Silicon-burning process

{{Short description|Very brief sequence of nuclear fusion reactions that occur in massive stars}} In astrophysics, '''silicon burning''' is a very brief<ref name="WoosleyJanka">{{cite journal | last1=Woosley | first1=S. | last2=Janka | first2=T. | title=The physics of core collapse supernovae | year=2006 | arxiv=astro-ph/0601261 | doi=10.1038/nphys172 | volume=1 | issue=3 | journal=Nature Physics | pages=147–154|bibcode = 2005NatPh...1..147W | citeseerx=10.1.1.336.2176 | s2cid=118974639 }}</ref> sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main sequence on the Hertzsprung–Russell diagram. It follows the previous stages of hydrogen, helium, carbon, neon and oxygen burning processes.

Silicon burning begins when gravitational contraction raises the star's core temperature to 2.7–3.5 billion kelvin (GK). The exact temperature depends on mass. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and may explode in what is known as a Type II supernova. The silicon-burning process is extremely brief: for a 25-solar mass star, the period lasts 5 days, compared to a prior 10 million years of hydrogen burning.<ref name="WoosleyJanka" /> Early stages of the silicon-burning process are theorized to occur in the accretion disk of stellar black holes, especially below 10 solar masses.<ref>{{Cite journal |last=Tang |first=Zifan |last2=Luo |first2=Yang |last3=Wang |first3=Jian-Min |date=2024-11-26 |title=Nuclear burning in an accretion flow around a stellar-mass black hole embedded within an AGN disc |url=https://academic.oup.com/mnras/article/535/4/3050/7899957 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=535 |issue=4 |pages=3050–3058 |doi=10.1093/mnras/stae2557 |issn=0035-8711|doi-access=free }}</ref>

==Nuclear fusion sequence and silicon photodisintegration==

After a star completes the oxygen-burning process, its core is composed primarily of silicon and sulfur.<ref name=Clayton>{{cite book|last=Clayton|first=Donald D.|title=Principles of Stellar Evolution and Nucleosynthesis|url=https://archive.org/details/principlesofstel0000clay|url-access=registration|year=1983|publisher=University of Chicago Press|isbn=9780226109534|pages=[https://archive.org/details/principlesofstel0000clay/page/519 519–524]}}</ref><ref>Woosley SE, Arnett WD, Clayton DD, "Hydrostatic oxygen burning in stars II. oxygen burning at balanced power", Astrophys. J. 175, 731 (1972)</ref> If it has sufficiently high mass, it further contracts until its core reaches temperatures in the range of 2.7–3.5 GK (230–300 keV). At these temperatures, silicon and other elements can photodisintegrate, emitting a proton or an alpha particle.<ref name=Clayton/> Silicon burning proceeds by photodisintegration rearrangement,<ref>Donald D. Clayton, ''Principles of stellar evolution and nucleosynthesis'', Chapter 7 (University of Chicago Press 1983)</ref> which creates new elements by the alpha process, adding one of these freed alpha particles<ref name=Clayton/> (the equivalent of a helium nucleus) per capture step in the following sequence (photoejection of alphas not shown):

:{| border="0" |- style="height:2em;" |{{nuclide|link=yes|silicon|28}} ||+ ||{{nuclide|link=yes|helium|4}} ||→ ||{{nuclide|link=yes|sulfur|32}} |- style="height:2em;" |{{nuclide|link=yes|sulfur|32}} ||+ ||{{nuclide|link=yes|helium|4}} ||→ ||{{nuclide|link=yes|argon|36}} |- style="height:2em;" |{{nuclide|link=yes|argon|36}} ||+ ||{{nuclide|link=yes|helium|4}} ||→ ||{{nuclide|link=yes|calcium|40}} |- style="height:2em;" |{{nuclide|link=yes|calcium|40}} ||+ ||{{nuclide|link=yes|helium|4}} ||→ ||{{nuclide|link=yes|titanium|44}} |- style="height:2em;" |{{nuclide|link=yes|titanium|44}} ||+ ||{{nuclide|link=yes|helium|4}} ||→ ||{{nuclide|link=yes|chromium|48}} |- style="height:2em;" |{{nuclide|link=yes|chromium|48}} ||+ ||{{nuclide|link=yes|helium|4}} ||→ ||{{nuclide|link=yes|iron|52}} |- style="height:2em;" |{{nuclide|link=yes|iron|52}} ||+ ||{{nuclide|link=yes|helium|4}} ||→ ||{{nuclide|link=yes|nickel|56}} |}

The chain could theoretically continue, as adding further alphas continues to be exothermic all the way to tin-100.{{AME2020 II|ref}} However, the steps after nickel-56 are much less exothermic and the temperature is so high that photodisintegration prevents further progress.<ref name="Clayton" />

The silicon-burning sequence lasts about one day before being struck by the shock wave that was launched by the core collapse. Burning then becomes much more rapid at the elevated temperature and stops only when the rearrangement chain has been converted to nickel-56 or is stopped by supernova ejection and cooling. The nickel-56 decays first to cobalt-56 and then to iron-56, with half-lives of 6 and 77 days respectively, but this happens later, because only minutes are available within the core of a massive star. The star has run out of nuclear fuel and within minutes its core begins to contract.<ref>{{cite journal |last1=Woosley |first1=S. E. |last2=Heger |first2=A. |last3=Weaver |first3=T. A. |title=The evolution and explosion of massive stars |journal=Reviews of Modern Physics |date=November 7, 2002 |volume=72 |url=https://scispace.com/pdf/the-evolution-and-explosion-of-massive-stars-57jc6k6s1i.pdf}}</ref>

