Editing Crab Nebula (section)

==Progenitor star==
[[File:Changes in the Crab Nebula.jpg|thumb|left|This sequence of [[Hubble Space Telescope|Hubble]] images shows features in the inner Crab Nebula changing over a period of four months.]]

The star that exploded as a supernova is referred to as the supernova's ''progenitor star''. Two types of stars explode as supernovae: [[white dwarf]]s and [[massive stars]]. In the so-called [[Type Ia supernova]]e, gases falling onto a 'dead' white dwarf raise its mass until it nears a critical level, the [[Chandrasekhar limit]], resulting in a [[thermal runaway|runaway nuclear fusion explosion]] that obliterates the star; in [[Type Ib and Ic supernovae|Type Ib/c]] and [[Type II supernova|Type II]] supernovae, the progenitor star is a massive star whose core runs out of fuel to power its [[nuclear fusion]] reactions and collapses in on itself, releasing [[gravitational potential energy]] in a form that blows away the star's outer layers. Type Ia supernovae do not produce pulsars,<ref name="Pasachoff">{{cite book|last1=Pasachoff|first1=Jay M.|url=https://books.google.com/books?id=yQMlAAAAQBAJ&pg=PA357|title=The Cosmos: Astronomy in the New Millennium|last2=Filippenko|first2=Alex|date=August 2013|publisher=Cambridge University Press|isbn=978-1-107-27695-6|page=357|name-list-style=amp}}</ref> so the pulsar in the Crab Nebula shows it must have formed in a core-collapse supernova.<ref name=Maoz>{{cite book |last1=Maoz |first1=Dan |title=Astrophysics in a Nutshell |publisher=Princeton University Press |page=90 |date=December 2011 |isbn=978-1-4008-3934-6 |url=https://books.google.com/books?id=yWidGMhCNX8C&pg=PA90}}</ref>

Theoretical models of supernova explosions suggest that the star that exploded to produce the Crab Nebula must have had a [[mass]] of between {{solar mass|9 and 11|link=y}}.<ref name="MacAlpineetal2007">{{cite journal |last1=MacAlpine |first1=Gordon M. |last2=Ecklund |first2=Tait C. |last3=Lester |first3=William R. |last4=Vanderveer |first4=Steven J. |last5=Strolger |first5=Louis-Gregory |display-authors=1 |title=A Spectroscopic Study of Nuclear Processing and the Production of Anomalously Strong Lines in the Crab Nebula |journal=[[The Astronomical Journal]] |volume=133 |issue=1 |pages=81–88 |date=2007 |bibcode=2007AJ....133...81M |doi=10.1086/509504 |arxiv=astro-ph/0609803|s2cid=18599459 }}</ref><ref name="Nomoto1985">{{cite conference |last1=Nomoto |first1=K. |chapter=Evolutionary models of the Crab Nebula's progenitor |title=The Crab Nebula and Related Supernova Remnants: Proceedings of a Workshop Held at George Mason University, Fairfax, Virginia, October 11–12, 1984 |journal=The Crab Nebula and Related Supernova Remnants |publisher=[[Cambridge University Press]] |pages=97–113 |date=January 1985 |bibcode=1985cnrs.work...97N |isbn=0-521-30530-6}}</ref> Stars with masses lower than {{solar mass|8}} are thought to be too small to produce supernova explosions, and end their lives by producing a [[planetary nebula]] instead, while a star heavier than {{solar mass|12}} would have produced a nebula with a different chemical composition from that observed in the Crab Nebula.<ref name="Davidsonetal1985">{{cite journal |last1=Davidson |first1=K. |last2=Fesen |first2=R. A. |name-list-style=amp |title=Recent developments concerning the Crab Nebula |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=23 |issue=507 |pages=119–146 |date=1985 |bibcode=1985ARA&A..23..119D |doi=10.1146/annurev.aa.23.090185.001003}}</ref> Recent studies, however, suggest the progenitor could have been a [[Asymptotic giant branch#Super-AGB stars|super-asymptotic giant branch]] star in the {{solar mass|8 to 10}} range that would have exploded in an [[Supernova#Electron-capture_supernovae|electron-capture supernova]].<ref name="Tominaga2013">{{cite journal |last1=Tominaga |first1=N. |last2=Blinnikov |first2=S. I. |last3=Nomoto |first3=Ken'Ichi |display-authors=1 |title=Supernova explosions of super-asymptotic giant branch stars: multicolor light curves of electron-capture supernovae |journal=[[The Astrophysical Journal Letters]] |volume=771 |issue=1 |page=L12 |bibcode=2013ApJ...771L..12T |doi=10.1088/2041-8205/771/1/L12 |arxiv=1305.6813 |year=2013|s2cid=118860608 }}</ref> 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 |vauthors=Hiramatsu D, Howell D, 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 }}</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]]}}</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]] |agency=[[Press Association|PA]]|date=28 June 2021|access-date=1 July 2021}}</ref> The 1054 supernova explosion that created the Crab Nebula 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/>

A significant problem in studies of the Crab Nebula is that the combined mass of the nebula and the pulsar add up to considerably less than the predicted mass of the progenitor star, and the question of where the 'missing mass' is, remains unresolved.<ref name="Fesenetal1997"/> Estimates of the mass of the nebula are made by measuring the total amount of light emitted, and calculating the mass required, given the measured temperature and density of the nebula. Estimates range from about {{solar mass|1–5}}, with {{solar mass|2–3}} being the generally accepted value.<ref name="Davidsonetal1985"/> The neutron star mass is estimated to be between {{solar mass|1.4 and 2}}.

The predominant theory to account for the missing mass of the Crab Nebula is that a substantial proportion of the mass of the progenitor was carried away before the supernova explosion in a fast [[stellar wind]], a phenomenon commonly seen in [[Wolf–Rayet star]]s. However, this would have created a shell around the nebula. Although attempts have been made at several wavelengths to observe a shell, none has yet been found.<ref>{{cite journal |last1=Frail |first1=D. A. |last2=Kassim |first2=N. E. |last3=Cornwell |first3=T. J. |last4=Goss |first4=W. M. |display-authors=1 |title=Does the Crab Have a Shell? |journal=[[The Astrophysical Journal Letters]] |volume=454 |issue=2 |pages=L129–L132 |date=1995 |doi=10.1086/309794 |bibcode=1995ApJ...454L.129F |arxiv=astro-ph/9509135|s2cid=14787898 }}</ref>