The Active Gravitational Mass of a Heat Conducting Sphere Out of   Hydrostatic Equilibrium
  We 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. 
L. Herrera, A. Di Prisco