Stellar evolution hydrogen 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. 

Figure 20. Upper panel Evolution of the equatorial rotation velocity with time during the core hydrogen burning phase of four 60 M sequences with different initial rotation rates (see at t = 0). The evolution of the critical rotation velocity (Eq. 5.31) is displayed for the sequence with Vrot.init. = 100 kms-1 by the triangles. It is very similar for the other sequences. For Vcrit — ( rot, the stars evolve at the Q-limit. Lower panel Evolution of the stellar mass with time for the same 60M sequences. The initial equatorial rotation velocities are given as labels. For comparison, the evolution of a non-rotating star is shown in addition.

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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