VLBI measurements of the shape, expansion and distance of supernova 1993J in M81.

SN 1993J is one of the closest supernovae discovered this century. Strong radio emission makes this an ideal source for very-long baseline interferometry (VLBI) investigations. We find that during the first three months the supernova is circularly symmetric and shows no sign of deceleration. The VLBI expansion velocities are consistent with the optical maximum line velocities, which suggests that the radio emission indeed arises from the shock front. We estimate a distance to M 81 of 4.0±0.6 Mpc.