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Unformatted text preview: 8. Stellar convection Reading: Shu, Vol.2, Ch. 10 8.1 Thermal convection We will note that in stratified media under a force (gravity!) a convection instability occurs when the temperature gradient is super-adiabatic. dT dr > dT dr adiab . (8 . 1) Gas that is left alone to relax to an equilibrium will tend to assume a stable stratification, known as the hydrostatic equilibrium. However, often the gas is subject to strong localized heating that can destroy stability. Examples are normal stars, in which nuclear fusion provides strong heating in the core, whereas radiation from the photosphere provides a cooling of the outer layers. Another example is the earth atmosphere that is heated by infrared emission from the surface of the earth. In the sun we posited that most of the energy transport from the core to the photosphere is provided by radiation that itself undergoes a random walk on acount of the high opacity. We expressed the diffusion coefficient and the transport timescale in terms of the Rosseland opacity κ R ≡ R ∞ dν ∂B ν ( T ) ∂T R ∞ dν 1 κ ν ∂B ν ( T ) ∂T (8 . 2) D = c 3 κ R = c l R 3 ⇒ τ dif ’ R 2 * D = 3 R * κ R R * c = 3 R * κ R τ free (8 . 3) If for some reason the opacity...
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