Nucleosynthesis is the process that creates newatomic nuclei frompre-existingnucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after theBig Bang through nuclear reactions in a process calledBig Bang nucleosynthesis.[1] After about 20 minutes, the universe had expanded and cooled to a point at which thesehigh-energy collisions among nucleons ended, so only the fastest and simplest reactions occurred, leaving our universe containinghydrogen andhelium, traces of other elements, such aslithium, and the hydrogenisotopedeuterium. Nucleosynthesis in stars and stellar events such asnovas andsupernovas later produced the variety of elements and isotopes that we have today, in a process called cosmic chemical evolution. The amounts of total mass in elements heavier than hydrogen and helium (called "metals" byastrophysicists) remain small (a few percent), so that theuniverse still has approximately the same composition.
Starsfuse light elements to heavier ones in theircores, giving off energy in the process known asstellar nucleosynthesis. Nuclear fusion reactions create many of the lighter elements, up to and includingiron andnickel in the most massive stars. Products of stellar nucleosynthesis remain trapped in stellar cores and remnants except if ejected throughstellar winds and explosions. Theneutron capture reactions of ther-process ands-process create heavier elements, from iron upwards.
Supernova nucleosynthesis within exploding stars is largely responsible for the elements betweenoxygen andrubidium: from the ejection of elements produced during stellar nucleosynthesis; through explosive nucleosynthesis during the supernova explosion; and from ther-process (absorption of multiple neutrons) during the explosion.
Neutron star mergers are a recently-identified major source of elements produced in ther-process. When two neutron stars collide, a significant amount ofneutron-rich matter may be ejected which then quickly forms heavy elements.
Cosmic ray spallation is a process whereincosmic rays impact nuclei and fragment them. It is a significant source of the lighter nuclei, particularly3He,9Be and10,11B, that are not created by stellar nucleosynthesis. Cosmic ray spallation can occur in theinterstellar medium, onasteroids andmeteoroids, or on Earth in theatmosphere or in the ground. This contributes to the presence on Earth ofcosmogenic nuclides.
On Earth new nuclei are also produced byradiogenesis, the decay oflong-lived,primordial radionuclides such as uranium, thorium, andpotassium-40.
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It is thought that the primordial nucleons themselves were formed from thequark-gluon plasma around13.8 billion years ago during theBig Bang, as it cooled below two trillionkelvins. A few minutes afterwards, starting with onlyprotons andneutrons, nuclei up tolithium andberyllium (both withmass number 7) were formed, but hardly any other elements. Someboron may have been formed at this time, but the process stopped before significantcarbon could be formed, as this element requires a far higher product of helium density and time than were present in the short nucleosynthesis period of the Big Bang. That fusion process essentially shut down at about 20 minutes, due to drops in temperature and density as the universe continued to expand. This first process,Big Bang nucleosynthesis, was the first type of nucleogenesis to occur in the universe, creating theso-calledprimordial elements.
A star formed in the early universe(aPopulation III star) produces heavier elements by combining its lighter nuclei – hydrogen,helium, lithium,beryllium, and boron – which were found in the initial composition of the interstellar medium and hence the star.Interstellar gas therefore contains declining abundances of these light elements, which are present only by virtue of their nucleosynthesis during the Big Bang, and alsocosmic ray spallation. These lighter elements in the present universe are therefore thought to have been produced through thousands of millions of years of cosmic ray-mediated breakup of heavier elements in interstellar gas and dust. The fragments of thesecosmic-ray collisions includehelium-3 and the stable isotopes of the light elements lithium, beryllium, and boron. Carbon was not made in the Big Bang, but was produced later in larger stars via thetriple-alpha process.
The subsequent nucleosynthesis of heavier elements( Z ≥ 6; carbon and heavier elements) requires the extreme temperatures and pressures found withinstars andsupernovae. These processes began as hydrogen and helium from the Big Bang collapsed into the first stars after about500 million years. Star formation has been occurring continuously in galaxies since that time. The primordial nuclides were created byBig Bang nucleosynthesis,stellar nucleosynthesis,supernova nucleosynthesis, and by nucleosynthesis in exotic events such asneutron star collisions. Other nuclides, such as40Ar, formed later through radioactive decay. On Earth, mixing and evaporation has altered the primordial composition to what is called the natural terrestrial composition. The heavier elements produced after the Big Bang range inatomic numbers fromZ = 6 (carbon) toZ = 94 (plutonium). Synthesis of these elements occurred through nuclear reactions involving the strong and weak interactions among nuclei, and callednuclear fusion (including both rapid and slow multiple neutron capture), and include alsonuclear fission and radioactive decays such asbeta decay. The stability of atomic nuclei of different sizes and composition(i.e., numbers of neutrons and protons) plays an important role in the possible reactions among nuclei. Cosmic nucleosynthesis, therefore, is studied among researchers of astrophysics and nuclear physics ("nuclear astrophysics").
