SN 1993J is a supernova that occurred in the galaxy Messier 81. It was discovered on March 28, 1993 by F. Garcia in Spain. When it occurred it was the second brightest type II supernova observed in the 20th century, behind SN 1987A.
Over time, the spectal characteristics of this supernova have changed. Initially it looked more like a type II supernova (a supernova formed by the explosion of a giant star), with a strong hydrogen spectral line emission, but later the hydrogen lines faded and spectral lines appear, making it appear to be more like a type Ib. In addition, the variations in SN 1993J's luminosity over time were not like the variations observed in other type II supernovae, but resembled the vartiaons observed in type Ib supernovae. The supernova has now been reclassifed as a type IIb supernova, an intermediate class between type II and type Ib. The scientific results from this supernova suggested that type Ib and Ic supernovae were actually formed through the explosions of giant stars through processes similar to what takes place in type II supernovae.
The supernova was used to estimate a distance of 8.5 million light years (2.6 megaprsecs) to Messier 81.
Light echoes from the explosion have subsequently been detected.
The progenitor of SN 1993J was identified in pre-explosion ground-based images. The progenitor was observed to be a K-type supergiant star, with an excess in the ultraviolet possibly due to surrounding hot stars or a hot binary companion. While the supernova is located in a region populated by young massive stars, late-time photometry with the Hubble Space Telescope and spectroscopy with the Keck 10m-telescope presented by Maund and collaborators revealed the presence of the long-suspected B-supergiant companion star.