More Info on novaSimilar Undetermined MusicSearch Artistopia
Biography
Other usesA nova (plural wiktionary:nova|novae ) is a Cataclysmic variable star|cataclysmic nuclear explosion in a star caused by the Accretion (astrophysics)|accretion of hydrogen on to the surface of a white dwarf star , which ignites and starts nuclear fusion in a runaway manner. Novae are not to be confused with supernova e or 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 companion'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 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. cite book|last=Prialnik |first=Dina |year=2001 |chapter=Novae |editor=Paul Murdin |title=Encyclopedia of Astronomy and Astrophysics |pages=1846–1856 |publisher= Institute of Physics Publishing / Nature Publishing Group |isbn=1-56159-268-4 (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. AAVSO Variable Star Of The Month: http://www.aavso.org/vstar/vsots/0501.shtml May 2001: Novae 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. cite book| first= Brian | last = Warner | year = 1995 | title = Cataclysmic Variable Stars | isbn = 0-521-41231-5 | publisher = Cambridge University Press
In spite of their violence, the amount of material ejected in novae is usually only about frac|10,000 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. cite book|last=Zeilik |first=Michael |year=1993 |title=Conceptual Astronomy |publisher= John Wiley & Sons |isbn=0-471-50996-5 In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope were surprised to discover, for the first time, that a nova can also emit gamma-rays (>100 MeV). cite web|author= JPL / NASA |date=12 August 2010 |title=Fermi detects 'shocking' surprise from supernova's little cousin |url= http://www.physorg.com/news200849593.html |work= PhysOrg |accessdate=15 August 2010
A helium nova is a proposed category of nova explosion that lacks hydrogen lines in the spectrum. This may be caused by the explosion of a helium shell on a white dwarf. It was proposed by Kato, Saio and Hachisu in 1989. The first candidate helium nova to be observed was V445 Puppis in 2000.cite journal | author=Kato, Mariko; Hachisu, Izumi | title=V445 Puppis: Helium Nova on a Massive White Dwarf | journal=The Astrophysical Journal | volume=598 | issue=2 | pages=L107& ndash;L110 | month=December | year=2003 | doi=10.1086/380597 | bibcode=2003ApJ...598L.107K |arxiv = astro-ph/0310351 Since then, four other novae explosions have been proposed as helium novae.cite journal | title=List of Helium Novae | last=Rosenbush | first=A. E. | booktitle=proceedings, Hydrogen-Deficient Stars ASP Conference Series, Vol. 391, | date=September 17–21, 2007 | location=Eberhard Karls University, Tübingen, Germany | editors=Klaus Werner and Thomas Rauch | month=July | year=2008 | bibcode=2008ASPC..391..271R
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 29 August 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 17 February 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. The number of novae discovered in the Milky Way each year is much lower, about 10. cite web|title=CBAT List of Novae in the Milky Way |url= http://cbat.eps.harvard.edu/nova_list.html |publisher=IAU Central Bureau for Astronomical Telegrams Roughly 25 novae brighter than about magnitude 20 are discovered in the Andromeda Galaxy each year and smaller numbers are seen in other nearby galaxies. cite web|title=M31 (Apparent) Novae Page |url= http://cbat.eps.harvard.edu/CBAT_M31.html |publisher=IAU Central Bureau for Astronomical Telegrams |accessdate=2009-02-24
Spectroscopy|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 frac|50 as much material to the Galaxy as supernovae, and only frac|200 as much as 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. cite book|last=Seeds |first=Michael A. |year=1998 |title=Horizons: Exploring the Universe |page=194 |edition=5th |publisher= Wadsworth Publishing Company |isbn=0-534-52434-6 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.
