Stellar evolution helium burning

Helium flash A rapid burst of nuclear reactions in the hydrogen-shell burning phase of stellar evolution. 

In this review we wish to discuss how observations of AGB stars can be used to determine the manner in which heavy elements are created during a thermal pulse, and how these heavy elements and carbon are transported to the stellar surface. In particular we wish to study how the periodic hydrogen and helium shell burning above a degenerate carbon-oxygen (C-0) core forms a neutron capture nucleosynthesis site that may eventually account for the observed abundance enhancements at the surfaces of AGB stars. In section II we discuss the nucleosynthesis provided by stellar evolution models (for a general review see [1]). In section III we discuss the isotopic abundances provided by nucleosynthesis reaction network calculations (see [2, 3]). In section IV we discuss how observations of AGB stars can be used to discriminate between the neutron capture nucleosynthesis sources (see [4]). And in section V we note some of the current uncertainty in this work. 

Fig. 9. A simplified view of the internal chemical structure of a star during the major phases of stellar evolution. The panels represent the main-sequence, through the giant-branch, helium-burning, asymptotic-giant branch and white dwarf phases. Filled circles and thick lines represent nuclear-burning regions. Not to scale...

Over the 13.7 billion years since the Big Bang, stars have burned nuclear fuel to maintain pressure support against gravitational contraction. In doing so, they have converted the hydrogen and helium left over from the Universe s earliest moments into the heavier elements that make nearly all of chemistry possible. This paper briefly reviews the evolution of stars, the mainline stages of stellar burning, and the side reactions that make Nature s heaviest elements. 

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