Editing Supernova (section)

===Core collapse===
[[Image:Evolved star fusion shells.svg|thumb|The layers of a massive, evolved star just before core collapse (not to scale)]]
Very massive stars can undergo core collapse when nuclear fusion becomes unable to sustain the core against its own gravity; passing this threshold is the cause of all types of supernova except type Ia. The collapse may cause violent expulsion of the outer layers of the star resulting in a supernova. However, if the release of gravitational potential energy is insufficient, the star may instead collapse into a [[black hole]] or neutron star with little radiated energy.<ref name="heger"/>

Core collapse can be caused by several different mechanisms: exceeding the [[Chandrasekhar limit]]; [[electron capture]]; [[Pair-instability supernova|pair-instability]]; or [[photodisintegration]].<ref name="heger">
{{Cite journal
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* When a massive star develops an iron core larger than the Chandrasekhar mass it will no longer be able to support itself by [[electron degeneracy pressure]] and will collapse further to a neutron star or black hole.
* Electron capture by magnesium in a [[degenerate matter|degenerate]] O/Ne/Mg core (8–10 solar mass progenitor star) removes support and causes [[gravitational collapse]] followed by explosive oxygen fusion, with very similar results.
* Electron-positron pair production in a large post-helium burning core removes thermodynamic support and causes initial collapse followed by runaway fusion, resulting in a pair-instability supernova.
* A sufficiently large and hot [[stellar core]] may generate gamma-rays energetic enough to initiate photodisintegration directly, which will cause a complete collapse of the core.

The table below lists the known reasons for core collapse in massive stars, the types of stars in which they occur, their associated supernova type, and the remnant produced. The [[metallicity]] is the proportion of elements other than hydrogen or helium, as compared to the Sun. The initial mass is the mass of the star prior to the supernova event, given in multiples of the Sun's mass, although the mass at the time of the supernova may be much lower.<ref name="heger"/>

Type IIn supernovae are not listed in the table. They can be produced by various types of core collapse in different progenitor stars, possibly even by type Ia white dwarf ignitions, although it seems that most will be from iron core collapse in luminous [[supergiant]]s or [[hypergiant]]s (including LBVs). The narrow spectral lines for which they are named occur because the supernova is expanding into a small dense cloud of circumstellar material.<ref name="moriya">
{{cite journal
 |last1=Moriya |first1=T. J.
 |date=2012
 |title=Progenitors of Recombining Supernova Remnants
 |journal=[[The Astrophysical Journal]]
 |volume=750 |issue=1 |pages=L13
 |arxiv=1203.5799
 |bibcode=2012ApJ...750L..13M
 |doi=10.1088/2041-8205/750/1/L13
|s2cid=119209527
 }}</ref> It appears that a significant proportion of supposed type IIn supernovae are supernova impostors, massive eruptions of LBV-like stars similar to the Great Eruption of [[Eta Carinae]]. In these events, material previously ejected from the star creates the narrow absorption lines and causes a shock wave through interaction with the newly ejected material.<ref name="smith">
{{Cite journal
 |last1=Smith |first1=N.
 |last2=Ganeshalingam |first2=M.
 |last3=Chornock |first3=R.
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 |last6=Silverman |first6=J. M.
 |last7=Steele |first7=T. N.
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 |last13=Winslow |first13=D. M.
 |year=2009
 |title=Sn 2008S: A Cool Super-Eddington Wind in a Supernova Impostor
 |journal=[[The Astrophysical Journal]]
 |volume=697 |issue=1|pages=L49
 |arxiv=0811.3929
 |bibcode=2009ApJ...697L..49S
 |doi=10.1088/0004-637X/697/1/L49
|s2cid=17627678
 }}</ref>

