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{{About|the astronomical event}}
[[Image:Keplers supernova.jpg|300px|thumb|Multiwavelength [[X-ray]], [[infrared]], and [[optical]] compilation image of [[Johannes Kepler|Kepler's]] [[supernova remnant|Supernova Remnant]], [[SN 1604]]. ([[Chandra X-ray Observatory]])]]
A '''supernova''' (plural '''supernovae''') is a [[Astronomy#Stellar astronomy|stellar]] [[explosion]] that is more energetic than a [[nova]]. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire [[galaxy]], before fading from view over several weeks or months. During this short interval a supernova can [[Radiation|radiate]] as much [[energy]] as the [[Sun]] is expected to emit over its entire life span.<ref>
{{cite journal
| last=Giacobbe | first=F. W. | year=2005
| title=How a Type II Supernova Explodes
| journal=[[Electronic Journal of Theoretical Physics]]
| volume=2 | issue=6 | pages=30–38
| bibcode=2005EJTP....2f..30G
}}</ref> The explosion expels much or all of a star's material<ref>
{{cite web
| date=27 July 2006
| title=Introduction to Supernova Remnants
| url=
| publisher=[[NASA]]/[[Goddard Space Flight Center]]
| accessdate=2006-09-07
</ref> at a velocity of up to {{val|30000|u=km/s}} (10% of the [[speed of light]]), driving a [[shock wave]]<ref>
{{cite journal
| author=Schawinski, K. ''et al'' | year=2008
| title=Supernova Shock Breakout from a Red Supergiant
| journal=[[Science (journal)|Science]]
| volume=321 | issue=5886 | pages=223
| doi=10.1126/science.1160456 | pmid=18556514
}}</ref> into the surrounding [[interstellar medium]]. This shock wave sweeps up an expanding shell of gas and dust called a [[supernova remnant]].
Several types of supernovae exist. Types I and II can be triggered in one of two ways, either turning off or suddenly turning on the production of energy through [[nuclear fusion]]. After the core of an [[stellar evolution#Massive stars|aging massive star]] ceases generating energy from nuclear fusion, it may undergo sudden [[gravitational collapse]] into a [[neutron star]] or [[black hole]], releasing [[potential energy#Gravitational potential energy|gravitational potential energy]] that heats and expels the star's outer layers. Alternatively a [[white dwarf]] star may accumulate sufficient material from a [[Binary star|stellar companion]] (either through [[Accretion (astrophysics)|accretion]] or via a merger) to raise its core temperature enough to [[Carbon detonation|ignite]] [[Carbon burning process|carbon fusion]], at which point it undergoes [[Thermal runaway|runaway]] nuclear fusion, completely disrupting it. Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the [[Chandrasekhar limit]], while accreting white dwarfs ignite as they approach this limit (roughly 1.38<ref name="Mazzali2007">
{{cite journal
| author=Mazzali, P. A.; K. Röpke, F. K.; Benetti, S.; Hillebrandt, W.
| year=2007
| title=A Common Explosion Mechanism for Type Ia Supernovae
| journal=[[Science (journal)|Science]]
| volume=315 | issue=5813 | pages=825–828
| doi=10.1126/science.1136259
| pmid=17289993
}}</ref> times the [[solar mass|mass of the sun]]). White dwarfs are also subject to a different, much smaller type of thermonuclear explosion [[CNO cycle|fueled by hydrogen]] on their surfaces called a nova. Solitary stars with a mass below approximately nine [[solar mass]]es, such as the [[Sun]], evolve into white dwarfs without ever becoming supernovae.
On average, supernovae occur about once every 50&nbsp;years in a galaxy the size of the [[Milky Way]].<ref name="supernova rate">
{{cite news
| date=4 January 2006
| title=Integral identifies supernova rate for Milky Way
| url=
| publisher=[[European Space Agency]]
| accessdate=2007-02-02
}}</ref> They play a significant role in enriching the interstellar medium with higher mass [[chemical element|elements]].<ref>
{{cite book
| first=Doug C. B. | last=Whittet | year=2003
| title=Dust in the Galactic Environment
| pages=45–46 | publisher=[[CRC Press]] | isbn=0750306246
}}</ref> Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.<ref name="aaa128" /><ref>
{{cite web
| last = Allen | first = J.
| date =2 February 1998
| title =Supernova Effects
| url =
| publisher=[[NASA]]/[[Goddard Space Flight Center]]
| accessdate = 2007-02-02
{{cite journal
| author=Boss, A. P. ''et al.''
| year=2008
| title=Simultaneous Triggered Collapse of the Presolar Dense Cloud Core and Injection of Short-Lived Radioisotopes by a Supernova Shock Wave
| journal=[[The Astrophysical Journal]]
| volume=686 | issue=2 | pages=L119–122
| doi=10.1086/593057
| bibcode=2008ApJ...686L.119B
''Nova'' (plural ''novae'') means "new" in [[Latin language|Latin]], referring to what appears to be a very bright new star shining in the [[celestial sphere]]; the [[Prefix (linguistics)|prefix]] "super-" distinguishes supernovae from ordinary [[nova]]e, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism. The word ''supernova'' was coined by Swiss astrophysicist and astronomer [[Fritz Zwicky]],<ref>
{{cite web
| name=Zwicky, F.
| title=Name NASA's Next Great Observatory
| url=
| publisher=[[NASA]]
| accessdate=2010-02-20
{{cite journal
| author=Baade, W.; Zwicky, F.
| year=1934
| title=On Super-novae
| journal=[[Proceedings of the National Academy of Sciences]]
| volume=20 | issue=5 | pages=254–259
| doi=10.1073/pnas.20.5.254
| bibcode=1934PNAS...20..254B
| pmid=16587881
| pmc=1076395
}}</ref> and was first used in print in 1926.<ref>
{{cite web
| title=supernova
| url=
| work=[[Merriam-Webster's Collegiate Dictionary|Merriam-Webster's Online Collegiate Dictionary]]
| accessdate=2009-06-10
==Observation history==
This section is written per Wikipedia:Summary style.
Please put detailed information about recent supernova
events on 'History of supernova observation'.
{{Main|History of supernova observation}}
[[Image:Crab Nebula.jpg|thumb|The [[Crab Nebula]] is a [[pulsar wind nebula]] associated with the [[SN 1054|1054 supernova]].]]
The earliest recorded supernova, [[SN 185]], was viewed by [[Chinese astronomy|Chinese astronomer]]s in 185 AD. The brightest recorded supernova was the [[SN 1006]], which was described in detail by [[China|Chinese]] and [[Islamic astronomy|Islamic astronomers]]. The widely observed supernova [[SN 1054]] produced the [[Crab Nebula]]. Supernovae [[SN 1572]] and [[SN 1604]], the last to be observed with the naked eye in the [[Milky Way]] galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the [[Aristotle|Aristotelian]] idea that the universe beyond the Moon and planets was immutable.<ref>
{{cite conference
| author=Clark, D. H.; Stephenson, F. R.
| date=29 June 1981
| title=The Historical Supernovae
| booktitle=Supernovae: A survey of current research; Proceedings of the Advanced Study Institute
| pages=355–370
| publisher=Dordrecht, D. Reidel Publishing Co.
| location=Cambridge, England
| bibcode=1982sscr.conf..355C
Since the development of the [[telescope]] the field of supernova discovery has extended to other galaxies, starting with the 1885 observation of supernova [[S Andromedae]] in the [[Andromeda galaxy]]. Supernovae provide important information on cosmological distances.<ref>
{{cite web
| last =van Zyl | first = J. E.
| year = 2003
| url =
| publisher = [[Astronomical Society of Southern Africa]]
| accessdate = 2006-09-27
}}</ref> During the twentieth century successful models for each type of supernova were developed, and scientists' comprehension of the role of supernovae in the star formation process {{As of|2010|alt=is growing}}. American astronomers [[Rudolph Minkowski]] and [[Fritz Zwicky]] developed the modern supernova classification scheme beginning in 1941.<ref>{{cite journal
| first=L. A. L. | last=da Silva
| title=The Classification of Supernovae
| journal=Astrophysics and Space Science | year=1993
| volume=202 | issue=2 | pages=215–236
| url=
| accessdate=2008-06-04
| doi=10.1007/BF00626878 }}</ref>
In the 1960s astronomers found that the maximum intensities of supernova explosions could be used as [[standard candles]], hence indicators of astronomical distances.<ref>
{{cite journal
| first=C. T. | last=Kowal
| year=1968
| title=Absolute magnitudes of supernovae
| journal=[[Astronomical Journal]]
| volume=73
| pages=1021–1024
| doi=10.1086/110763
| bibcode=1968AJ.....73.1021K
}}</ref> Some of the most distant supernovae recently observed appeared dimmer than expected. This supports the view that the expansion of the [[Accelerating universe|universe is accelerating]].<ref name="Leibundgut">
{{cite journal
| author=Leibundgut, B.; Sollerman, J.
