Super Novas
A supernova is a STAR that explodes. It suddenly increases in brightness by a factor of many billions, and within a few weeks it slowly fades. In terms of the human life span, such explosions are rare occurrences. In our Milky Way galaxy, for example, a supernova may be observed by the naked eye only once every few hundred years. Four such explosions have been recorded--in 1054, in 1572, in 1604, and, as described below, in 1987. The CRAB NEBULA consists of material ejected by the supernova of 1054. Such
materials, known as supernova remnants, are common in the heavens.
Almost all supernovas have been observed in modern times only with the availability of powerful telescopes to detect them. About 15 are detected each year in this way in distant galaxies--the most distant yet having been noted in 1988 in a galaxy 5 billion light-years away. Therefore by far the most interesting supernova seen recently was the one visible to the naked eye that was detected in the relatively nearby Large MAGELLANIC CLOUD, on Feb. 23, 1987, by an astronomer at Chile's Las Campanas Observatory.
It quickly became an object of intense study by all the means available to modern astronomy. Its radiations could be observed across the entire electromagnetic spectrum, from radio waves to gamma rays, and its NEUTRINO emissions could also be studied.
A supernova may radiate more energy in a few days than the Sun does in 100 million years, and the energy expended in ejecting material is much greater even than this. In many cases, including the Crab nebula supernova, the stellar remnant left behind after the explosion is a NEUTRON STAR--a star only a few kilometers in diameter having an enormously large density and consisting mainly of neutrons--or a PULSAR, a pulsating neutron star.
There are two common types of supernovas, called type I and type II. Type I occurs among old stars of small mass, probably white dwarfs, whereas type II occurs among very young stars of large mass. It is not known how a small-mass star can release the very large amounts of energy needed to explain type I supernovas. Scientists generally believe that this must involve binary systems--two stars revolving around each other. In such a system one of the stars is a white dwarf, a small, dense
star that is near the end of its nuclear burning phase. After attracting matter from the companion star for some time, the white dwarf eventually collapses with a great rush, becoming a neutron star, and ejecting matter outward. This rebound of matter is thought to be the supernova.
Stars with large masses burn their nuclear fuel very rapidly. Within a million years or less, such stars build cores containing much iron. When the iron eventually burns, energy is quickly drained from the core, and the star cannot continue to support itself against gravity. It suffers a mighty collapse analogous to that of a type I supernova, and the rebound causes matter to be ejected in a type II supernova explosion. Stars ending in this way are typically red super giants, but the one that
exploded as 1987A was not. It was a blue star, already known and photographed and named Sanduleak, about 50 times larger than the Sun but only about 20 times more massive. The reason for this discrepancy was later determined to be that the Large Magellanic Cloud where the star was born contains a much smaller proportion of heavy elements than does our own Galaxy. Such elements absorb much of the radiation welling up from within a star, causing it to expand and become a red super giant. Sanduleak, lacking
these elements, could become a smaller and hotter blue giant. Sanduleak's pattern of brightening and fading also varied notably from that of typical type II supernovas. In 1989 astronomers thought that they had detected a neutron star--in the form of a rapidly spinning pulsar--at the former site of Sanduleak, but this proved to be a mistaken reading. The remains of the star may instead have become a BLACK HOLE.
Cosmologists estimate that the universe came into existence about 15 billion years ago. This involved the initial creation of hydrogen and helium. Since then nuclear fusion in stars has changed some of the original hydrogen and helium into heavier elements. Supernovas have played an important role both in producing the heavy elements and in ejecting material back into space, where it has been used to make new stars and, probably, planetary systems. It is possible that one or more
supernovas exploded shortly before the formation of our solar system. Elements ejected from these explosions could have mixed with the solar nebula, eventually becoming part of the structures of the Sun, the Earth, and all living things.
The following was copied directly for a Grollier Encyclopedia dealing with the subject of "Constellations" It's by no means the purpose of the author of this web page to misrepresent them or take credit for their work.