Editing Supernova (section)

===Thermal runaway===
{{Main|Type Ia supernova}}
[[File:Progenitor IA supernova.svg|thumb|upright=1.7|Formation of a type Ia supernova]]
A white dwarf star may accumulate sufficient material from a stellar companion 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. There are three avenues by which this detonation is theorised to happen: stable [[accretion (astrophysics)|accretion]] of material from a companion, the collision of two white dwarfs, or accretion that causes ignition in a shell that then ignites the core. The dominant mechanism by which type Ia supernovae are produced remains unclear.<ref name="Piro2014">
{{Cite journal
 |last1=Piro |first1=A. L.
 |last2=Thompson |first2=T. A.
 |last3=Kochanek |first3=C. S.
 |year=2014
 |title=Reconciling 56Ni production in Type Ia supernovae with double degenerate scenarios
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=438 |issue=4 |pages=3456
 |arxiv=1308.0334
 |bibcode=2014MNRAS.438.3456P
 |doi=10.1093/mnras/stt2451
|doi-access=free
 |s2cid=27316605
 }}</ref> Despite this uncertainty in how type Ia supernovae are produced, type Ia supernovae have very uniform properties and are useful [[Cosmic distance ladder|standard candles]] over intergalactic distances. Some calibrations are required to compensate for the gradual change in properties or different frequencies of abnormal luminosity supernovae at high redshift, and for small variations in brightness identified by light curve shape or spectrum.<ref name="chen">
{{Cite journal
 |last1=Chen |first1=W.-C.
 |last2=Li |first2=X.-D.
 |year=2009
 |title=On the Progenitors of Super-Chandrasekhar Mass Type Ia Supernovae
 |journal=[[The Astrophysical Journal]]
 |volume=702 |issue=1|pages=686–691
 |arxiv=0907.0057
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 |doi=10.1088/0004-637X/702/1/686
|s2cid=14301164
 }}</ref><ref>
{{Cite journal
 |last1=Howell |first1=D. A.
 |last2=Sullivan |first2=M.
 |last3=Conley |first3=A. J.
 |last4=Carlberg |first4=R. G.
 |date=2007
 |title=Predicted and Observed Evolution in the Mean Properties of Type Ia Supernovae with Redshift
 |journal=[[Astrophysical Journal Letters]]
 |volume=667 |issue=1 |pages=L37–L40
 |arxiv=astro-ph/0701912
 |bibcode=2007ApJ...667L..37H
 |doi=10.1086/522030
|s2cid=16667595
 }}</ref>

====Normal type Ia====
There are several means by which a supernova of this type can form, but they share a common underlying mechanism. If a [[carbon]]-[[oxygen]] white dwarf accreted enough matter to reach the [[Chandrasekhar limit]] of about 1.44 [[solar mass]]es<ref name="Mazzali2007">
{{Cite journal
 |last1=Mazzali |first1=P. A.
 |last2=Röpke |first2=F. K.
 |last3=Benetti |first3=S.
 |last4=Hillebrandt |first4=W.
 |date=2007
 |title=A Common Explosion Mechanism for Type Ia Supernovae
 |journal=[[Science (journal)|Science]]
 |volume=315 |issue=5813 |pages=825–828
 |arxiv=astro-ph/0702351
 |bibcode=2007Sci...315..825M
 |doi=10.1126/science.1136259
 |pmid=17289993
|s2cid=16408991
 }}</ref> (for a non-rotating star), it would no longer be able to support the bulk of its mass through [[electron degeneracy pressure]]<ref name="Chandrasekhar">{{Cite journal
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 |last2=Yau
 |first2=H.-T.
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|archive-date=3 March 2020
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 }}</ref><ref name=canal1997>
{{Cite book
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 |last2=Gutiérrez |first2=J. L.
 |date=1997
 |chapter=The possible white dwarf-neutron star connection
 |editor1-last=Isern |editor1-first=J.
 |editor2-last=Hernanz |editor2-first=M.
 |editor3-last=Gracia-Berro |editor3-first=E.
 |title=White Dwarfs: Proceedings of the 10th European Workshop on White Dwarfs
 |series=Astrophysics and Space Science Library
 |volume=214 |page=49
 |publisher=[[Kluwer Academic Publishers]]
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 |doi=10.1007/978-94-011-5542-7_7
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|s2cid=9288287
 }}</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 ignite carbon fusion as the star approaches the limit (to within about 1%)<ref>
{{Cite book
 |last=Wheeler |first=J. C.
 |date=2000
 |title=Cosmic Catastrophes: Supernovae, Gamma-Ray Bursts, and Adventures in Hyperspace
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 |publisher=[[Cambridge University Press]]
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 |archive-date=10 September 2015
}}</ref> before collapse is initiated.<ref name="Mazzali2007"/> In contrast, 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.<ref name=canal1997/>