| year=2001
| title=A cosmological surprise: the universe accelerates
| url=
| journal=[[Europhysics News]]
| volume=32 | issue=4 | pages=121
| doi=10.1051/epn:2001401
}}</ref><ref name="CNRS">
{{cite news
| date=19 September 2003
| title=Confirmation of the accelerated expansion of the Universe
| url=
| publisher=[[Centre National de la Recherche Scientifique]]
| accessdate=2006-11-03
}}</ref> Techniques were developed for reconstructing supernova explosions that have no written records of being observed. The date of the [[Cassiopeia A]] supernova event was determined from [[light echo]]es off [[nebula]]e,<ref>
{{cite journal
| last=Fabian | first=A.C.
| year=2008
| title=A Blast from the Past
| journal=[[Nature (journal)|Nature]]
| volume=320 | issue=5880 | pages=1167–1168
| doi=10.1126/science.1158538
}}</ref> while the age of supernova remnant [[RX J0852.0-4622]] was estimated from temperature measurements<ref>
{{cite journal
| last=Aschenbach | first=B.
| date=12 November 1998
| title=Discovery of a young nearby supernova remnant
| journal=[[Nature (journal)|Nature]]
| volume=396 | pages=141–142
| doi=10.1038/24103
}}</ref> and the [[gamma ray]] emissions from the decay of [[titanium-44]].<ref>
{{cite journal
| author=Iyudin, A. F. ''et al.''
| year=1998
| title=Emission from <sup>44</sup>Ti associated with a previously unknown Galactic supernova
| journal=[[Nature (journal)|Nature]]
| volume=396 | issue=6707 | pages=142–144
| doi=10.1038/24106
}}</ref> In 2009 [[nitrate]]s were discovered in Antarctic ice deposits that matched the times of past supernova events.<ref>
{{cite web
| author=
| date=4 March 2009
| title=Ancient supernovae found written into the Antarctic ice
| url=
| work=[[New Scientist]]
| volume= | issue=2698 | pages=
| doi =
Because supernovae are relatively rare events within a galaxy, occurring about once every 50&nbsp;years in the Milky Way,<ref name="supernova rate" /> obtaining a good sample of supernovae to study requires regular monitoring of many galaxies.
Supernovae in other galaxies cannot be predicted with any meaningful accuracy. Normally, when they are discovered, they are already in progress.<ref>{{cite web
| last = Bishop | first = David
| url =
| title = Latest Supernovae
| publisher = Rochester's Astronomy Club
| accessdate = 2006-11-28 }}
</ref> Most scientific interest in supernovae—as [[standard candle]]s for measuring distance, for example—require an observation of their peak luminosity. It is therefore important to discover them well before they reach their maximum. [[Amateur astronomy|Amateur astronomers]], who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an [[optical telescope]] and comparing them to earlier photographs.
Towards the end of the 20th century astronomers increasingly turned to computer-controlled telescopes and [[charge-coupled device|CCDs]] for hunting supernovae. While such systems are popular with amateurs, there are also professional installations such as the [[Katzman Automatic Imaging Telescope]].<ref>{{cite web
| last=Evans | first=Robert O.
| year=1993
| url =
| title = Supernova Search Manual, 1993
| publisher = [[American Association of Variable Star Observers]] (AAVSO)
| accessdate = 2006-10-05 }}
</ref> Recently the [[Supernova Early Warning System]] (SNEWS) project has begun using a network of [[neutrino detector]]s to give early warning of a supernova in the Milky Way galaxy.<ref>{{cite journal
| author = Antonioli, P. ''et al.''
| title=SNEWS: the SuperNova Early Warning System
| journal=New Journal of Physics
| year=2004 | volume=6 | pages=114
| url=
| accessdate=2006-11-28
| doi = 10.1088/1367-2630/6/1/114
}}</ref><ref>{{cite web
| url =
| title = SNWES: Supernova Early Warning System
| publisher = [[National Science Foundation]]
| accessdate = 2006-11-28 }}</ref> [[Neutrino]]s are [[Subatomic particle|particles]] that are produced in great quantities by a supernova explosion,<ref>
{{cite journal
| first=J. F. | last=Beacom
| title=Supernova Neutrinos and the Neutrino Masses
| year=1999
| url=
| accessdate=2006-11-28 }}
</ref> and they are not significantly absorbed by the interstellar gas and dust of the galactic disk.
Supernova searches fall into two classes: those focused on relatively nearby events and those looking for explosions farther away. Because of the [[Metric expansion of space|expansion of the universe]], the distance to a remote object with a known emission spectrum can be estimated by measuring its [[Doppler shift]] (or [[redshift]]); on average, more distant objects recede with greater velocity than those nearby, and so have a higher redshift. Thus the search is split between high redshift and low redshift, with the boundary falling around a redshift range of ''z''&nbsp;=&nbsp;0.1–0.3<ref>{{cite web
| last = Frieman | first = Josh | year=2006
| url =
| title =SDSS Supernova Survey | publisher=SDSS
| accessdate = 2006-08-10 }}</ref>—where ''z'' is a dimensionless measure of the spectrum's frequency shift.
High redshift searches for supernovae usually involve the observation of supernova light curves. These are useful for standard or calibrated candles to generate Hubble diagrams and make cosmological predictions. Supernova spectroscopy, used to study the physics and environments of supernovae, is more practical at low than at high redshift.<ref>{{cite web
| last = Perlmutter | first = Saul
| url =
| title = High Redshift Supernova Search
| publisher = [[Lawrence Berkeley National Laboratory]]
| accessdate = 2006-10-09 }}
</ref><ref>{{cite journal
| author=Linder, E. V.; Huterer, D.
| title=Importance of supernovae at z>1.5 to probe dark energy
| journal=Physical Review D
| year=2003 | volume=67 | issue=8 | url=
| accessdate = 2007-02-01 | pages = 081303
| doi = 10.1103/PhysRevD.67.081303 }}</ref> Low redshift observations also anchor the low-distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies.<ref>{{cite journal
| author=Perlmutter, S. ''et al.''
| title=Measurements of the Cosmological Parameters Ω and Λ from the First Seven Supernovae at z >= 0.35
| journal=[[Astrophysical Journal]]
| year=1997 | volume=483 | url=
| accessdate = 2007-02-01 | pages=565
| doi=10.1086/304265
}}</ref><ref>{{cite web
| last = Aldering | first = Greg | date = 2005-12-01
| url =
| title = The Nearby Supernova Factory
| publisher = [[Lawrence Berkeley National Laboratory]] | accessdate = 2006-12-01 }}</ref>
{{See also|Hubble's law}}
==Naming convention==
[[Image:SN1994D.jpg|thumb|[[SN 1994D]] in the [[NGC 4526]] galaxy (bright spot on the lower left). Image by [[NASA]], [[ESA]], The Hubble Key Project Team, and The High-Z Supernova Search Team]]
Supernova discoveries are reported to the [[International Astronomical Union]]'s [[Central Bureau for Astronomical Telegrams]], which sends out a circular with the name it assigns to it. The name is the year of discovery, immediately followed by a one or two-letter designation. The first 26 supernovae of the year are designated with a capital letter from A to Z. Afterward pairs of lower-case letters are used: aa, ab, and so on.<ref>
{{cite web
| url =
| title = List of Recent Supernovae
| publisher = [[Harvard-Smithsonian Center for Astrophysics]] | accessdate = 2007-10-16 }}</ref> Professional and amateur astronomers find several hundreds of supernovae each year (367 in 2005, 551 in 2006 and 572 in 2007). For example, the last supernova of 2005 was SN 2005nc, indicating that it was the 367th<ref group="nb">The value is obtained by converting the suffix "nc" from [[Base (mathematics)|base 26]], with a=1, b=2, c=3, ... n=14, ... z=26. Thus nc = n&times;26+c = 14&times;26+3 = 367.</ref> supernova found in 2005.<ref>{{cite web | url =
| title = List of Supernovae
| publisher = [[International Astronomical Union]] (IAU) Central Bureau for Astronomical Telegrams
| accessdate = 2007-10-16 }}
</ref><ref>{{cite web
| url =
| title = The Padova-Asiago supernova catalogue
| publisher = Astronomical Observatory of Padua
| accessdate = 2006-11-28 }}
Historical supernovae are known simply by the year they occurred: [[SN 185]], [[SN 1006]], [[SN 1054]], [[SN 1572]] (Tycho's Nova) and [[SN 1604]] (Kepler's Star). Since 1885 the letter notation has been used, even if there was only one supernova discovered that year (e.g. SN 1885A, 1907A, etc.)—this last happened with SN 1947A. "SN", for SuperNova, is a standard prefix.
