  A spherically symmetric distribution of classical blackbody radiation is
considered, at conditions in which gravitational self-interaction effects
become not negligible. Static solutions to Einstein field equations are
searched for, for each choice of the assumed central energy density. Spherical
cavities at thermodynamic equilibrium, i.e. filled with blackbody radiation,
are then studied, in particular for what concerns the relation among the mass M
of the ball of radiation contained in them and their temperature at center and
at the boundary. For these cavities it is shown, in particular, that: i) there
is no absolute limit to M as well to their central and boundary temperatures;
ii) when radius R is fixed, however, limits exist both for mass and for
boundary energy density rho_B: M <= K M_S(R) and rho_B <= Q/R^2, with K = 0.493
and Q = 0.02718, dimensionless, and M_S(R) the Schwarzschild mass for that
radius. Some implications of the existence and the magnitude of these limits
are considered. Finally the radial profiles for entropy for these systems are
studied, in their dependence on the mass (or central temperature) of the ball
of radiation.

General Relativity and Quantum Cosmology