Question

describe the two cycles in hydrogen burning

Answer 1

Hydrogen burning is the fusion of 4 hydrogen nuclei (protons) into a single helium nucleus (2 protons + 2 neutrons). The fusion process takes place via a series of reactions. Exactly which reactions take place in a given star depends on its mass, and therefore its core temperature and density. In the Sun, this process is called the proton-proton chain. In stars somewhat more massive than the Sun, a different sequence of reactions, called the C-N-O cycle Carbon-Nitrogen-Oxygen, serves to fuse hydrogen into helium.