During this phase of the contraction, the potential energy of gravitational contraction heats the interior to 5 GK (430 keV), which opposes and delays the contraction.<ref>{{cite journal |last1=Janka |first1=H.-Th. |last2=Marek |first2=A. |last3=Martinez-Pinedo |first3=G. |last4=Müller |first4=B. |title=Theory of core-collapse supernovae |journal=Physics Reports |date=December 4, 2006 |volume=442 |issue=1–6 |pages=38–74 |doi=10.1016/j.physrep.2007.02.002 |arxiv=astro-ph/0612072v1}}</ref> However, since no additional heat energy can be generated via new fusion reactions, the final unopposed contraction rapidly accelerates into a collapse lasting only a few seconds.<ref name="collapse" /> The central portion of the star is now crushed into a neutron core with the temperature soaring further to 100 GK (8.6 MeV)<ref>{{cite book | last=Mann | first=Alfred K. | title=Shadow of a star: The neutrino story of Supernova 1987A | publisher=W. H. Freeman | date=1997 | location=New York | page=122 | url=http://www.whfreeman.com/GeneralReaders/book.asp?disc=TRAD&id_product=1058001008&@id_course=1058000240 | isbn=978-0-7167-3097-2 | access-date=2007-11-19 | archive-date=2008-05-05 | archive-url=https://web.archive.org/web/20080505003516/http://www.whfreeman.com/GeneralReaders/book.asp?disc=TRAD&id_product=1058001008&@id_course=1058000240 | url-status=dead }}</ref> that quickly cools down<ref>{{cite journal |first=I. |last=Bombaci |date=1996 |title=The Maximum Mass of a Neutron Star |journal=Astronomy and Astrophysics |volume=305 | pages=871–877 |bibcode=1996A&A...305..871B}}</ref> into a neutron star if the mass of the star is below {{Solar mass|20}}.<ref name="collapse"> {{cite web |last1 = Fryer |first1 = C. L. |last2 = New |first2 = K. C. B. |date = 2006-01-24 |url = http://relativity.livingreviews.org/Articles/lrr-2003-2/ |title = Gravitational Waves from Gravitational Collapse |publisher = Max Planck Institute for Gravitational Physics |access-date = 2006-12-14 |url-status = dead |archive-url = https://web.archive.org/web/20061213120144/http://relativity.livingreviews.org/Articles/lrr-2003-2/ |archive-date = 2006-12-13 }} </ref> Between {{Solar mass|20}} and {{Solar mass|40–50}}, fallback of the material will make the neutron core collapse further into a black hole.<ref>{{cite journal | last=Fryer | first=Chris L. | title=Black Hole Formation from Stellar Collapse | journal=Classical and Quantum Gravity | date=2003 | volume=20 | issue=10 | pages=S73–S80 | bibcode=2003CQGra..20S..73F | doi=10.1088/0264-9381/20/10/309 | s2cid=122297043 | url=https://zenodo.org/record/1235744 | access-date=2019-11-29 | archive-date=2020-10-31 | archive-url=https://web.archive.org/web/20201031082511/https://zenodo.org/record/1235744 | url-status=live }}</ref> The outer layers of the star are blown off in an explosion known as a Type II supernova that lasts days to months. The supernova explosion releases a large burst of neutrons, which may synthesize in about one second roughly half of the supply of elements in the universe that are heavier than iron, via a rapid neutron-capture sequence known as the ''r''-process (where the "r" stands for "rapid" neutron capture).<ref> {{Cite journal |last1=Qian |first1=Y.-Z. |last2=Vogel |first2=P. |last3=Wasserburg |first3=G. J. |date=1998 |title=Diverse Supernova Sources for the r-Process |journal=Astrophysical Journal |volume=494 |issue=1 |pages=285–296 |bibcode=1998ApJ...494..285Q |doi=10.1086/305198 |arxiv=astro-ph/9706120|s2cid=15967473 }}</ref>

==See also== * Alpha nuclide * Alpha process * Stellar evolution * Supernova nucleosynthesis * Neutron capture: ** p-process ** ''r''-process ** ''s''-process

==References== {{reflist}}

==External links== * [http://www.umich.edu/~gs265/star.htm ''Stellar Evolution: The Life and Death of Our Luminous Neighbors,'' by Arthur Holland and Mark Williams of the University of Michigan] * [https://web.archive.org/web/20110814005149/http://cosserv3.fau.edu/~cis/AST2002/Lectures/C13/Trans/Trans.html ''The Evolution and Death of Stars,'' by Ian Short] * ''[https://web.archive.org/web/20110926210917/http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_stel_6.html Origin of Heavy Elements],'' by [https://www.tufts.edu Tufts University] * ''[http://schools.qps.org/hermanga/images/Astronomy/chapter_21___stellar_explosions.htm Chapter 21: Stellar Explosions],'' by G. Hermann *Arnett, W. D., [http://adsabs.harvard.edu/full/1977ApJS...35..145A Advanced evolution of massive stars. VII – Silicon burning] / Astrophysical Journal Supplement Series, vol. 35, Oct. 1977, p. 145–159. * {{cite journal|last1=Hix|first1=W. Raphael|last2=Thielemann|first2=Friedrich-Karl|title=Silicon Burning. I. Neutronization and the Physics of Quasi-Equilibrium|journal=The Astrophysical Journal|date=1 April 1996|volume=460|pages=869|bibcode=1996ApJ...460..869H|access-date=29 July 2015|arxiv=astro-ph/9511088v1|url=http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996ApJ...460..869H&link_type=ARTICLE&db_key=AST&high=|doi=10.1086/177016|s2cid=119422051 }}

Category:Nucleosynthesis