{|class="wikitable"
|+ Core collapse scenarios by mass and metallicity<ref name="heger"/>
|-
! Cause of collapse !! Progenitor star approximate initial mass ([[solar mass]]es) !! Supernova type !! Remnant
|-
| [[Electron capture supernova|Electron capture in a degenerate O+Ne+Mg core]] || 9–10 || Faint II-P ||Neutron star
|-
|rowspan="7"| Iron core collapse || 10–25 || Faint II-P || Neutron star
|-
| 25–40 with low or solar metallicity || Normal II-P || Black hole after fallback of material onto an initial neutron star
|-
| 25–40 with very high metallicity || II-L or II-b || Neutron star
|-
| 40–90 with low metallicity || None || Black hole
|-
| ≥&nbsp;40 with near-solar metallicity || Faint Ib/c, or [[hypernova]] with [[gamma-ray burst]] (GRB) || Black hole after fallback of material onto an initial neutron star
|-
| ≥&nbsp;40 with very high metallicity || Ib/c || Neutron star
|-
| ≥&nbsp;90 with low metallicity || None, possible GRB || Black hole
|-
| Pair instability{{anchor|type_II_P_anchor}} || 140–250 with low metallicity || II-P, sometimes a hypernova, possible GRB || No remnant
|-
| Photodisintegration || ≥&nbsp;250 with low metallicity || None (or luminous supernova?), possible GRB || Massive black hole
|}

====Detailed process====
[[Image:Core collapse scenario.svg|upright=1.4|thumb|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, likely by [[Supernova neutrinos|neutrino heating]]. The surrounding material is blasted away (f), leaving only a degenerate remnant.<ref name="Janka-2007"/>]]

When a stellar core is no longer supported against gravity, it collapses in on itself with velocities reaching 70,000&nbsp;km/s (0.23[[Speed of light|''c'']]),<ref name="grav_waves">
{{cite journal
 |last1=Fryer |first1=C. L.
 |last2=New |first2=K. C. B.
 |year=2003
 |title=Gravitational Waves from Gravitational Collapse
 |journal=[[Living Reviews in Relativity]]
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 |bibcode=2003LRR.....6....2F
 |doi=10.12942/lrr-2003-2
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}}</ref> resulting in a rapid increase in temperature and density. What follows depends on the mass and structure of the collapsing core, with low-mass degenerate cores forming neutron stars, higher-mass degenerate cores mostly collapsing completely to black holes, and non-degenerate cores undergoing runaway fusion.<ref name="Janka-2007"/><ref name="Hurley-2000">{{Cite journal |last1=Hurley |first1=J. R. |last2=Pols |first2=O. R. |last3=Tout |first3=C. A. |date=1 July 2000 |title=Comprehensive analytic formulae for stellar evolution as a function of mass and metallicity |doi-access=free |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=315 |issue=3 |pages=543–569 |doi=10.1046/j.1365-8711.2000.03426.x |bibcode=2000MNRAS.315..543H |arxiv=astro-ph/0001295 |issn=0035-8711}}</ref>

The initial collapse of degenerate cores is accelerated by [[beta decay]], photodisintegration and electron capture, which causes a burst of [[electron neutrino]]s. As the density increases, [[Supernova neutrinos|neutrino emission]] is cut off as they become trapped in the core. The inner core eventually reaches typically 30&nbsp;[[kilometre|km]] in diameter<ref name="WoosleyJanka"/> with a density comparable to that of an [[atomic nucleus]], and neutron [[degeneracy pressure]] tries to halt the collapse. If the core mass is more than about 15 solar masses then neutron degeneracy is insufficient to stop the collapse and a black hole forms directly with no supernova.<ref name=renzon2018/>

In lower mass cores the collapse is stopped and the newly formed neutron core has an initial temperature of about 100&nbsp;billion [[kelvin]], 6,000 times the temperature of the [[Sun's core]].<ref name="Janka-2007">
{{Cite journal
 |last1=Janka |first1=H.-T.
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 }}</ref> At this temperature, neutrino-antineutrino pairs of all [[Neutrino oscillation|flavours]] are efficiently formed by [[Thermal radiation|thermal emission]]. These thermal neutrinos are several times more abundant than the electron-capture neutrinos.<ref>
{{Cite book
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}}</ref> About 10<sup>46</sup> joules, 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="barwick">
{{Cite arXiv
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}}</ref> The suddenly halted core collapse rebounds and produces a shock wave that stalls in the outer core within milliseconds<ref>
{{Cite journal
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 }}</ref> as energy is lost through the dissociation of heavy elements. A process that is {{As of|2023|alt=not clearly understood}} is necessary to allow the outer layers of the core to reabsorb around 10<sup>44</sup> joules<ref name="barwick"/> (1 [[Foe (unit)|foe]]) from the [[Supernova neutrinos|neutrino pulse]], producing the visible brightness, although there are other theories that could power the explosion.<ref name="WoosleyJanka"/>

