Nova

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Artist's conception of a white dwarf, right, accreting hydrogen from the Roche lobe of its larger companion star Making a Nova.jpg
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", 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.

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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.

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 several times per century. The last bright nova was V1369 Centauri reaching 3.3 magnitude on 14 December 2013. [1]

Etymology

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.

Although the term "stella nova" means "new star", novae most often take place as a result of white dwarfs, which are remnants of extremely old stars.

Stellar evolution of novae

Nova Eridani 2009 (apparent magnitude ~8.4) Nova-Eridani-2009-LB4.jpg
Nova Eridani 2009 (apparent magnitude ~8.4)

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 does not inflate, but its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K, initiating nuclear burning, via the CNO cycle. [3]

Hydrogen fusion may occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is unstable thermally 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 surface and produces an extremely bright outburst of light.

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]

Despite their violence, usually the amount of material ejected in novae is only about 110,000 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 50,000–100,000 times solar. [2] [6] In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that a nova also can emit gamma-rays (>100 MeV). [7]

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.

Helium novae

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]

Occurrence rate and astrophysical significance

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 50±27. [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]

Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. [2] The contribution of novae to the interstellar medium is not great; novae supply only 150 as much material to the Galaxy as do supernovae, and only 1200 as much as red giant and supergiant stars. [2] Classical novae explosions are galactic producers of the element lithium. [13] [14]

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]

V Sagittae is unusual in that we can predict now that it will go nova in approximately 2083, plus or minus about 11 years. [16]

Subtypes

Novae are classified according to the light curve development speed, so in

Remnants

GK Persei: Nova of 1901 GKPersei-MiniSuperNova-20150316.jpg
GK Persei: Nova of 1901

Some novae leave behind visible nebulosity, material expelled in the nova explosion or in multiple explosions. [19]

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). Comparisons of nova-based distance estimates to various nearby galaxies and galaxy clusters with those measured with Cepheid variable stars, have shown them to be of comparable accuracy. [20]

Recurrent nova

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.

Full name
Discoverer
Magnitude
range
Days to drop
3 magnitudes
from peak
Known eruption yearsTime span (years)Years since latest eruption
CI Aquilae K. Reinmuth 8.6–16.3401917, 1941, 200024–5923
V394 Coronae Australis L. E. Erro 7.2–19.761949, 19873836
T Coronae Borealis J. Birmingham 2.5–10.861866, 19468077
IM Normae I. E. Woods 8.5–18.5701920, 2002≤8222
RS Ophiuchi W. Fleming 4.8–11141898, 1907, 1933, 1958, 1967, 1985, 2006, 20219–262
V2487 Ophiuchi K. Takamizawa (1998)9.5–17.591900, 19989825
T Pyxidis H. Leavitt 6.4–15.5621890, 1902, 1920, 1944, 1967, 201112–4412
V3890 Sagittarii H. Dinerstein 8.1–18.4141962, 1990, 201928–294
U Scorpii N. R. Pogson 7.5–17.62.61863, 1906, 1917, 1936, 1979, 1987, 1999, 2010, 2022,8–431
V745 Scorpii L. Plaut 9.4–19.371937, 1989, 201425–529

Extragalactic novae

Nova in Andromeda Galaxy Nova in M31.jpg
Nova in Andromeda Galaxy

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]

See also

Related Research Articles

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<span class="mw-page-title-main">Cataclysmic variable star</span> Stars with irregular large fluctuations in brightness

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<span class="mw-page-title-main">HR Lyrae</span> Nova that appeared in 1919

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.

<span class="mw-page-title-main">V838 Herculis</span> 1991 Nova seen in the constellation Hercules

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.

<span class="mw-page-title-main">V1494 Aquilae</span> Nova seen in 1999 in the constellation of Aquila

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<span class="mw-page-title-main">IK Pegasi</span> Star in the constellation Pegasus

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<span class="mw-page-title-main">V339 Delphini</span> 2013 nova seen in the constellation Delphinus

V339 Delphini or Nova Delphini 2013 (PNV J20233073+2046041) is a bright nova star in the constellation Delphinus. It was discovered on 14 August 2013 by amateur astronomer Koichi Itagaki in Japan and confirmed by the Liverpool Telescope on La Palma. The nova appeared with a magnitude 6.8 when it was discovered and peaked at magnitude 4.3 on 16 August 2013. A nova is produced by the fusion of accumulated material on the white dwarf nova progenitor acquired from its companion star. The nova system is thus a binary star, and a classical nova. The white dwarf is a carbon-oxygen white dwarf, with an estimated mass of 1.04±0.02 M. There is not yet a consensus about what the binay's orbital period is; estimates range from 3.15 hours to 6.43 hours.

<span class="mw-page-title-main">RW Ursae Minoris</span> Nova that appeared in 1956

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<span class="mw-page-title-main">V906 Carinae</span> 2018 nova that occurred in the constellation Carina

V906 Carinae, also known as Nova Carinae 2018, was a nova in the Milky Way galaxy which appeared in the constellation Carina, near the 5th magnitude star HD 92063. It was discovered on images taken on 20.32 March 2018 by the All Sky Automated Survey for SuperNovae (ASAS-SN] telescope at the Cerro Tololo Inter-American Observatory. The ASAS-SN group assigned the name ASASSN-18fv to the object. The discovery image was saturated, allowing researchers to determine only that the object was brighter than apparent magnitude 10. An earlier image obtained by ASAS-SN on 26.32 March 2018 showed the nova was a magnitude ~10.4 object at that time, and the object was not detected on ASAS-SN images taken on 15.34 March 2018 and earlier.

<span class="mw-page-title-main">V392 Persei</span> Nova in the constellation Perseus

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<span class="mw-page-title-main">V1370 Aquilae</span> Nova that occurred in 1982

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.

<span class="mw-page-title-main">Nova Cassiopeiae 2021</span> Nova in the constellation Cassiopeia seen in 2021

Nova Cassiopeiae 2021, also known V1405 Cassiopeiae, was a nova in the constellation Cassiopeia. It reached a peak brightness of magnitude 5.449 on May 9, 2021, making it visible to the naked eye. It was discovered by Japanese amateur astronomer Yuji Nakamura of Kameyama, Japan, at 10:10 UT on March 18, 2021. The nova was first seen by Nakamura in four 15 second CCD exposures with a 135mm F/4 lens, when it was at magnitude 9.3. Nothing was seen brighter than magnitude 13.0 with the same equipment in exposures taken at 10:12 UT on March 14, 2021. For the first seven months after discovery, the nova's brightness stayed at a rough plateau, fading and rebrightening at least eight times; it is considered a very slow nova. After the seven month long series of peaks, Nova Cassiopeiae began a linear decline in brightness. This nova has been detected throughout the electromagnetic spectrum, from radio to gamma rays.

References

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