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NOVA
A nova (plural novae or novas) is a cataclysmic nuclear explosion on a white dwarf, which causes a sudden brightening of the star. Novae are not to be confused with other brightening phenomena such as supernovae or luminous red novae. Novae are thought to occur on the surface of a white dwarf in a binary system when they are sufficiently near to one another, allowing material (mostly hydrogen) to be pulled from the companion star's surface onto the white dwarf. The nova is the result of the rapid fusion of the accredited hydrogen on the surface of the star, commencing a runaway fusion reaction.

Life Cycle
Novae occur in systems where an accreting white dwarf is receiving matter from a binary companion.

The matter accreting from the binary companion builds up on the surface of the white dwarf, creating a layer of hydrogen is created. This layer of hydrogen sits on top of the rest of the white dwarf, which is composed mainly of carbon and oxygen. As the hydrogen builds up, the pressure and temperature at the white dwarf's surface grows and grows. Eventually, it may become hot enough to fuse the hydrogen into helium.

When it gets this hot, the hydrogen fuses very quickly. The entire surface of hydrogen gets blown off the white dwarf because of this. The explosion which obliterates the surface is called a nova.

Only the built up surface gets blown away. The white dwarf is left intact underneath. As the blown off material floats away into space to become part of the intersteller medium, the accreting white dwarf is left behind to start the cycle all over again.

The sub types of classical Novae
Novae are classified according to the light curve development speed, so in

NA: Fast novae, with a rapid brightness increase, followed by a brightness decline of 3 magnitudes — to about  1⁄16 brightness — within 100 days.[12]

NB: Slow novae, with a 3 magnitudes decline in 150 days or more.

NC: Very slow novae, staying at maximum light for a decade or more, fading very slowly. It is possible that NC type novae are objects differing physically very much from normal novae, for example planetary nebulae in formation, exhibiting Wolf-Rayet star like features.

NR/RN: Recurrent novae, novae with two or more outbursts separated by 10–80 years have been observed.

Novae as distance indicators
Novae have some promise for use as standard candle measurements of distances. For instance, the distribution of their absolute magnitude is bimodal, with a main peak at magnitude −8.8, and a lesser one at −7.5. Novae also have roughly the same absolute magnitude 15 days after their peak (−5.5). Comparisons of nova-based distance estimates to various nearby galaxies and galaxy clusters with those done with Cepheid variable stars have shown them to be of comparable accuracy.