  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.
