Hydrogen Burning

Processes by which hydrogen (1H) is fused into helium (4He) with in a star. The five possible fusion paths can be divided into two sets of processes: the Proton-Proton (PP) process, which depends only on the amount of H and He in the star, and the CNO cycle (carbon-nitrogen-oxygen), which depends on the amount of these elements in addition to the amount of H and He in the star. There are three branches to the PP process of converting hydrogen (1H) into helium (4He). The first branch does the conversion without creating any nuclei heavier than He. The remaining two branches go through a step that creates Be. The core temperature determines which of these branches is dominant. The first PP process branch (shown) dominates in the production of He for core temperatures below roughly 15 million degrees (1.3 keV).The other two PP processes occur at higher core temperatures (15–25+ million degrees).

 

Image Source: outreach.atnf.csiro.au

The core temperature of a star rises with its mass, so the PP process dominates at low masses, and the CNO cycle dominates at high masses. For main-sequence stars with elemental abundances similar to the Sun, the conversion of H into He is equal for the two processes when a star is ~2 Msun. Below ~1.2 Msun, the contribution to the energy production from the CNO cycle is insignificant; this means that the Sun is powered only by the PP process. Above ~3 Msun, virtually all of the energy generated in a star comes from the CNO cycle. The minimum mass of a star is ~0.075 Msun. Below this mass, the core temperature never rises high enough for hydrogen fusion to begin.


Some or all content above used with permission from J. H. Wittke.