How the energy is transferred from core to outside of a star?
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Energy is transported by three mechanisms
Radiation - photons carry energy away from the star's center
Convection - cells of hot gas move up, cells of cool gas move down
Conduction - collisions between electrons can move energy outwards
In most stars, conduction is not important.
So we have a situation where a lots of energy is being created in the middle of a star. And it's coming out.
Radiation
Photons carry energy away. But photons also can be absorbed (and re-emitted) by atoms. So photons "random walk" their way out of a star, constantly being absorbed and re-emitted.
The efficiency of this process is clearly linked to how much absorbing is going on. If a photon is frequently absorbed, the efficiency of radiative transport is low -- the photon can't travel very quickly. The absorbing power of material is called its opacity.
If the opacity is high, or if the density is high, or if the flux of photons is high, radiative transport alone is not sufficient to get the energy out fast enough. What is meant by "fast enough?"
Under these situations, convection takes over.
Convection
Hot gas rises, cool gas falls. Bulk motion inside the star moves energy outwards. If a parcel of gas is heated (ie by energy flux from deeper inside the star) to temperatures which are larger than the surrounding gas, it will rise. As it rises, it expands and cools . Once it thermalizes, it will stop rising.
Radiation - photons carry energy away from the star's center
Convection - cells of hot gas move up, cells of cool gas move down
Conduction - collisions between electrons can move energy outwards
In most stars, conduction is not important.
So we have a situation where a lots of energy is being created in the middle of a star. And it's coming out.
Radiation
Photons carry energy away. But photons also can be absorbed (and re-emitted) by atoms. So photons "random walk" their way out of a star, constantly being absorbed and re-emitted.
The efficiency of this process is clearly linked to how much absorbing is going on. If a photon is frequently absorbed, the efficiency of radiative transport is low -- the photon can't travel very quickly. The absorbing power of material is called its opacity.
If the opacity is high, or if the density is high, or if the flux of photons is high, radiative transport alone is not sufficient to get the energy out fast enough. What is meant by "fast enough?"
Under these situations, convection takes over.
Convection
Hot gas rises, cool gas falls. Bulk motion inside the star moves energy outwards. If a parcel of gas is heated (ie by energy flux from deeper inside the star) to temperatures which are larger than the surrounding gas, it will rise. As it rises, it expands and cools . Once it thermalizes, it will stop rising.
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Due to the photons. The photons carry out the energy from the core of a star
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