radius of a blackbody

  1. Oct 31, 2011 #1
    1. The problem statement, all variables and given/known data
    We are asked to calculate the flux at a specific wavelength (f_5500), the surface flux(F) and the radius of a star (Sirius A and then B)
    We are given the following:
    Brightness of Sirius A: V=-1.47
    Effective temperature: 9870K
    Central wavelength: 5510 Angstroms
    Filter constant: Cv=3.58 x 10^-2 W m^-2 m^-1

    We calculated distance earlier to be: 8.14 x 10^16 m

    2. Relevant equations
    f_5500=Cv x 10^-0.4V
    F=((2hc^2)/(lambda)^5)(1/(exp(hc/lambda k T)-1))

    3. The attempt at a solution
    I calculated the first two parts correctly, getting values of:
    f_5500=1.37x10^-1 W/m^2 m
    F=1.804x10^14 W/m^2m

    I am stuck on how to calculate the radius, as i thought have thought taking a ratio of the two fluxes would work, but i end up with R= d sqrt (f_5500/F)
    This gives the wrong answer, the correct answer is found by using the same equation but with the F being (pi F)
    I am unsure as to how this equation can be found, and so have no idea how to continue with the question

    Any help would be greatly appreciated :D

  2. jcsd
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