Editing Stellar evolution (section)

====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]].