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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. Novae are thought to occur with the dense atmosphere above the surface of a white dwarf in a close binary system when they are sufficiently near enough to allow material (mostly hydrogen) to be drawn from the companion star's surface onto the white dwarf. A nova is the result of the rapid fusion of the very dense accreted hydrogen just above the surface of the star, which on ignition commences a very rapid runaway fusion reaction. This material is dramatically expelled into space, whose brightened envelope from the energy produced is seen as the light we visually observe from the nova.
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Etymology
During the 16th 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 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.
Stellar Evolution of Novae
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 captures matter in an accretion disk steadily from the companion's outer atmosphere, and in turn, falls into the atmosphere. As the white dwarf consists of degenerate matter, so the accreted hydrogen does not inflate but its temperature increases. Rapid, an all at once, uncontrolled fusion occurs when the temperature of this atmospheric layer reaches ~20 million Kelvins, initiating nuclear burning via the CNO cycle.
Hydrogen fusion can occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, but 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. This blows away the remaining gases away from the white dwarf's surface and producing an extremely bright outburst of light. 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. Time taken for a nova to decay by around 2 or 3 magnitudes from maximum optical brightness is used to classify novae via its speed class. Fast nova will typically take less than 25 days to decay by 2 magnitudes while 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 novae are 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 .
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 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 do 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.
Subtypes
Novae are classified according to the light curve development speed, so in
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.
Recurrent novae
Recurrent novae are objects that have been seen to experience multiple nova eruptions. 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. The ten known recurrent novae are listed below.
Extragalactic novae
Novae in the Andromeda galaxy (M31) are relatively common. There are roughly several 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.