Structural changes in CAS A, TYCHO and other young supernova remnants

Structural changes have been found and investigated in a handful of young supernova remnants (SNRs). The variations in the Crab Nebula (Trimble, 1968) have been known for some time. Here the emphasis is on young shell remnants, of which the best studies is Cas A. It is, like the Crab Nebula, undergoing an overall radial expansion, although other forms of motion have also been observed. Simple expansion has been found in Tycho (3C10) and the remnant of SN 1006. Optical measurements of the remnant of Kepler's supernova (SN 1604), however, have failed to show motion of the magnitude expected from this relatively young SNR.