Nucleosynthesis is the production of new atomic nuclei through nuclear reactions. It explains how the universe acquired its chemical elements and their isotopes, from primordial hydrogen and helium to carbon, iron, and uranium. Its principal settings include the early universe, stellar interiors, stellar explosions, and collisions involving compact stellar remnants. Different environments produce different combinations of nuclei, so the observed abundance of an element usually reflects several production and destruction processes. (energy.gov)
Nuclear foundations
A nucleus contains protons and neutrons. Its proton count identifies the element, while nuclei with the same proton count but different neutron counts are isotopes. Nucleosynthesis rearranges these constituents through fusion, particle capture, fragmentation, and radioactive transformations. Unlike ordinary chemical reactions, which reorganize electrons and bonds, these processes change nuclear composition. (energy.gov)
In nuclear fusion, lighter nuclei combine into heavier ones. Fusion can release energy when the products are more tightly bound, with the mass difference converted into energy according to mass–energy equivalence. Nuclear binding energy per nucleon reaches its highest values around the iron–nickel region; consequently, building progressively heavier nuclei does not provide an indefinitely available stellar energy source. Heavy-element production therefore involves pathways beyond ordinary energy-generating fusion. (energy.gov)
Primordial nucleosynthesis
Big Bang nucleosynthesis occurred during the first few minutes of cosmic expansion. Before substantial nuclear assembly began, weak interactions controlled the neutron-to-proton ratio. As the universe expanded and cooled, these reactions became too slow to maintain equilibrium, and some free neutrons subsequently decayed. (pdg.lbl.gov)
Nuclear assembly was delayed because energetic photons repeatedly destroyed newly formed deuterium, the hydrogen isotope containing one proton and one neutron. Once deuterium survived, reactions rapidly produced helium-4, together with smaller quantities of deuterium, helium-3, and mass-seven nuclei that ultimately contributed lithium-7. Approximately one quarter of primordial ordinary matter by mass became helium-4; most of the remainder was hydrogen. Expansion, declining density, and the absence of stable nuclei with mass numbers five and eight prevented substantial production of heavier elements. (pdg.lbl.gov)
Primordial deuterium and helium abundances provide tests of cosmology independent of measurements of the cosmic microwave background. An important unresolved discrepancy is the “lithium problem”: standard calculations predict more lithium-7 than is inferred from old, metal-poor stars. Stellar depletion, observational uncertainties, nuclear physics, and possible departures from standard cosmology have been investigated as explanations. (pdg.lbl.gov)
Stellar nucleosynthesis
Inside stars, sustained nuclear reactions generate energy and transform their chemical composition. During the main-sequence stage, hydrogen is converted into helium. In the Sun, the dominant route is the proton–proton chain; hotter stars make greater use of the carbon–nitrogen–oxygen cycle, in which these heavier nuclei act as catalysts for hydrogen burning. (journals.aps.org)
After core hydrogen exhaustion, contraction can raise the temperature sufficiently for helium burning. The triple-alpha process combines three helium-4 nuclei into carbon-12 through an unstable beryllium-8 intermediate. Carbon can then capture another helium nucleus to form oxygen-16. These reactions establish much of the carbon and oxygen later incorporated into planets and living organisms. (journals.aps.org)
Stellar mass strongly influences the subsequent sequence. Stars like the Sun do not proceed through all advanced burning stages. Massive stars can undergo carbon, neon, oxygen, and silicon burning, producing nuclei extending into the iron group. Silicon burning is a network of nuclear rearrangements, including photodisintegration and particle capture, rather than simply the fusion of two silicon nuclei. Stellar winds and later explosions disperse processed material into space. (imagine.gsfc.nasa.gov)
Explosions and neutron capture
Supernova explosions both eject previously synthesized material and create additional nuclei through explosive burning. Core-collapse explosions rapidly heat stellar layers, while thermonuclear explosions involving a white dwarf can convert carbon and oxygen into intermediate-mass and iron-group elements. Newly produced nickel-56 decays through cobalt-56 into iron-56, supplying radioactive energy to supernova emission. (science.nasa.gov)
Many nuclei heavier than iron form through neutron capture. In the s-process, or slow neutron-capture process, captures are generally separated by enough time for unstable nuclei to undergo beta decay. Its main component operates in evolved asymptotic giant branch stars; a weaker component occurs in massive stars. The resulting abundances depend on neutron sources, stellar mixing, and the initial supply of seed nuclei. (archive.jinaweb.org)
In the r-process, captures occur rapidly enough to drive nuclei far toward neutron-rich compositions before decay brings them back toward stability. Mergers involving neutron stars provide an established production environment. Their ejecta can produce a kilonova, whose radiation is powered by radioactive decay. Observations of the 2017 merger GW170817, including identification of strontium, supplied direct evidence for heavy-element production. The relative contributions of mergers and other proposed sites remain under investigation. (nas.nasa.gov)
Fragmentation and observational evidence
Not all synthesis builds heavier nuclei. Cosmic-ray spallation fragments heavier nuclei in energetic collisions and contributes substantially to lithium, beryllium, and boron production. These comparatively fragile nuclei are readily destroyed in stellar interiors, making their origins distinct from those of many neighboring elements. (arxiv.org)
Researchers test nucleosynthesis models using spectroscopy, isotope measurements, and laboratory studies of nuclear reaction rates. The influential 1957 paper Synthesis of the Elements in Stars, by Margaret Burbidge, Geoffrey Burbidge, William Fowler, and Fred Hoyle, organized stellar reaction pathways into a framework connecting nuclear physics with astronomical abundances. Modern calculations combine these pathways with models of stellar evolution, explosions, and the transport of enriched matter through galaxies. (journals.aps.org)