Some material from the outer envelope falls back onto the neutron star, and, for cores beyond about {{Solar mass|8}}, there is sufficient fallback to form a black hole. This fallback will reduce the kinetic energy created and the mass of expelled radioactive material, but in some situations, it may also generate [[relativistic jet]]s that result in a [[gamma-ray burst]] or an exceptionally luminous supernova.<ref name=piram2019>{{cite journal |bibcode=2019ApJ...871L..25P |title=Relativistic Jets in Core-collapse Supernovae |last1=Piran |first1=Tsvi |last2=Nakar |first2=Ehud |last3=Mazzali |first3=Paolo |last4=Pian |first4=Elena |journal=The Astrophysical Journal |year=2019 |volume=871 |issue=2 |pages=L25 |doi=10.3847/2041-8213/aaffce |s2cid=19266567 |doi-access=free |arxiv=1704.08298 }}</ref>

The collapse of a massive non-degenerate core will ignite further fusion.<ref name="Hurley-2000"/> When the core collapse is initiated by [[Pair-instability supernova|pair instability]] ([[photon]]s turning into [[electron]]-[[positron]] pairs, thereby reducing the radiation pressure) oxygen fusion begins and the collapse may be halted. For core masses of {{Solar mass|40–60}}, the collapse halts and the star remains intact, but collapse will occur again when a larger core has formed. For cores of around {{Solar mass|60–130}}, the fusion of oxygen and heavier elements is so energetic that the entire star is disrupted, causing a supernova. At the upper end of the mass range, the supernova is unusually luminous and extremely long-lived due to many solar masses of ejected <sup>56</sup>Ni. For even larger core masses, the core temperature becomes high enough to allow photodisintegration and the core collapses completely into a black hole.<ref name="kasen">
{{Cite journal
 |last1=Kasen |first1=D.
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 |last3=Heger |first3=A.
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====Type II====
{{Main|Type II supernova}}
[[Image:SN 1997D.jpg|thumb|The atypical subluminous type II [[SN 1997D]]]]
Stars with initial masses less than about {{Solar mass|8}} never develop a core large enough to collapse and they eventually lose their atmospheres to become white dwarfs. Stars with at least {{Solar mass|9}} (possibly as much as {{Solar mass|12}}<ref name="superagb">
{{Cite journal
 |last1=Poelarends |first1=A. J. T.
 |last2=Herwig |first2=F.
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 |last4=Heger |first4=A.
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 }}</ref>) evolve in a complex fashion, progressively burning heavier elements at hotter temperatures in their cores.<ref name="WoosleyJanka">
{{Cite journal
 |last1=Woosley |first1=S. E.
 |last2=Janka |first2=H.-T.
 |date=2005
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 |volume=1 |issue=3 |pages=147–154
 |arxiv=astro-ph/0601261
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{{cite journal
 |last1=Gilmore |first1=G.
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 |volume=304 |issue=5679 |pages=1915–1916
 |doi=10.1126/science.1100370
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 }}</ref> The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells.<ref name=heger/><ref name="hinshaw">
{{cite book
 |last1=Faure |first1=G.
 |last2=Mensing |first2=T. M.
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 |title=Introduction to Planetary Science
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 |doi=10.1007/978-1-4020-5544-7_4
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}}</ref> Although popularly described as an onion with an iron core, the least massive supernova progenitors only have oxygen-[[neon]](-[[magnesium]]) cores. These [[super-AGB star]]s may form the majority of core collapse supernovae, although less luminous and so less commonly observed than those from more massive progenitors.<ref name="superagb"/>

