Supernova, exploding star that may increase in brightness by as much as a billion times its original state in just a few days, after which it gradually fades back to less than its original brightness. It is thought that supernovas are caused by the gravitational collapse of extremely massive stars. Although many presumed remnants of supernovas have been detected, only four of these have been definitely matched to explosions that were seen and recorded in human history. These occurred in 1006, 1054, 1572, and 1604. In more recent times (since the 19th century) many supernovas have been observed by astronomers.