Artist's conception of a white dwarf, right,accreting hydrogen from theRoche lobe of its larger companion star
Anova is atransient 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 involvewhite dwarfs in closebinary systems, 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), anddwarf novae. They are all considered to becataclysmic 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 amain sequence,subgiant, orred 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 drawaccretedmatter onto its surface, creating a dense but shallowatmosphere. This atmosphere, mostly consisting ofhydrogen, is heated by the hot white dwarf and eventually reaches a critical temperature, causing ignition of rapidrunawayfusion. 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-livednova remnants, 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 starT 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 aType Ia supernova.
Novae most often occur in the sky along the path of theMilky Way, especially near the observedGalactic Center in Sagittarius; however, they can appear anywhere in the sky. They occur far morefrequently than galacticsupernovae, averaging about ten per year in the Milky Way. Most are found telescopically, perhaps only one every 12–18 months reachingnaked-eye visibility. Novae reaching first or secondmagnitude occur only a few times per century. The last bright nova wasV1369 Centauri, which reached 3.3 magnitude on 14 December 2013.[1]
During the sixteenth century, astronomerTycho Brahe observed thesupernovaSN 1572 in theconstellationCassiopeia. He described it in his bookDe nova stella (Latin for "concerning the new star"), giving rise to the adoption of the namenova. In this work he argued that a nearby object should be seen to move relative to the fixed stars, and thus the nova had to be very far away. Although SN 1572 was later found to be a supernova and not a nova, the terms were considered interchangeable until the 1930s.[2] After this, novae were calledclassical novae to distinguish them fromsupernovae, as their causes and energies were thought to be different, based solely on the observational evidence.
Although the term "stella nova" means "new star", novae most often take place onwhite dwarfs, which are remnants of extremely old stars.
Evolution of potential novae begins with twomain sequence stars in a binary system. One of the twoevolves into ared giant, leaving its remnant white dwarf core in orbit with the remaining star. The second star—which may be either amain-sequence star or an aginggiant—begins to shed its envelope onto its white dwarf companion when it overflows itsRoche lobe. As a result, the white dwarf steadily captures matter from the companion's outer atmosphere in an accretion disk, and in turn, the accreted matter falls into the atmosphere. As the white dwarf consists ofdegenerate matter, theaccretedhydrogen is unable to expand even though its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 millionK, initiating nuclear burning via theCNO cycle.[3]
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 asuper 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,heavierchemical elements in arunaway reaction,[2] 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.[4] 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.[5]
Despite its violence, usually the amount ofmaterial ejected in a nova is only about1⁄10,000 of asolar 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.[2] 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 inluminosity from a few times solar to 50,000–100,000 times solar.[2][6] In 2010 scientists using NASA'sFermi Gamma-ray Space Telescope discovered that a nova also can emitgamma rays (>100 MeV).[7]
Potentially, awhite dwarf can generate multiple novae over time as additionalhydrogen continues to accrete onto its surface from itscompanion star. Where this repeated flaring is observed, the object is called a recurrent nova. An example isRS Ophiuchi, which is known to have flared seven times (in 1898, 1933, 1958, 1967, 1985, 2006, and 2021). Eventually, thewhite dwarf canexplode as aType Ia supernova if it approaches theChandrasekhar limit.
Occasionally, novae are bright enough and close enough to Earth to be conspicuous to the unaided eye. The brightest recent example wasNova Cygni 1975. This nova appeared on 29 August 1975, in the constellationCygnus about 5 degrees north ofDeneb, and reachedmagnitude 2.0 (nearly as bright asDeneb). The most recent wereV1280 Scorpii, which reached magnitude 3.7 on 17 February 2007, andNova Delphini 2013.Nova Centauri 2013 was discovered 2 December 2013 and so far is the brightest nova of thismillennium, reaching magnitude 3.3.
Astronomers have estimated that theMilky Way experiences roughly 25 to 75 novae per year.[10] The number of novae actually observed in the Milky Way each year is much lower, about 10,[11] probably because distant novae are obscured by gas and dust absorption.[11] As of 2019, 407 probable novae had been recorded in the Milky Way.[11] In theAndromeda Galaxy, roughly 25 novae brighter than about 20th magnitude are discovered each year, and smaller numbers are seen in other nearby galaxies.[12]
Spectroscopic observation of nova ejectanebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium.[2] Classical novaexplosions are galactic producers of the elementlithium.[13][14] The contribution of novae to theinterstellar medium is not great; novae supply only1⁄50 as much material to the galaxy as do supernovae, and only1⁄200 as much asred giant andsupergiant stars.[2]
Observed recurrent novae such asRS 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.[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]
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.[16]
Novae have some promise for use asstandard candle measurements of distances. For instance, the distribution of theirabsolute magnitude isbimodal, with a main peak at magnitude −8.8, and a lesser one at −7.5. Novae also have roughly the same absolute magnitude 15 days after their peak (−5.5). Nova-based distance estimates to various nearbygalaxies andgalaxy clusters have been shown to be of comparable accuracy to those measured withCepheidvariable stars.[21]
Arecurrent nova (RN) is an object that has been seen to experience repeated nova eruptions. The recurrent nova typically brightens by about 9 magnitudes, whereas a classical nova may brighten by more than 12 magnitudes.[22]
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.[23]
On 20 April 2016, theSky & Telescope website reported a sustained brightening ofT 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.[24] 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.[25] A similar dimming occurred in the year before the 1945 outburst, indicating that it would likely erupt between March and September 2024.[26] As of November 24, 2025, this predicted outburst has not yet occurred.
^Rosenbush, A. E. (17–21 September 2007). Klaus Werner; Thomas Rauch (eds.). "List of Helium Novae".Hydrogen-Deficient Stars.391. Eberhard Karls University, Tübingen, Germany (published July 2008): 271.Bibcode:2008ASPC..391..271R.
^Liimets, T.; Corradi, R.L.M.; Santander-García, M.; Villaver, E.; Rodríguez-Gil, P.; Verro, K.; Kolka, I. (2014). "A Dynamical Study of the Nova Remnant of GK Persei / Stella Novae: Past and Future Decades.".Stellar Novae: Past and Future Decades. ASP Conference Series. Vol. 490. pp. 109–115.arXiv:1310.4488.Bibcode:2014ASPC..490..109L.
^Schaefer, B.E.; Kloppenborg, B.; Waagen, E.O."Announcing T CrB pre-eruption dip".AAVSO. American Association of Variable Star Observers. Retrieved18 January 2024.
^Shafter, Allen W.; Hornoch, Kamil; Kučáková, Hana; Fatka, Petr; Zhao, Jingyuan; Gao, Xing; Yaqup, Shahidin; Zhong, Tuhong; Esamdin, Ali (10 January 2024), "M31N 2013-10c: A Newly Identified Recurrent Nova in M31",Research Notes of the AAS,8 (1): 5,arXiv:2401.05573,Bibcode:2024RNAAS...8....5S,doi:10.3847/2515-5172/ad19de
Hernanz, Margarita; José, Jordi, eds. (2002).Classical Nova Explosions: International Conference on Classical Nova Explosions, Sitges, Spain, 20-24 May, 2002. AIP conference proceedings. Melville, N.Y:American Institute of Physics.ISBN978-0-7354-0092-4.
