Nucleosynthesis
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Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons (protons and neutrons). The primordial preexisting nucleons were formed from the quark-gluon plasma of the Big Bang as it cooled below ten million degrees. This first process may be called nucleogenesis, the genesis of nucleons in the universe. The subsequent nucleosynthesis of the elements occurs primarily either by nuclear fusion or nuclear fission.
Processes
There are a number of astrophysical processes which are believed to be responsible for nucleosynthesis in the universe. Most of these occur within the hot matter inside of the stars. The processes bear the names Hydrogen Burning, Helium Burning, Carbon burning, Oxygen and Neon burning, Silicon Burning, and Equilibrium. These create the elements lighter than zinc within stars. The elements heavier than zinc are assembled within stars from those already present by processes called the R-process, S-process, and P-process. These three involve the interactions of the nucleosynthesis products with free neutrons inside of stars.
Historically this science began by two major discoveries. Hans Bethe showed before World War II how hydrogen can be fused to helium and the power source for the stars. Fred Hoyle's original work on nucleosynthesis of heavier elements in stars occurred just after World War II. Subsequently Hoyle's picture was expanded by creative contributions primarily by William A. Fowler, Alistair G. W. Cameron, and Donald D. Clayton, and then by many others.
Types of nucleosynthesis
The four basic types of nucleosynthesis known of are:
- Big Bang nucleosynthesis occurred within the first three minutes of the universe and is responsible for most of the helium-4 and deuterium in the universe. The 4He, 3He, 2H and 7Li nuclei are fused in the cooling of the Big Bang from the primordial nucleons, which were created by the cooling of the quark-gluon plasma. Because of the very short period in which Big Bang nucleosynthesis occurred, no elements heavier than lithium could be formed.
- Stellar nucleosynthesis occurs in stars. It creates the elements heavier than carbon from the remnants, hydrogen and helium, of the Big Bang. The stars are the nuclear furnaces in which the H and He are fused into all heavy nuclei. Particularly important is carbon, because its formation from He was the bottleneck to the entire process. Carbon is also the main element used in the production of free neutrons within the stars, giving rise to the S-process which involves the slow absorption of neutrons. The products of stellar nucleosynthesis are generally distributed as planetary nebulae.
- Supernova nucleosynthesis produces most of the elements heavier than oxygen, because the sequence of burning processes within stars occur more rapidly and reach heavier elements within supernova explosions. Supernovae are also the most favored candidate of R-process, in which elements heavier than zinc are produced by rapid absorption of free neutrons, although there are still some major unanswered questions about this process.
- Cosmic ray spallation produces some lighter elements such as lithium and boron through high speed bombardment of matter by particles. This process results from the impact of cosmic rays against other materials in space, including other cosmic rays.
Theories of nucleosynthesis are tested by calculating isotope abundances and comparing with observed results. Isotope abundances are typically calculated by calculating the transition rates between isotopes in a network. Often these calculations can be simplified as a few key reactions control the rate of other reactions.bg:Нуклеосинтез de:Nukleosynthese fr:Nucléosynthèse ko:핵합성 hu:Nukleoszintézis nl:Nucleosynthese ru:Нуклеосинтез