The first ideas on nucleosynthesis were simply that thechemical elements were created at the beginning of the universe, but no rational physical scenario for this could be identified. Gradually it became clear that hydrogen and helium are much more abundant than any of the other elements. All the rest constituteless than 2% of the mass of theSolar System, and of other star systems as well. At the same time it was clear that oxygen and carbon were the next two most common elements, and also that there was a general trend toward high abundance of the light elements, especially those with isotopes composed of whole numbers ofhelium-4 nuclei(alpha nuclides).
Arthur Stanley Eddington first suggested in 1920 that stars obtain their energy by fusing hydrogen into helium and raised the possibility that the heavier elements may also form in stars.[2][3] This idea was not generally accepted, as the nuclear mechanism was not understood. In the years immediately beforeWorld War II,Hans Bethe first elucidated those nuclear mechanisms by which hydrogen is fused into helium.
Fred Hoyle's original work on nucleosynthesis of heavier elements in stars, occurred just afterWorld War II.[4] His work explained the production of all heavier elements, starting from hydrogen. Hoyle proposed that hydrogen is continuously created in the universe from vacuum and energy, without need for universal beginning.
Hoyle's work explained how the abundances of the elements increased with time as the galaxy aged. Subsequently, Hoyle's picture was expanded during the 1960s by contributions fromWilliam A. Fowler,Alastair G. W. Cameron, andDonald D. Clayton, followed by many others. Theseminal 1957 "B2FH" review paper byE. M. Burbidge,G. R. Burbidge, Fowler and Hoyle[5] is awell-known summary of the state of the field in 1957. That paper defined new processes for the transformation of one heavy nucleus into others within stars, processes that could be documented by astronomers.
The Big Bang itself had been proposed in 1931, long before this period, byGeorges Lemaître, a Belgian physicist, who suggested that the evident expansion of the Universe in time required that the Universe, if contracted backwards in time, would continue to do so until it could contract no further. This would bring all the mass of the Universe to a single point, a "primeval atom", to a state before which time and space did not exist. Hoyle is credited with coining the term "Big Bang" during a 1949 BBC radio broadcast, saying thatLemaître's theory was "based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past". It is popularly reported that Hoyle intended this to bepejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.Lemaître's model was needed to explain the existence of deuterium and nuclides between helium and carbon, as well as the fundamentally high amount of helium present, not only in stars but also ininterstellar space. As it happened, both Lemaître and Hoyle's models of nucleosynthesis would be needed to explain the elemental abundances in the universe.
The goal of the theory of nucleosynthesis is to explain the vastly differing abundances of the chemical elements and their several isotopes from the perspective of natural processes. The primary stimulus to the development of this theory was the shape of a plot of the abundances versus the atomic number of the elements. Those abundances, when plotted on a graph as a function of atomic number, have a jaggedsawtooth structure that varies by factors up to ten million. A very influential stimulus to nucleosynthesis research was an abundance table created byHans Suess andHarold Urey that was based on the unfractionated abundances of thenon-volatile elements found within unevolvedmeteorites.[6] Such a graph of the abundances is displayed on alogarithmic scale(see graph, below), where the dramatically jagged structure is visually suppressed by the many powers of ten spanned in the vertical scale of this graph.
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There are severalastrophysical processes which are believed to be responsible for nucleosynthesis. The majority of these occur within stars, and the chain of those nuclear fusion processes are known ashydrogen burning (via theproton–proton chain or theCNO cycle),helium burning,carbon burning,neon burning,oxygen burning andsilicon burning. These processes are capable of creating elements up to and including iron and nickel. This is the region of nucleosynthesis within which the isotopes with the highestbinding energy per nucleon are created.
