supernova (pl. supernovae)
A catastrophic stellar explosion in which so much energy is released that the supernova alone can outshine an entire galaxy of billions of stars. In addition to the radiant energy produced, ten times as much energy goes into the kinetic energy of the material blown out by the explosion, and a hundred times as much is carried off by neutrinos.
A supernova explosion occurs when an evolved massive star has exhausted its nuclear fuel. Under these circumstances, the core becomes unstable against collapse.
Two distinct kinds of supernova are recognized, known as Type I and Type II. They are distinguished by the presence of hydrogen features in the spectrum of Type II supernovae which are absent from Type I. The light curves of Type I supernovae are all very similar: the luminosity increases steadily for about three weeks then declines systematically over six months or longer. The light curves of Type II supernovae are more varied.
Type I supernovae are subdivided into Types Ia and Ib, according to thestrength of a particular silicon absorption line in the optical spectrum. The line is strong in Ia and weak in Ib.
Type Ia supernovae are thought to be white dwarfs in binary systems, where mass transfer from the companion takes place. A wave of carbon burning through the newly acquired material could account for the energy released. The explosion may represent the total disintegration of the white dwarf. The nuclear reactions create about one solar mass of the unstable isotope 56Ni, which decays to 56Co and finally 56Fe over a period of months. This radioactive decay would take place at a rate consistent with the observed decline in light output. The difference in mechanism between Types Ia and Ib is not yet clear.
Type II supernovae appear to be stars of eight solar masses or more that have run the course of stellar evolution and totally exhausted the nuclear fuel available in their cores. At this stage their structure is like that of an onion, consisting of concentric spherical shells in which different nuclear reactions are taking place. Once silicon burning starts in the central core, instability develops within a day because the iron created cannot fuse into heavier elements without an input of energy. In the absence of energy generation, the pressure balancing the weight of the overlying layers is removed.
When the crunch comes, the core collapses in less than a second. The rate accelerates as iron nuclei break up and neutrons form. However, implosion cannot continue indefinitely. When the density of nuclear matter is reached, there is a sudden strong resistance to further pressure, the imploding material bounces back and an outward shock wave is generated. The outer layers of the star are blown outwards at thousands of kilometres per second, leaving the core exposed as a neutron star.
The material ejected in the explosion forms an expanding supernova remnant. The neutron stars can be detected as pulsars through their radio emission and, in some cases, by pulsed light and X-ray emission as well.
The explosion of supernovae serves to enrich the chemical composition of the interstellar medium from which subsequent generations of stars are created. Very old stars contain much lower quantities of the elements heavier than hydrogen and helium than are found in the Sun and solar system and many of these heavier elements can be created naturally only in the explosion of a supernova.
Supernovae are fairly rare events: only five have been observed visually in our own Galaxy in the last thousand years. Others have taken place, and radio emission from their remnants has been detected, but the outbursts were concealed behind obscuring dust. However, Supernova 1987A in the nearby Large Magellanic Cloud provided an opportunity unprecedented in modern times, enabling astronomers to study a supernova at relatively close hand. Numerous supernovae are detected each year in galaxies beyond our own.

See also: Crab Pulsar.