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 lifespan, such explosions are rare occurrences.
In our Milky Way galaxy, for example, a supernova may be observed every few
hundred years. Three such explosions are recorded in history: in 1054, in
1572, and in 1604. The CRAB NEBULA consists of material ejected by the
supernova of 1054. Such materials, known as supernova remnants, are common
in the heavens.
The supernovas observed in modern times have all occurred in other
galaxies, the most distant yet having been detected in 1988 in a galaxy 5
billion light-years away. The most interesting supernova of recent times
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
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, 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 SUPERGIANTS, but the one that exploded as 1987A was a
blue star, named Sanduleak, with a mass only about 15 times that of the Sun.
Its pattern of brightening and fading also varied notably from that of
typical type II supernovas, and an as yet unexplained "mystery spot"
appeared some time after the explosion, apparently near to Sanduleak's
former location. In 1989 astronomers thought that they had detected an
extremely fast-spinning pulsar at that location, but much further data is
still needed before this finding is confirmed.
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 (see STELLAR EVOLUTION).
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.
Bibliography: Clark, D. W., and Stephenson, F. R., eds.,
Historical Supernovae (1977); Jastrow, Robert, and Thompson,
Malcolm, Astronomy (1984); Marschall, Laurence A., The
Supernova Story (1988); Murdin, Paul and Leslie, Supernovae
(1985); Shy, Frank, The Physical Universe (1982); Woosley,
Stan, and Weaver, Tom, "The Great Supernova of 1987,"
Scientific American, August 1989; Zeilik, Michael, and Gaustad,
John, Astronomy (1983).
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