Heavier elements can be assembled within stars mainly by the slow neutron capture process known as thes-process or in explosive environments, such assupernovae andneutron star mergers, by ther-process, which involves rapid neutron captures (faster than thehalf-lifes of the intermediate isotopes). There is also a minor contribution from processes involving proton capture, such as therp-process, and thep-process. These processes allow the synthesis of some proton-rich isotopes that cannot be created by neutron capture and subsequentbeta-decays.[8]
Big Bang nucleosynthesis[9] occurred within the first three minutes of the beginning of the universe and is responsible for much of the abundance of1
H (protium),2
H (D,deuterium),3
He (helium-3), and4
He (helium-4). Although4
He continues to be produced by stellar fusion andalpha decays and trace amounts of1
H continue to be produced byspallation and certain types of radioactive decay, most of the mass of the isotopes in the universe are thought to have been produced in the Big Bang. The nuclei of these elements, along with some7
Li and7
Be are considered to have been formed between 100 and 300 seconds after the Big Bang when the primordialquark–gluon plasma froze out to form protons and neutrons. Because of the very short period in which nucleosynthesis occurred before it was stopped by expansion and cooling (about 20 minutes), no elements heavier thanberyllium (or possiblyboron) could be formed. Elements formed during this time were in theplasma state, and did not cool to the state of neutral atoms until much later.[citation needed][10]
Stellar nucleosynthesis is the nuclear process by which new nuclei are produced. It occurs in stars duringstellar evolution. It is responsible for the galactic abundances of elements from carbon to iron. Stars are thermonuclear furnaces in which hydrogen and helium are fused into heavier nuclei by increasingly high temperatures as the composition ofthe core evolves.[11] Of particular importance is carbon because its formation from He is a bottleneck in the entire process. Carbon is produced by thetriple-alpha process in all stars. Carbon is also the main element that causes the release of free neutrons within stars, giving rise to thes-process, in which the slow absorption of neutrons converts iron into elements heavier than iron and nickel.[12][13]
The products of stellar nucleosynthesis are generally dispersed into theinterstellar gas through mass loss episodes and thestellar winds oflow-mass stars. The mass loss events can be witnessed today in theplanetary nebula phase oflow-mass star evolution, and the explosive ending of stars, calledsupernovae, of those with more than eight times the mass of the Sun.
The first direct proof that nucleosynthesis occurs in stars was the astronomical observation that interstellar gas has become enriched with heavy elements as time passed. As a result, stars that were born from it late in the galaxy, formed with much higher initial heavy element abundances than those that had formed earlier. The detection oftechnetium in the atmosphere of ared giant star in 1952,[14] byspectroscopy, provided the first evidence of nuclear activity within stars. Because technetium is radioactive, with ahalf-life much less than the age of the star, its abundance must reflect its recent creation within that star. Equally convincing evidence of the stellar origin of heavy elements is the large overabundances of specific stable elements found instellar atmospheres ofasymptotic giant branch stars. Observation ofbarium abundances some20–50 times greater than found in unevolved stars is evidence of the operation of thes-process within such stars. Many modern proofs of stellar nucleosynthesis are provided by theisotopic compositions ofstardust, solid grains that have condensed from the gases of individual stars and which have been extracted from meteorites. Stardust is one component ofcosmic dust and is frequently calledpresolar grains. The measured isotopic compositions in stardust grains demonstrate many aspects of nucleosynthesis within the stars from which the grains condensed during the star'slate-lifemass-loss episodes.[15]
Supernova nucleosynthesis occurs in the energetic environment in supernovae, in which the elements betweensilicon andnickel are synthesized in quasiequilibrium[16] established during fast fusion that attaches by reciprocating balanced nuclear reactions to28Si. Quasiequilibrium can be thought of asalmost equilibrium except for a high abundance of the28Si nuclei in the feverishly burning mix. This concept[13] was the most important discovery in nucleosynthesis theory of theintermediate-mass elements since Hoyle's 1954 paper because it provided an overarching understanding of the abundant and chemically important elements between silicon( A = 28 ) and nickel( A = 60 ). It replaced the incorrect althoughmuch-citedalpha process of theB2FH paper, which inadvertently obscured Hoyle's 1954 theory.[17] Further nucleosynthesis processes can occur, in particular ther-process (rapid process) described by theB2FH paper and first calculated by Seeger, Fowler and Clayton,[18] in which the mostneutron-rich isotopes of elements heavier than nickel are produced by rapid absorption of free neutrons. The creation of free neutrons byelectron capture during the rapid compression of the supernova core along with the assembly of someneutron-rich seed nuclei makes ther-process aprimary process, and one that can occur even in a star of pure H and He. This is in contrast to the B2FH designation of the process as asecondary process. This promising scenario, though generally supported by supernova experts, has yet to achieve a satisfactory calculation ofr-process abundances. The primaryr-process has been confirmed by astronomers who had observed old stars born when galacticmetallicity was still small, that nonetheless contain their complement ofr-process nuclei; thereby demonstrating that the metallicity is a product of an internal process. Ther-process is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the mostneutron-rich isotopes of each heavy element.
Therp-process (rapid proton) involves the rapid absorption of free protons as well as neutrons, but its role and its existence are less certain.
