Hydrogen burning

Stellar nucleosynthesis is the process by which the natural abundances of the chemical elements within stars change due to nuclear fusion reactions in the cores and their overlying mantles. Stars are said to evolve (age) with changes in the abundances of the elements within. Core fusion increases the atomic weight of elements and reduces the number of particles, which would lead to a pressure loss except that gravitation leads to contraction, an increase of temperature, and a balance of forces. A star loses most of its mass when it is ejected late in the star's stellar lifetimes, thereby increasing the abundance of elements heavier than helium in the interstellar medium. The term supernova nucleosynthesis is used to describe the creation of elements during the evolution and explosion of a presupernova star, a concept put forth by Fred Hoyle in 1954. A stimulus to the development of the theory of nucleosynthesis was the discovery of variations in the abundances of elements found in the universe. Those abundances, when plotted on a graph as a function of atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions. This suggested a natural process other than random. Such a graph of the abundances can be seen at History of nucleosynthesis theory article. Of the several processes of nucleosynthesis, stellar nucleosynthesis is the dominating contributor to elemental abundances in the universe.

A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it was realized that the energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source of heat and light. The fusion of nuclei in a star, starting from its initial hydrogen and helium abundance, provides it energy and the synthesis of new nuclei is a byproduct of that fusion process. This became clear during the decade prior to World War II. The fusion-produced nuclei are restricted to those only slightly heavier than the fusing nuclei; thus they do not contribute heavily to the natural abundances of the elements. Nonetheless, this insight raised the plausibility of explaining all of the natural abundances of elements in this way. The prime energy producer in our Sun is the fusion of hydrogen to form helium, which occurs at a solar-core temperature of 14 million kelvin.

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