supernova, a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than iron are produced in supernovas. Supernovas also are the principal heat source for interstellar matter and may be a source of cosmic rays. Recent discoveries have confirmed an underlying connection between supernovas and gamma-ray bursts (GRBs). Both are associated with the deaths of massive stars and they often happen nearly simultaneously. There is no generally agreed upon model for how a massive star explodes. However, the association with gamma rays has renewed interest in the role played by stellar rotation and magnetic fields.
At peak intensity, a supernova can shine as brightly as the entire galaxy in which it occurs. Novas are less spectacular and more common; they increase in brightness only by a few thousand times, and several occur in our galaxy every year. Supernovas can occur in that small percentage of stars having a mass greater than 8 to 10 times the mass of the sun and perhaps in certain binary stars.
More than five supernovas have been observed to have occurred in our galaxy in the last thousand years, including the "guest star" in Taurus described by Chinese astronomers in 1054; Tycho's star in Cassiopeia, observed by Tycho Brahe in 1572; and Kepler's supernova in 1604. In 1885 the first extragalactic supernova was discovered telescopically in the Andromeda Galaxy; some 700 others have been observed since. In 1987 Supernova 1987A appeared in the Large Magellanic Cloud. It was the first supernova visible to the unaided eye since 1604, and its eruption marked the first time that neutrinos were detected on earth from such an event (see neutrino astronomy).
In the 1930s Fritz Zwicky, Walter Baade, and Rudolph Minkowski developed several models of supernova events. In a star about to become a Type I supernova, the star's hydrogen is exhausted, and the star's gravity pulling inward overcomes the forces of its thermonuclear fires pushing the material outward. As the core begins to contract, the remaining hydrogen ignites in a shell, swelling the star into a giant and beginning the process of helium burning. Eventually the star is left with a still contracting core of carbon and oxygen. If the star, now a white dwarf, has a nearby stellar companion, it will begin to pull matter from the companion. In many stars the excess matter is blown off periodically as a nova; if it is not, the star continues to get more and more massive until the matter in the core begins to contract again. When the star gets so massive that it passes Chandrasekhar's limit (1.44 times the sun's mass), it collapses very quickly and all of its matter explodes.
Type II supernovas involve massive stars that burn their gases out within a few million years. If the star is massive enough, it will continue to undergo nucleosynthesis after the core has turned to helium and then to carbon. Heavier elements such as phosphorus, aluminum, and sulfur are created in shorter and shorter periods of time until silicon results. It takes less than a day for the silicon to fuse into iron; the iron core gets hotter and hotter and in less than a second the core collapses. Electrons are forced into the nuclei of their atoms, forming neutrons and neutrinos, and the star explodes, throwing as much as 90% of its material into space at speeds exceeding 18,630 mi (30,000 km) per sec. After the supernova explosion, there remains a small, hot neutron star, possibly visible as a pulsar, surrounded by an expanding cloud, such as that seen in the Crab Nebula.
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