The first step is the fusion of four hydrogen nuclei to make one helium nucleus. This "hydrogen-burning" phase supplies energy to stars on the main sequence of the Hertzsprung-Russell diagram. There are two chains of reactions by which the conversion of hydrogen to helium is effected: the proton-proton cycle and the carbon-nitrogen-oxygen cycle (sometimes referred to simply as the carbon cycle). They were both first studied and proposed as sources of stellar energy by H. Bethe and independently by C. von Weiszäcker. The proton-proton cycle operates in less massive and luminous stars like the sun, while the carbon-nitrogen-oxygen cycle (which speeds up dramatically at higher temperatures) dominates in more massive and luminous stars.The Proton-Proton Cycle
In the proton-proton cycle, two hydrogen nuclei (protons) are fused and one of these protons is converted to a neutron by beta decay (see radioactivity) to make a deuterium nucleus (one proton and one neutron). Then a third proton is added to deuterium to form the light isotope of helium, helium-3. When two helium-3 nuclei collide, they form a nucleus of ordinary helium, helium-4 (two protons and two neutrons), and release two protons. In each of these steps considerable energy is also released.
The carbon-nitrogen-oxygen cycle requires minute traces of carbon as a catalyst. Four protons are added, one by one, to a carbon nucleus to form a succession of excited (unstable) nuclei of carbon, nitrogen, and oxygen. The intermediate nuclei shed their excess electric charge via beta decay and the final oxygen nucleus spontaneously splits into the original carbon nucleus and a helium-4 nucleus, releasing energy. The net effect is again the combination of four hydrogen nuclei to form one helium-4 nucleus; the carbon is free to begin the cycle over again.