Subtypes
Novae are classified according to the light curve development speed, thus in
NA : Fast novae, with a rapid brightness increase, followed by a brightness decline of 3 magnitudes — to circa frac|16 brightness — within 100 days.
cite web|date=31 March 2010 |title=Ritter Cataclysmic Binaries Catalog (7th Edition, Rev. 7.13) |publisher= High Energy Astrophysics Science Archive Research Center |url= http://heasarc.gsfc.nasa.gov/W3Browse/all/rittercv.html |accessdate=2010-09-25
NB : Slow novae, with a 3 magnitudes decline in 150 days or more.
NC : Very slow novae, staying at maximum light for a decade or more, fading very slowly. It is possible that NC type novae are objects differing physically very much from normal novae, for example planetary nebulae in formation. Exhibiting Wolf-Rayet star like features.
NR / RN : Recurrent novae, novae with two or more outbursts (instead of a single one) separated by 10–80 years have been observed.ftp://cdsarc.u-strasbg.fr/pub/cats/B/gcvs/vartype.txt GCVS' vartype.txt at VizieR
Etymology
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.
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 -8.8, and a lesser one at -7.5. 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. cite book|last=Robert |first=Gilmozzi |last2=Della Valle |first2=Massimo |year=2003 |chapter=Novae as Distance Indicators |editor-last=Alloin |editor-first=D. |editor2-last=Gieren |title=Stellar Candles for the Extragalactic Distance Scale |pages=229–241 |publisher= Springer (publisher)|Springer |isbn=3-540-20128-9 |editor2-first=W.
Bright novae since 1890
Main|List of novaeExpand section|date=March 2010 Over 53 novae have been registered since 1890.
Recurrent novae
There are ten known galactic recurrent novae. cite arxiv|last=Schaefer |first=Bradley E. |year=2009 |title=Comprehensive Photometric Histories of All Known Galactic Recurrent Novae |class=astro-ph.SR |eprint=0912.4426 The recurrent nova typically brightens by about 8.6 magnitude, whereas a classic nova brightens by more than 12 magnitude. Some of the better known and more easily observed recurrent novae are listed below.
Full name
Short name
Magnitude range
Days to drop 3 magnitude from peak
Eruption years
RS Ophiuchi
RS Oph
4.8–11
14
2006, 1985, 1967, 1958
T Coronae Borealis
T CrB
2.5–10.8
6
1946, 1866
T Pyxidis
T Pyx
6.4–15.5
62
2011, 1967, 1944, 1920, 1902
U Scorpii
U Sco
7.5–17.6
2.6
2010, 1999, 1987, 1979
Extragalactic novae
Novae in Messier 31|M31 are relatively common. There are roughly a couple dozen novae to be discovered (brighter than about apparent magnitude 20) in M31 each year. The Central Bureau for Astronomical Telegrams (CBAT) tracks novae in M31, Triangulum Galaxy|M33 , and Messier 81|M81 . cite web|first=David | last = Bishop |title=Extragalactic Novae |publisher=International Supernovae Network |url= http://www.supernovae.net/novae.html |accessdate=2010-09-11
cite arxiv |eprint=0912.4426 |title=Comprehensive Photometric Histories of All Known Galactic Recurrent Novae |author1=Schaefer |class=astro-ph.SR |year=2009
cite arxiv |eprint=1104.0222 |title=A Spectroscopic and Photometric Survey of Novae in M31|author1=Shafter et al. |class=astro-ph.SR |year=2011
http://www.sai.msu.su/groups/cluster/gcvs/gcvs/ General Catalog of Variable Stars, Sternberg Astronomical Institute , Moscow
http://www.aavso.org/vstar/vsots/0501.shtml AAVSO Variable Star of the Month. Novae: May 2001
http://www.universetoday.com/2010/01/28/long-anticipated-eruption-of-u-scorpii-has-begun/ Long Anticipated Eruption of U Scorpii Has Begun (Universe Today 2010-Jan-28)
http://www.supernovae.net/novae.html Extragalactic Novae
white dwarfVariable star topics Category:Novae| Category:Stellar phenomena
