Spring 2006 10 Misc

Stars in mechanical equilibrium can sustain a net outward-directed photon flux up to a limiting value, beyond which the rate of momentum transfer from photons to electrons and, hence, to protons via the Coulomb interaction, exceeds the local force of gravity. For a star of mass M = 1 M ≈ 2 × 1033 g what is the luminosity (total energy per unit time radiated from the stellar surface) required to produce this critical flux? Express your result in units of erg s−1 ; an order of magnitude estimate is fine. You may find it useful to know that the Thomson cross section for photon-electron scattering is σT ≈ 6.65 × 10−25 cm2 , the gravitational constant is G ≈ 6.67 × 10−8 erg cm g−2 , and the rest mass of a proton is mp ≈ 938.26 MeV ≈ 1 amu.

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Answer

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