Discovery of shell-like radio-structure in SN1993J

SUPERNOVA explosions are poorly understood, partly because of difficulties in modelling them theoretically¹, and partly because there have been no supernovae observed in our Galaxy since the invention of the telescope. But the recent discovery² of supernova SN1993J in the nearby galaxy M81 offers an opportunity to investigate the evolution of the remnant, and its interaction with the surrounding interstellar medium, at high resolution. Here we present radio observations of SN1993J, made using very-long-baseline interferometry, which show the development of a shell structure. This 8-month-old radio shell is the youngest ever discovered in a supernova. The data suggest that the supernova explosion and the expanding shell of the remnant have nearly spherical symmetry, with small deviations where some parts of the shell are brighter than others. If these deviations arise because of variations in the density of the shell, this may reconcile earlier reports of symmetric radio emission³ with the observed optical asymmetry^{4, 5}, as the density variations could easily cause the latter. We infer that the radio emission is generated at the interface^6-9, where the surrounding gas is shocked by the ejecta.