The total radiant energy per unit area, normal to the direction of...

If $\mathrm{r}$ is the radius of the star and $T$ its temperature, then the energy emitted by the star per second through radiation in accordance with Stefan's law will be given by $A \sigma \mathrm{T}^4=4 \pi \mathrm{r}^2 \sigma \mathrm{T}^4$ In reaching a distance $R$ this energy will spread over a sphere of radius $R$; so the intensity of radiation will be given by S=$\frac{P}{4 \pi R^2}=\frac{4 \pi r^2 \sigma T^4}{4 \pi \mathrm{R}^2}$