Silicon burning process 217726 224328182 2008-07-08T10:48:37Z SkyLined 6429027 /* Nuclear fusion sequence and the alpha process */ Added formulae for reactions In [[astrophysics]], '''[[silicon]] burning''' is a two week<ref name="WoosleyJanka">[http://arxiv.org/pdf/astro-ph/0601261 The physics of core collapse supernovae, Woosley and Janka]</ref> sequence of [[nuclear fusion]] reactions that occur in massive [[star]]s with a minimum of about 8–11 solar masses. Silicon burning is an end-of-life process for stars that have run out of the fuels that power them for the long periods while they are the ''main sequence'' on the [[Hertzsprung-Russell diagram]]. Silicon burning begins when gravitational contraction raises the star’s core to a temperature of 2.7–3.5 billion kelvin ([[Kelvin#SI_prefixed_forms_of_kelvin|GK]]). The exact temperature depends on mass. When a star has completed the silicon-burning phase, it can explode in what is known as a [[Type II supernova|Type II]] [[supernova]]. ==Nuclear fusion sequence and the alpha process== Stars with normal mass (no greater than about three solar masses) run out of fuel after their [[hydrogen]] has been consumed and fused into [[helium]]. If the star has intermediate mass (greater than three solar masses but less than about eight) the star can “burn” (fuse) helium into [[carbon]]. These stars end their lives when their helium has been exhausted and they have a carbon core. High mass stars (>8–11 solar masses) are able to burn carbon because of the extraordinarily high gravitational [[potential energy]] bound in their mass. As a massive star contracts, its core heats up to 600 [[Kelvin#SI_prefixed_forms_of_kelvin|MK]] and carbon burning begins which creates new [[Chemical element|elements]] as follows: <!-- Autogenerated using Phykiformulae 0.12 [[User:SkyLined#Phykiformulae]] C-11 + He-4 -> O-16 O-16 + He-4 -> Ne-20 Ne-20 + He-4 -> Mg-24 -->:{| border="0" |- style="height:2em;" |{{Nuclide|Link|carbon|11}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|oxygen|16}} |- style="height:2em;" |{{Nuclide|Link|oxygen|16}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|neon|20}} |- style="height:2em;" |{{Nuclide|Link|neon|20}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|magnesium|24}} |} The chemical elements are defined by the number of [[protons]] in their nucleus. In the elements listed above, the suffix denotes a particular [[isotope]] (form of a chemical element having a different number of [[neutron]]s) in terms of its [[molar mass]]. After a high-mass star has burned all its carbon, it contracts, gets hotter, and begins burning the oxygen, neon, and magnesium as follows: <!-- Autogenerated using Phykiformulae 0.12 [[User:SkyLined#Phykiformulae]] Mg-24 + He-4 -> Si-28 Si-28 + He-4 -> S-32 -->:{| border="0" |- style="height:2em;" |{{Nuclide|Link|magnesium|24}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|silicon|28}} |- style="height:2em;" |{{Nuclide|Link|silicon|28}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|sulfur|32}} |} After high-mass stars have nothing but sulfur and silicon in their cores, they further contract until their cores reach in the range of 2.7–3.5 GK; silicon burning starts at this point. Silicon burning entails the ''[[alpha process]]'' which creates new elements by adding the equivalent of one helium nucleus (two protons plus two neutrons) per step in the following sequence: <!-- Autogenerated using Phykiformulae 0.12 [[User:SkyLined#Phykiformulae]] S-32 + He-4 -> Ar-36 Ar-36 + He-4 -> Ca-40 Ca-40 + He-4 -> Ti-44 Ti-44 + He-4 -> Cr-48 Cr-48 + He-4 -> Fe-52 Fe-52 + He-4 -> Ni-56 -->:{| border="0" |- style="height:2em;" |{{Nuclide|Link|sulfur|32}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|argon|36}} |- style="height:2em;" |{{Nuclide|Link|argon|36}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|calcium|40}} |- style="height:2em;" |{{Nuclide|Link|calcium|40}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|titanium|44}} |- style="height:2em;" |{{Nuclide|Link|titanium|44}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|chromium|48}} |- style="height:2em;" |{{Nuclide|Link|chromium|48}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|iron|52}} |- style="height:2em;" |{{Nuclide|Link|iron|52}}&nbsp;||+&nbsp;||{{Nuclide|Link|helium|4}}&nbsp;||&rarr;&nbsp;||{{Nuclide|Link|nickel|56}} |} The