If core collapse occurs during a supergiant phase when the star still has a hydrogen envelope, the result is a type II supernova.<ref name=Horiuchi_et_al_2014>{{cite journal |bibcode=2014MNRAS.445L..99H |arxiv=1409.0006 |doi=10.1093/mnrasl/slu146|title=The red supergiant and supernova rate problems: Implications for core-collapse supernova physics |year=2014 |last1=Horiuchi |first1=S. |last2=Nakamura |first2=K. |last3=Takiwaki |first3=T. |last4=Kotake |first4=K. |last5=Tanaka |first5=M. |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=445 |pages=L99–L103 |doi-access=free }}</ref> The rate of mass loss for luminous stars depends on the metallicity and [[luminosity]]. Extremely luminous stars at near solar metallicity will lose all their hydrogen before they reach core collapse and so will not form a supernova of type II.<ref name=Horiuchi_et_al_2014/> At low metallicity, all stars will reach core collapse with a hydrogen envelope but sufficiently massive stars collapse directly to a black hole without producing a visible supernova.<ref name="heger"/>

Stars with an initial mass up to about 90 times the Sun, or a little less at high metallicity, result in a type II-P supernova, which is the most commonly observed type. At moderate to high metallicity, stars near the upper end of that mass range will have lost most of their hydrogen when core collapse occurs and the result will be a type II-L supernova.<ref>{{cite journal |doi=10.1093/mnras/stu1760 |bibcode=2014MNRAS.445..554F |arxiv=1409.1536 |title=A sample of Type II-L supernovae |year=2014 |last1=Faran |first1=T. |last2=Poznanski |first2=D. |last3=Filippenko |first3=A. V. |last4=Chornock |first4=R. |last5=Foley |first5=R. J. |last6=Ganeshalingam |first6=M. |last7=Leonard |first7=D. C. |last8=Li |first8=W. |last9=Modjaz |first9=M. |last10=Serduke |first10=F. J. D. |last11=Silverman |first11=J. M. |journal=Monthly Notices of the Royal Astronomical Society |volume=445 |issue=1 |pages=554–569 |doi-access=free }}</ref> At very low metallicity, stars of around {{Solar mass|140–250}} will reach core collapse by pair instability while they still have a hydrogen atmosphere and an oxygen core and the result will be a supernova with type II characteristics but a very large mass of ejected <sup>56</sup>Ni and high luminosity.<ref name="heger"/><ref>{{cite journal|title=Mind the Gap: The Location of the Lower Edge of the Pair-instability Supernova Black Hole Mass Gap |first1=R. |last1=Farmer |first2=M. |last2=Renzo |first3=S. E. |last3=de Mink |first4=P. |last4=Marchant |first5=S. |last5=Justham |doi=10.3847/1538-4357/ab518b |bibcode=2019ApJ...887...53F |arxiv=1910.12874 |journal=The Astrophysical Journal |year=2019 |volume=887 |number=1 |page=53 |doi-access=free }}</ref>

====Type Ib and Ic====
{{Main|Type Ib and Ic supernovae}}
[[Image:Supernova 2008D.jpg|thumb|upright=1.2|Type Ib SN 2008D<ref>
{{cite journal
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 |year=2009
 |title=Early Spectroscopic Identification of SN 2008D
 |journal=[[The Astrophysical Journal Letters]]
 |volume=692 |issue=2 |pages=L84
 |arxiv=0805.1188
 |bibcode=2009ApJ...692L..84M
 |doi=10.1088/0004-637X/692/2/L84
|s2cid=1435322
 }}</ref> at the far upper end of the galaxy, shown in [[X-ray]] (left) and visible light (right),<ref>
{{cite journal
 |last1=Svirski |first1=G.
 |last2=Nakar |first2=E.
 |year=2014
 |title=Sn 2008D: A Wolf-Rayet Explosion Through a Thick Wind
 |journal=[[The Astrophysical Journal]]
 |volume=788 |issue=1|pages=L14
 |arxiv=1403.3400
 |bibcode=2014ApJ...788L..14S
 |doi=10.1088/2041-8205/788/1/L14
|s2cid=118395580
 }}</ref> with the brighter SN 2007uy closer to the centre]]

