Peel burning

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Layers of fusion processes in a star with sufficiently large mass; from top to bottom; white: layer without fusion processes; gray: layer with fusion processes; other elements generated in the respective process in ()
1: hydrogen burning : H → He
2: helium burning : He → C (Be)
3: carbon burning : C → O (Mg, Na, Ne)
4: neon burning : Ne → O (Mg )
5: Oxygen burning : O → Si (Mg, Ne, P, S)
6: Silicon burning : Si → Fe (Co, Ni)

A process in an aging star is called shell burning . The generation of energy through hydrogen burning shifts from the innermost volume to the periphery, while in the core initially helium to carbon and later, if necessary, other heavier elements are fused.

Overview of the successive fusion processes within massive stars

procedure

When most of the hydrogen nuclei in the innermost core of the star have fused to form helium nuclei , this first stage of nuclear fusion is extinguished. This reduces the star's radiation pressure, which was generated by the energy released during hydrogen burning . The radiation pressure has by this time together with the gas pressure of gravity counteracted and kept the star in hydrostatic equilibrium of the three forces. Because of the now prevailing gravity, the star is now beginning to shrink. Due to the laws of gas , the temperature and density inside rise, so that the next stage of fusion, the burning of helium , can start in the core , provided the star has a sufficiently large mass.

As the fusion begins anew in the core, the temperature of a spherical shell around the core will also rise until the hydrogen still present there begins to fuse to form helium, just as it did in the core of the star.

This process (exhaustion of the nuclear fuel in the core, contraction, next fusion stage) is repeated in the next time segments, provided that the star has a sufficiently large mass for the next stage. If it has more than 4 (or 5, see below) solar masses , the next stage is carbon burning . If the star has more than 8 (or 11, see below) solar masses, neon burns , oxygen burns and, as the last stage, silicon burns follow . The interior of the star now resembles an onion with skins that consist of heavier and heavier elements on the inside .

The burning of silicon represents the end of the fusion processes. The supply of nuclear fuel inside is used up in a few hours to a few days, depending on the mass of the star, and the collapse of gravity is followed by the star's explosion in a supernova .

Required masses

In order to be able to initiate the next stage in this chain of fusion processes, a star needs at least the following masses (all data in solar masses M ☉ ):

  • Hydrogen burning : at least 0.08 M ☉
  • Helium burning : at least 0.25 or 0.5 M ☉
  • Carbon burning : at least 4 or 5 M ☉ . According to the following sources, the mass of the core after the helium burning is actually decisive, which must be at least 1 or 1.06 M ☉ .
  • Neon light : at least 8 or 11 M ☉ . According to the following sources, the limit masses are not precisely determined.
  • Oxygen burning : at least 8 or 11 M ☉
  • Silicon firing : at least 8 or 11 M ☉

Required temperatures in the core

In order to be able to initiate the next stage in this chain of fusion processes, at least the following temperature is required in the core of the star:

  • Hydrogen burning: between 1 and 15 or 35 or 40 or 60 million Kelvin
  • Helium burning: at least 100 or 180 or 200 million K
  • Carbon burning: at least 500 or 600 or 800 or 900 million K
  • Neon burning: at least 1.2, 1.6 or 1.7 billion K.
  • Oxygen burning: at least 1.5 or 1.9 or between 1.5 and 2 or 2.3 billion K
  • Silicon burning: at least 2.7, 3.3, or 4.1 billion K.

Duration of the burn phases

The duration of the respective burning phase in the core of the star is:

Star with 15 M ☉ with 25 M ☉
Hydrogen burning 11 million years 7 million years
Helium burning 2 million years 500 or 700 thousand years
Carbon burning 2,000 years 600 years
Burning neon 0.7 years 1 year
Oxygen burn 2.6 years 6 months
Silicon firing 18 days 1 day

Density in the core

Density in the innermost core of the star
Burn phase Star with 15 M ☉ Star with 25 M ☉
Density [g / cm³]
Hydrogen burning 5.8 50
Helium burning 1,390 700
Carbon burning 2.8the5 2.0e5
Burning neon 1.2e6th 4the6th
Oxygen burn 8th.8the6th 1e7th
Silicon firing 4th.8the7th 3e7th

For comparison: the density of nuclear matter is about 2e14  g / cm³; the density of gold is19.32 g / cm³ at 20 ° C.

See also

literature

  • Joachim Krautter et al .: Meyers Handbuch Weltall . 7th edition. Meyers Lexikonverlang, Mannheim / Leipzig / Vienna / Zurich 1993, ISBN 3-411-07757-3 , p. 356 ff .

Individual evidence

  1. ↑ a b c d e f Astro-Lexikon T 3. In: Wissenschaft-online.de. Retrieved September 19, 2016 .
  2. ↑ a b c d e f g h A. Weiss: Nucleosynthesis. (PDF; 1.6 MB) Max Planck Institute for Astrophysics, July 20, 2012, pp. 80–84 (79-83) , accessed on September 19, 2016 .
  3. ↑ a b A. Weiss: Nucleosynthesis. (PDF; 1.6 MB) Max Planck Institute for Astrophysics, July 20, 2012, pp. 82–83 (81-82) , accessed on September 19, 2016 : “The decisive factor is actually the helium core mass, which is used in 1 M ☉ must lie; the total mass strongly depends on the mass loss "
  4. ↑ a b Chapter 11 Pre-supernova evolution of massive stars. (PDF; 1 MB) Argelander Institute for Astronomy (AIfA), p. 1 (153) , accessed on September 19, 2016 (English): “This requires a certain minimum mass for the CO core after central He burning, which detailed evolution models put at M CO-core > 1.06 M ☉ . "
  5. ↑ a b c Chapter 11 Pre-supernova evolution of massive stars. (PDF; 1 MB) (AIfA), pp. 1 (153) and 10 (162) , accessed on September 19, 2016 (English).
  6. ↑ Chapter 11 Pre-supernova evolution of massive stars. (PDF; 1 MB) (AIfA), p. 1 (153) , accessed on September 19, 2016 (English): "The fate of stars in the approximate mass range 8 - 11 M ☉ is still somewhat uncertain."
  7. ↑ XI. Star evolution. (PDF 1.3; MB) Eberhard Karls University of Tübingen Institute for Astronomy & Astrophysics, p. 6 , accessed on September 19, 2016 : “The limit masses at 8 and 10 M ☉ are not exactly known ( 1-2 M ☉ ) , since z. B. Loss of mass is dependent on metallicity. "
  8. ↑ a b c d e f g h Stan Woosley, Thomas Janka: The Physics of Core-Collapse Supernovae . S. 3 , arxiv : astro-ph / 0601261 .
  9. ↑ a b c d e f g h Nuclear Burning in High Mass Stars. Cornell University, accessed September 19, 2016 .
  10. ↑ a b c d e f g h i 7. Element synthesis and stellar evolution 7.2 Fusion and element synthesis in massive stars. (PDF; 1.1 MB) Ruprecht-Karls-Universität Heidelberg , Physikalisches Institut, p. 8 , accessed on September 19, 2016 .
  11. ^ A b The Evolution of the Sun. Cornell University , accessed September 19, 2016 .
  12. ↑ D. Meschede: Gerthsen Physik . 22nd edition, 2004, p. 630.