Stellar core

For an ordinary main sequence star, the core region is the volume where the temperature and pressure conditions allow for energy production through thermonuclear fusion of hydrogen into helium.

This energy in turn counterbalances the mass of the star pressing inward; a process that self-maintains the conditions in thermal and hydrostatic equilibrium.

[1] Main sequence stars are distinguished by the primary energy-generating mechanism in their central region, which joins four hydrogen nuclei to form a single helium atom through thermonuclear fusion.

Once stars with the mass of the Sun form, the core region reaches thermal equilibrium after about 100 million (108)[2][verification needed] years and becomes radiative.

The low-mass end of the VLMS range reaches about 0.075 M☉, below which ordinary (non-deuterium) hydrogen fusion does not take place and the object is designated a brown dwarf.

For stars above this mass, the energy generation comes increasingly from the CNO cycle, a hydrogen fusion process that uses intermediary atoms of carbon, nitrogen, and oxygen.

[7] The longest-lived stars are fully convective red dwarfs, which can stay on the main sequence for hundreds of billions of years or more.

[9] Stars with masses between about 0.4 M☉ and 1 M☉ have small non-convective cores on the main sequence and develop thick hydrogen shells on the subgiant branch.

They spend several billion years on the subgiant branch, with the mass of the helium core slowly increasing from the fusion of the hydrogen shell.

When the mass exceeds that limit, the core collapses, and the outer layers of the star expand rapidly to become a red giant.

[9] Once the supply of hydrogen at the core of a low-mass star with at least 0.25 M☉[8] is depleted, it will leave the main sequence and evolve along the red giant branch of the Hertzsprung–Russell diagram.

This event is not observed outside the star, as the unleashed energy is entirely used up to lift the core from electron degeneracy to normal gas state.

[12] Hence, as the star ages, the core continues to contract and heat up until a triple alpha process can be maintained at the center, fusing helium into carbon.