A nova (pl. novae) is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star.

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If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the star's outer atmosphere. The companion may be a main sequence star, or one that is aging and expanding into a red giant. The captured gases consist primarily of hydrogen and helium, the two principal constituents of ordinary matter in the universe. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so is largely unresponsive to heat, while the accreted hydrogen is not. Hydrogen fusion can occur in a stable manner on the surface through the CNO cycle, but the dependence of the helium fusion rate on temperature and pressure mean that it is only when helium is compressed and heated near the surface of the white dwarf to a temperature of some 20 million K that a nuclear fusion reaction rapidly converts a large amount of the helium into other heavier elements.Prialnik, Dina. "Novae", pp. 1846-56, in Paul Murdin, ed. Encyclopedia of Astronomy and Astrophysics. London: Institute of Physics Publishing Ltd and Nature Publishing Group, 2001. ISBN 1561592684 The enormous amount of energy liberated by this process blows the remaining gases away from the white dwarf's surface and produces an extremely bright outburst of light. The rise to peak brightness can be very rapid (as in fast novae) or gradual (as in slow novae); after the peak the brightness declines steadily.AAVSO Variable Star Of The Month: May 2001: Novae

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