entire silicon-burning sequence lasts about one day and stops when nickel–56 has been produced. Nickel–56 (which has 28 protons) has a [[half-life]] of 6.02 days and decays via [[Beta particle|beta radiation]] (beta plus decay, which is the emission of a positron) to [[cobalt]]–56 (27 protons), which in turn has a half-life of 77.3 days as it decays to iron–56 (26 protons). However, only minutes are available for the nickel–56 to decay within the core of a massive star. At the end of the day-long silicon-burning sequence, the star can no longer convert mass into energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per [[nucleon]] (proton and neutron) of all the elements in the alpha process sequence. Although iron–58 and nickel–62 have slightly less mass per nucleon than iron–56,<ref>Citation: ''[http://adsabs.harvard.edu/abs/1995AmJPh..63..653F The atomic nuclide with the highest mean binding energy]'', Fewell, M. P., American Journal of Physics, Volume 63, Issue 7, pp. 653-658 (1995). Click [http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html#c1 here] for a high-resolution graph, ''The Most Tightly Bound Nuclei,'' which is part of the [http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html Hyperphysics] project at [http://www.gsu.edu/ Georgia State University.]</ref> the next step up in the alpha process would be [[zinc]]–60, which has slightly ''more'' mass per nucleon and thus, would actually ''consume'' energy in its production rather than release any. The star has run out of nuclear fuel and within minutes begins to contract. The potential energy of gravitational contraction heats the interior to 5&nbsp;GK and this opposes and delays the contraction. However, since no additional heat energy can be generated via new fusion reactions, the contraction rapidly accelerates into a collapse lasting only a few seconds. The central portion of the star gets crushed into either a [[neutron star]] or, if the star is massive enough, a [[black hole]]. The outer layers of the star are blown off in an explosion known as a [[Type II supernova|Type&nbsp;II]] [[supernova]] that lasts days to months. The supernova explosion releases a large burst of neutrons, which synthesizes in about one second roughly half the elements heavier than iron, via a neutron-capture mechanism known as the ''[[r-process]]'' (where the “r” stands for rapid neutron capture). ==Binding energy== The graph below shows the binding energy of various elements. Increasing values of binding energy can be thought of in two ways: 1) it is the energy ''required'' to remove a nucleon from a nucleus, and 2) it is the energy ''released'' when a nucleon is added to a nucleus. As can be seen, light elements such as hydrogen release large amounts of energy (a big increase in binding energy) as nucleons are added—the process of fusion. Conversely, heavy elements such as uranium release energy when nucleons are ''removed''—the process of [[nuclear fission]]. Although nuclei with 58 and 62 nucleons have the very lowest binding energy, fusing four nucleons to nickel–56 to produce the next element — zinc–60 — actually ''requires'' energy rather than releases any. Accordingly, nickel–56 is the last fusion product produced in the core of a high-mass star. [[Image:Binding energy curve - common isotopes.svg|90%|center|Curve of binding energy]] ==See also== * [[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] * [http://cosserv3.fau.edu/~cis/AST2002/Lectures/C13/Trans/Trans.html ''The Evolution and Death of Stars,'' by Ian Short] * [http://www.nasa.gov/worldbook/star_worldbook.html ''Star,'' by World Book @ NASA] * ''[http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_stel_6.html Origin of Heavy Elements,]'' by [http://www.tufts.edu/main.php?p=flash Tufts University] * ''[http://schools.qps.org/hermanga/images/Astronomy/chapter_21___stellar_explosions.htm Chapter 21: Stellar Explosions,]'' by G. Hermann {{Nuclear_processes}} [[Category:Nucleosynthesis]] [[de:Siliciumbrennen]] [[it:Processo di fusione del silicio]] [[lt:Silicio degimo procesas]] [[tr:Silikonun yanma süreci]] [[zh:矽燃燒過程]]