[[File:An isolated neutron star in the Small Magellanic Cloud.jpg|thumb|The blue spot at the centre of the red ring is an isolated neutron star in the [[Small Magellanic Cloud]].]]
Within a few seconds of the collapse process, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (1–{{val|2|e=44|ul=J}})<ref name="aaa270">
{{Cite journal
 |last1=Khokhlov |first1=A. M.
 |last2=Mueller |first2=E.
 |last3=Höflich |first3=P. A.
 |date=1993
 |title=Light curves of Type IA supernova models with different explosion mechanisms
 |journal=[[Astronomy and Astrophysics]]
 |volume=270 |issue=1–2 |pages=223–248
 |bibcode=1993A&A...270..223K
}}</ref> to [[gravitational binding energy|unbind]] the star in a supernova.<ref name="ropke">
{{Cite journal
 |last1=Röpke |first1=F. K.
 |last2=Hillebrandt |first2=W.
 |date=2004
 |title=The case against the progenitor's carbon-to-oxygen ratio as a source of peak luminosity variations in type Ia supernovae
 |journal=[[Astronomy and Astrophysics Letters]]
 |volume=420 |issue=1 |pages=L1–L4
 |arxiv=astro-ph/0403509
 |bibcode=2004A&A...420L...1R
 |doi=10.1051/0004-6361:20040135
|s2cid=2849060
 }}</ref> An outwardly expanding [[shock wave]] is generated, with matter reaching velocities on the order of 5,000–20,000 [[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&nbsp;billion times brighter than the Sun), with little variation.<ref name="explosion_model">
{{Cite journal
 |last1=Hillebrandt |first1=W.
 |last2=Niemeyer |first2=J. C.
 |date=2000
 |title=Type IA Supernova Explosion Models
 |journal=[[Annual Review of Astronomy and Astrophysics]]
 |volume=38 |issue=1 |pages=191–230
 |arxiv=astro-ph/0006305
 |bibcode=2000ARA&A..38..191H
 |doi=10.1146/annurev.astro.38.1.191
|s2cid=10210550
 }}</ref>

The 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 [[Stellar evolution|evolve]] off the [[main sequence]], and it expands to form a [[red giant]]. 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
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 |date=1976
 |title=Common Envelope Binaries
 |book-title=Structure and Evolution of Close Binary Systems
 |conference=IAU Symposium No. 73
 |editor1-last=Eggleton |editor1-first=P.
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 |editor3-last=Whelan |editor3-first=J.
 |pages=75–80
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}}</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. The exact details of initiation and of the heavy elements produced in the catastrophic event remain unclear.<ref>{{Cite journal |last1=Poludnenko |first1=Alexei Y. |last2=Chambers |first2=Jessica |last3=Ahmed |first3=Kareem |last4=Gamezo |first4=Vadim N. |last5=Taylor |first5=Brian D. |date=November 2019 |title=A unified mechanism for unconfined deflagration-to-detonation transition in terrestrial chemical systems and type Ia supernovae |url=https://www.science.org/doi/10.1126/science.aau7365 |journal=Science |language=en |volume=366 |issue=6465 |pages=eaau7365 |bibcode=2019Sci...366.7365P |doi=10.1126/science.aau7365 |pmid=31672866 |arxiv=1911.00050 |s2cid=207817150 |issn=0036-8075 |quote=Theoretical models of SNIa have remained limited because of uncertainties in the explosion mechanisms. [...] SNIa explosions are driven by fast thermonuclear burning in <sup>12</sup>C/<sup>16</sup>O white dwarf (WD) stars with a mass close to, or below, the Chandrasekhar mass limit of ≈1.4 solar masses [...] Beyond this general statement, however, the exact mechanisms of SNIa remain unclear, with a number of possible scenarios.}}</ref>

Type Ia supernovae produce a characteristic light curve—the graph of luminosity as a function of time—after the event. 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 is extremely consistent across normal type Ia supernovae, having a maximum absolute magnitude of about −19.3. This is because typical type Ia supernovae arise from a consistent type of progenitor star by gradual mass acquisition, and explode when they acquire a consistent typical mass, giving rise to very similar supernova conditions and behaviour. This allows them to be used as a secondary<ref>
{{Cite journal
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}}</ref>