As part of the attempt to understand supernovae, astronomers have classified them according to the [[absorption line]]s of different chemical elements that appear in their [[Astronomical spectroscopy|spectra]]. The first element for a division is the presence or absence of a line caused by [[hydrogen]]. If a supernova's spectrum contains a line of hydrogen (known as the [[Balmer series]] in the visual portion of the spectrum) it is classified ''Type&nbsp;II''; otherwise it is ''Type&nbsp;I''. Among those types, there are subdivisions according to the presence of lines from other elements and the shape of the [[light curve]] (a graph of the supernova's [[apparent magnitude]] as a function of time).<ref name="types">{{cite conference
| author=Cappellaro, E.; Turatto, M.
| title=Supernova Types and Rates
| booktitle=Influence of Binaries on Stellar Population Studies
| publisher=Dordrecht: Kluwer Academic Publishers
| date=2000-08-08 | location=Brussels, Belgium | url=
| accessdate=2006-09-15 }}</ref>
{| class="wikitable"
|+Supernova taxonomy<ref name="taxonomy">
{{cite web | last = Montes | first = M.
| date = 2002-02-12 | url =
| title = Supernova Taxonomy
| publisher = [[Naval Research Laboratory]]
| accessdate = 2006-11-09 }}</ref>
|colspan="2" style="background: #EEEEEE; text-align: center"|Type&nbsp;I
|[[Type Ia supernova|Type&nbsp;Ia]]
|Lacks hydrogen and presents a singly [[ionization|ionized]] [[silicon]] (Si II) line at 615.0&nbsp;[[Nanometre|nm]]&nbsp;(nanometers), near peak light.
|[[Type Ib and Ic supernovae|Type&nbsp;Ib]]
|Non-ionized [[helium]] (He I) line at 587.6&nbsp;nm and no strong silicon absorption feature near 615&nbsp;nm.
|[[Type Ib and Ic supernovae|Type&nbsp;Ic]]
|Weak or no helium lines and no strong silicon absorption feature near 615&nbsp;nm.
|colspan="2" style="background: #EEEEEE; text-align: center"|Type&nbsp;II
|[[Type II supernova|Type&nbsp;IIP]]
|Reaches a "plateau" in its light curve
|[[Type II supernova|Type&nbsp;IIL]]
|Displays a "linear" decrease in its light curve (linear in magnitude versus time).<ref name="comparative_study">{{cite journal
| author=Doggett, J. B.; Branch, D.
| title=A Comparative Study of Supernova Light Curves
| journal=[[Astronomical Journal]]
| year=1985 | volume=90 | pages=2303–2311
| url=
| accessdate = 2007-02-01
| doi=10.1086/113934 }}</ref>
The supernovae of Type&nbsp;II can also be sub-divided based on their spectra. While most Type&nbsp;II supernova show very broad [[emission line]]s which indicate expansion velocities of many thousands of [[kilometres per second]], some have relatively narrow features. These are called Type&nbsp;IIn, where the 'n' stands for 'narrow'. Supernovae that do not fit into the normal classifications are designated peculiar, or 'pec'.<ref name="taxonomy" />
A few supernovae, such as SN 1987K and SN 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type&nbsp;IIb" is used to describe the combination of features normally associated with Types&nbsp;II and Ib.<ref name="taxonomy" />
==Current models==
===Type Ia===
[[File:Progenitor IA supernova.svg|thumb|240px|Formation of a type Ia supernova]]
{{Main|Type Ia supernova}}
There are several means by which a supernova of this type can form, but they share a common underlying mechanism. If a [[carbon]]-[[oxygen]]<ref group="nb">For a core primarily composed of oxygen, neon and magnesium, the collapsing white dwarf will typically form a [[neutron star]]. In this case, only a fraction of the star's mass will be ejected during the collapse.<br />See: {{cite web
| author=Fryer, C. L.; New, K. C. B.
| date =2006-01-24 | url = | title =2.1 Collapse scenario
| work=Gravitational Waves from Gravitational Collapse | publisher =Max-Planck-Gesellschaft
| accessdate = 2007-06-07 }}</ref> [[white dwarf]] accreted enough matter to reach the [[Chandrasekhar limit]] of about 1.38&nbsp;[[solar mass]]es<ref name="Mazzali2007"/> (for a non-rotating star), it would no longer be able to support the bulk of its plasma through [[electron degeneracy pressure]]<ref name="Chandrasekhar">
{{cite journal
| author=Lieb, E. H.; Yau, H.-T.
| title=A rigorous examination of the Chandrasekhar theory of stellar collapse
| journal=[[Astrophysical Journal]]
| year=1987 | volume=323 | issue=1 | pages=140–144
| url=
| accessdate = 2007-02-01
| doi=10.1086/165813
}}</ref><ref>{{cite journal
| author=Canal, R.; Gutiérrez, J.
| title=The possible white dwarf-neutron star connection
| journal=Astrophysics and Space Science Library
| year=1997 | volume=214 | pages=49
| url=
| accessdate = 2007-02-01 }}</ref> and would begin to collapse. However, the current view is that this limit is not normally attained; increasing temperature and density inside the core [[Carbon detonation|ignite]] [[Carbon burning process|carbon fusion]] as the star approaches the limit (to within about 1%<ref>{{cite book
| last=Wheeler | first=J. Craig
| title=Cosmic Catastrophes: Supernovae, Gamma-Ray Bursts, and Adventures in Hyperspace
| publisher=[[Cambridge University Press]]
| date =2000-01-15 | location = Cambridge, UK
| pages = 96 | url =
| isbn = 0521651956}}</ref>), before collapse is initiated.<ref name="Mazzali2007"/> Within a few seconds, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (1–2 &times; 10<sup>44</sup>&nbsp;[[joule]]s)<ref name="aaa270">{{cite journal
| author=Khokhlov, A.; Mueller, E.; Hoeflich, P.
| title=Light curves of Type IA supernova models with different explosion mechanisms
| journal=Astronomy and Astrophysics
| year=1993 | volume=270 | issue=1–2 | pages=223–248
| url=
| accessdate=2007-05-22 }}</ref> to unbind the star in a supernova explosion.<ref name="ropke">{{cite journal
| author =Röpke, F. K.; Hillebrandt, W.
| title = The case against the progenitor's carbon-to-oxygen ratio as a source of peak luminosity variations in Type&nbsp;Ia supernovae
| journal = [[Astronomy and Astrophysics]]
| volume = 420 | pages = L1–L4 | year = 2004
| doi = 10.1051/0004-6361:20040135 }}</ref> An outwardly expanding [[shock wave]] is generated, with matter reaching velocities on the order of 5,000–20,000&nbsp;[[kilometers per second|km/s]], or roughly 3% of the speed of light. There is also a significant increase in luminosity, reaching an [[absolute magnitude]] of -19.3 (or 5 billion times brighter than the Sun), with little variation.<ref name="explosion_model">
{{cite journal
| author=Hillebrandt, W.; Niemeyer, J. C.
| title=Type IA Supernova Explosion Models
| journal=Annual Review of Astronomy and Astrophysics
| year=2000 | volume=38 | pages=191–230
| url=
| accessdate = 2007-02-01
| doi = 10.1146/annurev.astro.38.1.191 }}</ref>
One model for the formation of this category of supernova is a close [[binary star]] system. The larger of the two stars is the first to evolve off the [[main sequence]], and it expands to form a [[red giant]].<ref>{{cite web
| last = Richmond | first = Michael
| url =
| title = Late stages of evolution for low-mass stars
| publisher = Rochester Institute of Technology
| accessdate = 2006-08-04 }}</ref> The two stars now share a common envelope, causing their mutual orbit to shrink. The giant star then sheds most of its envelope, losing mass until it can no longer continue [[nuclear fusion]]. At this point it becomes a white dwarf star, composed primarily of carbon and oxygen.<ref>
{{cite conference
| first = B. | last = Paczynski
| title = Common Envelope Binaries
| booktitle = Structure and Evolution of Close Binary Systems
| pages = 75–80
| publisher = Dordrecht, D. Reidel Publishing Co.
| date = July 28 – August 1, 1975
| location = Cambridge, England
| url =
| accessdate = 2007-01-08
}}</ref><ref>{{cite web
| author=Postnov, K. A.; Yungelson; L. R. | year = 2006
| url =
| title = The Evolution of Compact Binary Star Systems
| publisher = Living Reviews in Relativity
| accessdate = 2007-01-08 }}</ref> Eventually the secondary star also evolves off the main sequence to form a red giant. Matter from the giant is accreted by the white dwarf, causing the latter to increase in mass.
Another model for the formation of a Type Ia explosion involves the merger of two white dwarf stars, with the combined mass momentarily exceeding the Chandrasekhar limit.<ref>{{cite web
| author=Staff
| url =
| title =Type Ia Supernova Progenitors
| publisher =Swinburne University
| accessdate = 2007-05-20 }}</ref> A white dwarf could also accrete matter from other types of companions, including a main sequence star (if the orbit is sufficiently close).
