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When a white dwarf has a close companion star, the companion will often begin to have its outer atmosphere drawn away from it by the white dwarf's gravity as the companion star ages and expands into a red giant. The captured gases consist primarily of hydrogen and helium, the two principal constituents of 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. Eventually, the pressures and temperatures within the hydrogen layer becomes great enough to trigger a nuclear fusion reaction that rapidly converts a large amount of the hydrogen into helium and other heavier elements.
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 but brief-lived outburst of light. This bright light, lasting only a matter of days, gave rise to the name nova, which is Latin for "new"; ancient astronomers would see a nova become visible in the night sky where no star was visible before, and believed that it was a "new star."
A white dwarf can potentially generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion star. An example is RS Ophiuchi , which is known to have flared five times (in 1898, 1933, 1958, 1967, and 1985). Eventually, however, either the companion star will either run out of material, or the white dwarf will undergo a nova so powerful that it is completely destroyed in the process. This is somewhat similar to a type Ia supernova. Supernovae in general, however, involve different processes as well as much higher energies, and should not be confused with ordinary novae.
Occasionally a nova is bright enough and close enough to be conspicuous to the unaided eye. The most recent example was Nova Cygni 1975 . This nova appeared on August 29, 1975 in the constellation Cygnus about five degrees north of Deneb and reached magnitude 2.0 (nearly as bright as Deneb).
Bright novae since 1900
- GK Persei in 1901 (max magnitude: 0.2) 
- V603 Aquilae in 1918 (max magnitude: −0.5 or −1.4)
- CP Puppis in 1942 (max magnitude: 0.3)
- V1500 Cygni in 1975 (max magnitude: 2.0) 
- V382 Velorum in 1999 (max magnitude: 2.6)
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