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. The core convection zone of these stars is overlaid by a radiation zone that is in thermal equilibrium and undergoes little or no mixing. In the most massive stars, the convection zone may reach all the way from the core to the surface.