Over the past several decades, a handful of familiar stars in nearby galaxies have exploded in supernovae. Most of these stars were red supergiants, which matches the theoretical expectation that most supernovae occur when the core of a red supergiant star collapses. By chance, however, the nearest recent supernova was a rarity: an exploding blue supergiant. This was the supernova SN 1987A, and it is the best studied of all supernovae explosions, having occurred in Large Magellanic Cloud, which is a neighboring galaxy to our own Milky Way Galaxy.
The basic theory behind core-collapse supernovae is that a massive star exhausts the thermonuclear fuel at its core; without a source of energy to compensate for the energy radiated away by the star, the core is unable to resist the force of gravity, and it collapses. The sudden release of gravitational potential energy as the core collapses to a neutron star blows the outer layers of the star away. These expanding layers emit the light we see in the supernovae. SN 1987A confirmed part of this theory when the neutrinos released during the collapse of the star's core were detected by underground neutrino detectors. Why the star was a blue supergiant rather than a red supergiant, however, is something of a puzzle. The page added with this issue discusses the many unusual characteristics of SN 1987A, and how these features are tied to the type of the star that exploded.
Next Issue: The next issue of this web site will present a page on the theories for the blue supergiant that created SN 1987A.
SN 1987A. In February of 1987, astronomers saw the closest supernova of modern times; it was in the Large Magellanic Cloud, a neighboring dwarf galaxy. This supernova, named SN 1987A, is incontrovertible proof that the collapse of the core of a massive star can produce a supernova. Not only were neutrinos detected from this explosion, as one expects in the birth of a neutron star from the collapse of a stellar core, but also the star that exploded was observed many times before the supernova and found to be massive. The surprise is that the star was a blue supergiant rather than the expected red supergiant. Other striking features of this supernova are its unusual chemical composition, its high expansion velocity, its low luminosity, and the unusual shape of its nebula. Some of these features are tied to the star being a blue supergiant, while others are clues to why the star was in a blue supergiant state when it exploded. (continue)