A second model for the formation of type Ia supernovae involves the merger of two white dwarf stars, with the combined mass momentarily exceeding the Chandrasekhar limit.<ref>
{{cite journal
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 |last2=Blinnikov |first2=S.
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 |year=2000
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}}</ref> This is sometimes referred to as the double-degenerate model, as both stars are degenerate white dwarfs. Due to the possible combinations of mass and chemical composition of the pair there is much variation in this type of event,<ref>
{{cite journal
 |last1=Dan |first1=M.
 |last2=Rosswog |first2=S.
 |last3=Guillochon |first3=J.
 |last4=Ramirez-Ruiz |first4=E.
 |year=2012
 |title=How the merger of two white dwarfs depends on their mass ratio: Orbital stability and detonations at contact
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=422|issue=3|pages=2417
 |arxiv=1201.2406
 |bibcode=2012MNRAS.422.2417D
 |doi=10.1111/j.1365-2966.2012.20794.x
 |doi-access=free
 | s2cid=119159904
}}</ref> and, in many cases, there may be no supernova at all, in which case they will have a less luminous light curve than the more normal SN type Ia.<ref>{{Cite journal |last1=Maoz |first1=Dan |last2=Mannucci |first2=Filippo |last3=Nelemans |first3=Gijs |date=18 August 2014 |title=Observational Clues to the Progenitors of Type Ia Supernovae |url=https://www.annualreviews.org/doi/10.1146/annurev-astro-082812-141031 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=52 |issue=1 |pages=107–170 |doi=10.1146/annurev-astro-082812-141031 |bibcode=2014ARA&A..52..107M |arxiv=1312.0628 |s2cid=55533680 |issn=0066-4146}}</ref>

====Non-standard type Ia====
Abnormally bright type Ia supernovae occur when the white dwarf already has a mass higher than the Chandrasekhar limit,<ref>
{{Cite journal
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 |last15=Pain |first15=R.
 |last16=Perrett |first16=K. M.
 |last17=Pritchet |first17=C. J.
 |year=2006
 |title=The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star
 |journal=[[Nature (journal)|Nature]]
 |volume=443 |issue=7109 |pages=308–311
 |arxiv=astro-ph/0609616
 |bibcode=2006Natur.443..308H
 |doi=10.1038/nature05103
 |pmid=16988705
|s2cid=4419069
 }}</ref> possibly enhanced further by asymmetry,<ref>
{{Cite journal
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 |last3=Yamanaka |first3=M.
 |last4=Maeda |first4=K.
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 |last6=Aoki |first6=K.
 |last7=Nomoto |first7=K. I.
 |last8=Iye |first8=M.
 |last9=Sasaki |first9=T.
 |last10=Mazzali |first10=P. A.
 |last11=Pian |first11=E.
 |year=2010
 |title=Spectropolarimetry of Extremely Luminous Type Ia Supernova 2009dc: Nearly Spherical Explosion of Super-Chandrasekhar Mass White Dwarf
 |journal=[[The Astrophysical Journal]]
 |volume=714 |issue=2 |pages=1209
 |arxiv=0908.2057
 |bibcode=2010ApJ...714.1209T
 |doi=10.1088/0004-637X/714/2/1209
|s2cid=13990681
 }}</ref> but the ejected material will have less than normal kinetic energy. This super-Chandrasekhar-mass scenario can occur, for example, when the extra mass is supported by [[differential rotation]].<ref>{{cite journal | title=Thermonuclear explosions of rapidly differentially rotating white dwarfs: Candidates for superluminous Type Ia supernovae? | last1=Fink | first1=M. | last2=Kromer | first2=M. | last3=Hillebrandt | first3=W. | last4=Röpke | first4=F. K. | last5=Pakmor | first5=R. | last6=Seitenzahl | first6=I. R. | last7=Sim | first7=S. A. | journal=Astronomy & Astrophysics | volume=618 | id=A124 | date=October 2018 | pages=A124 | doi=10.1051/0004-6361/201833475 | arxiv=1807.10199 | bibcode=2018A&A...618A.124F | s2cid=118965737 }}</ref>