Type Ia supernovae follow a characteristic [[light curve]]—the graph of luminosity as a function of time—after the explosion. This luminosity is generated by the [[radioactive decay]] of [[nickel]]-56 through [[cobalt]]-56 to [[iron]]-56.<ref name="explosion_model" /> The peak luminosity of the light curve was believed to be consistent across Type Ia supernovae (the vast majority of which are initiated with a uniform mass via the accretion mechanism), having a maximum [[absolute magnitude]] of about -19.3. This would allow them to be used as a secondary<ref>
{{cite journal
| author=Macri, L. M. ''et al.''
| title=A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant
| journal=[[Astrophysical Journal]]
| year=2006 | volume= 652 | issue=2 | pages= 1133–1149
| url=
| accessdate = 2007-02-01
| doi=10.1086/508530 }}</ref> [[standard candle]] to measure the distance to their host
[[galaxy|galaxies]].<ref>{{cite journal
| first=S. A. | last=Colgate
| title=Supernovae as a standard candle for cosmology
| journal=[[Astrophysical Journal]]
| year=1979 | volume=232 | issue=1 | pages=404–408
| url=
| accessdate = 2007-02-01
| doi=10.1086/157300 }}</ref> However, recent discoveries reveal that there is some evolution in the average lightcurve width, and thus in the intrinsic luminosity of supernovae, although significant evolution is found only over a large redshift baseline.<ref>{{cite journal
| last=Howell | first=D. Andrew
| coauthors=Sullivan, M.; Conley, A.; Carlberg, R.
| title=Predicted and Observed Evolution in the Mean Properties of Type Ia Supernovae with Redshift
| journal=The Astrophysical Journal
| year=2007 | volume=667 | pages=L37–L40
| url=
| accessdate=2008-03-19
| doi=10.1086/522030 }}</ref>
===Type Ib and Ic===
{{Main|Type Ib and Ic supernovae}}
[[Image:Supernova 2008D.jpg|thumb|SN 2008D, a Type Ib<ref>{{cite web
| author=Malesani, D. et al. | year=2008
| url=
| title=Early spectroscopic identification of SN 2008D
| publisher=Cornell University
| accessdate=2008-05-22 }}</ref> supernova, shown in [[X-ray]] (left) and visible light (right) at the far upper end of the galaxy. ''NASA image.''<ref>{{cite web
| last=Naeye | first=Robert
| coauthors=Gutro, Rob | date=2008-05-21 | url=
| title=NASA's Swift Satellite Catches First Supernova in the Act of Exploding
| publisher=NASA/Goddard Space Flight Center
| accessdate=2008-05-22 }}</ref>]]
These events, like supernovae of Type&nbsp;II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types&nbsp;Ib and Ic have lost most of their outer (hydrogen) envelopes due to strong [[stellar wind]]s or else from interaction with a companion.<ref>{{cite conference
| last = Pols | first = Onno
| title = Close Binary Progenitors of Type Ib/Ic and IIb/II-L Supernovae
| booktitle = Proceedings of The Third Pacific Rim Conference on Recent Development on Binary Star Research
| pages = 153–158 | date = October 26 – November 1, 1995
| location = Chiang Mai, Thailand
| url =
| accessdate = 2006-11-29 }}</ref> Type&nbsp;Ib supernovae are thought to be the result of the collapse of a massive [[Wolf-Rayet star]]. There is some evidence that a few percent of the Type&nbsp;Ic supernovae may be the progenitors of [[gamma ray bursts]] (GRB), though it is also believed that any hydrogen-stripped, Type&nbsp;Ib or Ic supernova could be a GRB, dependent upon the geometry of the explosion.<ref>{{cite journal
| author=Ryder, S. D. ''et al.''
| title=Modulations in the radio light curve of the Type IIb supernova 2001ig: evidence for a Wolf-Rayet binary progenitor?
| journal=Monthly Notices of the [[Royal Astronomical Society]]
| year=2004 | volume=349 | issue=3 | pages=1093–1100
| url=
| accessdate = 2007-02-01
| doi=10.1111/j.1365-2966.2004.07589.x }}</ref>
===Type II===
{{Main|Type II supernova}}
[[Image:Evolved star fusion shells.png|thumb|The onion-like layers of a massive, evolved star just prior to core collapse. (Not to scale.)]]
NOTE: This section is written summary style. Please place more
detailed information on the "Type II supernova" main article
linked above.
Stars with at least nine solar masses of material evolve in a complex fashion.<ref name="science304">{{cite journal
| last = Gilmore | first = Gerry
| title=The Short Spectacular Life of a Superstar
| journal=Science
| year=2004 | volume=304 | issue=5697 | pages=1915–1916
| url=;304/5679/1915
| accessdate=2007-05-01 | doi=10.1126/science.1100370
| pmid=15218132 }}</ref> In the core of the star, hydrogen is fused into helium and the [[thermal energy]] released creates an outward pressure, which maintains the core in [[hydrostatic equilibrium]] and prevents collapse.
When the core's supply of hydrogen is exhausted, this outward pressure is no longer created. The core begins to [[gravitational collapse|collapse]], causing a rise in temperature and pressure which becomes great enough to ignite the helium and start a helium-to-[[carbon]] fusion cycle, creating sufficient outward pressure to halt the collapse. The core expands and cools slightly, with a hydrogen-fusion outer layer, and a hotter, higher pressure, helium-fusion center. (Other elements such as [[magnesium]], [[sulfur]] and [[calcium]] are also created and in some cases burned in these further reactions.)
This process repeats several times; each time the core collapses, and the collapse is halted by the ignition of a further process involving more massive nuclei and higher temperatures and pressures. Each layer is prevented from collapse by the heat and outward pressure of the fusion process in the next layer inward; each layer also burns hotter and quicker than the previous one—the final burn of silicon to nickel consumes its fuel in just a few days at most.<ref name="WoosleyJanka">
{{cite journal | first=Stan | last=Woosley
| coauthors=Janka, Hans-Thomas
| title=The Physics of Core-Collapse Supernovae
| journal=[[Nature Physics]]
| volume=1 | issue=3 | pages=147–154 | month=December | year=2005
| url=
| format=PDF | doi=10.1038/nphys172 }}</ref> The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells.<ref name="late stages">{{cite web
| last = Richmond | first = Michael
| url =
| title = Late stages of evolution for low-mass stars
| publisher = [[Rochester Institute of Technology]]
| accessdate = 2006-08-04
}}</ref><ref name="hinshaw">{{cite web
| last = Hinshaw | first = Gary | date = 2006-08-23
| url =
| title = The Life and Death of Stars
| publisher = [[NASA]] [[Wilkinson Microwave Anisotropy Probe]] (WMAP) Mission
| accessdate = 2006-09-01 }}</ref>
In the later stages increasingly heavier elements with higher [[binding energy]] undergo nuclear fusion. Fusion produces progressively less energy, and also at higher core energies [[photodisintegration]] and [[electron capture]] occur which cause further energy loss in the core, requiring a general acceleration of the fusion processes to maintain [[hydrostatic equilibrium]].<ref name="WoosleyJanka" /> This escalation culminates with the [[silicon burning process|production of nickel-56]], which is unable to produce energy through fusion (but does produce iron-56 through radioactive decay).<ref>{{cite journal
| last = Fewell | first = M. P.
| title=The atomic nuclide with the highest mean binding energy
| journal=[[American Journal of Physics]]
| year=1995 | volume=63 | issue=7 | pages=653–658 | url=
| accessdate = 2007-02-01
| doi=10.1119/1.17828 }}</ref> As a result, a nickel-iron core<ref>{{cite web
| last=Fleurot | first=Fabrice | url=
| title=Evolution of Massive Stars
| publisher=Laurentian University
| accessdate=2007-08-13 }}</ref> builds up that cannot produce further outward pressure on the scale needed to support the rest of the structure. It can only support the overlaying mass of the star through the [[degeneracy pressure]] of [[electron]]s in the core. If the star is sufficiently large, then the iron-nickel core will eventually exceed the [[Chandrasekhar limit]] (1.38&nbsp;solar masses), at which point this mechanism catastrophically fails. The forces holding atomic nuclei apart in the innermost layer of the core suddenly give way, the core [[Implosion (mechanical process)|implodes]] due to its own mass, and no further fusion process is available to ignite and prevent collapse this time.<ref name="Chandrasekhar" />
====Core collapse====
{{See also|Gravitational collapse}}
The core collapses in on itself with velocities reaching 70,000&nbsp;km/s (0.23[[Speed of light|c]]),<ref name="grav_waves">{{cite web
| author=Fryer, C. L.; New, K. C. B.
| date = 2006-01-24 | url = | title = Gravitational Waves from Gravitational Collapse | publisher = [[Max Planck Institute for Gravitational Physics]]
| accessdate = 2006-12-14 }}</ref> resulting in a rapid increase in temperature and density. The energy loss processes operating in the core cease to be in equilibrium. Through [[photodisintegration]], [[gamma ray]]s decompose iron into helium nuclei and free [[neutron]]s, absorbing energy, whilst [[electron]]s and [[proton]]s merge via [[electron capture]], producing neutrons and electron [[neutrino]]s, which escape.
