In 1993, a star inside of galaxy M81 exploded in a supernova named SN 1993J. M81, located twelve million light years from Earth, is located in the constellation of Ursa Major. A supernova occurs when a massive star ceases to generate fusion energy in its core and collapses inward under the force of its own gravity, resulting in a cataclysmic explosion. SN 1993J is only the second supernova for which the progenitor star has been identified.