The gaseous remnants of historical supernova explosions provide astronomers with a valuable class of objects for investigating the dynamical evolution of supernova remnants (SNRs). For these remnants one knows the average expansion rate and thus a determination of the current expansion rate yields an indication of the remnant's deceleration. However the expected motions are small, ;SPMlt;1 yr , and thus significant measurements of current expansion rates in SNRs require high angular resolution, long time baselines, and deep observations.
Tycho's SNR is an ideal target for such studies. Indeed, the expansion of Tycho's SNR has been measured previously in both the optical (Kamper & van den Bergh 1978) and radio bands (Tan & Gull 1985). The observed proper motion of the optical filaments indicate expansion rates ranging from 0.18 yr to 0.28 yr . Since the amount of material probed by the optical emission is a tiny fraction of the total mass of Tycho's SNR and filaments appear only toward the north and east near the remnant's periphery, observations in the optical band are unable to provide a comprehensive picture of the expansion of the remnant. Radio data reveal a nearly circular, limb-brightened, clumpy shell of emission. The overall expansion of the entire radio remnant has been determined to be % yr , although azimuthal sectors of the remnant are observed to expand at rates varying from 0.05 % yr to 0.15 % yr or more. Again, the radio band probes material different from that in the X-ray or optical: in this case regions of compressed magnetic field and relativistic electron acceleration.