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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", which is 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. 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. Such were taken in past centuries to be 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.
During the sixteenth 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 adoption of 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. Although this event was a supernova and not a nova, the terms were considered interchangeable until the 1930s.[2] After this, novae were classified as classical novae to distinguish them from supernovae, as their causes and energies were thought to be different, based solely in the observational evidence.
The rise to peak brightness may be very rapid, or gradual. This is related to the speed class of the nova; yet after the peak, the brightness declines steadily.[4] The time taken for a nova to decay by around 2 or 3 magnitudes from maximum optical brightness is used for classification, via its speed class. Fast novae typically will take fewer than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days.[5]
Potentially, a white dwarf can 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 seven times (in 1898, 1933, 1958, 1967, 1985, 2006, and 2021). Eventually, the white dwarf could explode as a Type Ia supernova if it approaches the Chandrasekhar limit.
Occasionally, novae are bright enough and close enough to Earth to be conspicuous to the unaided eye. The brightest recent example was Nova Cygni 1975. This nova appeared on 29 August 1975, in the constellation Cygnus about five degrees north of Deneb, and reached magnitude 2.0 (nearly as bright as Deneb). The most recent were V1280 Scorpii, which reached magnitude 3.7 on 17 February 2007, and Nova Delphini 2013. Nova Centauri 2013 was discovered 2 December 2013 and so far, is the brightest nova of this millennium, reaching magnitude 3.3.
A helium nova (undergoing a helium flash) is a proposed category of nova events that lacks hydrogen lines in its spectrum. This may be caused by the explosion of a helium shell on a white dwarf. The theory was first proposed in 1989, and the first candidate helium nova to be observed was V445 Puppis in 2000.[8] Since then, four other novae have been proposed as helium novae.[9]
Astronomers estimate that the Milky Way experiences roughly 30 to 60 novae per year, but a recent examination has found the likely improved rate of about 5027.[10] The number of novae discovered in the Milky Way each year is much lower, about 10,[11] probably due to distant novae being obscured by gas and dust absorption.[11] Roughly 25 novae brighter than about the twentieth magnitude are discovered in the Andromeda Galaxy each year and smaller numbers are seen in other nearby galaxies.[12] As of 2019, 407 probable novae are recorded in the Milky Way.[11]
Observed recurrent novae such as RS Ophiuchi (those with periods on the order of decades) are rare. Astronomers theorize, however, that most, if not all, novae are recurrent, albeit on time scales ranging from 1,000 to 100,000 years.[15] The recurrence interval for a nova is less dependent on the accretion rate of the white dwarf than on its mass; with their powerful gravity, massive white dwarfs require less accretion to fuel an eruption than lower-mass ones.[2] Consequently, the interval is shorter for high-mass white dwarfs.[2]
A recurrent nova (RNe) is an object that has been seen to experience repeated nova eruptions. as well as several extragalactic ones (in the Andromeda Galaxy (M31) and the Large Magellanic Cloud). One of these extragalactic novae, M31N 2008-12a, erupts as frequently as once every 12 months. The recurrent nova typically brightens by about 8.6 magnitudes, whereas a classic nova may brighten by more than 12 magnitudes.[21] Although it is estimated that as many as a quarter of nova systems experience multiple eruptions, only ten recurrent novae have been observed in the Milky Way.[22] The ten known galactic recurrent novae are listed below.
Novae are relatively common in the Andromeda Galaxy (M31).[12] Approximately several dozen novae (brighter than about apparent magnitude 20) are discovered in M31 each year.[12] The Central Bureau for Astronomical Telegrams (CBAT) tracked novae in M31, M33, and M81.[23]
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