Photon stars

We discuss numerical solutions of Einstein's field equation describing stat ic, spherically symmetric conglomerations of a photon gas. These equations imply a back reaction of the metric on the energy density of the photon gas according to Tolman's equation. The 3-fold of solutions corresponds to a c lass of physically different solutions which is parameterized by only two q uantities, i.e. mass and surface temperature. The energy density is typical ly concentrated on a shell because the center contains a repelling singular ity, which can, however, not be reached by timelike geodesics and only by r adial null geodesics. The physical relevance-of these solutions is complete ly open, although their existence may raise some doubts w.r.t. the stabilit y of black holes.