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Unformatted text preview: 7. Plane-parallel atmospheres Reading: Shu, Vol.I, Ch.4 7.1 LTE atmospheres We have seen that the photon mean free path in stars is many orders of magnitude smaller that the system size, the stellar radius R * . In the preceding chapter we have used the near isotropy of radiation thus implied to study the properties of the radiative energy transport in stars. When we now turn our attention to the stellar surface, we can use other simplifications that are justified if l R ¿ R * . • When integrating from infinity (the observer’s position) inwards, the transition zone from optically thin to optically thick regions will be a narrow layer in comparison with R * . In fact the stellar radius is defined by the location of the transition zone, where F rad = b T 4 = a 4 T 4 (7 . 1) after integration over the outward-oriented half-sphere. The resulting temperature, at which the star radiates as if it were a black-body, is called the effective temperature T eff ....
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