2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/198243We obtain an expression for the active gravitational mass of a relativistic heat conducting fluid, just after its departure from hydrostatic equilibrium, on a time scale of the order of relaxation time. It is shown that an increase of a characteristic parameter leads to larger (smaller) values of active gravitational mass of collapsing (expanding) spheres, enhacing thereby the instability of the system.To appear in General Relativity and GravitationGeneral Relativity and Quantum CosmologyAstrophysicstext