A nova (pl. novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", Latin for "new") that slowly fades over weeks or months. Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve white dwarfs in close binary systems. 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. This type is likely created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star. If the orbital period of the system 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 its surface, creating a dense but shallow atmosphere. This atmosphere, mostly consisting of hydrogen, is 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. In past centuries such an event was thought to be a new star. A few novae produce short-lived nova remnants, lasting for perhaps several centuries. A recurrent nova involves the same processes as the classical nova, except that the fusion ignition is 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 Center in Sagittarius; however, they can appear anywhere in the sky. They occur far more frequently than galactic supernovae, averaging about ten per year in the Milky Way. Most are found telescopically, perhaps only one every 12–18 months reaching naked-eye visibility. Novae reaching first or second magnitude occur only a few times per century. The last bright nova was V1369 Centauri, which reached 3.3 magnitude on 14 December 2013.
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