Artist's conception of a white dwarf, right, accreting hydrogen from the Roche lobe of its larger companion star
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. All observed novae involve white dwarfs in close binary 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), and dwarf novae. 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 runawayfusion. 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 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 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.[1]
Etymology
During the sixteenth century, astronomer Tycho Brahe observed the supernovaSN 1572 in the constellationCassiopeia. 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 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 called classical novae to distinguish them from supernovae, 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 on white dwarfs, which are remnants of extremely old stars.
Evolution of potential novae begins with two main sequence stars in a binary system. One of the two evolves into a red giant, leaving its remnant white dwarf core in orbit with the remaining star. The second star—which may be either a main-sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche 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 of degenerate matter, the accreted hydrogen is unable to expand even though its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20million K, initiating nuclear burning via the CNO 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 a supersoft 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, heavier chemical elements in a runaway 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 3magnitudes from maximum optical brightness is used for grouping novae into speed classes. Fast novae typically will take less than 25days to decay by 2magnitudes, while slow novae will take more than 80days.[5]
Despite its violence, usually the amount of material ejected in a nova is only about 1⁄10,000 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.[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 in luminosity from a few times solar to 50,000–100,000times solar.[2][6] In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that a nova also can emit gamma rays (>100MeV).[7]
Potentially, a white dwarf can generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion 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 explode as a TypeIa 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 29August 1975, in the constellation Cygnus about 5 degrees north of Deneb, and reached magnitude2.0 (nearly as bright as Deneb). The most recent were V1280 Scorpii, which reached magnitude3.7 on 17February 2007, and Nova Delphini 2013. Nova Centauri 2013 was discovered 2December 2013 and so far is the brightest nova of this millennium, reaching magnitude3.3.
Helium novae
A helium nova (undergoing a helium flash) is a proposed category of nova event that lacks hydrogen lines in its spectrum. The absence of hydrogen lines 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]
Occurrence rate and astrophysical significance
Astronomers have estimated that the Milky 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 the Andromeda 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 ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium.[2] Classical nova explosions are galactic producers of the element lithium.[13][14] The contribution of novae to the interstellar medium is not great; novae supply only 1⁄50 as much material to the galaxy as do supernovae, and only 1⁄200 as much as red giant and supergiant stars.[2]
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.[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]
Subtypes
Novae are classified according to the light curve decay speed, referred to as either type A, B, C and R,[17] or using the prefix "N":
NA: fast novae, with a rapid brightness increase, followed by a brightness decline of 3 magnitudes—to about 1⁄16 brightness—within 100 days.[18]
NB: slow novae, with a brightness decline of 3 magnitudes in 150 days or more.
NC: very slow novae, also known as symbiotic novae, staying at maximum light for a decade or more and then fading very slowly.
NR/RN: recurrent novae, where two or more eruptions separated by 80 years or less have been observed.[19] These are generally also fast.
Some novae leave behind visible nebulosity, material expelled in the nova explosion or in multiple explosions.[20]
Novae as distance indicators
Novae have some promise for use as standard candle measurements of distances. For instance, the distribution of their absolute magnitude is bimodal, 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 nearby galaxies and galaxy clusters have been shown to be of comparable accuracy to those measured with Cepheidvariable stars.[21]
Recurrent novae
A recurrent 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, 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.[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 5October2024,[update] this predicted outburst has not yet occurred.
In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae, since ones with an outburst brightness visible to the naked eye and an invisible quiescent brightness appeared as new stars in the sky.
T Pyxidis is a recurrent nova and nova remnant in the constellation Pyxis. It is a binary star system and its distance is estimated at 4,783 parsecs from Earth. It contains a Sun-like star and a white dwarf. Because of their close proximity and the larger mass of the white dwarf, the latter draws matter from the larger, less massive star. The influx of matter on the white dwarf's surface causes periodic thermonuclear explosions to occur.
T Coronae Borealis, nicknamed the Blaze Star, is a binary star and a recurrent nova about 3,000 light-years away in the constellation Corona Borealis. It was first discovered in outburst in 1866 by John Birmingham, though it had been observed earlier as a 10th magnitude star. It may have been observed in 1217 and in 1787 as well.
V476 Cygni or Nova Cygni 1920 was a nova which occurred in the constellation Cygnus in 1920. It was discovered by William Frederick Denning, an English amateur astronomer, at 09:30 GMT on 20 August 1920, at which time it had a magnitude of 3.7. It reached a peak brightness of magnitude 1.7 on 23 August 1920. Its quiescent brightness is magnitude 17.09.
RS Ophiuchi is a recurrent nova system approximately 5,000 light-years away in the constellation Ophiuchus. In its quiet phase it has an apparent magnitude of about 12.5. It has been observed to erupt in 1898, 1933, 1958, 1967, 1985, 2006 and 2021 and reached about magnitude 5 on average. A further two eruptions, in 1907 and 1945, have been inferred from archival data. The recurrent nova is produced by a white dwarf star and a red giant in a binary system. About every 15 years, enough material from the red giant builds up on the surface of the white dwarf to produce a thermonuclear explosion. The white dwarf orbits close to the red giant, with an accretion disc concentrating the overflowing atmosphere of the red giant onto the white dwarf.
