The seminal review paper by E. Burbidge , G. Burbidge , Fowler and Hoyle  is a well-known summary of the state of the field in That paper defined new processes for the transformation of one heavy nucleus into others within stars, processes that could be documented by astronomers. 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. 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 jagged sawtooth structure that varies by factors up to ten million. A very influential stimulus to nucleosynthesis research was an abundance table created by Hans Suess and Harold Urey that was based on the unfractionated abundances of the non-volatile elements found within unevolved meteorites.
There are a number of astrophysical 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 as hydrogen burning via the proton-proton chain or the CNO cycle , helium burning , carbon burning , neon burning , oxygen burning and silicon burning. These processes are able to create elements up to and including iron and nickel. This is the region of nucleosynthesis within which the isotopes with the highest binding energy per nucleon are created.
Heavier elements can be assembled within stars by a neutron capture process known as the s-process or in explosive environments, such as supernovae and neutron star mergers , by a number of other processes. Some of those others include the r-process , which involves rapid neutron captures, the rp-process , and the p-process sometimes known as the gamma process , which results in the photodisintegration of existing nuclei.
Big Bang nucleosynthesis  occurred within the first three minutes of the beginning of the universe and is responsible for much of the abundance of 1 H protium , 2 H D, deuterium , 3 He helium-3 , and 4 He helium Although 4 He continues to be produced by stellar fusion and alpha decays and trace amounts of 1 H continue to be produced by spallation 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 some 7 Li and 7 Be are considered to have been formed between and seconds after the Big Bang when the primordial quark—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 than beryllium or possibly boron could be formed. Elements formed during this time were in the plasma state, and did not cool to the state of neutral atoms until much later.
Stellar nucleosynthesis is the nuclear process by which new nuclei are produced. It occurs in stars during stellar evolution. It is responsible for the galactic abundances of elements from carbon to iron. Stars are thermonuclear furnaces in which H and He are fused into heavier nuclei by increasingly high temperatures as the composition of the core evolves.
Carbon is produced by the triple-alpha process in all stars. Carbon is also the main element that causes the release of free neutrons within stars, giving rise to the s-process , in which the slow absorption of neutrons converts iron into elements heavier than iron and nickel. The products of stellar nucleosynthesis are generally dispersed into the interstellar gas through mass loss episodes and the stellar winds of low mass stars.
The mass loss events can be witnessed today in the planetary nebulae phase of low-mass star evolution, and the explosive ending of stars, called supernovae , 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 of technetium in the atmosphere of a red giant star in ,  by spectroscopy, provided the first evidence of nuclear activity within stars.
Because technetium is radioactive, with a half-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 in stellar atmospheres of asymptotic giant branch stars.
Observation of barium abundances some times greater than found in unevolved stars is evidence of the operation of the s-process within such stars. Many modern proofs of stellar nucleosynthesis are provided by the isotopic compositions of stardust , solid grains that have condensed from the gases of individual stars and which have been extracted from meteorites. Stardust is one component of cosmic dust and is frequently called presolar grains. The measured isotopic compositions in stardust grains demonstrate many aspects of nucleosynthesis within the stars from which the grains condensed during the star's late-life mass-loss episodes.
Supernova nucleosynthesis occurs in the energetic environment in supernovae, in which the elements between silicon and nickel are synthesized in quasiequilibrium  established during fast fusion that attaches by reciprocating balanced nuclear reactions to 28 Si. Quasiequilibrium can be thought of as almost equilibrium except for a high abundance of the 28 Si nuclei in the feverishly burning mix. It replaced the incorrect although much cited alpha process of the B 2 FH paper , which inadvertently obscured Hoyle's theory.
The creation of free neutrons by electron capture during the rapid compression of the supernova core along with the assembly of some neutron-rich seed nuclei makes the r-process a primary process , and one that can occur even in a star of pure H and He. This is in contrast to the B 2 FH designation of the process as a secondary process. This promising scenario, though generally supported by supernova experts, has yet to achieve a satisfactory calculation of r-process abundances.
The primary r-process has been confirmed by astronomers who had observed old stars born when galactic metallicity was still small, that nonetheless contain their complement of r-process nuclei; thereby demonstrating that the metallicity is a product of an internal process. The r-process is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most neutron-rich isotopes of each heavy element. The rp-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.
Such multiple-alpha-particle nuclides are totally stable up to 40 Ca 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 radioactive isobars 44 Ti, 48 Cr, 52 Fe, and 56 Ni, which except 44 Ti are created in abundance but decay after the explosion and leave the most stable isotope of the corresponding element at the same atomic weight. The most abundant and extant isotopes of elements produced in this way are 48 Ti, 52 Cr, and 56 Fe.
These decays are accompanied by the emission of gamma-rays radiation from the nucleus , whose spectroscopic lines can be used to identify the isotope created by the decay. The detection of these emission lines were an important early product of gamma-ray astronomy. The most convincing proof of explosive nucleosynthesis in supernovae occurred in when those gamma-ray lines were detected emerging from supernova A.
Gamma-ray lines identifying 56 Co and 57 Co nuclei, whose radioactive half-lives limit their age to about a year, proved that their radioactive cobalt parents created them. This nuclear astronomy observation was predicted in  as a way to confirm explosive nucleosynthesis of the elements, and that prediction played an important role in the planning for NASA's Compton 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, radioactive 44 Ti was measured to be very abundant within supernova stardust grains at the time they condensed during the supernova expansion. Other unusual isotopic ratios within these grains reveal many specific aspects of explosive nucleosynthesis. Neutron star collisions are believed to be the main source of r-process elements. Nucleosynthesis may happen in accretion disks of black holes. Cosmic ray spallation process reduces the atomic weight of interstellar matter by the impact with cosmic rays, to produce some of the lightest elements present in the universe though not a significant amount of deuterium.
Most notably spallation is believed to be responsible for the generation of almost all of 3 He and the elements lithium , beryllium , and boron , although some 7 Li and 7 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 the interstellar 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 within solar atmospheres.
The quantities of the light elements 1 H and 4 He produced by spallation are negligible relative to their primordial abundance. Beryllium and boron are not significantly produced by stellar fusion processes, since 8 Be is not particle-bound. Theories of nucleosynthesis are tested by calculating isotope 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. Tiny amounts of certain nuclides are produced on Earth by artificial means.
Nucleosynthesis - Wikipedia
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.
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The 'stuff' of the universe keeps changing
For the song by Vangelis, see Albedo 0. Main article: Big Bang nucleosynthesis. Chief nuclear reactions responsible for the relative abundances of light atomic nuclei observed throughout the universe. Main articles: Stellar nucleosynthesis , Proton-proton chain , Triple-alpha process , CNO cycle , s-process , p-process , and photodisintegration.