Other meanings of Nucleosynthesis
Astrophysics
Nucleosynthesis is the process by which new atomic nuclei are created from pre-existing nucleons (protons and neutrons). It encompasses the cosmic origins of the chemical elements, from the lightest hydrogen and helium forged in the Big Bang to the heavy elements synthesized in stars and stellar explosions.
Primordial nucleosynthesis occurred in the first few minutes after the Big Bang, when the universe was hot and dense enough for protons and neutrons to fuse. This process produced mostly hydrogen (about 75% by mass) and helium-4 (about 25%), with trace amounts of deuterium, helium-3, and lithium-7. The measured abundances of these light elements match predictions from Big Bang nucleosynthesis (BBN) remarkably well, providing strong evidence for the hot Big Bang model.1
BBN ceased after about 20 minutes, when the universe cooled below the threshold for nuclear fusion. Heavier elements were not made then because there is no stable nucleus with mass number 5 or 8, which acts as a bottleneck. Thus, the universe was left with only the lightest elements until stars formed.
Stellar nucleosynthesis is the sum of nuclear reactions inside stars that build heavier elements from lighter ones. In main-sequence stars like the Sun, hydrogen fuses into helium via the proton-proton chain and the CNO cycle. When a star exhausts its core hydrogen, it expands into a red giant and begins helium burning, producing carbon and oxygen via the triple-alpha process.
In more massive stars, successive stages of fusion (carbon, neon, oxygen, and silicon burning) create elements up to iron. Iron-56 has the highest binding energy per nucleon, so fusion beyond it consumes energy rather than releasing it. Thus, elements heavier than iron are not produced by ordinary stellar fusion but by neutron capture processes.
The slow neutron-capture process (s-process) occurs in asymptotic giant branch (AGB) stars, where neutrons are produced by reactions like 13C(α,n)16O. The s-process builds about half of the heavy elements beyond iron, following the valley of beta stability.2 It operates over thousands of years, allowing unstable nuclei to decay before capturing another neutron.
The rapid neutron-capture process (r-process) happens in explosive environments such as supernovae and neutron star mergers. Here, neutron densities are so high that nuclei capture many neutrons before decaying, producing very neutron-rich nuclei that later beta-decay to stable isotopes. The r-process is responsible for about half of the heavy elements, including gold and platinum. The detection of a kilonova associated with the neutron star merger GW170817 confirmed that such mergers are a major r-process site.
Explosive nucleosynthesis refers to the rapid nuclear reactions that occur during supernovae and other stellar explosions. In core-collapse supernovae, the shock wave and intense neutrino flux drive nuclear reactions that produce elements like silicon, sulfur, and nickel-56, which decays to iron.3 The extreme temperatures and neutron fluxes also enable the p-process (proton capture) and the γ-process (photodisintegration), which create rare proton-rich isotopes.
Type Ia supernovae, which are thermonuclear explosions of white dwarfs, produce large amounts of iron-peak elements, especially nickel-56. These events are crucial for the chemical evolution of galaxies, enriching the interstellar medium with heavy elements that later incorporate into new stars and planets.
Beyond the main processes, nucleosynthesis includes several niche mechanisms. The p-process, for instance, produces about 30 proton-rich isotopes that cannot be made by neutron capture, such as 92Mo and 144Sm, likely in supernova explosions.4 The ν-process, driven by neutrino interactions in supernovae, creates rare isotopes like 11B and 19F.
Cosmic ray spallation, where high-energy particles fragment interstellar nuclei, produces light elements like lithium, beryllium, and boron, which are otherwise destroyed in stars. Additionally, the rp-process (rapid proton capture) occurs on the surfaces of accreting neutron stars, producing elements up to mass 100. These processes contribute to the full tapestry of elemental abundances, though they are less prominent than the s- and r-processes.
Nucleosynthesis is a cornerstone of astrophysics, linking nuclear physics to the cosmic history of matter.
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