At its peak, SN 1993J was one of the brightest supernovae in this century, and it is being studied more thoroughly than any supernova except SN 1987A. It is proving to be similar to the transition object SN 1987K, which metamorphosed from being a hydrogen-rich Type II near peak to having a hydrogen-deficient nebular phase. SN 1993J has been observed throughout the electromagnetic spectrum and with optical spectropolarimetry. It is interacting with a dense circumstellar nebula and is generating

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... adio and X-ray flux, but it has probably not been detected in gamma rays. The photometric and spectral evolution are consistent with a star of original mass ∼ 15 Mʘthat lost appreciable mass to a binary companion leaving an extended, helium-rich hydrogen envelope of ≲ 0.5 Mʘand a helium core of ∼ 4 Mʘ. The spectral evolution will put strong constraints on the mixing of66Ni and other species.