Silicon firing

In astrophysics, silicon burning is a group of nuclear fusion reactions inside heavy stars with an initial mass of at least eleven solar masses , in which energy is released through the conversion of the initial material silicon . The silicon firing typically only lasts a few weeks, followed by the oxygen firing . Silicon burning is the final fusion step for stars that have used up the nuclear fuel that energized them in their long lifetime on the main sequence of the Hertzsprung-Russell diagram .

The silicon burn begins after the core temperature has risen to 2.7 · 10 ⁹   - 3.5 · 10 ⁹   Kelvin due to contraction . The exact temperature depends on the mass, the density is at least 3 · 10 ¹⁰  kg / m³. After the silicon burns, no further fusion reactions are possible, so that the star finally collapses.

Reactions

After the oxygen burns, the core of the star consists mainly of silicon and sulfur . If the star has a sufficiently large mass, it will contract until its core temperature is in the 2.8-4.1 GK range.

The direct fusion of two silicon atoms is not possible because of the high Coulomb barrier . Instead, photo disintegration enables a complex reaction network made up of more than 100 individual nuclear reactions. Individual core components are extracted from silicon and other elements; these are either single protons, neutrons or whole alpha particles. A temperature of 4 GK only corresponds to an average energy of 344 keV, i.e. too little compared to the several MeV required for nuclear fission, but the photons follow a Planck distribution , in whose high-energy tail there are enough photons at these temperatures to to let the photo-disintegration run fast enough.

During the silicon burning process, atomic nuclei capture the protons, neutrons or alpha particles released by photo-disintegration. Heavy nuclei with a mass number A = 50-65 are successively formed via the following reaction chain, for example :

${\displaystyle {}_{14}^{28}\mathrm {Si} +{}_{2}^{4}\mathrm {He} \rightarrow {}_{16}^{32}\mathrm {S} }$

${\displaystyle {}_{16}^{32}\mathrm {S} +{}_{2}^{4}\mathrm {He} \rightarrow {}_{18}^{36}\mathrm {Ar} }$

${\displaystyle {}_{18}^{36}\mathrm {Ar} +{}_{2}^{4}\mathrm {He} \rightarrow {}_{20}^{40}\mathrm {Ca} }$

${\displaystyle {}_{20}^{40}\mathrm {Ca} +{}_{2}^{4}\mathrm {He} \rightarrow {}_{22}^{44}\mathrm {Ti} }$

${\displaystyle {}_{22}^{44}\mathrm {Ti} +{}_{2}^{4}\mathrm {He} \rightarrow {}_{24}^{48}\mathrm {Cr} }$

${\displaystyle {}_{24}^{48}\mathrm {Cr} +{}_{2}^{4}\mathrm {He} \rightarrow {}_{26}^{52}\mathrm {Fe} }$

${\displaystyle {}_{26}^{52}\mathrm {Fe} +{}_{2}^{4}\mathrm {He} \rightarrow {}_{28}^{56}\mathrm {Ni} }$

${\displaystyle {}_{28}^{56}\mathrm {Ni} +{}_{2}^{4}\mathrm {He} \rightarrow {}_{30}^{60}\mathrm {Zn} }$

Due to the high temperatures, the accumulation of α-particles, protons and neutrons takes place sufficiently quickly that, despite the photo-disintegration of the lighter nuclei, heavy nuclei can also arise. Since these heavy nuclei have a higher binding energy per nucleon, there are not enough photons with high energy in the stellar nucleus to be able to split them again immediately. As a result, more heavy elements are formed than destroyed. Large quantities of nickel-56 are produced during silicon firing, as this has the highest binding energy of all nuclei with the same number of protons as neutrons. Since nickel-56 is radioactive, it decays through two beta plus decays to form the stable core iron-56 (with the third highest binding energy per nucleon); Zinc-60 decays in the same way to form the stable nickel-60, which has the highest binding energy per nucleon. After the silicon firing has ended, energy can no longer be released by nuclear fusion. In summary, the main mechanism of silicon firing is therefore:

28 Si + 28 Si	→ 56 Ni + γ
56 Ni	→ 56 Co + e + + ν e	(β + decay)
56 Co	→ 56 Fe + e + + ν e .	(β + decay)

See also

• Peel burning

Web links

• Arthur Holland, Mark Williams: Stellar Evolution: The Life and Death of Our Luminous Neighbors. University of Michigan.
• Ian Short: The Evolution and Death of Stars.
• Origin of Heavy Elements. Tufts University
• G. Hermann: Chapter 21: Stellar Explosions.
• WD Arnett: Advanced evolution of massive stars. VII - Silicon burning In: Astrophysical Journal. Supplement Series, vol. 35, Oct. 1977, pp. 145-159.

Individual evidence

1. ^ Christian Iliadis: Nuclear Physics of Stars . 2nd Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33648-7 , pp. 23 (English).
2. ^ S. Woosley, T. Janka: The physics of core collapse supernovae . In: Nature Physics . tape 1 , 2006, p. 147–154 , doi : 10.1038 / nphys172 , arxiv : astro-ph / 0601261 , bibcode : 2005NatPh ... 1..147W .
3. ↑ Donald D. Clayton: Principles of Stellar Evolution and Nucleosynthesis . University of Chicago Press , 1983, ISBN 978-0-226-10953-4 , pp. 519-524 .
4. ^ SE Woosley, WD Arnett, DD Clayton: Hydrostatic oxygen burning in stars II. Oxygen burning at balanced power. In: Astrophys. J. 175, 1972, p. 731.
5. ^ ^{A b} Christian Iliadis: Nuclear Physics of Stars . 2nd Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33648-7 , pp. 420-432 (English).
6. ^ Christian Iliadis: Nuclear Physics of Stars . 2nd Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33648-7 , pp. 142-143 (English).
7. ↑ Donald D. Clayton: Principles of stellar evolution and nucleosynthesis. University of Chicago Press, 1983, Chapter 7.
8. ^ ^{A b} Christian Iliadis: Nuclear Physics of Stars . 2nd Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33648-7 , pp. 33-34 (English).
9. ↑ Hannu Karttunen, Pekka Kröger, Heikki Oja, Markku Poutanen, Karl Johan Donner: Fundamental Astronomy . 5th edition. Springer, Berlin / Heidelberg / New York 2007, ISBN 978-3-540-34143-7 , 10.3 Stellar Energy Sources, p. 237 (English, Finnish: Tähtitieteen perusteet . Helsinki 2003.).