| nova (pl. novae) |
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A star that suddenly increases in brightness by about ten magnitudes, then declines gradually over a period of months. The word nova is a shortening of the Latin "nova stella" - a new star. Observations have demonstrated that novae are close binary stars of which one component is a white dwarf. When the companion star evolves and expands to fill its Roche lobe, material streams towards the white dwarf, forming an accretion disc around it. The accepted theory of nova outbursts is that material accumulates in a layer on the surface of the white dwarf until the temperature and pressure at the base of the layer become high enough for the carbon cycle nuclear reactions to be initiated. The energy produced is unable to escape as more material is deposited in the overlying layers. The temperature may rise to 100 million degrees and, at some point, explosive nuclear reactions are triggered, producing the observed nova outburst. Some novae have been seen surrounded by an expanding envelope of gas. At speeds of up to 1,500 km/s, the envelope soon disperses into space. It is estimated that the mass of material lost is about one ten-thousandth (10-4) the mass of the Sun, and the energy released is only a millionth of that released in a supernova. The ejected material is rich in the elements carbon, nitrogen and oxygen, and the observed ratios of the isotopes 13C / 12C and 15N/14N are consistent with the theory. Classical novae are observed to erupt only once, though it is believed that outbursts may recur every 10,000 to 100,000 years. Recurrent novae, such as P Cygni, have been observed to repeat their outbursts on timescales of ten to a hundred years. In any one galaxy, typically a few tens of novae occur in a year. See also: dwarf nova. |