SUPERNOVA

plural supernovas, or supernovae, any of a class of violently exploding stars whose luminosity after eruption suddenly increases millions or even billions of times its normal level. The term supernova is derived from nova (Latin: new), the name for another type of exploding star. Supernovas resemble novas in several respects. Both are characterized by a tremendous, rapid brightening lasting for a few weeks, followed by a slow dimming. Spectroscopically, they show blue-shifted emission lines, which imply that hot gases are blown outward. But a supernova explosion, unlike a nova outburst, is a cataclysmic event for a star, one that essentially ends its active (i.e., energy-generating) lifetime. When a star goes supernova, considerable amounts of its matter, equaling the material of several Suns, may be blasted into space with such a burst of energy as to enable the exploding star to outshine its entire home galaxy. Historically, only seven supernovas are known to have been recorded before the early 17th century, with the most famous occurring in AD 1054. It was seen in one of the horns of the constellation Taurus. The remnants of this explosion are visible today as the Crab Nebula, which is composed of glowing ejecta of gases flying outward in an irregular fashion and a rapidly spinning, pulsating neutron star, called a pulsar, in the centre. The supernova of 1054 was recorded by Chinese and Korean observers; it also may have been seen by southwestern American Indians, as suggested by certain rock paintings discovered in Arizona and New Mexico. It was bright enough to be seen during the day, and its great luminosity lasted for weeks. Other prominent supernovas are known to have been observed from Earth in 185, 393, 1006, 1181, 1572, and 1604. Supernova 1987A in the Large Magellanic Cloud. This picture shows the faint A knot in the central ring of Supernova 1987A, as observed by the Hubble Space Telescope The closest and most easily observed of the hundreds of supernovas that have been recorded since 1604 was first sighted on the morning of Feb. 24, 1987, by the Canadian astronomer Ian K. Shelton while working at the Las Campanas Observatory in Chile. Designated SN 1987A, this formerly extremely faint object attained a magnitude of 4.5 within just a few hours, thus becoming visible to the unaided eye. The newly appearing supernova was located in the Large Magellanic Cloud at a distance of about 50,000 parsecs. It immediately became the subject of intense observation by astronomers throughout the Southern Hemisphere and has been observed by the Hubble Space Telescope. SN 1987A's brightness peaked in May with a magnitude of about 3 and slowly declined in the following months. Supernovas may be divided into two broad classes, Type I and Type II, according to the way in which they detonate. Type I supernovas may be up to three times brighter than Type II; they also differ from Type II supernovas in that their spectra contain no hydrogen lines and they expand about twice as rapidly. The so-called classic explosion, associated with Type II supernovas, has as progenitor a very massive star (a Population I star) of at least eight solar masses that is at the end of its active lifetime. (These are seen only in spiral galaxies, most often near the arms.) Until this stage of its evolution, the star has shone by means of the nuclear energy released at and near its core in the process of squeezing and heating lighter elements such as hydrogen or helium into successively heavier elementsi.e., in the process of nuclear fusion. Forming elements heavier than iron absorbs rather than produces energy, however, and, since energy is no longer available, an iron core is built up at the centre of the aging, heavyweight star. When the iron core becomes too massive, its ability to support itself by means of the outward explosive thrust of internal fusion reactions fails to counteract the tremendous pull of its own gravity. Consequently, the core collapses until it reaches a point at which its constituent nuclei and free electrons are crushed together into a hard, rapidly spinning core. This core consists almost entirely of neutrons, which are compressed in a volume only 20 km (12 miles) across but whose combined weight equals that of several Suns. A teaspoonful of this extraordinarily dense material would weigh 50 billion tons on Earth. The supernova detonation occurs when material falls in from the outer layers of the star and then rebounds off the core, which has stopped collapsing and suddenly presents a hard surface to the infalling gases. The shock wave generated by this collision propagates outward and blows off the star's outer gaseous layers. The amount of material blasted outward depends on the star's original mass. In some cases, the core collapse may be too great to produce a supernova and the imploding star is compressed into an even smaller and denser body than a neutron starnamely, a black hole. Infalling material disappears into the black hole, the gravitational field of which is so intense that not even light can escape. The entire star is not taken in by the black hole, since much of the falling envelope of the star either rebounds from the temporary formation of a spinning neutron core or misses passing through the very centre of the core and is spun off instead. Three distant Type Ia supernovas, as observed by the Hubble Space Telescope in Type I supernovas have only recently become explicable, though not without some uncertainty, on the basis of observational data. It appears that the immediate process resulting in the Type I explosionthe collapsing core suddenly becoming rigid and causing the infalling material to reboundis similar to what occurs with Type II supernovas. The progenitor of the Type I variety, however, is a lighter-weight star (a Population II star) of only four to eight solar masses. Type I supernovas occur in all kinds of galaxies and may result when the extremely massive white dwarf component in a binary star system draws so much matter from its companion that it collapses to a neutron core. There is less infalling material in the case of Type I supernovas. As a consequence, radioactive elements, most notably nickel-56, that are formed during the rebound near the core's surface may be blasted out through the material above. Moreover, when nickel-56 decays to cobalt-56 and the latter to iron-56, significant amounts of energy are released. This energy is the source of much of the light emitted during the weeks following the explosion. Supernova explosions release not only tremendous amounts of radio radiation and X-radiation but also cosmic rays and many of the heavier elements that make up the components of the solar system, including the Earth, into the interstellar medium. Spectral analyses show that abundances of the heavier elements are greater than normal, indicating that these elements do indeed form during the course of the explosion. The shell of remnants continues to expand until, at a very advanced stage, it dissolves into the interstellar medium. Compare nova. See also black hole.