Explosive nucleosynthesis occurs too rapidly for radioactive decay to decrease the number of neutrons, so that many abundant isotopes with equal and even numbers of protons and neutrons are synthesized by the silicon quasi-equilibrium process.[16] During this process, the burning of oxygen and silicon fuses nuclei that themselves have equal numbers of protons and neutrons to produce nuclides which consist of whole numbers ofhelium nuclei,up to 15 (representing60Ni). Such multiple-alpha-particle nuclides are totally stable up to40Ca (made of 10 helium nuclei), but heavier nuclei with equal and even numbers of protons and neutrons are tightly-bound but unstable. The quasi-equilibrium produces radioactiveisobars44Ti,48Cr,52Fe, and56Ni, which (except44Ti) are created in abundance but decay after the explosion and leave the most stable isotope of the corresponding element at the sameatomic weight. The most abundant and extant isotopes of elements produced in this way are48Ti,52Cr, and56Fe. These decays are accompanied by the emission ofgamma rays (radiation from the nucleus), whosespectroscopic lines can be used to identify the isotope created by the decay. The detection of these emission lines was an important early product ofgamma-ray astronomy.[19]
The most convincing proof of explosive nucleosynthesis in supernovae occurred in 1987 when thosegamma-ray lines were detected emerging fromsupernova 1987A.Gamma-ray lines identifying56Co and57Co nuclei, whosehalf-lives limit their age to about a year, proved that their radioactive cobalt parents created them. This nuclear astronomy observation was predicted in 1969[19] as a way to confirm explosive nucleosynthesis of the elements, and that prediction played an important role in the planning for NASA'sCompton Gamma Ray Observatory.
Other proofs of explosive nucleosynthesis are found within the stardust grains that condensed within the interiors of supernovae as they expanded and cooled. Stardust grains are one component of cosmic dust. In particular, radioactive44Ti was measured to be very abundant within supernova stardust grains at the time they condensed during the supernova expansion.[15] This confirmed a 1975 prediction of the identification of supernova stardust ("SUNOCONs"), which became part of the pantheon ofpresolar grains. Other unusual isotopic ratios within these grains reveal many specific aspects of explosive nucleosynthesis.
Another type of explosive nucleosynthesis through ther-process was suggested in the flaring ofmagnetars. Some direct evidence for this was published in 2025. It is estimated that this kind of event has created~1%–10% of the heavier elements in the universe.[20]
As of the mid-2020s, themerger of binary neutron stars (BNSs) is believed to be the main source ofr-process elements.[21] Beingneutron-rich by definition, mergers of this type had been suspected of being a source of such elements, but definitive evidence was difficult to obtain. In 2017 strong evidence emerged, whenLIGOTooltip Laser Interferometer Gravitational-Wave Observatory,Virgo, theFermiGamma-ray Space Telescope andINTEGRALTooltip INTErnational Gamma-Ray Astrophysics Laboratory, along with a collaboration of many observatories around the world, detected bothgravitational wave and electromagnetic signatures of a likely neutron star merger,GW170817, and subsequently detected signals of numerous heavy elements such as gold as the ejecteddegenerate matter decayed and cooled.[22] The first detection of the merger of a neutron star andblack hole (NSBHs) came in July 2021 and more after but analysis seem to favorBNSs overNSBHs as the main contributors to heavy metal production.[23][24]
Nucleosynthesis may happen inaccretion disks ofblack holes.[25][26][27][28][29][30][31]
Cosmic ray spallation process reduces the atomic weight of interstellar matter by the impact withcosmic rays, to produce some of the lightest elements present in the universe (though not a significant amount ofdeuterium). Most notably spallation is believed to be responsible for the generation of almost all of3He and the elementslithium,beryllium, and boron, although some7
Li and7
Be are thought to have been produced in the Big Bang. The spallation process results from the impact of cosmic rays (mostly fast protons) against theinterstellar medium. These impacts fragment carbon, nitrogen, and oxygen nuclei present. The process results in the light elements beryllium, boron, and lithium in the cosmos at much greater abundances than they are found withinstellar atmospheres. The quantities of the light elements1H and4He produced by spallation are negligible relative to their primordial abundance.
Beryllium and boron are not significantly produced by stellar fusion processes, since8Be has an extremely shorthalf-life of8.2×10−17 seconds.[32]
Theories of nucleosynthesis are tested by calculatingisotope abundances and comparing those results with observed abundances. Isotope abundances are typically calculated from 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.[citation needed]
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Tiny amounts of certain nuclides are produced on Earth by artificial means. Those are our primary source, for example, of technetium. However, some nuclides are also produced by a number of natural means that have continued after primordial elements were in place. These often act to create new elements in ways that can be used to date rocks or to trace the source of geological processes. Although these processes do not produce the nuclides in abundance, they are assumed to be the entire source of the existing natural supply of those nuclides.
These mechanisms include:
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