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A nova 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. All observed novae involve in close binary star, but causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. The main sub-classes of novae are classical novae, recurrent novae (RNe), and . They are all considered to be cataclysmic variable stars.
Classical nova eruptions are the most common type. This type is usually 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 is a few days or less, the white dwarf is close enough to its companion star to draw 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 thermal runaway nuclear 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 , lasting for perhaps several centuries.
A recurrent nova involves the same processes as a classical nova, except that the nova event repeats in cycles of a few decades or less as the companion star again feeds the dense atmosphere of the white dwarf after each ignition, as in the star T Coronae Borealis.
Under certain conditions, mass accretion can eventually trigger runaway fusion that destroys the white dwarf rather than merely expelling its atmosphere. In this case, the event is usually classified as a Type Ia supernova.
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 , 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.
Although the term "stella nova" means "new star", novae most often take place on , which are remnants of extremely old stars.
If the accretion rate is just right, hydrogen fusion may occur in a stable manner on the surface of the white dwarf, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other, Weight in a thermal runaway reaction,Dina Prialnik (2025). 9781561592685, Institute of Physics Publishing/Nature Publishing Group. ISBN 9781561592685 liberating an enormous amount of energy. This blows the remaining gases away from the surface of the white dwarf and produces an extremely bright outburst of light.
The rise to peak brightness may be very rapid, or gradual; after the peak, the brightness declines steadily.AAVSO Variable Star Of The Month: May 2001: Novae The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used for grouping novae into speed classes. Fast novae typically will take less than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days.
Despite its violence, usually the amount of material ejected in a nova is only about of a solar mass, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused during the power outburst. Nonetheless, this is enough energy to accelerate nova ejecta to velocities as high as several thousand kilometers per second—higher for fast novae than slow ones—with a concurrent rise in luminosity from a few times solar to 50,000–100,000 times solar.
Potentially, a white dwarf can generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its Binary star. Where this repeated flaring is observed, the object is called a recurrent nova. 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 can Explosion 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 5 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.
Spectroscopy observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. Classical nova are galactic producers of the element lithium. The contribution of novae to the interstellar medium is not great; novae supply only as much material to the galaxy as do supernovae, and only as much as red giant and supergiant stars.
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 recur, albeit on time scales ranging from 1,000 to 100,000 years. 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. Consequently, the interval is shorter for high-mass white dwarfs.
V Sagittae is unusual in that the time of its next eruption can be predicted fairly accurately; it is expected to recur in approximately 2083, plus or minus about 11 years.
Although it is estimated that as many as a quarter of nova systems experience multiple eruptions, only ten recurrent novae (listed below) have been observed in the Milky Way.
Several extragalactic recurrent novae have been observed 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.
On 20 April 2016, the Sky & Telescope website reported a sustained brightening of T Coronae Borealis from magnitude 10.5 to about 9.2 starting in February 2015. A similar event had been reported in 1938, followed by another outburst in 1946. By June 2018, the star had dimmed slightly but still remained at an unusually high level of activity. In March or April 2023, it dimmed to magnitude 12.3. A similar dimming occurred in the year before the 1945 outburst, indicating that it would likely erupt between March and September 2024. As of October 14, 2025, this predicted outburst has not yet occurred.
| CI Aquilae | K. Reinmuth | 8.6–16.3 | 40 | 1917, 1941, 2000 | 24–59 | ||
| V394 Coronae Australis | L. E. Erro | 7.2–19.7 | 6 | 1949, 1987 | 38 | ||
| T Coronae Borealis | J. Birmingham | 2.5–10.8 | 6 | 1217, 1787, 1866, 1946 | 79–82 | ||
| IM Normae | I. E. Woods | 8.5–18.5 | 70 | 1920, 2002 | ≤82 | ||
| RS Ophiuchi | W. Fleming | 4.8–11 | 14 | 1898, 1907, 1933, 1958, 1967, 1985, 2006, 2021 | 9–26 | ||
| V2487 Ophiuchi | Kesao Takamizawa (1998) | 9.5–17.5 | 9 | 1900, 1998 | 98 | ||
| T Pyxidis | H. Leavitt | 6.4–15.5 | 62 | 1890, 1902, 1920, 1944, 1967, 2011 | 12–44 | ||
| V3890 Sagittarii | H. Dinerstein | 8.1–18.4 | 14 | 1962, 1990, 2019 | 28–29 | ||
| U Scorpii | N. R. Pogson | 7.5–17.6 | 2.6 | 1863, 1906, 1917, 1936, 1979, 1987, 1999, 2010, 2022, | 8–43 | ||
| V745 Scorpii | Lukas Plaut | 9.4–19.3 | 7 | 1937, 1989, 2014 | 25–52 |
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