Nova

A nova (plural 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 with the dense atmosphere above the surface of a white dwarf in a close binary system when they are sufficiently near enough to allow material (mostly hydrogen) to be drawn from the companion star's surface onto the white dwarf. A nova is the result of the rapid fusion of the very dense accreted hydrogen just above the surface of the star, which on ignition commences a very rapid runaway fusion reaction. This material is dramatically expelled into space, whose brightened envelope from the energy produced is seen as the light we visually observe from the nova.

During the 16th century, astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia. He described it in his book De nova stella (Latin for "concerning the new star"), giving rise to the name nova. In this work he argued that a nearby object should be seen to move relative to the fixed stars, and that the nova had to be very far away. Though this was a supernova and not a classical nova, the terms were considered interchangeable until the 1930s.

Evolution of potential novae begins with two main sequence stars in a binary system. One of the two evolves into a red giant leaving its remnant white dwarf core in orbit with the remaining star. The second star—which may be either a main sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche lobe. As a result, the white dwarf captures matter in an accretion disk steadily from the companion's outer atmosphere, and in turn, falls into the atmosphere. As the white dwarf consists of degenerate matter, so the accreted hydrogen does not inflate but its temperature increases. Rapid, an all at once, uncontrolled fusion occurs when the temperature of this atmospheric layer reaches ~20 million Kelvins, initiating nuclear burning via the CNO cycle.

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