Convection zone

The Schwarzschild criterion expresses the conditions under which a region of a star is unstable to convection.

In main sequence stars more than 1.3 times the mass of the Sun, the high core temperature causes nuclear fusion of hydrogen into helium to occur predominantly via the carbon-nitrogen-oxygen (CNO) cycle instead of the less temperature-sensitive proton–proton chain.

The high temperature gradient in the core region forms a convection zone that slowly mixes the hydrogen fuel with the helium product.

This combination of circumstances produces an outer convection zone, the top of which is visible in the Sun as solar granulation.

This causes dredge-up events, short-lived very deep convection zones that transport fusion products to the surface of the star.

An illustration of the structure of the Sun
An illustration of the structure of the Sun and a red giant star, showing their convective zones. These are the granular zones in the outer layers of the stars.