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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).

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