A nova (pl. novae) is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star.

Development

If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the star'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 two principal constituents of ordinary matter in the universe. 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 is largely unresponsive to heat, while the accreted hydrogen is not. Hydrogen fusion can occur in a stable manner on the surface through the CNO cycle, but the dependence of the helium fusion rate on temperature and pressure mean that it is only when helium is compressed and heated near the surface of the white dwarf to a temperature of some 20 million K that a nuclear fusion reaction rapidly converts a large amount of the helium into other heavier elements.Prialnik, Dina. "Novae", pp. 1846-56, in Paul Murdin, ed. Encyclopedia of Astronomy and Astrophysics. London: Institute of Physics Publishing Ltd and Nature Publishing Group, 2001. ISBN 1561592684 The enormous amount of energy liberated by this process blows the remaining gases away from the white dwarf's surface and produces an extremely bright outburst of light. The rise to peak brightness can be very rapid (as in fast novae) or gradual (as in slow novae); after the peak the brightness declines steadily.AAVSO Variable Star Of The Month: May 2001: Novae

In spite of their violence, the amount of material ejected in novae is usually only about 1/10,000th of a solar mass, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused to power the 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.Zeilik, Michael. Conceptual Astronomy. New York: John Wiley & Sons, Inc., 1993. ISBN 0471509965,

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 again in 2006). Eventually, however, either the white dwarf will run out of material, or collapse into a neutron star, or explode as a type Ia supernova.

Occasionally a nova is bright enough and close enough to be conspicuous to the unaided eye. The most recent example was Nova Cygni 1975. This nova appeared on August 29, 1975 in the constellation Cygnus about five degrees north of Deneb and reached magnitude 2.0 (nearly as bright as Deneb). Another recent instance was Nova Cygni 1992, though it was considerably fainter.

Occurrence rate, and astrophysical significance

Astronomers estimate that the Milky Way experiences roughly 20 to 60 novae per year, with a likely rate of about 40. The number of novae discovered each year is much lower, probably due to great distance and observational biases.Muirden, James. "Searching for Novae", pp. 259-79. In James Muirden, ed., Sky Watcher's Handbook. New York: W.H. Freeman and Company Ltd., 1993. ISBN 071674502X By comparison, the number of novae discovered each year in the sibling galaxy M31 is much lower; roughly 1/2 to 1/3 that of the Milky Way.W. Liller, B. Mayer, July 1987, "The rate of nova production in the Galaxy", Publications Astronomical Society of the Pacific, vol. 99, pp. 606-609.

Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. Though it would seem that the contributions of novae to the Galaxy might be large over astronomical time scales, this is not the case; in fact, novae supply only 1/50th the amount of material to the interstellar medium as supernovae do, and only 1/200th that of 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.Seeds, Michael A. Horizons: Exploring the Universe, 5th ed. Belmont: Wadsworth Publishing Company, 1998, ISBN 0534524346, p.194. 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.

Historical significance

The astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, and 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 for many years; indeed, no distinction was made between the two phenomena until the 1930s.

Novae as distance indicators

Novae have some promise for use as standard candles. For instance, the distribution of their absolute magnitude is bimodal, with a main peak at magnitude -7.5, and a lesser one at -8.8. 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.Alloin, D., and W. Gieren, eds. Stellar Candles for the Extragalactic Distance Scale. Robert Gilmozzi and Massimo Della Valle, "Novae as Distance Indicators", pp. 229-241. Berlin: Springer, 2003. ISBN 3540201289.

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