A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently new star, that slowly fades over several weeks or many months. Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve a white dwarf in a close binary system. The main sub-classes of novae are classical novae, recurrent novae (RNe), and dwarf novae. They are all considered to be cataclysmic variable stars.Classical nova eruptions are the most common type of nova. They are likely created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star. When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. This atmosphere, mostly consisting of hydrogen, is thermally heated by the hot white dwarf and eventually reaches a critical temperature causing ignition of rapid runaway fusion.The sudden increase in energy expels the atmosphere into interstellar space creating the envelope seen as visible light during the nova event and previously mistaken as a new star. A few novae produce short-lived nova remnants, lasting for perhaps several centuries. Recurrent nova processes are the same as the classical nova, except that the fusion ignition may be repetitive because the companion star can again feed the dense atmosphere of the white dwarf.Novae most often occur in the sky along the path of the Milky Way, especially near the observed galactic centre in Sagittarius; however, they can appear anywhere in the sky. They occur far more frequently than galactic supernovae, averaging about ten per year. Most are found telescopically, perhaps only one every 12–18 months reaching naked-eye visibility. Novae reaching first or second magnitude occur only several times per century. The last bright nova was V1369 Centauri reaching 3.3 magnitude on 14 December 2013.
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