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A nova (pl. novae or novas) is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. Novae are not to be confused with Type Ia supernovae, or another form of stellar explosion first announced by Caltech in May 2007, Luminous Red Novae.
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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 40.1 The number of novae discovered each year is much lower, probably due to great distance and observational biases.2 By comparison, the number of novae discovered each year in the nearby Andromeda Galaxy is much lower; roughly ½ to ⅓ that of the Milky Way.3
Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium.1 The contribution of novae to the interstellar medium is not great; novae supply only 1/50th the amount of material to the Galaxy as supernovae, and only 1/200th that of red giant and supergiant stars.1
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.4 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.1 Consequently, the interval is shorter for high-mass white dwarfs.1
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 until the 1930s.1
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.5
Bright novae since 1890
Recurrent novae
See also
- Dwarf nova
- Supernova
- Hypernova
- Luminous Red Nova
- Nova remnant
- Cataclysmic variable
- Crab Nebula
- Guest star (astronomy)
- Extragalactic Distance Scale
Notes
- Under the Morgan-Keenan spectral classification scheme, novae are classified as Type-Q.
References
- ^ a b c d e f 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 1-56159-268-4
- ^ 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 0-7167-4502-X
- ^ 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.
- ^ Seeds, Michael A. Horizons: Exploring the Universe, 5th ed. Belmont: Wadsworth Publishing Company, 1998, ISBN 0-534-52434-6, p.194.
- ^ 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 3-540-20128-9.
- ^ A higher magnitude means a lower brightness. i.e. T Aurigae (+3.8) was a brighter nova than HR Lyrae (+6.5)
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