HR Lyrae or Nova Lyrae 1919 was a nova which occurred in the constellation Lyra in 1919. Its discovery was announced by Johanna C. Mackie on 6 December 1919. She discovered it while examining photographic plates taken at the Harvard College Observatory. The bulletin announcing the discovery states "Between December 4 and 6 it rose rapidly from the sixteenth magnitude or fainter, to a maximum of about 6.5". It was the first nova ever reported in Lyra, and Mackie was awarded the AAVSO gold medal for her discovery. Its peak magnitude of 6.5 implies that it might have been visible to the naked eye, under ideal conditions.
V838 Herculis, also known as Nova Herculis 1991, was a nova which occurred in the constellation Hercules in 1991. It was discovered by George Alcock of Yaxley, Cambridgeshire, England at 4:35 UT on the morning of 25 March 1991. He found it with 10×50 binoculars, and on that morning its apparent visual magnitude was 5. Palomar Sky Survey plates showed that before the outburst, the star was at photographic magnitude 20.6 and 18.25.
V1494 Aquilae or Nova Aquilae 1999 b was a nova which occurred during 1999 in the constellation Aquila and reached a brightness of magnitude 3.9 on 2 December 1999. making it easily visible to the naked eye. The nova was discovered with 14×100 binoculars by Alfredo Pereira of Cabo da Roca, Portugal at 18:50 UT on 1 December 1999, when it had a visual magnitude of 6.0.
V606 Aquilae was a nova, which lit up in the constellation Aquila in 1899. The brightest reported magnitude for this nova was apparent magnitude 5.5, making it a naked eye object. It was discovered by Williamina Fleming on a photographic plate taken on 21 April 1899 at the Harvard College Observatory. On the discovery plate, its photographic magnitude was later determined to be 6.75. It was not seen on the plate taken on 1 November 1898, and there were no reported observations of the region around the star during the 171 day interval before Fleming's discovery, so it is possible that the actual maximum of the event was missed. By 27 October 1899 it had faded to 10th magnitude, and on 9 July 1900 Oliver Wendell reported its brightness to be between magnitude 11.5 and 12.0.
P Cygni is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in Uranometria as a nova. Located about 5,300 light-years from Earth, it is a hypergiant luminous blue variable (LBV) star of spectral type B1-2 Ia-0ep that is one of the most luminous stars in the Milky Way.
V605 Aquilae, in the constellation Aquila, is the variable central star of the planetary nebula Abell 58. It is a highly unusual hydrogen-deficient carbon-rich star.
RW Ursae Minoris is a cataclysmic variable star system that flared up as a nova in the constellation Ursa Minor in 1956.
IM Normae is a recurrent nova in the constellation Norma, one of only ten known in the Milky Way. It has been observed to erupt in 1920 and 2002, reaching magnitude 8.5 from a baseline of 18.3. It was poorly monitored after the first eruption, so it is possible that it erupted in between these dates.
V392 Persei, also known as Nova Persei 2018, is a bright nova in the constellation Perseus discovered on April 29, 2018. It was previously known as a dwarf nova.
OS Andromedae, known also as Nova Andromedae 1986, is a classical nova that appeared in the constellation Andromeda during 1986. It was discovered at 10:34 UT on 5 December 1986 by Mitsuri Suzuki, a 28-year-old school teacher living in Ena, Japan. He photographed the portion of the Milky Way that passes through northern Andromeda with a 200-mm telephoto lens, and found the nova when its apparent magnitude was 8.0. Two days later it reached a peak apparent visual magnitude of 6.3.
YZ Reticuli, also known as Nova Reticuli 2020 was a naked eye nova in the constellation Reticulum discovered on July 15, 2020. Previously it was known as a VY Sculptoris type object with the designation MGAB-V207.
QZ Aurigae, also known as Nova Aurigae 1964, was a nova which occurred in the constellation Auriga during 1964. It was discovered by Nicholas Sanduleak on an objective prism photographic plate taken at the Warner and Swasey Observatory on 4 November 1964. Examination of pre-discovery plates from Sonneberg Observatory showed that the eruption occurred in early February 1964, and it had a photographic magnitude of 6.0 on 14 February 1964. Its brightness declined in images taken after the 14th, suggesting that its peak brightness was above 6.0. It was probably visible to the naked eye for a short time.
LV Vulpeculae, also known as Nova Vulpeculae 1968 no. 1, was the first of two novae in the constellation of Vulpecula which erupted in 1968. It was discovered by George Alcock who observed it from the back garden of his home in Farcet, England, on the morning of 15 April 1968. The next night it was independently discovered by Midtskoven in Norway. It reached a peak apparent magnitude of 4.79 on 17 April 1968. It was visible to the naked eye at the same time HR Delphini was a naked eye object, and the two novae were less than 15 degrees apart on the sky.
V1370 Aquilae, also known as Nova Aquilae 1982, is a nova that appeared in the constellation Aquila during 1982. It was discovered by Minoru Honda of Kurashiki, Japan at 20:30 UT on 27 January 1982. At that time the Sun had moved just far enough from Aquila to allow the nova to be seen in the morning sky. Although it was discovered photographically, its apparent magnitude was 6–7, making it potentially visible to the naked eye under ideal conditions. A possible magnitude 20 progenitor was located on the Palomar Sky Survey prints. Spectra of the object were taken in February 1982 at Asiago Astrophysical Observatory, which confirmed that it is a nova.
↑ 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. Retrieved 18 January 2024.
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.
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