These supernovae, like those of type II, are massive stars that undergo core collapse. Unlike the progenitors of type II supernovae, the stars which become types Ib and Ic supernovae have lost most of their outer (hydrogen) envelopes due to strong [[stellar wind]]s or else from interaction with a companion.<ref>
{{cite conference
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 |title=Close Binary Progenitors of Type Ib/Ic and IIb/II-L Supernovae
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 |book-title=Proceedings of the Third Pacific Rim Conference on Recent Development on Binary Star Research
 |volume=130 |pages=153–158
 |series=[[ASP Conference Series]]
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}}</ref> These stars are known as [[Wolf–Rayet stars]], and they occur at moderate to high metallicity where continuum driven winds cause sufficiently high mass-loss rates. Observations of type Ib/c supernova do not match the observed or expected occurrence of Wolf–Rayet stars. Alternate explanations for this type of core collapse supernova involve stars stripped of their hydrogen by binary interactions. Binary models provide a better match for the observed supernovae, with the proviso that no suitable binary helium stars have ever been observed.<ref name="eldridge">
{{cite journal
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Type Ib supernovae are the more common and result from Wolf–Rayet stars of [[Wolf-Rayet star#Classification|type WC]] which still have helium in their atmospheres. For a narrow range of masses, stars evolve further before reaching core collapse to become [[Wolf-Rayet star#Classification|WO stars]] with very little helium remaining, and these are the progenitors of type Ic supernovae.<ref>{{cite journal |doi=10.1093/mnras/stx1496|title=Towards a better understanding of the evolution of Wolf–Rayet stars and Type Ib/Ic supernova progenitors |year=2017 |last1=Yoon |first1=Sung-Chul |journal=Monthly Notices of the Royal Astronomical Society |volume=470 |issue=4 |pages=3970–3980 |doi-access=free |arxiv=1706.04716 |bibcode=2017MNRAS.470.3970Y }}</ref>

A few percent of the type Ic supernovae are associated with [[gamma-ray burst]]s (GRB), though it is also believed that any hydrogen-stripped type Ib or Ic supernova could produce a GRB, depending on the circumstances of the geometry.<ref>
{{Cite journal
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 |last6=Stockdale |first6=C. J.
 |date=2004
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 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=349 |issue=3 |pages=1093–1100
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|doi-access=free
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 }}</ref> The mechanism for producing this type of GRB is the jets produced by the magnetic field of the rapidly spinning [[magnetar]] formed at the collapsing core of the star. The jets would also transfer energy into the expanding outer shell, producing a [[super-luminous supernova]].<ref name=piram2019/><ref>
{{cite journal
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Ultra-stripped supernovae occur when the exploding star has been stripped (almost) all the way to the metal core, via mass transfer in a close binary.<ref>
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In 2022 a team of astronomers led by researchers from the Weizmann Institute of Science reported the first supernova explosion showing direct evidence for a Wolf-Rayet progenitor star. [[SN 2019hgp]] was a type Icn supernova and is also the first in which the element neon has been detected.<ref>{{cite journal |doi=10.1038/s41586-021-04155-1 |title=A WC/WO star exploding within an expanding carbon–oxygen–neon nebula |year=2022 |last1=Gal-Yam |first1=A. |last2=Bruch |first2=R. |last3=Schulze |first3=S. |last4=Yang |first4=Y. |last5=Perley |first5=D. A. |last6=Irani |first6=I. |last7=Sollerman |first7=J. |last8=Kool |first8=E. C. |last9=Soumagnac |first9=M. T. |last10=Yaron |first10=O. |last11=Strotjohann |first11=N. L. |last12=Zimmerman |first12=E. |last13=Barbarino |first13=C. |last14=Kulkarni |first14=S. R. |last15=Kasliwal |first15=M. M. |last16=De |first16=K. |last17=Yao |first17=Y. |last18=Fremling |first18=C. |last19=Yan |first19=L. |last20=Ofek |first20=E. O. |last21=Fransson |first21=C. |last22=Filippenko |first22=A. V. |last23=Zheng |first23=W. |last24=Brink |first24=T. G. |last25=Copperwheat |first25=C. M. |last26=Foley |first26=R. J. |last27=Brown |first27=J. |last28=Siebert |first28=M. |last29=Leloudas |first29=G. |last30=Cabrera-Lavers |first30=A. L. |journal=Nature |volume=601 |issue=7892 |pages=201–204 |pmid=35022591 |arxiv=2111.12435 |bibcode=2022Natur.601..201G |s2cid=244527654 }}</ref><ref>{{Cite web|title=Astronomers discover first supernova explosion of a Wolf-Rayet star|url=https://www.iac.es/en/outreach/news/astronomers-discover-first-supernova-explosion-wolf-rayet-star|access-date=9 February 2022|website=Instituto de Astrofísica de Canarias • IAC|date=12 January 2022 |language=en}}</ref>

