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.

Distribution of Supernovas

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 ).

Theoretical Models of Supernovas

Type I Supernovas

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

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 .

Supernova

As stars age, many use up their fuel and fade away to oblivion. Others, however, go out with a bang as supernovae, releasing energies of up to 10⁴⁴joules—an amount of energy equivalent to 30 times the power of a typical nuclear bomb. The explosions of low-mass stars can be triggered by the accretion of mass from a companion star in a binary system to create classical, or Type Ia, supernovae. These supernovae show no hydrogen in their spectra . Massive stars, on the other hand, proceed through normal nuclear fusion but then, when their energy supply runs out, there is no outward pressure to hold them up and they rapidly collapse. The core is crushed into a neutron star or black hole, and the outer layers bounce and are then hurled outward into the surroundings at many million kilometers per hour. These are Type Ib and II supernovae. The Type II supernovae still eject some hydrogen from the unprocessed atmosphere of the star. During a supernova explosion, temperatures are so high that all the known elements can be produced by nuclear fusion.

The most recent supernova that was close enough to be seen without a telescope occurred in early 1987 within a nearby galaxy , the Large Magellanic Cloud. Known as 1987A, it is the only supernova for which there is accurate data on the progenitor star before it exploded. It has been a tremendous help in understanding how stars explode and expand.

The rapidly growing surface of the star can brighten by up to 100 billion times. Then, as the material gets diluted, it becomes transparent and the brightness fades on time scales of a few years. The ejecta are still moving rapidly, however, and quickly sweep up surrounding matter to form a shell that slows down as mass gets accumulated, an action similar to that of a snowplow. This is the beginning of the supernova remnant that can be visible for tens of thousands of years. 1987A is starting to show such interaction with its surroundings.

Supernova remnants emit various forms of radiation. The material is moving highly supersonically and creates a shock wave ahead of it. The shock heats the material in the shell to temperatures over 1 million degrees, producing bright X rays . In the presence of interstellar magnetism, shocks also accelerate some electrons to almost the speed of light, to produce strong synchrotron radiation at radio wavelengths . Sometimes, even high-energy gamma rays can be produced. Dense areas can also cool quickly and we observe filaments of cool gas, at about 10,000 degrees, in various spectral lines at optical wavelengths.

In 1054 astronomers in China and New Mexico observed a famous example of the explosion of a massive star. What remains is a large volume of material that, with a lot of imagination, looks like a crab and, hence, is named the Crab Nebula. The object is being stimulated by jets from a rapidly spinning (about thirty times a second) neutron star called a pulsar. In most supernova remnants, this pulsar wind nebula is surrounded by the shell discussed above, but remarkably, no one has yet detected the shell around the Crab Nebula. Oppositely, the young supernova remnant Cassiopeia A has a shell and a neutron star but no pulsar wind nebula. Astronomers hope to explain these and many other mysteries about supernovae and their remnants using more multiwavelength observations with new telescopes.

see also Black Holes (volume 2); Cosmic Rays (volume 2); Galaxies (volume 2); Pulsars (volume 2); Stars (volume 2).

John R. Dickel

Bibliography

Robinson, Leif. "Supernovae, Neutrinos, and Amateur Astronomers." Sky and Telescope 98, no. 2 (1999):31-37.

Wheeler, J. Craig. Cosmic Catastrophes. Cambridge, UK: Cambridge University Press,2000.

Zimmerman, Robert. "Into the Maelstrom." Astronomy 26, no. 11 (1998):44-49.

supernova A violent explosion in which certain stars end their lives; given the variable-star type designation SN. In a supernova explosion, the star may become over a billion times brighter than the Sun, and for weeks may outshine the entire galaxy in which it lies. However, the optical luminosity represents only 0.01% of the energy released in the explosion. Most of the energy emerges in the form of neutrinos, and 1% goes into the kinetic energy of the gas expelled. The last supernova seen in our Galaxy was in 1604 (Kepler's Star), although Supernova 1987A in the Large Magellanic Cloud reached naked-eye brightness. However, two or three supernovae are expected to occur every century in a typical spiral galaxy like ours, which suggests that many have been missed due to absorption of light by dust in the galactic plane.

 Supernovae are classified into Types I and II, the latter type showing hydrogen in its spectrum, whereas the spectrum of a Type I supernova shows no hydrogen. There is further subclassification into Types Ia, Ib, and Ic according to other details of the spectrum. Type Ia supernovae reach a maximum magnitude of about −19, while Types Ib and Ic are about 1.5 magnitudes fainter. Type II have a wide range of peak magnitudes, but on average are similar to Types Ib and Ic. Types II, Ib, and Ic occur in young stars of Population I, and thus are concentrated in the disks of spiral galaxies. Type Ia supernovae occur among old stars of Population II, as are found in elliptical galaxies and the halos of spirals.

