9416904 Katz The objective of this project is to understand better the physical processes which produce gamma-ray bursts. Models have been developed in which a debris shell from a relativistic fireball collides with interstellar gas. Analytic estimates have shown that this model may lead to gamma-ray bursts consistent with those actually observed. Dr. Katz will test these models by replacing the estimates previously made with quantitative calculations. The method to be employed is that of numerical hydrodynamics, which permits the quantitative calculation of the flow of fireball debris and interstellar matter in almost arbitrarily complicated flows.