Published online by Cambridge University Press: 08 February 2017
Theoretical methods applicable to the study of line formation in steady-state extended atmospheres are reviewed. The formal solution of the transfer equation is considered, as well as numerical and analytical methods of determining the source function. Topics discussed include: the local frequency transformation, geometrical effects, and the case of large velocity gradients. A new plane-parallel approximation for spherically symmetric moving atmospheres is given that takes account of transverse velocity gradients.