1993J

1993J is a supernova in the nearby spiral galaxy M81 (also known as Bode's Galaxy). It was discovered in 1993 and is one of the best-studied examples of a Type IIb supernova, a transitional class between Type II and Type Ib. The host galaxy lies at a distance of about 3.6 megaparsecs (roughly 12 million light-years) from Earth.

Early spectra showed strong hydrogen emission from the expanding envelope, while the hydrogen lines faded over time and helium lines became prominent, indicating that the progenitor had lost most of its hydrogen envelope prior to explosion. The light curve featured an initial bright peak followed by a rapid decline and then a slower tail, and was extensively monitored across optical, radio, and X-ray wavelengths, making 1993J a benchmark for IIb evolution.

The prevailing view is that the progenitor was a massive star in a binary system that transferred most of its hydrogen to a companion, leaving a compact helium-rich core to explode. Pre-explosion images from the Hubble Space Telescope identified a candidate progenitor compatible with this scenario.

1993J has influenced theories of stellar evolution and supernova classification by illustrating how binary interactions can strip hydrogen and produce Type IIb explosions. It remains a reference object for studying the transition from hydrogen-rich to hydrogen-poor spectra and for calibrating models of envelope structure and mass loss.