AST 341 Lecture Notes - Lecture 13: Triple-Alpha Process, Nuclear Reaction, Cno Cycle

• School: Stony Brook University
• Department: Astronomy
• Course: AST 341
• Professor: Rosalba Perna

Document Summary

Let"s now look in more detail at the speci c nuclear reactions that occur in stars. The most abundant element in newly born stars is hydrogen, with z = 1. Fusion of hydrogen into the next element, helium, with z = 2, would require an encounter of three or four protons - hydrogen nuclei - within a distance on the order of 1013cm. The probability of such a multiple encounter is vanishingly small. Therefore, the process by which hydrogen is, eventually, turned into helium does not happen at once but gradually, through a chain of reactions, each involving the close encounter of only two particles. Here we see two protons (hydrogen nucleus) on the left, resulting in a deuterium (proton. + neutron), a positron, and an electron neutrino. This is the critical reaction in the p-p chain (the one that determines its rate). It is slow because forming deuterium from two protons requires transforming a proton into a neutron.