2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/44810The equations determining the energy density $ρ$ 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 $r$, $ρ\sim 1/r^2$ where the proportionality constant is a {\it fundamental constant\/} and is the same for all spherical masses at all temperatures.Plain TeX 16 pages, IP-ASTP-12-93High Energy Physics - PhenomenologyAstrophysicsGeneral Relativity and Quantum Cosmologytext