Editing Supernova (section)

====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
 |last1=Lieb
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 |last2=Yau
 |first2=H.-T.
 |date=1987
 |title=A rigorous examination of the Chandrasekhar theory of stellar collapse
 |journal=[[The Astrophysical Journal]]
 |volume=323
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|archive-date=3 March 2020
|archive-url=https://web.archive.org/web/20200303072644/https://dash.harvard.edu/handle/1/32706795
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 }}</ref><ref name=canal1997>
{{Cite book
 |last1=Canal |first1=R.
 |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]]
 |location=Dordrecht
 |arxiv=astro-ph/9701225
 |bibcode=1997ASSL..214...49C
 |doi=10.1007/978-94-011-5542-7_7
 |isbn=978-0-7923-4585-5
|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
 |url=https://books.google.com/books?id=s3SFQgAACAAJ
 |page=96
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-65195-0
 |url-status=live
 |archive-url=https://web.archive.org/web/20150910190113/https://books.google.com/books?id=s3SFQgAACAAJ
 |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
 |last=Paczyński |first=B.
 |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.
 |editor2-last=Mitton |editor2-first=S.
 |editor3-last=Whelan |editor3-first=J.
 |pages=75–80
 |publisher=[[D. Reidel]]
 |location=Dordrecht
 |bibcode=1976IAUS...73...75P
}}</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
 |last1=Macri |first1=L. M.
 |last2=Stanek |first2=K. Z.
 |last3=Bersier |first3=D.
 |last4=Greenhill |first4=L. J.
 |last5=Reid |first5=M. J.
 |date=2006
 |title=A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant
 |journal=[[The Astrophysical Journal]]
 |volume=652 |issue=2 |pages=1133–1149
 |arxiv=astro-ph/0608211
 |bibcode=2006ApJ...652.1133M
 |doi=10.1086/508530
|s2cid=15728812
 }}</ref> standard candle to measure the distance to their host galaxies.<ref>
{{Cite journal
 |last=Colgate |first=S. A.
 |date=1979
 |title=Supernovae as a standard candle for cosmology
 |journal=[[The Astrophysical Journal]]
 |volume=232 |issue=1 |pages=404–408
 |bibcode=1979ApJ...232..404C
 |doi=10.1086/157300
}}</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
 |last1=Ruiz-Lapuente |first1=P.
 |last2=Blinnikov |first2=S.
 |last3=Canal |first3=R.
 |last4=Mendez |first4=J.
 |last5=Sorokina |first5=E.
 |last6=Visco |first6=A.
 |last7=Walton |first7=N.
 |year=2000
 |title=Type IA supernova progenitors
 |journal=Memorie della Societa Astronomica Italiana
 |volume=71 |pages=435
 |bibcode=2000MmSAI..71..435R
}}</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>