Some interacting, short period binary stars have orbits so small that one member, a main sequence star, transfers mass to its companion white dwarf, triggering an explosion with an energy equal to 10,000 years' worth of the sun's output. Such systems are visible as optical novae or as X-ray or gamma-ray binaries. The name ``nova'' comes from the Latin "new" and refers to the fact that the pre-nova star was probably invisible to the naked eye. Suddenly a star would appear where none had been visible before.
A nova is a member of a binary star system. One of the stars expands to ``fill its Roche lobe'', and material is dumped from one star onto the other.
``Cataclysmic Variable Stars'', Webbink, R.F. 1989, Amer. Sci. 77, 248.
``Very High Energy Gamma--Ray Binary Stars'', Lamb, R.C. and Weekes, T.C. 1987, Science 238, 1528.
``Black Hole in the Making?'', in News Notes, 1989, Sky and Telescope November, 460.
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