Relativistic gravitational collapse in non-comoving coordinates: The   post-quasistatic approximation
  A general, iterative, method for the description of evolving self-gravitating relativistic spheres is presented. Modeling is achieved by the introduction of an ansatz, whose rationale becomes intelligible and finds full justification within the context of a suitable definition of the post--quasistatic approximation. As examples of application of the method we discuss three models, in the adiabatic case. 
L. Herrera, W.Barreto, A. Di Prisco and N. O. Santos