Convection zone
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The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation. 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 everyday example of convection current is a lava lamp.
In a star, convection occurs when the temperature gradient is steep (i. e. the temperature changes rapidly with distance from the center of the star). All stars are hottest at their cores and coolest at the photosphere. Thus anywhere in a star, a parcel of gas that rises slightly will be hotter than the surrounding gas. If the rising parcel can exchange heat quickly enough with its cooler surroundings, it will cool and rise no further. However, if the temperature gradient is steep enough, the rising, warm parcel of gas will continue to rise because of its buoyancy faster than it can cool. The region of the star in which this happens is the convective zone.
In intermediate mass, main sequence stars like the Sun, the convection zone is located in the outer (roughly) 30% of the star. Once hot gas convects up to the photosphere, it cools and settles back into the star. Hot gas emits light more intensely than cool gas, so the rising and falling parcels of gas cause stellar granules. In these stars, the convective zone is an envelope around a radiative core. Thus there is no mixing of the core region and the helium produced in nuclear fusion accumulates.
In stars more than 1.1 times the mass of the Sun, the nuclear fusion of hydrogen into helium occurs via CNO cycle instead of the proton-proton chain. The CNO process is very temperature sensitive, so the core is very hot but the temperature falls off rapidly. Therefore, the core region forms a convection zone that uniformly 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.[1]
Low mass main sequence stars, such as red dwarfs, and red giants are convective throughout and do not contain a radiation zone.
[edit] See also
[edit] References
- ^ Brainerd, Jim (February 16, 2005). Main Sequence Stars. The Astrophysics Spectator. Retrieved on 2007-11-25.
[edit] General References
- Hansen, C. J., Kawaler, S. D., & Trimble, V. (2004). Stellar Interiors. Springer. ISBN 0387200894.
- Zeilik, M. & Gregory, S. A. (1998). Introductory Astronomy and Astrophysics. Brooks Cole. ISBN 9780030062285.
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