  We present high-resolution X-ray proper motion measurements of Cassiopeia A
using Chandra observations from 2000 and 2002. We separate the emission into
four spectrally distinct classes: Si-dominated, Fe-dominated,
low-energy-enhanced, and continuum-dominated. These classes also represent
distinct spatial and kinematic components. The Si- and Fe-dominated classes are
ejecta and have a mean expansion rate of 0.2%/yr. This is the same as for the
forward shock filaments but less than the 0.3%/yr characteristic of optical
ejecta. The low-energy-enhanced spectral class possibly illuminates a clumpy
circumstellar component and has a mean expansion rate of 0.05%/yr. The
continuum-dominated emission likely represents the forward shock and consists
of diffuse circumstellar material which is seen as a circular ring around the
periphery of the remnant as well as projected across the center.

Astrophysics