Supernova

A supernova is the massive explosion of a star , and is one of the most violent events in the Universe. There are two types of supernovae. A Type I supernova happens when a dead star called a white dwarf accretes so much matter from a companion star that it becomes unstable and explodes. A Type II supernova occurs when a high-mass star runs out of thermonuclear fuel. In this case, the star's core collapses and becomes tremendously hard and rigid. The collapsing outer layers of the star bounce off the core and are flung outward with a burst of energy that can rival the output of an entire galaxy .

Guest stars

In a.d. 1054, a brilliant new star blazed into view into the constellation of Taurus, the Bull. The Chinese astronomers who observed it called it a "guest star," and at its peak brightness they could see it even during the day. Over the following months it gradually faded and disappeared. When we train our telescopes on the location of the former guest star, we see an angry-looking cloud of gas called the Crab Nebula. Several other "guest stars" appear in the historical record, usually separated by intervals of a few hundred years. They are therefore quite uncommon. Two appeared in a.d. 1572 and a.d. 1604, but after then astronomers would have to wait more than 380 years before the next one.

Studies of the Crab Nebula show that it is expanding, as if the enormous cloud of gas had been flung outward from a central point. Modern stellar evolution theory has provided a reason for this: the "guest stars" were the final acts in the lives of a massive star. These stars end their lives as supernovae, massive explosions that blast the stars' outer layers into space . For a short time they can rival the brightness of a small galaxy; later, like a dying coal in a fire, they fade away.

Types of supernovae

A supernova is the explosion of a star. In a single cataclysm, a massive star may blow itself to bits, releasing as much energy, for a brief time, as an entire galaxy. There are two types of supernovae.

A Type I supernova is the explosion and complete destruction of a dead star called a white dwarf. (The Sun , after it dies, will become a white dwarf.) If the white dwarf is made of carbon (the end product of the thermonuclear reactions that took place during the star's life), and if it is a member of a binary system, a Type I supernova can potentially occur.

Two important effects contribute to a Type I supernova. First, a white dwarf cannot be more massive than about 1.4 solar masses and remain stable. Second, if the white dwarf's companion star expands to become a red giant, some of its matter may be drawn away and sucked onto the surface of the white dwarf. If you could hover in a spacecraft at a safe distance from such a system, you would see a giant stream of matter flowing from the large, bloated star to its tiny companion, swirling into an accretion disk which then trickles onto its surface. A white dwarf that is almost 1.4 solar masses may be pushed over the critical mass limit by the constant influx of material. If this happens, the white dwarf will explode in an instantaneous nuclear reaction that involves all the mass of the star.

The popular image of a massive, supergiant star ending its life in one final, dazzling, swan song is what astronomers classify as a Type II supernova. In a Type II supernova, a massive star runs out of thermonuclear fuel and can no longer sustain itself against the inward pull of its own gravity. In a matter of seconds, the star collapses. The core is crushed into a tiny object called a neutron star , which may be no more than 6 mi (10 km) across. The outer layers collapse as well, but when they encounter the extremely hard, rigid, collapsed core, they bounce off it. An immense cloud of glowing gas rushes outward, and some of the nebulae visible in small telescopes are these dispersed outer layers of stars.

Astronomers can tell the type of a supernova by observing its total brightness as well as its spectrum . Type I supernovae release more energy and therefore have a lower absolute magnitude (about -19 at peak brightness, which is as bright as a small galaxy). Since a Type I supernova is the explosion of a dead star made largely of carbon, there is little evidence in its spectrum for the element hydrogen . Type II supernovae, however, have prominent hydrogen lines in their spectra, for hydrogen is the primary element in the exploding star.

Type I supernovae are useful to astronomers trying to determine the distances to other galaxies, which is a very difficult task. Since all Type I supernovae have about the same absolute brightness, astronomers can calculate how far away a Type I supernova is by measuring its apparent brightness and then calculating how far away it must be to appear that bright. Type I supernovae therefore serve as one of several kinds of distance indicators that help us determine the size of the Universe.

Why a supernova explodes

If a Type II supernova is the collapse of a massive star, why does a huge explosion result? We do not have the answer, partly because of the extreme physical conditions that exist in the temperatures (about a trillion degrees) and pressures in a collapsing stellar core, and partly because everything happens very rapidly. The more rapidly the situation is changing, the more difficult it is to simulate it on a computer.

The usual explanation is that the outer layers "bounce" off the collapsed core. Try this experiment: hold two superballs, one larger than the other, about 5 ft (1.5 m) off the floor. Hold the smaller superball so that it is on top of and touching the larger one. Drop them simultaneously, so they fall with the little ball just above the big one (this takes some practice). If you do it right, the large ball (the "core") will stop dead on the floor, and the little ball (the "outer layers") will be flung high in the air. Something akin to this is thought to happen in a supernova. As the core collapses, a shock wave develops as the material gets jammed together. The incredible energy involved blasts the star's outer layers far into space.

Supernova 1987A

Supernovae ought to happen in our galaxy about once every 30 years. Until 1987, however, no bright supernova had been seen since 1604. By no means were we "due for one"—that is an all-too-frequent abuse of statistics—but nevertheless a supernova did explode on February 23, 1987. (To be precise, that was the date that light from the explosion first reached us; the actual explosion took place 170,000 years earlier.) It was not in our galaxy, but in one of the small satellite galaxies orbiting it, the Large Magellanic Cloud, visible in the southern hemisphere. It was a distance of 170,000 light-years from Earth , and became known as SN 1987A. The star that exploded was a blue supergiant, probably 20 times more massive than the Sun, and when it exploded it was visible to the naked eye . Supernova 1987A became one of the most studied events in astronomical history, as observations of the expanding blast revealed numerous exciting results.

As a star's core collapses, the protons and electrons in it are smashed together to form a neutron , and every time such a reaction happens, an evanescent particle called a neutrino is created. The neutrinos travel outward from the core at the speed of light, and they are created in such vast numbers that they carry off most of the energy produced during the supernova. Just before SN 1987A became visible, surges of neutrinos were indeed detected here on Earth. Since the neutrinos escape from the star before the visible explosion, the timing of the event provided important observational support for the idea that core collapse and the subsequent bounce of infalling outer layers drives some supernova explosions.

There are several stars in Earth's celestial neighborhood that are prime supernova candidates. Betelgeuse, the red supergiant that marks Orion's right shoulder, and Antares, the brightest star in Scorpio, are two notable examples.

Resources

books

Mark, Hans, Maureen Salkin, and Ahmed Yousef, eds. Encyclopedia of Space Science & Technology. New York: John Wiley & Sons, 2001.

Seeds, M.A. Horizons: Discovering the Universe. Chap. 10. Wiley, 1991.

periodicals

Filippenko, A. "A Supernova with an Identity Crisis." Sky &Telescope (December 1993): 30.

Naeye, R. "Supernova 1987A Revisited." Sky & Telescope (February 1993): 39.

Thorpe, A. "Giving Birth to Supernovae." Astronomy (December 1992): 47.

Jeffrey C. Hall

KEY TERMS

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Core collapse

—The sudden collapse of a star's central region when the star's last fuel reserves are exhausted. With no energy being produced to sustain it against its own gravity, the core collapses in a fraction of a second, triggering a supernova explosion.

Type I supernova

—The explosion of a white dwarf in a binary system. Often the white dwarf, which is the dead remnant of a star originally about as massive as the Sun, has a stream of matter being dumped onto its surface by its companion. The white dwarf may be pushed past the limit of 1.4 solar masses, after which it will become unstable and explode.

Type II supernova

—The explosion of a massive star that has run out of thermonuclear fuel.