Editing Stellar evolution (section)

===Massive stars===
{{Main|Supergiant}}
[[Image:VLTI reconstructed view of the surface of Antares.jpg|thumb|left|Reconstructed image of [[Antares]], a red supergiant]]
In massive stars, the core is already large enough at the onset of the hydrogen burning shell that helium ignition will occur before electron degeneracy pressure has a chance to become prevalent. Thus, when these stars expand and cool, they do not brighten as dramatically as lower-mass stars; however, they were more luminous on the main sequence and they evolve to highly luminous supergiants.  Their cores become massive enough that they cannot support themselves by [[electron degeneracy]] and will eventually collapse to produce a [[neutron star]] or [[black hole]].{{Citation needed|date=May 2021}}

====Supergiant evolution====
Extremely massive stars (more than approximately {{Solar mass|40}}), which are very luminous and thus have very rapid stellar winds, lose mass so rapidly due to radiation pressure that they tend to strip off their own envelopes before they can expand to become [[red supergiant]]s, and thus retain extremely high surface temperatures (and blue-white color) from their main-sequence time onwards. The largest stars of the current generation are about {{Solar mass|100-150}} because the outer layers would be expelled by the extreme radiation. Although lower-mass stars normally do not burn off their outer layers so rapidly, they can likewise avoid becoming red giants or red supergiants if they are in binary systems close enough so that the companion star strips off the envelope as it expands, or if they rotate rapidly enough so that convection extends all the way from the core to the surface, resulting in the absence of a separate core and envelope due to thorough mixing.<ref>{{cite journal |first1=D. |last1=Vanbeveren |title=Massive stars |journal=The Astronomy and Astrophysics Review |date=1998 |volume=9 |issue=1–2 |pages=63–152 |doi=10.1007/s001590050015 |last2=De Loore |first2=C. |last3=Van Rensbergen |first3=W. |bibcode=1998A&ARv...9...63V|s2cid=189933559 }}</ref>

[[Image:Evolved star fusion shells.svg|right|thumb|The onion-like layers of a massive, evolved star just before core collapse (not to scale)]]
The core of a massive star, defined as the region depleted of hydrogen, grows hotter and denser as it accretes material from the fusion of hydrogen outside the core.  In sufficiently massive stars, the core reaches temperatures and densities high enough to fuse carbon and heavier elements via the [[alpha process]].  At the end of helium fusion, the core of a star consists primarily of carbon and oxygen.  In stars heavier than about {{solar mass|8}}, the carbon ignites and [[Carbon-burning process|fuses]] to form neon, sodium, and magnesium.  Stars somewhat less massive may partially ignite carbon, but they are unable to fully fuse the carbon before [[electron degeneracy]] sets in, and these stars will eventually leave an oxygen-neon-magnesium [[white dwarf]].<ref name=jones>{{cite journal |doi=10.1088/0004-637X/772/2/150 |title=Advanced Burning Stages and Fate of 8-10M☉Stars |journal=The Astrophysical Journal |volume=772 |issue=2 |pages=150 |year=2013 |last1=Jones |first1=S. |last2=Hirschi |first2=R. |last3=Nomoto |first3=K. |last4=Fischer |first4=T. |last5=Timmes |first5=F. X. |last6=Herwig |first6=F. |last7=Paxton |first7=B. |last8=Toki |first8=H. |last9=Suzuki |first9=T. |last10=Martínez-Pinedo |first10=G. |last11=Lam |first11=Y. H. |last12=Bertolli |first12=M. G. |arxiv=1306.2030 |bibcode=2013ApJ...772..150J |s2cid=118687195 }}</ref><ref name=woosley>{{cite journal |doi=10.1103/RevModPhys.74.1015 |title=The evolution and explosion of massive stars |journal=Reviews of Modern Physics |volume=74 |issue=4 |pages=1015–1071 |year=2002 |last1=Woosley |first1=S. E. |last2=Heger |first2=A. |last3=Weaver |first3=T. A. |bibcode=2002RvMP...74.1015W |s2cid=55932331 }}</ref>

