Nova 21784 225717549 2008-07-15T01:42:03Z Headbomb 1461430 moved refs {{alternateuses}} [[Image:Making a Nova.jpg|frame|right|Artist's conception of a white dwarf star accreting hydrogen from a larger companion]] A '''nova''' (pl. ''[[wiktionary:nova|novae]]'') is a [[cataclysm]]ic nuclear explosion caused by the [[Accretion (astrophysics)|accretion]] of hydrogen onto the surface of a [[white dwarf]] [[star]]. Novae are not to be confused with [[Type Ia supernova]]e, or another form of stellar explosion first announced by [[Caltech]] in May 2007, [[Luminous Red Nova]]e. ==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 [[Baryonic matter|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 does not inflate at increased heat, while the accreted hydrogen is compressed upon the surface. The dependence of the hydrogen fusion rate on temperature and pressure means that it is only when it is compressed and heated at the surface of the white dwarf to a temperature of some 20 million [[Kelvin|K]] that a [[nuclear fusion]] reaction occurs; at these temperatures, hydrogen burns via the [[CNO cycle]]. For most binary system parameters, the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other heavier [[Chemical element|element]]s in a [[thermal runaway|runaway]] reaction.<ref name="encyc">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</ref> (Hydrogen fusion can occur in a stable manner on the surface, but only for a narrow range of accretion rates.) 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 or gradual which is related to the speed class of the nova; after the peak, the brightness declines steadily.<ref>[[AAVSO]] Variable Star Of The Month: [http://www.aavso.org/vstar/vsots/0501.shtml May 2001: Novae]</ref> The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used to classify a nova via its speed class. A fast nova will typically take less than 25 days to decay by 2 magnitudes and a slow nova will take over 80 days.<ref>{{cite book | title = Cataclysmic Variable Stars | author = Brian Warner | id = 052154209X}}</ref> 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.<ref name="encyc" /> 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.<ref name="encyc" /><ref>Zeilik, Michael. ''Conceptual Astronomy''. New York: John Wiley & Sons, Inc., 1993. ISBN 0-471-50996-5</ref> 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, the white dwarf could explode as a type Ia [[supernova]] if it exceeds the Chandrasekhar limit. Occasionally a nova is bright enough and close enough to be conspicuous to the unaided eye. The brightest recent example was [[V1500 Cygni|Nova Cygni 1975]]. This nova appeared on [[August 29]], [[1975]] in the constellation [[Cygnus (constellation)|Cygnus]] about five degrees north of [[Deneb]] and reached [[Apparent magnitude|magnitude]] 2.0 (nearly as bright as Deneb). The most recent was [[V1280 Scorpii]] which reached magnitude 3.7 on [[February 17]], [[2007]]. ==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.<ref name="encyc" /> The number of novae discovered each year is much lower, probably due to great distance and [[Selection bias|observational biases]].<ref>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</ref> 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.<ref>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.</ref> [[Spectroscopy|Spectroscopic]] observation of nova ejecta [[nebulae]] has shown that they are enriched in elements such as helium, [[carbon]], [[nitrogen]], [[oxygen]], [[neon]], and [[magnesium]].