Editing Nucleosynthesis (section)

==History==
{{unreferenced section|date=April 2021}}
[[File:Nucleosynthesis periodic table.svg|thumb|600px|Periodic table showing the currently believed origins of each element. Elements from carbon up to sulfur may be made in stars of all masses by charged-particle fusion reactions. Iron group elements originate mostly from the nuclear-statistical equilibrium process in thermonuclear supernova explosions. Elements beyond iron are made in high-mass stars with slow neutron capture ([[s-process]]), and by rapid neutron capture in the r-process, with origins being debated among rare supernova variants and compact-star collisions. Note that this graphic is a first-order simplification of an active research field with many open questions.]]

===Timeline===
It is thought that the primordial nucleons themselves were formed from the [[quark–gluon plasma]] around 13.8 billion years ago during the [[Big Bang]] as it cooled below two trillion degrees. A few minutes afterwards, starting with only [[proton]]s and [[neutron]]s, nuclei up to [[lithium]] and [[beryllium]] (both with [[mass number]] 7) were formed, but hardly any other elements. Some [[boron]] may have been formed at this time, but the process stopped before significant [[carbon]] 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 the so-called [[primordial element]]s.

A star formed in the early universe produces heavier elements by combining its lighter nuclei{{snd}}[[hydrogen]], [[helium]], lithium, [[beryllium]], and boron{{snd}}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 also [[cosmic 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 (mostly high-energy proton) mediated breakup of heavier elements in interstellar gas and dust. The fragments of these cosmic-ray collisions include [[helium-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 the [[triple-alpha process]].

The subsequent nucleosynthesis of heavier elements (''Z''&nbsp;≥&nbsp;6, carbon and heavier elements) requires the extreme temperatures and pressures found within [[star]]s and [[supernova]]e. These processes began as hydrogen and helium from the Big Bang collapsed into the first stars after about 500 million years. Star formation has been occurring continuously in galaxies since that time. The primordial nuclides were created by [[Big Bang nucleosynthesis]], [[stellar nucleosynthesis]], [[supernova nucleosynthesis]], and by nucleosynthesis in exotic events such as neutron star collisions. Other nuclides, such as {{sup|40}}Ar, 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 in [[atomic number]]s from ''Z''&nbsp;=&nbsp;6 (carbon) to ''Z''&nbsp;=&nbsp;94 ([[plutonium]]). Synthesis of these elements occurred through nuclear reactions involving the strong and weak interactions among nuclei, and called [[nuclear fusion]] (including both rapid and slow multiple neutron capture), and include also [[nuclear fission]] and radioactive decays such as [[beta 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]]").

===History of nucleosynthesis theory===
The first ideas on nucleosynthesis were simply that the [[chemical 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 constitute less than 2% of the mass of the [[Solar 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 of helium-4 nuclei ([[alpha nuclide]]s).

[[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.<ref>{{cite journal |last1=Eddington |first1=A. S. |date=1920 |title=The Internal Constitution of the Stars |journal=[[The Observatory (journal)|The Observatory]] |volume=43 |issue= 1341|pages=233–40 |doi=10.1126/science.52.1341.233 |pmid=17747682 |bibcode=1920Obs....43..341E|url=https://zenodo.org/record/1429642 }}</ref><ref>{{cite journal |last1=Eddington |first1=A. S. |date=1920 |title=The Internal Constitution of the Stars |journal=[[Nature (journal)|Nature]] |volume=106 |issue=2653 |pages=14–20 |bibcode=1920Natur.106...14E |doi=10.1038/106014a0|pmid=17747682 |doi-access=free }}</ref> This idea was not generally accepted, as the nuclear mechanism was not understood. In the years immediately before World 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 after World War II.<ref>Actually, before the war ended, he learned about the problem of spherical implosion of [[plutonium]] in the [[Manhattan project]]. He saw an analogy between the plutonium fission reaction and the newly discovered supernovae, and he was able to show that exploding super novae produced all of the elements in the same proportion as existed on Earth. He felt that he had accidentally fallen into a subject that would make his career. [http://nobelprize.org/nobel_prizes/physics/laureates/1983/fowler-autobio.html Autobiography William A. Fowler]</ref> 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 from [[William A. Fowler]], [[Alastair G. W. Cameron]], and [[Donald D. Clayton]], followed by many others. The [[B2FH paper|seminal 1957 review paper]] by [[Margaret Burbidge|E. M. Burbidge]], [[Geoffrey Burbidge|G. R. Burbidge]], Fowler and Hoyle<ref>{{cite journal |last1=Burbidge |first1=E. M. |last2=Burbidge |first2=G. R. |last3=Fowler |first3=W. A. |last4=Hoyle |first4=F. |year=1957 |title=Synthesis of the Elements in Stars |journal=[[Reviews of Modern Physics]] |volume=29 |issue=4 |pages=547–650 |bibcode=1957RvMP...29..547B |doi=10.1103/RevModPhys.29.547|doi-access=free }}</ref> is a well-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, by [[Georges 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 that Lemaître'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 be pejorative, 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 in interstellar 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 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.<ref>{{cite journal |last1=Suess |first1=Hans E. |last2=Urey |first2=Harold C. |title=Abundances of the Elements |journal=[[Reviews of Modern Physics]] |date=1956 |volume=28 |issue=1 |pages=53–74 |bibcode=1956RvMP...28...53S |doi=10.1103/RevModPhys.28.53}}</ref> Such a graph of the abundances is displayed on a logarithmic scale below, where the dramatically jagged structure is visually suppressed by the many powers of ten spanned in the vertical scale of this graph.

[[Image:SolarSystemAbundances.svg|thumb|center|800px|Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, residuals within the paradigm of the Big Bang.<ref>{{cite book |last1=Stiavelli |first1=Massimo |year=2009 |title=From First Light to Reionization the End of the Dark Ages |url=https://books.google.com/books?id=iCLNBElRTS4C&pg=PA8 |page=8 |publisher=[[Wiley-VCH]] |location=Weinheim, Germany |isbn=9783527627370}}</ref> The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance of elements according to whether they have even or odd atomic numbers, and (2) a general decrease in abundance, as elements become heavier. Within this trend is a peak at abundances of iron and nickel, which is especially visible on a logarithmic graph spanning fewer powers of ten, say between logA=2 (A=100) and logA=6 (A=1,000,000).]]