Nucleosynthesis - History of Nucleosynthesis Theory

History of Nucleosynthesis Theory

The first ideas on nucleosynthesis were simply that the chemical elements were created at the beginnings of the universe, but no successful physical scenario for this could be identified. Hydrogen and helium were clearly far more abundant than any of the other elements (all the rest of which constituted less than 2% of the mass of the solar system, and presumably other star systems as well). At the same time it was clear that carbon was the next most common element, and also that there was a general trend toward abundance of light elements, especially those composed of whole numbers of helium-4 nuclei.

Arthur Stanley Eddington first suggested in 1920 that stars obtain their energy by fusing hydrogen to helium, but this idea was not generally accepted because it lacked nuclear mechanisms. In the years immediately before World War II Hans Bethe first provided those nuclear mechanisms by which hydrogen is fused into helium. However, neither of these early works on stellar power addressed the origin of the elements heavier than helium.

Fred Hoyle's original work on nucleosynthesis of heavier elements in stars occurred just after World War II. This work attributed production of all heavier elements formed in stars during the nuclear evolution of their compositions, 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 creative contributions by William A. Fowler, Alastair G. W. Cameron, and Donald D. Clayton, and then by many others. The creative 1957 review paper by E. M. Burbidge, G. R. Burbidge, Fowler and Hoyle (see Ref. list) is a well-known summary of the state of the field in 1957. That paper defined new processes for changing one heavy nucleus into others within individual 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 and Roman Catholic priest, who suggested that the evident expansion of the Universe in forward time required that the Universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the Universe into a single point, a "primeval atom", at a point in time before which time and space did not exist. Hoyle later gave Lemaître's model the derisive term of Big Bang, not realizing that 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 gas. As it happened, both Lemaître and Hoyle's models of nucleosynthesis would be needed to explain elemental abundances in the universe.

The goal of nucleosynthesis is to understand the vastly differing abundances of the chemical elements and their several isotopes as being a result of natural history. The primary stimulus to the development of this theory was the shape of the natural abundances. Those abundances, when plotted on a graph as a function of atomic number of the element, have a jagged sawtooth structure varying by factors up to ten million. A very influential stimulus to nucleosynthesis was an abundance table by Hans Suess and Harold Urey based on the unfractionated abundances of the non-volatile elements within unevolved meteorites. Such data, itself hard won by scientists, demanded a natural explanation to scientists, rather than as a given by God. 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 this graph. See Handbook of Isotopes in the Cosmos for more data and discussion of abundances of the isotopes.