supernova

Explosive death of a star, which temporarily attains a brightness of 100 million Suns or more, so that it can shine as brilliantly as a small galaxy for a few days or weeks. Very approximately, it is thought that a supernova explodes in a large galaxy about once every 100 years. Many supernovae – astronomers estimate some 50% – remain undetected because of obscuring by interstellar dust.

The name ‘supernova’ was coined in 1934 by Swiss Astronomer Fritz Zwicky and German-born US astronomer Walter Baade. Zwicky was also responsible for the division into types I and II. Type I supernovae are thought to occur in binary star systems, in which gas from one star falls on to a white dwarf, causing it to explode. Type II supernovae occur in stars ten or more times as massive as the Sun, which suffer runaway internal nuclear reactions at the ends of their lives, leading to explosions. These are thought to leave behind neutron stars and black holes. Gas ejected by such an explosion causes an expanding radio source, such as the Crab nebula. Supernovae are thought to be the main source of elements heavier than hydrogen and helium.

A supernova at maximum attains an absolute magnitude between −14 and −20, over 10,000 times brighter than an ordinary nova. When one occurs in a galaxy its light quite often equals or exceeds the combined light of all the other stars and nebulae in the system, while its spectrum indicates very violent internal motions. Over a dozen possible supernovae in our own Galaxy that are still observable as radio sources have been identified among the ‘guest’ stars recorded in Chinese and other annals. Of these the best known are the one that appeared in Taurus in 1054 and gave rise to the Crab nebula; the one observed in 1572 by Danish astronomer Tycho in Cassiopeia, which for some days outshone Venus and was visible in broad daylight; and the one described by both German mathematician and astronomer Johannes Kepler and the Italian astronomer and physicist Galileo, which appeared in Serpens in 1604. The first supernova was recorded in AD 185 in China. The most recent supernova in our Galaxy was seen in 1604, but many others have been seen since in other galaxies. In 1987 a supernova visible to the unaided eye occurred in the Large Magellanic Cloud, a small neighbouring galaxy.