The Energy Density of a Gas of Photons Surrounding a Spherical Mass $M$ at a Non-Zero Temperature

The equations determining the energy density $\rho$ of a gas of photons in thermodynamic equilibrium with a spherical mass $M$ at a non-zero temperature $T_s>0$ is derived from Einstein's equations. It is found that for large $r$, $\rho \sim 1/r^2$ where the proportionality constant is a {\it fundamental constant\/} and is the same for all spherical masses at all temperatures.