NOVA

NOVA AND SUPERNOVA

NOVA AND SUPERNOVA(Lat. novus,"new"), in astronomy, names of two kinds of explosive events that take place in some stars. A nova is a star that suddenly increases greatly in brightness and then slowly fades, but may continue to exist for some time. A supernova exhibits the same pattern of behavior, but the causative explosion destroys or profoundly alters the star. Supernovas are much rarer than novas, which are observed fairly frequently in photographs of the sky.

Novas

Before the era of modern astronomy, a star that appeared suddenly where none had been seen before was called a nova, or "new star". This is a misnomer, as the stars involved had existed long before they became visible to the naked eye. Astronomers estimate that perhaps about a dozen novas occur in the Milky Way, or the earth's galaxy, each year, but two or three are too distant to be seen or are obscured by interstellar matter. Indeed, novas are often more easily observed in other, nearby galaxies rather than in the earth's. Novas are named according to the year of their occurrence and the constellation in which they appear. Typically, a nova flares up to several thousand times its original brightness in a matter of days or hours. It next enters a transition stage, during which it may fade and grow bright again and then fade gradually to or near its original level of brightness.

Novas may be considered variable stars in a late stage of evolution. They apparently behave as they do because their outer layers have built up an excess of helium through nuclear reactions and expand too rapidly to be contained. The star explosively emits a small fraction of its mass as a shell of gas--the cause of the increase in brightnes--and then settles down. Such a star is typically a white dwarf and is commonly thought to be the smaller member of a binary (two-star) system, subject to a continuous infall of matter from the larger star. This is perhaps always the case with dwarf novas, which erupt repeatedly at regular intervals of a few to hundreds of days.

Novas in general show a relationship between their maximum brightness and the time they take to fade a certain number of magnitudes. By means of measurements of nearer novas of known distance and magnitude, astronomers can use novas in other galaxies as indicators of the distance to those galaxies.

Supernovas

A supernova explosion is far more spectacular and destructive than a nova and much rarer. Such events may occur no more than once every few years in the Galaxy; and despite their increase in brilliance by a factor of billions, only a few are ever observable to the naked eye. Until 1987, only three had been positively identified in recorded history, the best known of which is the one that occurred in ad 1054 and is now known as the Crab nebula. Supernovas, like novas, are more often seen in other galaxies. Thus, the most recent supernova, which appeared in the southern hemisphere on Feb. 24, 1987, was found located in a companion galaxy, the Large Magellanic Cloud. This supernova, which exhibits some unusual traits, is now the object of intense astronomical scrutiny.

The mechanisms that produce supernovas are less certain than those of novas, particularly in the case of stars approximately as massive as the earth's sun, an average star. Stars that are much more massive, however, sometimes explode in the late stages of their rapid evolution as a result of gravitational collapse, when the pressure created by nuclear processes within the star is no longer able to withstand the weight of the star's outlying layers. Little may remain after the explosion except the expanding shell of gases. The Crab nebula has left behind a pulsar, or rapidly rotating neutron star. Supernovas are significant contributors to the interstellar material that forms new stars.