Editing Stellar evolution (section)

====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/>