A nova visible to the naked eye.

Novas erupt frequently throughout the heavens, but the brilliant stellar outburst recently observed in the constellation Cygnus has special significance. It presents the first nova visible to the naked eye since 1975.

Discovered Feb. 18 by amateur astronomer Peter Collins of Boulder, Colo., the new nova--a Milky Way outburst dubbed Nova Cygni 1992--has lasted longer than many such flare-ups, which typically fade from view in a few days.

Nova Cygni peaked in luminosity on Feb. 20 and remained nearly that bright for three days, after which it began to dim. Observations with the International Ultraviolet Explorer (IUE) satellite on Feb. 26, however, revealed a puzzling finding: After days of dimming, the nova had brightened somewhat in both ultraviolet and visible light. (This week, though, observers may need binoculars to see it.)

Novas occur in double-star systems that feature a dense star, called a white dwarf, which steals mass from a closely orbiting companion. The accumulation of mass on the surface of the dwarf sparks a thermonuclear explosion, hurling a shell of hot, luminous gas into space. Unlike a supernova explosion, which destroys the entire star, a nova leaves the white dwarf intact, enabling it to later repeat its violent behavior.

Sumner G. Starrfield of Arizona State University in Tempe and his colleagues, who are studying the nova's spectra with the IUE, report that light emissions from iron now mask the presence of other elements in the newly ejected gas shell. He says the nova may contain relatively little carbon, since this element would have shortened the duration of the outburst.