A nova (plural novae or novas) is a cataclysmic nuclear explosion on a white dwarf, which causes a sudden brightening of the star. Novae are not to be confused with other brightening phenomena such as supernovae or luminous red novae. A nova is caused by the accretion of hydrogen onto the surface of the star, which ignites and starts nuclear fusion in a raging manner. Novae are thought to occur on the surface of a white dwarf in a binary system. If the two stars are close enough, material can be pulled from the companion star's surface onto the white dwarf.
If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the companion's outer atmosphere. The companion may be a main sequence star, or one that is aging and expanding into a red giant. The captured gases consist primarily of hydrogen and helium. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so does not inflate as its temperature increases, while the accreted hydrogen is compressed upon the surface. The dependence of the hydrogen fusion rate on temperature and pressure means that it is only when it is compressed and heated at the surface of the white dwarf to a temperature of some 20 million kelvin that a nuclear fusion reaction occurs; at these temperatures, hydrogen burns via the CNO cycle.
While hydrogen fusion can occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, for most binary system parameters the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other heavier elements in a runaway reaction, liberating an enormous amount of energy, blowing the remaining gases away from the white dwarf's surface and producing an extremely bright outburst of light. The rise to peak brightness can be very rapid or gradual and is related to the speed class of the nova; after the peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used to classify a nova via its speed class. A fast nova will typically take less than 25 days to decay by 2 magnitudes and a slow nova will take over 80 days.
In spite of their violence, the amount of material ejected in novae is usually 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. 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. In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope were surprised to discover, for the first time, that a nova can also emit gamma-rays (>100 MeV).
A white dwarf can potentially 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 six times (in 1898, 1933, 1958, 1967, 1985, and 2006). Eventually, the white dwarf could explode as a type Ia supernova if it approaches the Chandrasekhar limit.
Occasionally a nova is bright enough and close enough 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 is so far the brightest nova of this millennium reaching magnitude 3.3.
A helium nova (or helium flash) is a proposed category of nova explosion that lacks hydrogen lines in the spectrum. This may be caused by the explosion of a helium shell on a white dwarf. It was proposed by Kato, Saio and Hachisu in 1989. The first candidate helium nova to be observed was V445 Puppis in 2000. Since then, four other novae explosions have been proposed as helium novae.
Occurrence rate and astrophysical significance
Astronomers estimate that the Milky Way experiences roughly 30 to 60 novae per year, with a likely rate of about 40. The number of novae discovered in the Milky Way each year is much lower, about 10. Roughly 25 novae brighter than about magnitude 20 are discovered in the Andromeda Galaxy each year and smaller numbers are seen in other nearby galaxies.
Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. The contribution of novae to the interstellar medium is not great; novae supply only 1⁄50 as much material to the Galaxy as supernovae, and only 1⁄200 as much as red giant and supergiant stars.
Recurrent novae like 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. The recurrence interval for a nova is less dependent on the white dwarf's accretion rate than on its mass; with their powerful gravity, massive white dwarfs require less accretion to fuel an outburst than lower-mass ones. Consequently, the interval is shorter for high-mass white dwarfs.
Novae are classified according to the light curve development speed, so in
- NA: Fast novae, with a rapid brightness increase, followed by a brightness decline of 3 magnitudes — to about 1⁄16 brightness — within 100 days.
- NB: Slow novae, with a 3 magnitudes decline in 150 days or more.
- NC: Very slow novae, staying at maximum light for a decade or more, fading very slowly. It is possible that NC type novae are objects differing physically very much from normal novae, for example planetary nebulae in formation, exhibiting Wolf-Rayet star like features.
- NR/RN: Recurrent novae, novae with two or more outbursts separated by 10–80 years have been observed.
During the 16th century, astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia. He described it in his book De stella nova (Latin for "concerning the new star"), giving rise to 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. Though this was a supernova and not a classical nova, the terms were considered interchangeable until the 1930s.
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 done with Cepheid variable stars have shown them to be of comparable accuracy.
Bright novae since 1890
Over 53 novae have been registered since 1890.
There are ten known galactic recurrent novae. The recurrent nova typically brightens by about 8.6 magnitude, whereas a classic nova brightens by more than 12 magnitude. Some of the better known and more easily observed recurrent novae are listed below.
Days to drop
|RS Ophiuchi||RS Oph||4.8–11||14||2006, 1985, 1967, 1958, 1933, 1898|
|T Coronae Borealis||T CrB||2.5–10.8||6||1946, 1866|
|T Pyxidis||T Pyx||6.4–15.5||62||2011, 1967, 1944, 1920, 1902, 1890|
|U Scorpii||U Sco||7.5–17.6||2.6||2010, 1999, 1987, 1979, 1936, 1917, 1906, 1863|
Novae in M31 are relatively common. There are roughly a couple dozen novae discovered (brighter than about apparent magnitude 20) in M31 each year. The Central Bureau for Astronomical Telegrams (CBAT) tracks novae in M31, M33, and M81.
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