The total radiant energy per unit area, normal to the direction of incidence, received at a distance $R$ from the centre of a star of radius $r$, whose outer surface radiates as a black body at a temperature $T\ K$ is given by
AIPMT 2010, Medium
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According to the Stefan Boltzmann law, the power radiated by the star whose surface radiates as a black body at temperature $T\,K$ is given by
$P = \sigma 4\pi {r^2}{T^4}$
Where, $r=radius\,of\,the\,star$
$\sigma=Stefan's\,constant$
The radiant power per unit area received at a distance $R$ from the centre of a star is
$S = \frac{P}{{4\pi {R^2}}} = \frac{{\sigma 4\pi {r^2}{T^4}}}{{4\pi {R^2}}} = \frac{{\sigma {r^2}{T^4}}}{{{R^2}}}$
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where $\sigma$ is the Stefan's constant.