In a typical Type II supernova the newly formed neutron core has an initial temperature of about 100&nbsp;billion [[kelvin]] (100&nbsp;GK), 6000 times the temperature of the sun's core. A further release of neutrinos carries away much of the thermal energy, allowing a stable neutron star to form (the neutrons would "boil away" if this cooling did not occur).<ref>{{cite book
| last=Mann | first=Alfred K.]
| title = Shadow of a star: The neutrino story of Supernova 1987A
| publisher = W. H. Freeman | year = 1997
| location = New York | pages = 122
| url =
| isbn = 0716730979 }}</ref> These 'thermal' neutrinos form as neutrino-antineutrino pairs of all [[Neutrino oscillation|flavors]], and total several times the number of electron-capture neutrinos.<ref>{{cite book
| last = Gribbin | first = John R.
| authorlink = John Gribbin
| last2 = Gribbin | first2 = Mary
| title = Stardust: Supernovae and Life - The Cosmic Connection
| publisher = [[Yale University Press]]
| year = 2000 | location = New Haven | pages = 173
| url =
| isbn = 9780300090970 }}</ref> About 10<sup>46</sup>&nbsp;joules of gravitational energy—approximately 10% of the star's rest mass—is converted into a ten-second burst of neutrinos, which is the main output of the event.<ref name="WoosleyJanka" /><ref name="APS_study">{{cite web
| author=Barwick, S.; Beacom, J. ''et al.''
| date = 2004-10-29 | url =
| title = APS Neutrino Study: Report of the Neutrino Astrophysics and Cosmology Working Group
| publisher = [[American Physical Society]]
| format=PDF | accessdate = 2006-12-12 }}</ref> These carry away energy from the core and accelerate the collapse, while some neutrinos are absorbed by the star's outer layers and provide energy to the supernova explosion.<ref name="hayakawa">{{cite journal
| author = Hayakawa, T. ''et al.''
| title = Principle of Universality of Gamma-Process Nucleosynthesis in Core-Collapse Supernova Explosions
| journal = [[The Astrophysical Journal]]
| volume = 648 | pages = L47–L50 | year = 2006
| doi = 10.1086/507703 }}</ref>
The inner core eventually reaches typically 30&nbsp;[[kilometer|km]] diameter,<ref name="WoosleyJanka" /> and a density comparable to that of an [[atomic nucleus]], and further collapse is abruptly stopped by [[strong force]] interactions and by [[degeneracy pressure]] of neutrons. The infalling matter, suddenly halted, rebounds, producing a [[shock wave]] that propagates outward. Computer simulations indicate that this expanding shock does not directly cause the supernova explosion;<ref name="WoosleyJanka" /> rather, it stalls within [[millisecond]]s<ref>{{cite journal
| first= Eric | last=S. Myra | coauthors = Burrows, Adam
| year = 1990
| title = Neutrinos from type II supernovae- The first 100 milliseconds
| journal = Astrophysical Journal
| volume = 364 | pages = 222–231
| url =
| doi = 10.1086/169405 }}</ref> in the outer core as energy is lost through the dissociation of heavy elements, and a process that is {{As of|2010|alt=not clearly understood}} is necessary to allow the outer layers of the core to reabsorb around 10<sup>44</sup>&nbsp;joules<ref group="nb">Per the [[American Physical Society]] Neutrino Study reference, Barwick, Beacom ''et al.'' (2004), roughly 99% of the gravitational potential energy is released as neutrinos of all flavors. The remaining 1% is equal to 10<sup>44</sup> J</ref> (1&nbsp;[[Foe (unit)|foe]]) of energy, producing the visible explosion.<ref name="collapse scenario">{{cite web
| author = Fryer, C. L.; New, K. B. C.
| date = 2006-01-24
| url =
| title = Gravitational Waves from Gravitational Collapse, section 3.1
| publisher = [[Los Alamos National Laboratory]]
| accessdate = 2006-12-09 }}</ref> {{As of|2010|alt=Current}} research focuses upon a combination of neutrino reheating, [[rotation]]al and [[magnetic field|magnetic]] effects as the basis for this process.<ref name="WoosleyJanka" />
[[Image:Core collapse scenario.png|480px|thumb|center| Within a massive, evolved star (a) the onion-layered shells of elements undergo fusion, forming an iron core (b) that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red). The shock starts to stall (e), but it is re-invigorated by a process that may include neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant.]]
When the progenitor star is below about 20&nbsp;solar masses (depending on the strength of the explosion and the amount of material that falls back), the degenerate remnant of a core collapse is a neutron star.<ref name="grav_waves" /> Above this mass the remnant collapses to form a [[black hole]].<ref name="hinshaw" /><ref>
{{cite journal
| last = Chris L. | first = Michael
| title=Black Hole Formation from Stellar Collapse
| journal=Classical and Quantum Gravity
| year=2003 | volume=20 | issue=10 | pages=S73–S80
| url=
| accessdate = 2007-02-01
| doi=10.1088/0264-9381/20/10/309 }}</ref> (This type of collapse is one of many candidate explanations for [[gamma ray burst]]s, possibly producing a large burst of [[gamma ray]]s through a [[hypernova]] explosion.)<ref>
{{cite news
| title=Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked
| publisher=[[European Organisation for Astronomical Research in the Southern Hemisphere]] (ESO) | date=2003-06-18
| url=
| accessdate=2006-10-30 }}</ref> The theoretical limiting mass for this type of core collapse scenario was estimated around 40–50&nbsp;solar masses.
Above 50&nbsp;solar masses stars were believed to collapse directly into a black hole without forming a supernova explosion,<ref name="fryer">
{{cite journal
| last = Fryer | first = Chris L.
| title=Mass Limits For Black Hole Formation
| journal=[[The Astrophysical Journal]]
| year=1999 | volume=522 | issue=1 | pages=413–418
| url=
| accessdate = 2007-02-01
| doi=10.1086/307647 }}</ref> although uncertainties in models of supernova collapse make accurate calculation of these limits difficult. Above about 140 solar masses stars may become [[Supernova#Pair-instability type|pair-instability supernovae]] that do not leave behind a black hole remnant.<ref name="SN2006gy">{{cite web
| last = Boen | first = Brooke | date =2007-05-05
| url =
| title = NASA's Chandra Sees Brightest Supernova Ever
| publisher = NASA | accessdate = 2007-08-09
}}</ref><ref>{{cite news
| first=Robert | last=Sanders
| title=Largest, brightest supernova ever seen may be long-sought pair-instability supernova
| publisher=University of California, Berkeley
| date=2007-05-07 | url=
| accessdate=2006-05-24 }}</ref>
====Light curves and unusual spectra====
[[Image:SNIIcurva.png|thumb|This graph of the luminosity as a function of time shows the characteristic shapes of the light curves for a Type&nbsp;II-L and II-P supernova.]]
The light curves for Type&nbsp;II supernovae are distinguished by the presence of hydrogen [[Balmer series|Balmer absorption lines]] in the spectra. These light curves have an average decay rate of 0.008&nbsp;[[absolute magnitude|magnitudes]] per day, much lower than the decay rate for Type&nbsp;I supernovae. Type II are sub-divided into two classes, depending on whether there is a plateau in their light curve (Type&nbsp;II-P) or a linear decay rate (Type&nbsp;II-L). The net decay rate is higher at 0.012&nbsp;magnitudes per day for Type&nbsp;II-L compared to 0.0075&nbsp;magnitudes per day for Type&nbsp;II-P. The difference in the shape of the Type&nbsp;II-L supernovae light curve is believed to be caused by the expulsion of most of the hydrogen envelope of the progenitor star.<ref name="comparative_study" />
The plateau phase in Type&nbsp;II-P supernovae is due to a change in the [[opacity (optics)|opacity]] of the exterior layer. The shock wave [[ionize]]s the hydrogen in the outer envelope, which greatly increases the opacity. This prevents photons from the inner parts of the explosion from escaping. Once the hydrogen cools sufficiently to recombine, the outer layer becomes transparent.<ref>
{{cite web
| url =
| title = Type II Supernova Light Curves
| publisher = [[Swinburne University of Technology]]
| accessdate = 2007-03-17 }}</ref>
Of the Type&nbsp;II supernovae with unusual features in their spectra, Type&nbsp;IIn supernovae may be produced by the interaction of the ejecta with circumstellar material.<ref>{{cite journal
| author=Pastorello, A. ''et al.''
| title=The type IIn supernova 1995G: interaction with the circumstellar medium
| journal=Monthly Notices of the [[Royal Astronomical Society]]
| year=2002 | volume=333 | issue=1 | pages=27–38 | url=
| accessdate = 2007-02-01
| doi=10.1046/j.1365-8711.2002.05366.x }}</ref> Type&nbsp;IIb supernovae are likely massive stars which have lost most, but not all, of their hydrogen envelopes through [[tidal force|tidal stripping]] by a companion star. As the ejecta of a Type&nbsp;IIb expands, the hydrogen layer quickly becomes [[optical depth|optically thin]] and reveals the deeper layers.<ref>{{cite journal
| last = Utrobin | first = V. P.
| title=Nonthermal ionization and excitation in Type IIb supernova 1993J
| journal=[[Astronomy and Astrophysics]]
| year=1996 | volume=306 | pages=219–231 | url=
| accessdate = 2007-02-01 }}</ref>
The peak [[absolute magnitude]] of Type II supernovae varies from one to another, but they are dimmer than Type Ia.<ref>{{cite journal
| author=Richardson, Dean; Branch, David; Casebeer, Darrin; Millard, Jennifer; Thomas, R. C.; Baron, E.
| title=A Comparative Study of the Absolute Magnitude Distributions of Supernovae
| journal=The Astronomical Journal | volume=123 | issue=2
| pages=745–752 | month=February | year=2002
| doi=10.1086/338318 }}</ref> For instance, the low-luminosity [[SN 1987A]] had a peak visual absolute magnitude of -15.5 (apparent magnitude +3 for a distance of 51 kpc), as compared to the standard -19.3 for Type Ia.
