Astronomers recognize two types of supernovae -- Type I and Type II. Type I supernovae probably occur in certain binary stars. A binary star is a pair of stars that are close together and orbit about each other. A Type I probably occurs in binaries in which one of the stars is a small, dense star called a white dwarf. If the two stars are close enough to each other, the gravitational pull of the white dwarf draws mass from the larger companion. When the white dwarf reaches a mass about 1.4 times that of the sun, it collapses and then explodes.
A Type II supernova results from the death of a single star much more massive than the sun. When such a star begins to burn out, its core quickly collapses. Tremendous energy is suddenly released in the form of neutrinos (a type of subatomic particle) and electromagnetic radiation (electric and magnetic energy). This energy causes the star to erupt into a supernova.
Most supernovae reach maximum brightness a few days after they occur and shine intensely for several weeks. Some fade within months. Others fade over a period of years. Supernovae also differ in the amount and composition of the material that they expel.
Supernovae can also leave behind different types of objects. After some supernova explosions, there remains a small, dense star composed mainly of neutrons or perhaps of elementary particles called quarks. Such a star is called a neutron star. Rapidly rotating, highly magnetized neutron stars are called pulsars. After other explosions, an invisible object called a black hole may be left behind. A black hole has such powerful gravitational force that not even light can escape it. In some cases, no object of any kind remains after a supernova explosion.
Scientists believe that supernovae created all the heavier elements, such as iron, gold, and uranium, that are found on earth and have been detected in objects outside the solar system. Also, there is evidence that some high-energy cosmic rays originate in supernovae.
In 1054, Chinese astronomers recorded a supernova so bright that it was visible during the day. The explosion left behind a pulsar and a huge cloud of gas and dust known as the Crab Nebula, which still can be seen today.
In 1987, a Type II supernova became visible in the Large Magellanic Cloud, the galaxy closest to the Milky Way. It was the first supernova to be visible to the naked eye in almost 400 years. It could be viewed only from the Southern Hemisphere.Contributor: Kenneth Brecher, Ph.D., Professor of Astronomy and Physics, Boston University.
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