Stellar Nucleosynthesis - Key Reactions - Hydrogen Burning

Hydrogen Burning

"Hydrogen burning" is an expression that astronomers sometimes use for the stellar process that results in the nuclear fusion of four protons to form a nucleus of helium-4. (This should not be confused with the combustion of hydrogen in an oxidizing atmosphere.) There are two predominant processes by which stellar hydrogen burning occurs.

In the cores of lower mass main sequence stars such as the Sun, the dominant process is the proton-proton chain reaction (pp-chain reaction). This creates a helium-4 nucleus through a sequence of chain reactions that begin with the fusion of two protons to form a nucleus of deuterium. The subsequent process of deuterium burning will consume any pre-existing deuterium found at the core. The pp-chain reaction cycle is relatively insensitive to temperature, so this hydrogen burning process can occur in up to a third of the star's radius and occupy half the star's mass. As a result, for stars above 35% of the Sun's mass, the energy flux toward the surface is sufficiently low that the core region remains a radiative zone, rather than becoming convective. In each complete fusion cycle, the p-p chain reaction releases about 26.2 MeV.

In higher mass stars, the dominant process is the CNO cycle, which is a catalytic cycle that uses nuclei of carbon, nitrogen and oxygen as intermediaries to produce a helium nucleus. During a complete CNO cycle, 25.0 MeV of energy is released. The difference in energy compared to the p-p chain reaction is accounted for by the energy lost through neutrino emission. The CNO cycle is very temperature sensitive, so it is strongly concentrated at the core. About 90% of the CNO cycle energy generation occurs within the inner 15% of the star's mass. This results in an intense outward energy flux that can not be sustained by radiative transfer. As a result, the core region becomes a convection zone, which stirs the hydrogen burning region and keeps it well mixed with the surrounding proton-rich region. This core convection occurs in stars where the CNO cycle contributes more than 20% of the total energy. As the star ages and the core temperature increases, the region occupied by the convection zone slowly shrinks from 20% of the mass down to the inner 8% of the mass.

The type of hydrogen burning process that dominates inside a star is determined by the temperature dependency differences between the two reactions. The pp-chain reaction starts at temperatures around 4×106 K, making it the dominant mechanism in smaller stars. A self-maintaining CNO chain requires a higher temperature of approximately 15×106 K, but thereafter it increases more rapidly in efficiency than the pp-chain reaction as the temperature grows. Above approximately 17×106 K, the CNO cycle becomes the dominant source of energy. This temperature is achieved in the cores of main sequence stars with at least 1.3 times the mass of the Sun. The Sun itself has a core temperature of around 15.7×106 K and only 0.8% of the energy being produced in the Sun comes from the CNO cycle. As a main sequence star ages, the core temperature will rise, resulting in a steadily increasing contribution from its CNO cycle.

Once a star with about 0.5–10 times the mass of the Sun has consumed nearly all the hydrogen at its core, it begins to evolve up the red giant branch. Hydrogen burning will occur along a shell surrounding an inert helium core. This will continue until the steadily increasing core temperature exceeds 1×108 K, at which point helium burning begins with a thermal runaway process called the helium flash. Hydrogen burning continues along a thin shell surrounding the helium core.