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
${{\sigma {r^2}{T^4}} \over {{R^2}}}$
${{\sigma {r^2}{T^4}} \over {4\pi {R^2}}}$
${{\sigma {r^2}{T^4}} \over {{R^4}}}$
${{4\pi \sigma {r^2}{T^4}} \over {{R^2}}}$
Solution
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