===Pair-instability type===
{{Main|Pair-instability supernova}}
The core temperature of a star of over about 140 solar masses can become so high that [[pair production|photons convert spontaneously to electron-positron pairs]], reducing the [[radiation pressure|photon pressure]] supporting the star's outer layers and triggering a collapse that vaporises the star. This pair-instability supernova creates a larger quantity of elements heavier than helium ("[[metallicity|metals]]") than other types of supernova and leaves no black hole as a remnant. Stars of this size can only form from interstellar gas with very low metal content, which is characteristic of the early universe before the first supernovae produced metals from the primordial hydrogen and helium. It is believed that supernova [[SN 2007bi]] was of this type; it was distinguished from other supernovae by very long duration&mdash;77 days to peak brightness, bright enough to observe for 555 days&mdash;and production of much more radioactive nickel. The pair-instability supernova was predicted by Gary S. Fraley in 1968.<ref>{{cite journal
| last=Fraley | first=G. S. | year=1968
| title=Supernovae Explosions Induced by Pair-Production Instability
| journal=Astrophysics and Space Science | volume=2 | pages=96–114 | month=August
| doi=10.1007/BF00651498 | bibcode=1968Ap&SS...2...96F
| issue=1 }}</ref>
A long-standing puzzle surrounding Type II supernovae is why the compact object remaining after the explosion is given a large velocity away from the core.<ref>
{{cite book
| editor=P. Hoflich, P. Kumar, J. C. Wheeler
| title=Cosmic explosions in three dimensions: asymmetries in supernovae and gamma-ray bursts
| chapter=Neutron star kicks and supernova asymmetry
| publisher=[[Cambridge University Press]]
| location=Cambridge
| year=2004 | pages=276 | url=
| accessdate = 2007-02-01 }}</ref> (Neutron stars are observed, as [[pulsar]]s, to have high velocities; black holes presumably do as well, but are far harder to observe in isolation.) The initial impetus can be substantial, propelling an object of more than a solar mass at a velocity of 500&nbsp;km/s or greater. This displacement indicates an asymmetry in the explosion, but the mechanism by which this momentum is transferred to the compact object {{As of|2010|alt=remains}} a puzzle. Proposed explanations for this kick include convection in the collapsing star and jet production during neutron star formation.
[[Image:Chandra-crab.jpg|thumb|This composite image shows [[X-ray]] (blue) and optical (red) radiation from the [[Crab Nebula]]'s core region. A [[pulsar]] near the center is propelling particles to almost the speed of light.<ref>
{{cite web
| author=Beasley, D.; Roy, S.; Watzke, M.; Villard, R.
| date =2002-09-19 | url =
| title = Space Movie Reveals Shocking Secrets of the Crab Pulsar
| publisher = [[NASA]]
| accessdate = 2006-08-10 }}</ref> This neutron star is travelling at an estimated 375&nbsp;km/s.<ref>{{cite journal
| author=Frail, D. A.; Giacani, E. B.; Goss, W. M.; Dubner, G.
| title=The Pulsar Wind Nebula Around PSR B1853+01 in the Supernova Remnant W44
| journal=[[The Astrophysical Journal]]
| year=1996 | volume=464 | issue=2
| pages=L165–L168 | url=
| accessdate = 2007-02-01
| doi=10.1086/310103
}}</ref> ''[[NASA]]/CXC/HST/ASU/J. Hester'' et al. ''image credit.'']]
One possible explanation for the asymmetry in the explosion is large-scale [[convection]] above the core. The convection can create variations in the local abundances of elements, resulting in uneven nuclear burning during the collapse, bounce and resulting explosion.<ref>{{cite journal
| last = Fryer | first = Chris L.
| title=Neutron Star Kicks from Asymmetric Collapse
| journal=[[The Astrophysical Journal]]
| year=2004 | volume=601 | issue=2 | pages=L175–L178
| url=
| accessdate = 2007-02-01
| doi=10.1086/382044 }}</ref>
Another possible explanation is that accretion of gas onto the central neutron star can create a [[accretion disk|disk]] that drives highly directional jets, propelling matter at a high velocity out of the star, and driving transverse shocks that completely disrupt the star. These jets might play a crucial role in the resulting supernova explosion.<ref>{{cite news
| title=Jets, Not Neutrinos, May Cause Supernova Explosions, Scientists Say
| publisher=[[McDonald Observatory]]
| date=2002-03-02 | url=
| accessdate=2006-12-11
}}</ref><ref>{{cite web
| last = Foust | first = Jeff | date = 2000-01-09
| url =
| title = Evidence presented for new supernova explosion model
| publisher = Spaceflight Now
| accessdate = 2006-12-13 }}</ref> (A similar model is now favored for explaining long [[gamma ray bursts]].)
Initial asymmetries have also been confirmed in Type&nbsp;Ia supernova explosions through observation. This result may mean that the initial luminosity of this type of supernova depends on the viewing angle. However, the explosion becomes more symmetrical with the passage of time. Early asymmetries are detectable by measuring the polarization of the emitted light.<ref>{{cite news
| title=The VLT Measures the Shape of a Type Ia Supernova
| publisher=[[European Organisation for Astronomical Research in the Southern Hemisphere]] (ESO)
| date=2003-08-06
| url=
| accessdate=2006-12-11 }}</ref>
===Energy output===
Because they have a similar functional model, Types&nbsp;Ib, Ic and various Types&nbsp;II supernovae are collectively called Core Collapse supernovae. A fundamental difference between Type&nbsp;Ia and Core Collapse supernovae is the source of energy for the radiation emitted near the peak of the light curve. The progenitors of Core Collapse supernovae are stars with extended envelopes that can attain a degree of transparency with relatively little expansion. Most of the energy powering the emission at peak light is derived from the shock wave that heats and ejects the envelope.<ref>
{{cite conference
| first = B. | last = Leibundgut
| title = Observations of Supernovae
| booktitle = Proceedings of the NATO Advanced Study Institute on the Lives of the Neutron Stars
| pages = 3
| publisher = Kluwer Academic
| date = August 29 – September 12, 1993
| location = Kemer, Turkey
| url =
| accessdate = 2006-12-18
| id = ISBN 0-7923-324-6-6 }}</ref>
The progenitors of Type&nbsp;Ia supernovae, on the other hand, are compact objects, much smaller (but more massive) than the Sun, that must expand (and therefore cool) enormously before becoming transparent. Heat from the explosion is dissipated in the expansion and is not available for light production. The radiation emitted by Type&nbsp;Ia supernovae is thus entirely attributable to the decay of [[radionuclide]]s produced in the explosion; principally [[nickel]]-56 (with a half-life of 6.1&nbsp;days) and its daughter [[cobalt]]-56 (with a half-life of 77&nbsp;days). Gamma rays emitted during this [[nuclear decay]] are absorbed by the ejected material, heating it to [[incandescence]].
As the material ejected by a Core Collapse supernova expands and cools, radioactive decay eventually takes over as the main energy source for light emission in this case also. A bright Type&nbsp;Ia supernova may expel 0.5–1.0&nbsp;solar masses of nickel-56,<ref>{{cite journal
| author = Matz, S. M.; Share, G. H.
| title=A limit on the production of Ni-56 in a type I supernova
| journal=[[Astrophysical Journal]]
| year=1990 | volume=362 | pages=235–24
| url=
| accessdate = 2007-02-01
| doi=10.1086/169259 }}</ref> while a Core Collapse supernova probably ejects closer to 0.1&nbsp;solar mass of nickel-56.<ref>{{cite journal
| author = Schlegel, E. M.; Kirshner, R. P.
| title=The type Ib supernova 1984L in NGC 991
| journal=[[Astrophysical Journal]]
| year=1989 | volume=98 | pages=577–589
| url=
| accessdate = 2007-02-01
| doi=10.1086/115158 }}</ref>
==Interstellar impact==
===Source of heavy elements===
{{Main|Supernova nucleosynthesis}}
Supernovae are a key source of [[chemical element|elements]] heavier than [[oxygen]].<ref>{{cite journal
| author=François, P.; Matteucci, F.; Cayrel, R.; Spite, M.; Spite, F.; Chiappini, C. | title=The evolution of the Milky Way from its earliest phases: Constraints on stellar nucleosynthesis | journal=Astronomy and Astrophysics
| volume=421 | pages=613&ndash;621 | year=2004 | month=July
| doi=10.1051/0004-6361:20034140
| bibcode=2004A&A...421..613F }}</ref> These elements are produced by [[nuclear fusion]] (for [[iron]]-56 and lighter elements), and by [[nucleosynthesis]] during the supernova explosion for elements heavier than iron.<ref>{{cite journal
| author=Woosley, S. E.; Arnett, W. David; Clayton, Donald D.
| title=The Explosive Burning of Oxygen and Silicon
| journal=Astrophysical Journal Supplement volume=26
| pages=231&ndash;312 | year=1973 | month=November
| doi=10.1086/190282 | bibcode=1973ApJS...26..231W
| volume=26
}}</ref> Supernovae are the most likely, although not undisputed, candidate sites for the [[r-process]], which is a rapid form of nucleosynthesis that occurs under conditions of high temperature and high density of neutrons. The reactions produce highly unstable [[atomic nucleus|nuclei]] that are rich in [[neutron]]s. These forms are unstable and rapidly [[beta decay]] into more stable forms.
