Editing Compact object (section)

==Neutron stars==
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[[Image:V838 Mon HST.jpg|160px|right|thumb|Formerly a white dwarf, [[V838 Monocerotis]] has [[accretion theory|accreted]] enough material to become a [[red supergiant]].]]
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{{Main|Neutron star}}
[[Image:Chandra-crab.jpg|upright|thumb|The [[Crab Nebula]] is a [[supernova remnant]] containing the [[Crab Pulsar]], a [[neutron star]].]]
In certain [[binary stars]] containing a white dwarf, mass is transferred from the companion star onto the white dwarf, eventually pushing it over the [[Chandrasekhar limit]]. Electrons react with protons to form neutrons and thus no longer supply the necessary pressure to resist gravity, causing the star to collapse. If the center of the star is composed mostly of carbon and oxygen then such a [[gravitational collapse]] will ignite runaway fusion of the carbon and oxygen, resulting in a [[Type Ia supernova]] that entirely blows apart the star before the collapse can become irreversible. If the center is composed mostly of magnesium or heavier elements, the collapse continues.<ref>
{{cite journal
|last1=Hashimoto |first1=M.
|last2=Iwamoto |first2=K.
|last3=Nomoto |first3=K.
|date=1993
|title=Type II supernovae from 8–10 solar mass asymptotic giant branch stars
|journal=[[The Astrophysical Journal]]
|volume=414 |pages=L105
|bibcode=1993ApJ...414L.105H
|doi=10.1086/187007
|doi-access=free
}}</ref><ref>
{{cite journal
|last1=Ritossa |first1=C.
|last2=Garcia-Berro |first2=E.
|last3=Iben |first3=I. Jr.
|date=1996
|title=On the Evolution of Stars That Form Electron-degenerate Cores Processed by Carbon Burning. II. Isotope Abundances and Thermal Pulses in a 10 M<sub>sun</sub> Model with an ONe Core and Applications to Long-Period Variables, Classical Novae, and Accretion-induced Collapse
|journal=[[The Astrophysical Journal]]
|volume=460 |pages=489
|bibcode=1996ApJ...460..489R
|doi=10.1086/176987
|doi-access=free
}}</ref><ref>
{{cite journal
|last1=Wanajo |first1=S.
|date=2003
|title=The r-Process in Supernova Explosions from the Collapse of O-Ne-Mg Cores
|journal=[[The Astrophysical Journal]]
|volume=593 |issue=2 |pages=968–979
|arxiv=astro-ph/0302262
|bibcode=2003ApJ...593..968W
|doi=10.1086/376617
|s2cid=13456130
|display-authors=etal}}</ref> As the density further increases, the remaining electrons react with the protons to form more neutrons. The collapse continues until (at higher density) the neutrons become degenerate. A new equilibrium is possible after the star shrinks by three [[orders of magnitude]], to a radius between 10 and 20&nbsp;km. This is a ''neutron star''.

Although the first neutron star was not observed until 1967 when the first radio [[pulsar]] was discovered, neutron stars were proposed by Baade and Zwicky in 1933, only one year after the neutron was discovered in 1932. They realized that because neutron stars are so dense, the collapse of an ordinary star to a neutron star would liberate a large amount of [[gravitational energy|gravitational potential energy]], providing a possible explanation for [[supernova]]e.<ref>{{cite journal
|last1=Osterbrock |first1=D. E.
|date=2001
|title=Who Really Coined the Word Supernova? Who First Predicted Neutron Stars?
|journal=[[Bulletin of the American Astronomical Society]]
|volume=33 |pages=1330
|bibcode=2001AAS...199.1501O
}}</ref><ref>
{{cite journal
|last1=Baade |first1=W.
|last2=Zwicky |first2=F.
|date=1934
|title=On Super-Novae
|journal=[[Proceedings of the National Academy of Sciences]]
|volume=20 |issue=5 |pages=254–9
|bibcode=1934PNAS...20..254B
|doi=10.1073/pnas.20.5.254 |pmid=16587881 |pmc=1076395
|doi-access=free
}}</ref><ref>
{{cite journal
|last1=Baade |first1=W.
|last2=Zwicky |first2=F.
|date=1934
|title=Cosmic Rays from Super-Novae
|journal=[[Proceedings of the National Academy of Sciences]]
|volume=20 |issue=5 |pages=259–263
|bibcode=1934PNAS...20..259B
|doi=10.1073/pnas.20.5.259
|pmid=16587882
|pmc=1076396
|doi-access=free
}}</ref> This is the explanation for supernovae of types [[Type Ib and Ic supernovae|Ib, Ic]], and [[Supernova#Type II|II]]. Such supernovae occur when the iron core of a massive star exceeds the Chandrasekhar limit and collapses to a neutron star.

Like electrons, neutrons are [[fermions]]. They therefore provide [[neutron degeneracy pressure]] to support a neutron star against collapse. In addition, repulsive neutron-neutron interactions{{Citation needed|date=July 2008}} provide additional pressure. Like the Chandrasekhar limit for white dwarfs, there is a limiting mass for neutron stars: the [[Tolman–Oppenheimer–Volkoff limit]], where these forces are no longer sufficient to hold up the star. As the forces in dense hadronic matter are not well understood, this limit is not known exactly but is thought to be between 2 and {{Solar mass|3}}. If more mass accretes onto a neutron star, eventually this mass limit will be reached. What happens next is not completely clear.