There is no formal sub-classification for non-standard type Ia supernovae. It has been proposed that a group of sub-luminous supernovae that occur when helium accretes onto a white dwarf should be classified as '''type Iax'''.<ref name=wang>
{{cite journal
 |last1=Wang |first1=B.
 |last2=Liu |first2=D.
 |last3=Jia |first3=S.
 |last4=Han |first4=Z.
 |year=2014
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 |journal=[[Proceedings of the International Astronomical Union]]
 |volume=9 |issue=S298 |pages=442
 |arxiv=1301.1047
 |bibcode=2014IAUS..298..442W
 |doi=10.1017/S1743921313007072
|s2cid=118612081
 }}</ref><ref name=foley>
{{cite journal
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 |last2=Challis |first2=P. J.
 |last3=Chornock |first3=R.
 |last4=Ganeshalingam |first4=M.
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 |last6=Marion |first6=G. H.
 |last7=Morrell |first7=N. I.
 |last8=Pignata |first8=G.
 |last9=Stritzinger |first9=M. D.
 |last10=Silverman |first10=J. M.
 |last11=Wang |first11=X.
 |last12=Anderson |first12=J. P.
 |last13=Filippenko |first13=A. V.
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 |last18=McCully |first18=C.
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 |last22=Soderberg |first22=A. M.
 |year=2013
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 |journal=[[The Astrophysical Journal]]
 |volume=767 |issue=1|pages=57
 |arxiv=1212.2209
 |bibcode=2013ApJ...767...57F
 |doi=10.1088/0004-637X/767/1/57
|s2cid=118603977
 }}</ref> This type of supernova may not always completely destroy the white dwarf progenitor and could leave behind a [[zombie star]].<ref name="mccully">
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 |last9=Stritzinger |first9=M. D.
 |year=2014
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 |arxiv=1408.1089
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 |doi=10.1038/nature13615
 |pmid=25100479
|s2cid=4464556
 }}</ref>

One specific type of supernova originates from exploding white dwarfs, like type Ia, but contains hydrogen lines in their spectra, possibly because the white dwarf is surrounded by an envelope of hydrogen-rich [[Circumstellar disc|circumstellar material]]. These supernovae have been dubbed '''type Ia/IIn''', '''type Ian''', '''type IIa''' and '''type IIan'''.<ref>
{{cite journal
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 |last16=Graham |first16=M. L.
 |last17=Griffith |first17=C. V.
 |last18=Horesh |first18=A.
 |last19=Kasliwal |first19=M. M.
 |last20=Kulkarni |first20=S. R.
 |last21=Leonard |first21=D. C.
 |last22=Li |first22=W.
 |last23=Matheson |first23=T.
 |last24=Miller |first24=A. A.
 |last25=Modjaz |first25=M.
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 |last27=Pan |first27=Y.-C.
 |last28=Perley |first28=D. A.
 |last29=Poznanski |first29=D.
 |last30=Quimby |first30=R. M.
 |year=2013
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 |arxiv=1304.0763
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|s2cid=51415846
 }}</ref>

The quadruple star [[HD 74438]], belonging to the open cluster [[IC 2391]] the [[Vela constellation]], has been predicted to become a non-standard type Ia supernova.<ref name="GIAISURVEY">{{cite journal |last1=Gilmore |first1=Gerry |last2=Randich |first2=Sofia |title=The Gaia-ESO Public Spectroscopic Survey |journal=The Messenger |date=March 2012 |volume=147 |issue=147 |pages=25–31 |publisher=European Southern Observatory |location=Garching, Germany |language=en |bibcode=2012Msngr.147...25G}}</ref><ref name="supernova">{{cite journal |last1=Merle |first1=Thibault |last2=Hamers |first2=Adrian S. |last3=Van Eck |first3=Sophie |last4=Jorissen |first4=Alain |last5=Van der Swaelmen |first5=Mathieu |last6=Pollard |first6=Karen |last7=Smiljanic |first7=Rodolfo |last8=Pourbaix |first8=Dimitri |last9=Zwitter |first9=Tomaž |last10=Traven |first10=Gregor |last11=Gilmore |first11=Gerry |last12=Randich |first12=Sofia |last13=Gonneau |first13=Anaïs |last14=Hourihane |first14=Anna |last15=Sacco |first15=Germano |last16=Worley |first16=C. Clare |title=A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar type Ia supernovae |journal=Nature Astronomy |date=12 May 2022 |volume=6 |issue=6 |pages=681–688 |doi=10.1038/s41550-022-01664-5|arxiv=2205.05045 |bibcode=2022NatAs...6..681M |s2cid=248665714 }}</ref>