The r-process reaction, which is likely to occur in type II supernovae, produces about half of all the element abundance beyond iron, including [[plutonium]], [[uranium]] and [[californium]].<ref>{{cite journal
| author=Qian, Y.-Z.; Vogel, P.; Wasserburg, G. J.
| title=Diverse Supernova Sources for the r-Process
| journal=[[The Astrophysical Journal]]
| year=1998 | volume=494 | issue=1 | pages=285–296
| url=
| accessdate = 2007-02-01
| doi=10.1086/305198 }}</ref> The only other major competing process for producing elements heavier than iron is the [[s-process]] in large, old red giant stars, which produces these elements much more slowly, and which cannot produce elements heavier than [[lead]].<ref>{{cite journal
| author=Gonzalez, G.; Brownlee, D.; Ward, P.
| title=The Galactic Habitable Zone: Galactic Chemical Evolution
| journal=[[Icarus (journal)|Icarus]]
| year=2001 | volume=152 | pages=185–200
| format=PDF
| url=
| accessdate = 2007-02-01
| doi=10.1006/icar.2001.6617 }}</ref>
===Role in stellar evolution===
{{Main|Supernova remnant}}
The remnant of a supernova explosion consists of a compact object and a rapidly expanding [[shock wave]] of material. This cloud of material sweeps up the surrounding [[interstellar medium]] during a free expansion phase, which can last for up to two centuries. The wave then gradually undergoes a period of [[adiabatic process|adiabatic expansion]], and will slowly cool and mix with the surrounding interstellar medium over a period of about 10,000&nbsp;years.<ref>{{cite web
| date = 2006-09-07 | url =
| title = Introduction to Supernova Remnants
| publisher = High Energy Astrophysics Science Archive Research Center, [[NASA]] (HEASARC)
| accessdate = 2006-10-20 }}</ref>
The [[Big Bang]] produced [[hydrogen]], [[helium]], and traces of [[lithium]], while all heavier elements are synthesized in stars and supernovae. Supernovae tend to enrich the surrounding [[interstellar medium]] with ''[[metallicity|metals]]''&mdash;elements other than hydrogen and helium.
[[Image:STScl-2005-15.png|left|thumb|Supernova remnant N 63A lies within a clumpy region of gas and dust in the [[Large Magellanic Cloud]]. ''[[NASA]] image''.]]
These injected elements ultimately enrich the [[molecular cloud]]s that are the sites of star formation.<ref>
{{cite web
| last = Kulyk | first = Christine L.
| date = 2006-06-19 | url =
| title = Explosive Debate: Supernova Dust Lost and Found
| publisher = [[]]
| accessdate = 2006-12-01 }}</ref> Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. Supernovae are the dominant mechanism for distributing these heavier elements, which are formed in a star during its period of nuclear fusion, throughout space. The different abundances of elements in the material that forms a star have important influences on the star's life, and may decisively influence the possibility of having [[planet]]s orbiting it.
The [[kinetic energy]] of an expanding supernova remnant can trigger star formation due to compression of nearby, dense molecular clouds in space.<ref>{{cite conference
| author=Preibisch, T.; Zinnecker, H.
| title=Triggered Star Formation in the Scorpius-Centaurus OB Association (Sco OB2) | booktitle=ASP Conference Proceedings, From Darkness to Light: Origin and Evolution of Young Stellar Clusters | page=791
| publisher=Astronomical Society of the Pacific | volume=243
| location=San Francisco | year=2001 | bibcode=2001ASPC..243..791P
| url=
| accessdate=2009-01-05 }}</ref> The increase in turbulent pressure can also prevent star formation if the cloud is unable to lose the excess energy.<ref name="aaa128">{{cite journal
| author=Krebs, J.; Hillebrandt, W.
| title=The interaction of supernova shockfronts and nearby interstellar clouds
| journal=[[Astronomy and Astrophysics]]
| year=1983 | volume=128 | issue=2 | pages=411–419
| url=
| accessdate = 2007-02-01 }}</ref>
Evidence from daughter products of short-lived [[radioactive isotope]]s shows that a nearby supernova helped determine the composition of the [[Solar System]] 4.5&nbsp;billion years ago, and may even have triggered the formation of this system.<ref>{{cite web
| last = Taylor | first = G. Jeffrey
| date = 2003-05-21 | url =
| title = Triggering the Formation of the Solar System
| publisher = Planetary Science Research
| accessdate = 2006-10-20 }}
</ref> Supernova production of heavy elements over astronomic periods of time ultimately made the [[biochemistry|chemistry of life]] on Earth possible.
===Impact on Earth===
{{Main|Near-Earth supernova}}
A '''near-Earth supernova''' is a supernova close enough to the Earth to have noticeable effects on its [[biosphere]]. This would need to be nearer than about 100 to 3000&nbsp;[[light-year]]s away, depending upon type and energy&mdash;different figures have been suggested. [[Gamma ray]]s from a supernova would induce a [[chemical reaction]] in the upper [[Earth's atmosphere|atmosphere]] converting molecular [[nitrogen]] into [[nitrogen oxide]]s, depleting the [[ozone layer]] enough to expose the surface to harmful [[solar radiation|solar]] and [[cosmic radiation]]. This has been proposed as the cause of the [[Ordovician-Silurian extinction events|end Ordovician extinction]], which resulted in the death of nearly 60% of the oceanic life on Earth.<ref>{{cite journal
|author=Melott, A. ''et al.''
|title=Did a gamma-ray burst initiate the late Ordovician mass extinction?
|journal=International Journal of Astrobiology
|year=2004 | volume=3 | issue=2 | pages=55–61
|accessdate = 2007-02-01
|doi = 10.1017/S1473550404001910 }}</ref>
In 1996 it was theorized that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in [[rock strata]]. [[Iron#Isotopes|Iron-60]] enrichment was later reported in deep-sea rock of the [[Pacific Ocean]].<ref>{{cite news
|first=Staff | pages=17
|title=Researchers Detect 'Near Miss' Supernova Explosion
|date=Fall/Winter 2005–2006
|publisher=[[University of Illinois at Urbana-Champaign|University of Illinois College of Liberal Arts and Sciences]]
|accessdate = 2007-02-01
}}</ref><ref>{{cite journal
|author=Knie, K. ''et al.''
|title=<sup>60</sup>Fe Anomaly in a Deep-Sea Manganese Crust and Implications for a Nearby Supernova Source
|journal=[[Physical Review Letters]]
|year=2004 | volume=93 | issue=17 | pages= 171103–171106
|doi=10.1103/PhysRevLett.93.171103}}</ref><ref name="Fields">{{cite journal
|author=Fields, B. D.; Ellis, J. | title=On Deep-Ocean Fe-60 as a Fossil of a Near-Earth Supernova
|journal=New Astronomy
|year=1999 | volume=4 | pages=419–430
|accessdate = 2007-02-01
|doi = 10.1016/S1384-1076(99)00034-2 }}</ref> In 2009, elevated levels of nitrate ions were found in Antarctic ice, which coincided with the 1006 and 1054 supernovae. Gamma rays from these supernovae could have boosted levels of nitrogen oxides, which became trapped in the ice.<ref>{{cite journal
| title=Supernovae in Ice
| journal=Scientific American
| month=May | year=2009 | page28 }}</ref>
Type&nbsp;Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because these supernovae arise from dim, common white dwarf stars, it is likely that a supernova that can affect the Earth will occur unpredictably and in a star system that is not well studied. One theory suggests that a Type&nbsp;Ia supernova would have to be closer than a thousand parsecs (3300&nbsp;light-years) to affect the Earth.<ref>{{cite web
| url=
| title=Will a Nearby Supernova Endanger Life on Earth?
| first=Michael | last=Richmond | date=2005-04-08
| format=TXT | accessdate=2006-03-30 }}—see section 4.</ref> The closest known candidate is [[IK Pegasi]] (see below).<ref>{{cite journal
| month=March | year=2007
| title=The Supernova Menace
| first=Mark | last=Gorelick
| journal=[[Sky & Telescope]] }}</ref>
Recent estimates predict that a Type&nbsp;II supernova would have to be closer than eight [[parsec]]s (26&nbsp;light-years) to destroy half of the Earth's ozone layer.<ref name=Gehrels>
{{Cite journal | url=
| title=Ozone Depletion from Nearby Supernovae
| first=Neil | last=Gehrels
| coauthors=Laird, Claude M. ''et al.''
| journal=[[Astrophysical Journal]]
| date=2003-03-10 | volume=585 | pages= 1169–1176
| accessdate = 2007-02-01 | doi=10.1086/346127 }}</ref>
==Milky Way candidates==
{{Main|List of supernova candidates}}
[[Image:Wolf rayet2.jpg|thumb| The [[Wolf-Rayet nebula|nebula]] around [[Wolf-Rayet star]] WR124, which is located at a distance of about 21,000 [[light year]]s.<ref>{{cite web
| author=van der Sluys, Marc; Lamers, H. J. G. L. M.
| year=2003
| url=
| title=The dynamics of the Wolf-Rayet ring nebula M1-67
| publisher=Astronomical Institute, Utrecht
| accessdate=2007-06-07 }}</ref> ''[[NASA]] image''.]]