The exact mass limit for full carbon burning depends on several factors such as metallicity and the detailed mass lost on the [[asymptotic giant branch]], but is approximately {{solar mass|8-9}}.<ref name=jones/>  After carbon burning is complete, the core of these stars reaches about {{solar mass|2.5}} and becomes hot enough for heavier elements to fuse.  Before oxygen starts to [[Oxygen-burning process|fuse]], neon begins to [[Electron capture|capture electrons]] which triggers [[Neon-burning process|neon burning]].  For a range of stars of approximately {{solar mass|8-12}}, this process is unstable and creates runaway fusion resulting in an [[electron capture supernova]].<ref name=nomoto1987>{{cite journal |author=Ken'ichi Nomoto |title=Evolution of 8–10 {{Solar mass}} stars toward electron capture supernovae. II – Collapse of an O + Ne + Mg core |journal=Astrophysical Journal |date=1987 |volume=322 |pages=206–214 |bibcode=1987ApJ...322..206N |doi=10.1086/165716}}</ref><ref name=woosley/>

In more massive stars, the fusion of neon proceeds without a runaway deflagration.  This is followed in turn by complete oxygen burning and [[Silicon-burning process|silicon burning]], producing a core consisting largely of [[iron-peak element]]s.  Surrounding the core are shells of lighter elements still undergoing fusion.  The timescale for complete fusion of a carbon core to an iron core is so short, just a few hundred years, that the outer layers of the star are unable to react and the appearance of the star is largely unchanged.  The iron core grows until it reaches an ''effective Chandrasekhar mass'', higher than the formal [[Chandrasekhar mass]] due to various corrections for the relativistic effects, entropy, charge, and the surrounding envelope.  The effective Chandrasekhar mass for an iron core varies from about {{solar mass|1.34}} in the least massive red supergiants to more than {{solar mass|1.8}} in more massive stars.  Once this mass is reached, electrons begin to be captured into the iron-peak nuclei and the core becomes unable to support itself.  The core collapses and the star is destroyed, either in a [[supernova]] or direct collapse to a [[black hole]].<ref name=woosley/>

====Supernova====
{{Main|Supernova}}
[[Image:Crab Nebula.jpg|thumb|left|The [[Crab Nebula]], the shattered remnants of a star which exploded as a supernova visible in 1054 AD]]
When the core of a massive star collapses, it will form a [[neutron star]], or in the case of cores that exceed the [[Tolman–Oppenheimer–Volkoff limit]], a [[black hole]].  Through a process that is not completely understood, some of the [[gravitational potential energy]] released by this core collapse is converted into a Type Ib, Type Ic, or Type II [[supernova]]. It is known that the core collapse produces a massive surge of [[neutrino]]s, as observed with supernova [[SN 1987A]]. The extremely energetic [[neutrinos]] fragment some nuclei; some of their energy is consumed in releasing [[nucleons]], including [[neutrons]], and some of their energy is transformed into heat and [[kinetic energy]], thus augmenting the [[shock wave]] started by rebound of some of the infalling material from the collapse of the core. Electron capture in very dense parts of the infalling matter may produce additional neutrons. Because some of the rebounding matter is bombarded by the neutrons, some of its nuclei capture them, creating a spectrum of heavier-than-iron material including the radioactive elements up to (and likely beyond) [[uranium]].<ref>[http://www.mpa-garching.mpg.de/HIGHLIGHT/2001/highlight0102_e.html How do Massive Stars Explode?<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20030627124651/http://www.mpa-garching.mpg.de/HIGHLIGHT/2001/highlight0102_e.html |date=2003-06-27 }}</ref> Although non-exploding red giants can produce significant quantities of elements heavier than iron using neutrons released in side reactions of earlier [[nuclear reactions]], the abundance of elements heavier than [[iron]] (and in particular, of certain isotopes of elements that have multiple stable or long-lived isotopes) produced in such reactions is quite different from that produced in a supernova. Neither abundance alone matches that found in the [[Solar System]], so both supernovae, [[neutron star merger]]s<ref>{{cite web |last=Stromberg |first=Joseph |date=16 July 2013 |title=All the Gold in the Universe Could Come from the Collisions of Neutron Stars |url=http://www.smithsonianmag.com/science-nature/all-the-gold-in-the-universe-could-come-from-the-collisions-of-neutron-stars-13474145/?page=1 |work=[[Smithsonian (magazine)|Smithsonian]] |access-date=27 April 2014}}</ref> and ejection of elements from red giants are required to explain the observed abundance of heavy elements and [[isotopes]] thereof.

