Novae and supernovae are apparently similar, but unrelated, stellar phenomena. A nova is a faint star that brightens suddenly by blowing off its surface layers before returning to its original form. A supernova is a star that explodes and scatters nearly all of its constituent material into space.
Stars are generally regarded as being unchanging on a human time scale, so the appearance of a newly visible star, or nova, is unusual. A nova may quickly brighten by hundreds or millions of times and often changes from beirfg invisible to the naked eye to a clearly defined star in the night sky. Generally novae brighten suddenly (within weeks or days), fade drastically during the following few weeks, then continue to fade more gradually for several years. Some novae are recurrent; they brighten and decline regularly with periods that vary from a few years to decades.
Spectroscopic studies of novae show that the absorption lines undergo a Doppler shift to the blue, indicating that the sudden increase of brightness corresponds to an explosive expansion that ejects the surface of the star into space. After several years, the ejected gas can be seen through a telescope as an expanding cloud around the star. The distance of the nova can be determined by correlating the Doppler shift of this gas cloud with the rate at which its angular size is increasing.
Most novae belong to double-star systems in which one of the pairs is a red giant and the other is a white dwarf, or condensed star. The internal gravitational forces of a red giant are relatively weak. Some of the red giant’s surface material may be pulled toward the nearby more massive white dwarf (which exerts a stronger gravitational pull). Matter falling toward the white dwarf may trigger off huge explosions, which may recur every few years, throwing off the accumulated matter from the white dwarf’s surface.
Each year there are two or three dozen nova outbursts in our galaxy. The brightest of recent years appeared in the constellation Cygnus in August 1975. Within two days, the nova had become brighter than the nearby star Deneb (one of the brightest stars in the sky), then, within a week, it had faded to below naked-eye visibility.
Sometimes, the brightening of a nova can be detected only in the X-ray part of the spectrum. One such X-ray nova was noticed in the constellation of Monoceros (the Unicorn) in 1975. When old photographs of the same area of the sky were examined, it was found that the star had been an optical nova in 1917, thus confirming the recurrent nature of nova flare-ups.
The very hot stars that lie in the center of an expanding shell of gas are believed to be related to novae. The central star is the contracting core of what would have been a red giant, and the tenuous halo of gas is the remnant of a previous explosion. They were originally misnamed planetary nebulae because of their resemblance to the greenish planetary disks of Uranus and Neptune when viewed through early telescopes.
One of the most spectacular and rare sights in the sky is a supernova explosion, which marks the death of a massive old star. On average, such an explosion occurs only once every 15 years in galaxies like the Milky Way. The conditions within a supergiant star suddenly become so unstable that the star violently explodes, ejecting into space a fast-moving cloud of material. In the following weeks, the supernova emits a huge amount of radiation, sometimes as much as that emitted by the rest of the entire galaxy in which the supernova lies.
A supernova explosion is thought to begin with the accumulation of an iron core (the remains of previous stages of nuclear burning). The core heats up until the iron undergoes a nuclear transformation. But unlike the previous elements that have been used in the star’s nuclear reactions, iron takes up energy while changing into other elements, leaving no extra energy for further heating of the core. As a result, the core shrinks and eventually becomes so unstable that it collapses, causing matter to fall into it from the surrounding layers of the star. Shock waves are produced that travel outward from the core, creating heavy elements such as uranium. Within seconds there is a cataclysmic explosion that destroys the star, blowing off its outer layers, leaving only remnants of the core to remain. The heavy elements (which are produced either near the center of the star or during the explosion itself) are blown off into space and enrich the interstellar gas.
When a new star forms out of such enriched gas, the heavy elements form part of a planet (or planets) that may become part of a new star system. Thus, all the heavy elements that are vital to life and make up our familiar world, were first formed by nuclear reactions that took place within super giant stars.
The gas cloud ejected by the explosion of a supernova spreads out into space, perhaps for millions of years. During this time, the stellar remnants radiate huge amounts of energy, not only as visible radiation (light), but also as X rays and radio waves. Scientists believe that subatomic particles—neutrinos— are also ejected. Neutrinos are believed to have so little mass that they can pass through matter undisturbed. The two most recent supernovas in our galaxy were seen by Tycho Brahe in 1572 and by Johannes Kepler in 1604. Although visually unspectacular, Kepler’s supernova appears as a striking astronomical feature when observed at radio wavelengths. Because of their rarity, our knowledge of supernovae is mainly gathered from galaxies other than our own. An international search program has been initiated to ensure that they are detected and recorded. In February, 1987, a Canadian astronomer, Ian Shelton, using a telescope in Chile, discovered Supernova 1987A, the brightest supernova and nearest to earth since Kepler’s observations in 1604. Studies of the neutrinos given off by Supernova 1987A seem to prove the theory of how a massive star becomes a supernova.