![]() In main sequence stars of less than about 1.3 solar masses, the outer envelope of the star contains a region where partial ionization of hydrogen and helium raises the heat capacity. In the most massive stars, the convection zone may reach all the way from the core to the surface. The core convection zone of these stars is overlaid by a radiation zone that is in thermal equilibrium and undergoes little or no mixing. The high temperature gradient in the core region forms a convection zone that slowly mixes the hydrogen fuel with the helium product. ![]() 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. See also: Radiation zone § Stability against convection The region of the star in which this happens is the convection zone. Its buoyancy will then cause it to continue to rise. its temperature changes relatively slowly as it expands) then the rising parcel of gas will remain warmer and less dense than its new surroundings even after expanding and cooling. the temperature changes rapidly with distance from the center of the star), or if the gas has a very high heat capacity (i.e. However, if the temperature gradient is steep enough (i.e. If the rising parcel cools to a lower temperature than its new surroundings, so that it has a higher density than the surrounding gas, then its lack of buoyancy will cause it to sink back to where it came from. As a result, the parcel will expand and cool. A parcel of gas that rises slightly will find itself in an environment of lower pressure than the one it came from. The Schwarzschild criterion expresses the conditions under which a region of a star is unstable to convection. Stellar convection consists of mass movement of plasma within the star which usually forms a circular convection current with the heated plasma ascending and the cooled plasma descending. ![]() In a radiation zone, energy is transported by radiation and conduction. Energy is primarily or partially transported by convection in such a region. These are the granular zones in the outer layers of the stars.Ī convection zone, convective zone or convective region of a star is a layer which is unstable due to convection. An illustration of the structure of the Sun and a red giant star, showing their convective zones.
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