X- and Gamma-ray Studies of Cas A: Exposing Core Collapse to the Core

In this review of X-ray and gamma-ray observations of Cas A, evidence is discussed that Cas A was a Type Ib supernova of a Wolf-Rayet star with a main sequence mass between 22-25 Msun, that exploded after stellar wind loss had reduced its mass to ~6 Msun. The observed kinematics and the high Ti-44 yield indicate that the supernova explosion was probably assymetric, with a kinetic energy of ~2x10^51 erg.