 Type Ia supernovae are believed to be due to the explosion of a white dwarf in a binary as a result of matter falling on to it from its companion star. When the mass of the white dwarf eventually exceeds the Chandrasekhar limit, it undergoes runaway carbon burning and explodes, ejecting about 1 solar mass.

 Type Ib and Ic supernovae are thought to result from the collapse of the cores of massive stars which have lost their hydrogen envelopes, either through a stellar wind or by transfer of matter to a companion in a binary. Types Ib and Ic show minor differences in spectra, indicating different compositions of the progenitor stars, which are probably Wolf–Rayet stars stripped of different amounts of their outer layers. Despite their classification, Type Ib and Ic supernovae are more closely related to Type II than to Type Ia.

 Type II supernovae arise from the explosion of stars of more than 8 solar masses. Nuclear reactions cease once the star's core consists of iron and heavier elements, because these elements cannot be burnt to produce energy. The stars then collapse under their own gravity, reaching densities so high that protons and electrons combine to form neutrons, producing a neutron star or even a black hole. The formation of a neutron star causes the overlying layers of material to rebound violently. In this process much of the envelope of the original star, amounting to many solar masses, is ejected at speeds of 2000–20 000 km/s.

 Type II supernovae can be subdivided into II-P, II-L, and IIb. Type II-P (for plateau) remain at near-constant brightness for 2–3 months after the outburst before fading. The rarer II-L (for linear) type fades more rapidly from an initial peak. Type IIb show a double maximum in brightness. Type IIb are thought to result from massive stars that have lost most, but not all, of their hydrogen envelope before exploding. The second peak, a few weeks after the initial outburst, is caused by the decay of radioactive nickel and cobalt in the supernova debris. See also supernova remnant.

Supernova

Ancient astronomers assigned the word nova, Latin for "new," to any bright star that suddenly appeared in the sky. They called an extremely bright new star a supernova.

Modern astronomers now know that a supernova, one of the most violent events in the universe, is the massive explosion of a star. Only relatively large stars (those having 1.5 times the mass of our Sun or more) explode in supernovae at the end of their lives. Once a star has used up all its nuclear fuel, it begins to collapse in on itself. During this process, energy is released and the outer layers of the star are pushed out. These layers are large and cool, and the star at this point is considered a red giant. The star continues to expand, however, and soon explodes outward with great force. As a result of the explosion, the star sheds its outer atmospheric layers and shines more brightly than the rest of the stars in the galaxy put together.

What happens next depends on the original mass of the star. Stars up to three times the mass of the Sun end up as densely packed neutron stars or pulsars (rapidly rotating stars that emit varying radio waves at precise intervals). Stars more than three times the mass of the Sun collapse, in theory, to form a black hole (an infinite abyss from which nothing can escape).

Words to Know

Black hole: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.

Chandrasekhar's limit: Theory that determines whether an exploding supernova will become either a neutron star or a black hole depending on its original mass.

Neutrino: High-energy subatomic particle with no electrical charge and no mass, or such a small mass as to be undetectable.

Neutron star: Extremely dense, neutron-filled remains of a star following a supernova.

Nuclear fusion: Merging of two hydrogen nuclei into one helium nucleus, with a tremendous amount of energy released in the process.

Pulsar: Rapidly spinning, blinking neutron star.

Radio waves: Electromagnetic radiation, or energy emitted in the form of waves or particles.

The formation of a supernova

Astronomers did not know what causes a star to explode in a super nova until the 1939, when Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910–1995) pieced together the sequence of events leading up to a supernova. He also calculated a figure for the mass of a star (known as Chandrasekhar's limit) that would determine if it would end up as a neutron star or a black hole.

Various theories have been proposed to explain the reasons a star explodes outward while collapsing inward. One theory is that the explosion is caused by a final burst of uncontrolled nuclear fusion. A more recent theory is that the explosion is due to the ejection of a wave of high-energy subatomic particles called neutrinos (electrically neutral particles in the lepton family). The neutrino theory gained greater acceptance following the 1987 supernova in the Large Magellanic Cloud, our galaxy's closest companion. Just before the supernova came into view, a surge of neutrinos was detected in laboratories around the world. This supernova, called Supernova 1987A, was the first visible to the naked eye since 1604.

[See also Star; White dwarf ]

supernova Stellar explosion in which virtually an entire star is disrupted. For a week or so, a supernova may outshine all the other stars in its galaxy. After a couple of years, the supernova expands so much it becomes thin and transparent. For hundreds or thousands of years, the ejected material remains visible as a supernova remnant. A supernova is c.1000 times brighter than a nova.

su·per·no·va / ˈsoōpərˌnōvə/ • n. (pl. -no·vae / -ˌnōvē/ or -no·vas ) Astron. a star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.