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When the core of a star is hot enough, due to gravitational contraction, atoms are stripped off their electrons and collisions between atomic nuclei trigger nuclear reactions: the star establishes its hydrostatic equilibrium by radiating away some of the nuclear energy, hence its specific surface temperature.Nucleosynthesis in main sequence stars involves fusion of 4 Hydrogen nuclei into Helium (He or α-particle) through a chain of reactions called the Proton-Proton chain (as first discovered by Hans Bethe in 1939).
At this stage Hydrogen occupies the outer layer which expands because of temperature increase. When Helium is exhausted energy generated by Helium burning ceases, giving way to gravitational contraction of the core.
Stellar nucleosynthesis provides clues not only to stellar evolution but also to space-time distribution of matter in the universe.
A probe to nucleosynthesis in our Galaxy is given by the chemical abundances in the solar system which testify for their abundance at the time of formation of the solar system.
The latter synthesizes the lightest, most neutron-poor, isotopes of the elements heavier than iron from preexisting heavier isotopes.
The theory that nucleosynthesis of the chemical elements occurred primarily during advanced evolution of massive stars was first proposed by Hoyle in 1954, in which he predicted the existence of the excited state in the C nucleus that enables the triple-alpha process to burn resonantly, enabling it to heat the helium cores of stars while synthesizing massive quantities of carbon and oxygen; and he introduced the thermonuclear sequels of carbon-burning synthesizing Ne, Mg and Na and of oxygen-burning synthesizing Si, Al and S.