Several large stars within the [[Milky Way]] have been suggested as possible supernovae within the next million years. These include [[Rho Cassiopeiae]],<ref>{{cite news
| author=Staff
| title=The William Herschel telescope finds the best candidate for a supernova explosion
| publisher=[[Particle Physics and Astronomy Research Council]]
| date=2003-01-31
| url=
| accessdate=2007-01-05 }}</ref> [[Eta Carinae]],<ref>{{cite web
| author=van Boekel, R.; Schöller, M.; Herbst, T.
| date = 2003-11-18 | url =
| title = Biggest Star in Our Galaxy Sits within a Rugby-Ball Shaped Cocoon
| publisher = [[European Organisation for Astronomical Research in the Southern Hemisphere]] (ESO)
| accessdate = 2007-01-08
}}</ref><ref>{{cite news
| first=Wil | last=Milan
| title=Possible Hypernova Could Affect Earth
| publisher=[[]] | date=2000-03-07 | url=
| accessdate=2007-01-06 }}</ref> [[RS Ophiuchi]],<ref>{{cite web
| last = Than | first = Ker
| date = 2006-07-19 | url =
| title = Mystery of Explosive Star Solved
| publisher = [[]]
| accessdate = 2007-01-08
}}</ref><ref>{{cite news | author=Staff
| title=Astronomers See Future Supernova Developing
| publisher=[[SpaceDaily]]
| date=2006-07-25 | url=
| accessdate=2006-12-01 }}</ref> [[U Scorpii]],<ref>{{cite conference
| author=Thoroughgood, T. D.; Dhillon, V. S.; Littlefair, S. P.; Marsh, T. R.; Smith, D. A. | title=The recurrent nova U Scorpii -- A type Ia supernova progenitor | booktitle=The Physics of Cataclysmic Variables and Related Objects
| volume=261 | bibcode=2002ASPC..261...77T | year=2002
| location=San Francisco, CA
| publisher=Astronomical Society of the Pacific | url=
| accessdate=2009-01-24 }}</ref> [[VY Canis Majoris]],<ref>{{cite web
| author=Weaver, D.; Humphreys, R.
| date = 2007-01-08 | url =
| title = Astronomers Map a Hypergiant Star's Massive Outbursts
| publisher = HubbleSite NewsCenter
| accessdate = 2007-01-16 }}</ref> [[Betelgeuse]], [[Antares]], and [[Spica]].<ref name="chandra_snr">{{cite web
| date = 2005-08-02 | url =
| title = Supernova Remnants and Neutron Stars
| publisher = [[Harvard-Smithsonian Center for Astrophysics]]
| accessdate = 2006-06-08 }}</ref> Many [[Wolf-Rayet star]]s, such as [[Gamma Velorum]],<ref>{{cite web
| last = Kaler | first = Jim | url =
| title = Regor
| publisher = [[University of Illinois at Urbana-Champaign|University of Illinois]]
| accessdate = 2007-01-08
}}</ref> [[WR 104]],<ref>{{cite web
| last = Kaler | first = Jim
| date = 1999-04-09 | url = | title = WR 104: Pinwheel Star
| publisher = [[Astronomy Picture of the Day]]
| accessdate = 2007-01-08 }}</ref> and those in the [[Quintuplet Cluster]],<ref>{{cite web
| last = Lloyd | first = Robin
| date = 2006-09-04 | url = | title = Strange Space Pinwheels Spotted
| publisher = [[]]
| accessdate = 2007-01-08 }}</ref> are also considered possible precursor stars to a supernova explosion in the 'near' future.
The nearest supernova candidate is [[IK Pegasi]] (HR 8210), located at a distance of 150&nbsp;light-years. This closely orbiting [[binary star system]] consists of a main sequence star<!-- not yet evolved into a red giant --> and a white dwarf 31&nbsp;million&nbsp;kilometres apart. The dwarf has an estimated mass 1.15 times that of the Sun.<ref>{{cite journal
| author=Landsman, W.; Simon, T.; Bergeron, P.
| title=The hot white-dwarf companions of HR 1608, HR 8210, and HD 15638
| journal=[[Astronomical Society of the Pacific]]
| year=1999 | volume=105 | issue=690 | pages=841–847
| url=
| accessdate = 2007-02-01
| doi=10.1086/133242 }}</ref> It is thought that several million years will pass before the white dwarf can accrete the critical mass required to become a Type&nbsp;Ia supernova.<ref>{{cite web
| last = Samuel | first = Eugenie
| date = 2002-05-23 | url =
| title = Supernova poised to go off near Earth
| publisher = [[New Scientist]]
| accessdate = 2007-01-12
}}</ref><ref>{{cite web
| author=Tzekova, S. Y. ''et al.'' | year=2004
| url =
| title = IK Pegasi (HR 8210)
| publisher = ESO | accessdate = 2007-01-12 }}</ref>
==See also==
{{Portal box|Astronomy|Star|Space}}
* [[Champagne Supernova (astronomy)]]
* [[Dwarf nova]]
* [[Guest star (astronomy)]]
* [[List of supernovae]]
* [[List of supernova remnants]]
* [[Quark nova]]
* [[Supernovae in fiction]]
* [[Timeline of white dwarfs, neutron stars, and supernovae]]
* [[Supernova impostor]]
==Further reading==
* {{cite journal
| first=Hans | last=Bethe
| authorlink=Hans Bethe
| title=SUPERNOVAE. By what mechanism do massive stars explode?
| journal=[[Physics Today]]
| volume=43 | issue=9 | pages=24–27
| month=September | year=1990 | format=PDF
| url=
| doi=10.1063/1.881256}}
Further reading internet link:
* {{cite book
| first=Ken | last=Croswell | authorlink=Ken Croswell | year=1996
| title=[[The Alchemy of the Heavens]]: Searching for Meaning in the Milky Way
| publisher=Anchor Books | isbn=0385472145 }} A popular-science account.
* {{cite journal | first=Alexi V. | last=Filippenko
| year=1997 | title=Optical Spectra of Supernovae
| journal=[[Annual Reviews|Annual Review of Astronomy and Astrophysics]]
| volume=35 | pages=309–355
| doi=10.1146/annurev.astro.35.1.309
}} An article describing spectral classes of supernovae.
* {{cite journal
| author=Takahashi, K.; Sato, K.; Burrows, A.; Thompson, T. A.
| title=Supernova Neutrinos, Neutrino Oscillations, and the Mass of the Progenitor Star
| journal=Physical Review D
| year=2003 | volume=68 | issue=11 | pages=77–81 | article=113009
| url=
| accessdate=2006-11-28
| doi = 10.1103/PhysRevD.68.113009
}} A good review of supernova events.
* {{cite journal
| title=How to Blow Up a Star
| first=Wolfgang | last=Hillebrandt
| coauthors=Janka, Hans-Thomas; Müller, Ewald
| journal=[[Scientific American]]
| month=October | year=2006 | volume=295 | issue=4 | pages=42–49
| url=
| doi=10.1038/scientificamerican1006-42 }}
* {{cite journal
| first=Stan | last=Woosley
| coauthors=Hans-Thomas Janka
| title=The Physics of Core-Collapse Supernovae
| journal=[[Nature Physics]]
| volume=1 | issue=3 | pages=147–154 | month=December | year=2005
| url=
| format=PDF
| doi=10.1038/nphys172
}}—link is to a pre-print of the article submitted to ''[[Nature (journal)|Nature]]''.
==External links==
{{Wikipedia-Books|Classes of supernovae}}
* [ List of Supernovae-related Web pages].
* {{cite web | url =
| title = RSS news feed
| publisher = The Astronomer's Telegram
| format = RSS
| accessdate = 2006-11-28 }}
* {{cite web
| author = Tsvetkov, D. Yu.; Pavlyuk, N. N.; Bartunov, O. S.; Pskovskii, Yu. P.
| url =
| title = Sternberg Astronomical Institute Supernova Catalogue
| publisher = [[Sternberg Astronomical Institute]], [[Moscow University]]
| accessdate = 2006-11-28
}} A searchable catalog.
* {{cite web | author=Anonymous
| date = 2007-01-18
| url =
| title = BoomCode
| publisher = [[WikiUniversity]]
| accessdate = 2007-03-17
}} Professional-grade type&nbsp;II supernova simulator on Wikiversity.
* {{cite web
| url =
| title = List All Supernovae
| publisher = IAU: Central Bureau for Astronomical Telegrams
| accessdate = 2008-03-24
*{{cite news
| url =
| title = Scientists See Supernova in Action
| work = [[The New York Times]]
| date=May 21, 2008
| accessdate = 2008-05-21
| first=Dennis
| last=Overbye}}
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Revision as of 22:34, 6 July 2010