NOVAS & SUPERNOVAS A nova is a VARIABLE STAR whose luminosity suddenly increases by a factor of thousands or tens of thousands. Two or three novas are discovered each year and are named for the constellation and the year in which they appear. By the late 20th century, many hundreds of novas had been observed within our own galaxy, the Milky Way, as well as in extragalactic systems. Although the term nova means "new star," novas are actually long-existing stars that suddenly flare into brilliance as seen in the Earth's sky. Novas are celestial phenomena distinct from SUPERNOVAS, which are in fact destructive explosions of massive stars; therefore supernovas, unlike novas, do not recur, because the steller material has been dispersed by the explosion. The sudden brightness of a nova occurs when a STAR's surface suddenly explodes, rapidly discharging an expanding shell of gas into space. Though the observed discharge appears spectacular, the star is not destroyed by its outburst. In fact, some novas are recurrent. Only a tiny fraction, perhaps 10 to the power of -7, of the star's entire mass is lost. Peak brightness tends to occur within 10 to 50 hours. Following a brief peak period, the nova begins to fade, reaching its original luminosity days, months, or perhaps years later. A transition stage early in the fading period is common. During this transition the luminosity may fluctuate between several different levels, drop sharply and then quickly recover, or simply decline gradually and evenly. Fading after the transition stage is characteristically gradual and slow. Current theories suggest that most novas are members of a binary-star system containing a white-dwarf star and a red main-sequence star. Material pulled from the main-sequence star becomes hotter as it approaches the white dwarf, until hydrogen fusion explosively releases energy. Bibliography: Friedjung, M., ed., Novae and Related Stars (1977); Payne-Gaposchkin, Cecilia, Stars and Clusters (1979); Petit, Michel, Variable Stars (1986).