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Convection is one of the two methods that energy from the core of a star is transported to the surface. (The other being thermal radiation.) Where convection occurs at a specific range of radii from the core, this is termed a convection zone. 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. A good example of convection current is a lava lamp.
For the Sun, the convection zone is located in the outer (roughly) 30% of the interior. Once hot gas convects up to the photosphere, it emits photons into space, cools, and settles back into the star. This convective flow is the origin of stellar granules, and the departing energy is the visible light and other electromagnetic radiation that the star emits into space.
Stars with the lowest temperatures, such as red dwarfs, are fully convective throughout. Medium-size stars, such as the Sun, are convective near the surface and the core region is in radiative equilibrium. Thus there is no mixing of the core region and the ashes of nuclear fusion accumulate. Stars above 1.1 times the mass of the Sun use a different nuclear process at their cores, called the CNO cycle. This process is very temperature sensitive, so the core region forms a convection zone that uniformly mixes the fuel with the ashes. The core convection zone of these stars is overlaid by a radiation zone that is in thermal equilibrium and undergoes little or no mixing.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Convection_zone". A list of authors is available in Wikipedia.|