Editing Nucleosynthesis (section)

===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).]]