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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 TK is given by

(a) $\sigma {r}^{2}{T}^{4}/{R}^{2}$                                       (b) $\sigma {r}^{2}{T}^{4}/4{\mathrm{\pi r}}^{2}$

(c) $\sigma {r}^{4}{T}^{4}/{r}^{4}$                                        (d)

(where $\sigma$ is Stefan's constant)

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