The energy transferred from collapse of the core to rebounding material not only generates heavy elements, but provides for their acceleration well beyond [[escape velocity]], thus causing a Type Ib, Type Ic, or Type II supernova. Current understanding of this energy transfer is still not satisfactory; although current computer models of Type Ib, Type Ic, and Type II supernovae account for part of the energy transfer, they are not able to account for enough energy transfer to produce the observed ejection of material.<ref>{{cite web|url=http://www.mpa-garching.mpg.de/HIGHLIGHT/2003/highlight0306_e.html|title=Supernova Simulations Still Defy Explosions|date=June 2003|author=Robert Buras|display-authors=etal|publisher=Max-Planck-Institut für Astrophysik|work=Research Highlights|url-status=dead|archive-url=https://web.archive.org/web/20030803015427/http://www.mpa-garching.mpg.de/HIGHLIGHT/2003/highlight0306_e.html|archive-date=2003-08-03}}</ref> However, neutrino oscillations may play an important role in the energy transfer problem as they not only affect the energy available in a particular flavour of neutrinos but also through other general-relativistic effects on neutrinos.<ref>{{cite journal|doi=10.1023/B:GERG.0000038633.96716.04|title=Addendum to: Gen. Rel. Grav. 28 (1996) 1161, First Prize Essay for 1996: Neutrino Oscillations and Supernovae|journal=General Relativity and Gravitation|volume=36|issue=9|pages=2183–2187|year=2004|last1=Ahluwalia-Khalilova|first1=D. V|bibcode=2004GReGr..36.2183A|arxiv=astro-ph/0404055|s2cid=1045277}}</ref><ref>{{cite journal|bibcode=2017PhRvD..96b3009Y|arxiv=1705.09723|title=GR effects in supernova neutrino flavor transformations|journal=Physical Review D|volume=96|issue=2|pages=023009|last1=Yang|first1=Yue|last2=Kneller|first2=James P|year=2017|doi=10.1103/PhysRevD.96.023009|s2cid=119190550}}
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Some evidence gained from analysis of the mass and orbital parameters of binary neutron stars (which require two such supernovae) hints that the collapse of an oxygen-neon-magnesium core may produce a supernova that differs observably (in ways other than size) from a supernova produced by the collapse of an iron core.<ref>{{cite journal | author=E. P. J. van den Heuvel | title=X-Ray Binaries and Their Descendants: Binary Radio Pulsars; Evidence for Three Classes of Neutron Stars? | journal=Proceedings of the 5th INTEGRAL Workshop on the INTEGRAL Universe (ESA SP-552) | volume=552 | date=2004 | pages=185–194 | bibcode=2004ESASP.552..185V |arxiv = astro-ph/0407451}}</ref>

The most massive stars that exist today may be completely destroyed by a supernova with an energy greatly exceeding its [[gravitational binding energy]]. This rare event, caused by [[pair-instability supernova|pair-instability]], leaves behind no black hole remnant.<ref name="Hammer">[http://www.mpa-garching.mpg.de/~hammer/lager/pair.pdf Pair Instability Supernovae and Hypernovae.], Nicolay J. Hammer, (2003), accessed May 7, 2007. {{webarchive |url=https://web.archive.org/web/20120608135141/http://www.mpa-garching.mpg.de/~hammer/lager/pair.pdf |date=June 8, 2012 }}</ref> In the past history of the universe, some stars were even larger than the largest that exists today, and they would immediately collapse into a black hole at the end of their lives, due to [[photodisintegration]].