Supernova

A supernova is the explosion of a star. In a single cataclysm, a massive star may blow itself to bits, releasing as much energy, for a brief time, as an entire galaxy. There are two types of supernovae.

A Type I supernova is the explosion and complete destruction of a dead star called a white dwarf. (The Sun, after it dies, will become a white dwarf.) If the white dwarf is made of carbon (the end product of the thermonuclear reactions that took place during the star's life), and if it is a member of a binary system, a Type I supernova can potentially occur.

Two important effects contribute to a Type I supernova. First, a white dwarf cannot be more massive than about 1.4 solar masses and remain stable. Second, if the white dwarf's companion star expands to become a red giant, some of its matter may be drawn away and sucked onto the surface of the white dwarf. If you could hover in a spacecraft at a safe distance from such a system, you would see a giant stream of matter flowing from the large, bloated star to its tiny companion, swirling into an accretion disk which then trickles onto its surface. A white dwarf that is almost 1.4 solar masses may be pushed over the critical mass limit by the constant influx of material. If this happens, the white dwarf will explode in an instantaneous nuclear reaction that involves all the mass of the star.

The popular image of a massive, supergiant star ending its life in one final, dazzling, swan song is what astronomers classify as a Type II supernova. In a Type II supernova, a massive star runs out of thermonuclear fuel and can no longer sustain itself against the inward pull of its own gravity. In a matter of seconds, the star collapses. The core is crushed into a tiny object called a neutron star, which may be no more than 6 mi (10 km) across. The outer layers collapse as well, but when they encounter the extremely hard, rigid, collapsed core, they bounce off it. An immense cloud of glowing gas rushes outward, and some of the nebulae visible in small telescopes are these dispersed outer layers of stars.

Astronomers can tell the type of a supernova by observing its total brightness as well as its spectrum. Type I supernovae release more energy and therefore have a lower absolute magnitude (about -19 at peak brightness, which is as bright as a small galaxy). Since a Type I supernova is the explosion of a dead star made largely of carbon, there is little evidence in its spectrum for the element hydrogen. Type II supernovae, however, have prominent hydrogen lines in their spectra, for hydrogen is the primary element in the exploding star.

Type I supernovae are useful to astronomers trying to determine the distances to other galaxies, which is a very difficult task. Since all Type I supernovae have about the same absolute brightness, astronomers can calculate how far away a Type I supernova is by measuring its apparent brightness and then calculating how far away it must be to appear that bright. Type I supernovae therefore serve as one of several kinds of distance indicators that help us determine the size of the Universe.