====<span class="anchor" id="Electron-capture supernovae"></span> Electron-capture supernovae ====
In 1980, a "third type" of supernova was predicted by [[Ken'ichi Nomoto]] of the [[University of Tokyo]], called an electron-capture supernova. It would arise when a star "in the transitional range (~8 to 10 solar masses) between white dwarf formation and iron core-collapse supernovae", and with a [[degenerate matter|degenerate]] O+Ne+Mg core,<ref name=HowellEtAl2021-01a/> imploded after its core ran out of nuclear fuel, causing gravity to compress the electrons in the star's core into their atomic nuclei,<ref name=KeckObsNews2021-06-28-01a/><ref name=RTE2021-06-28-01a/> leading to a supernova explosion and leaving behind a neutron star.<ref name="heger"/> In June 2021, a paper in the journal ''[[Nature Astronomy]]'' reported that the 2018 supernova [[SN 2018zd]] (in the galaxy [[NGC 2146]], about 31 million light-years from Earth) appeared to be the first observation of an electron-capture supernova.<ref name=HowellEtAl2021-01a>{{cite journal |author1=Hiramatsu D |author2=Howell D |author3=Van S |display-authors=etal |date=28 June 2021 |title=The electron-capture origin of supernova 2018zd |url=https://www.nature.com/articles/s41550-021-01384-2 |journal=Nat Astron |volume=5 |issue=9 |pages=903–910 |doi=10.1038/s41550-021-01384-2 |arxiv=2011.02176 |bibcode=2021NatAs...5..903H |s2cid=226246044 |access-date=1 July 2021 |archive-date=30 June 2021 |archive-url=https://web.archive.org/web/20210630023436/https://www.nature.com/articles/s41550-021-01384-2 |url-status=live }}</ref><ref name=KeckObsNews2021-06-28-01a>{{cite web |url=https://keckobservatory.org/electron-capture-supernova |title=New, Third Type Of Supernova Observed |date=28 June 2021 |website=[[W. M. Keck Observatory]] |access-date=1 July 2021 |archive-date=29 June 2021 |archive-url=https://web.archive.org/web/20210629155228/https://www.keckobservatory.org/electron-capture-supernova |url-status=live }}</ref><ref name=RTE2021-06-28-01a>{{cite news|url=https://www.rte.ie/news/newslens/2021/0628/1231824-supernova-crab-nebula/|title=Astronomers discover new type of supernova|publisher=[[RTE News]]|author=|agency=[[Press Association|PA]]|date=28 June 2021|access-date=1 July 2021|quote=In 1980, Ken'ichi Nomoto of the University of Tokyo predicted a third type called an electron capture supernova. ... In an electron capture supernova, as the core runs out of fuel, gravity forces electrons in the core into their atomic nuclei, causing the star to collapse in on itself.|archive-date=30 June 2021|archive-url=https://web.archive.org/web/20210630165238/https://www.rte.ie/news/newslens/2021/0628/1231824-supernova-crab-nebula/|url-status=live}}</ref> The 1054 supernova explosion that created the Crab Nebula in our galaxy had been thought to be the best candidate for an electron-capture supernova, and the 2021 paper makes it more likely that this was correct.<ref name=KeckObsNews2021-06-28-01a/><ref name=RTE2021-06-28-01a/>