<ref name="encyc" /> 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.<ref name="encyc" /> 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.<ref>Seeds, Michael A. ''Horizons: Exploring the Universe'', 5th ed. Belmont: Wadsworth Publishing Company, 1998, ISBN 0-534-52434-6, p.194.</ref> 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.<ref name="encyc" /> Consequently, the interval is shorter for high-mass white dwarfs.<ref name="encyc" /> ==Historical significance== The astronomer [[Tycho Brahe]] observed the [[supernova]] [[SN 1572]] in the constellation [[Cassiopeia (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.<ref name="encyc" /> ==Novae as distance indicators== Novae have some promise for use as [[standard candle]]s. For instance, the distribution of their [[absolute magnitude]] is [[bimodal distribution|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.<ref>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.</ref> === Bright novae since 1890 === {| class="wikitable" !Year !! Nova !! Maximum brightness |- |1891 |[[T Aurigae]] |3.8 mag |- |1898 |[[V1059 Sagittarii]] |4.5 mag |- |1899 |[[V606 Aquilae]] |5.5 mag |- |1901 |[[GK Persei]] |0.2 mag |- |1903 |[[Nova Geminorum 1903]] |6 mag |- |1905 |[[Nova Aquilae 1905]] |7.3 mag |- |1910 |[[Nova Lacertae 1910]] |4.6 mag |- |1912 |[[Nova Geminorum 1912]] |3.5 mag |- |1918 |[[V603 Aquilae]] | &minus;1.4 mag |- |1919 |[[Nova Lyrae 1919]] |7.4 mag |- |1919 |[[Nova Ophiuchi 1919]] |7.4 mag |- |1920 |[[Nova Cygni 1920]] |2.0 mag |- |1925 |[[RR Pictoris]] |1.2 mag |- |1934 |[[DQ Herculis]] |1.4 mag |- |1936 |[[CP Lacertae]] |2.1 mag |- |1939 |[[BT Monocerotis]] |4.5 mag |- |1942 |[[CP Puppis]] |0.3 mag |- |1943 |[[Nova Aquilae 1943]] |6.1 mag |- |1950 |[[DK Lacertae]] |5.0 mag |- |1960 |[[V446 Herculis]] |2.8 mag |- |1963 |[[V533 Herculis]] |3 mag |- |1970 |[[FH Serpentis]] |4 mag |- |1975 |[[V1500 Cygni]] |2.0 mag |- |1975 |[[V373 Scuti]] |6 mag |- |1976 |[[NQ Vulpeculae]] |6 mag |- |1978 |[[V1668 Cygni]] |6 mag |- |1984 |[[QU Vulpeculae]] |5.2 mag |- |1986 |[[V842 Centauri]] |4.6 mag |- |1991 |[[V838 Herculis]] |5.0 mag |- |1992 |[[V1974 Cygni]] |4.2 mag |- |1999 |[[V1494 Aquilae]] |5.03 mag |- |1999 |[[V382 Velorum]] |2.6 mag |- |2006 |[[RS Ophiuchi]] |4.5 mag |- |2007 |[[V1280 Scorpii]] |3.9 mag [http://www.aavso.org/publications/alerts/alert346.shtml],[http://www.aavso.org/cgi-bin/newql.pl?name=V1280%20Sco&output=html] |} <small>''Note: Please add all novae brighter than 6 mag'' [http://www.tsm.toyama.toyama.jp/curators/aroom/var/nova/1600.htm ]</small> === Recurrent novae === * [[RS Ophiuchi]] * [[T Coronae Borealis]] * [[T Pyxidis]] == See also == * [[Dwarf nova]] * [[Supernova]] * [[Hypernova]] * [[Luminous Red Nova]] * [[Nova remnant]] * [[Cataclysmic variable star|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== <references /> == External links == {{commons|Nova|Nova}} * [http://www.sai.msu.su/groups/cluster/gcvs/gcvs/ General Catalog of Variable Stars], [[Sternberg Astronomical Institute]], [[Moscow]] * [http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_4a.html NASA Observatorium: Classical Nova] [[Category:Novae| ]] [[Category:Stellar phenomena]] {{Link FA|fi}} [[ar:مستعر]] [[bn:নবতারা]] [[ca:Nova]] [[cs:Nova]] [[de:Nova (Stern)]] [[el:Καινοφανείς αστέρες]] [[es:Nova]] [[eo:Nova]] [[fa:نواختر]] [[fr:Nova]] [[ko:신성 (천체)]] [[it:Nova]] [[he:נובה]] [[lv:Nova]] [[lt:Nova (astronomija)]] [[hu:Nóva]] [[nl:Nova (sterrenkunde)]] [[ja:新星]] [[pl:Nowa klasyczna]] [[pt:Nova]] [[ro:Novă]] [[ru:Новая звезда]] [[sk:Nova]] [[fi:Nova]] [[sv:Nova]] [[tr:Nova]] [[zh:新星]]