In this chapter we deal with the material function κ(\(\varrho \), T ). While for the equation of state it was possible to use certain approximations (for instance, that of a perfect gas) without introducing too much error, this is almost impossible for the opacity. Although there are similar approximations (such as those for electron scattering or free–free transitions) they never hold for the whole star and are used only in simplifying approaches. Therefore nowadays, when solving the stellar-structure equations, one uses numerical opacity tables for different chemical mixtures, which give κ(\(\varrho \), T ) in the full range of \(\varrho \) and T.
KeywordsElectron Scattering Free Transition Chemical Mixture Neutral Hydrogen Atom Gaunt Factor
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