nova
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Any of a class of stars whose luminosity temporarily increases by several thousand up to a million times normal. Most appear to be close binary stars, one of which is a white dwarf star drawing in matter from the other until it becomes unstable, causing an outburst in which the outer layer of material is shed. A nova reaches maximum luminosity within hours after its outburst and may shine intensely for several days or even a few weeks; it then slowly returns to its former level. The process can repeat at intervals ranging from a few dozen to hundreds of thousands of years. Stars that become novas are usually too faint to see with the unaided eye until their sudden increase in luminosity, sometimes great enough to make them readily visible in the night sky. To observers, such objects may appear to be new stars; hence their name (Latin for “new”). Seealso supernova.
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Province (pop., 2006: 913,462), Canada, one of the Maritime Provinces. It comprises the peninsula of Nova Scotia, Cape Breton Island, and a few small adjacent islands, and it is bounded by the Northumberland Strait, the Gulf of St. Lawrence, the Atlantic Ocean, the Bay of Fundy, and New Brunswick. Its capital is Halifax. After centuries of occupation by the Mi'kmaq, the region was possibly visited by Vikings circa AD 1000; it certainly came to the attention of Europeans by the late 15th century. French settlers established trading posts in the early 17th century, and they named the region Acadia (Acadie). English and Scottish colonists arrived by 1621. The conflict between France and England over control of the area was ended by the 1713 Peace of Utrecht, which awarded it to England. In the 1750s the British expelled most of the French settlers. Following the American Revolution, many loyalists emigrated there. It joined the Dominion of Canada in 1867 as one of the original members. Tourism and other service industries have grown in importance as traditional economic activities such as fishing, forestry, and coal mining have declined.
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A nova (pl. nova 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.
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 each year is much lower, probably due to great distance and observational biases. 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.
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 1/50th the amount of material to the Galaxy as supernovae, 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. 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 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.
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
External links
- General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow
- NASA Observatorium: Classical Nova
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Last updated on Friday October 10, 2008 at 08:43:51 PDT (GMT -0700)
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