Chapter 5.2 - Models of Solar and Stellar Atmospheres

Abstract

This chapter addresses the basic principles of the radiative transfer theory and nonequilibrium plasma physics, which were developed during the past century and are now widely used to model solar and stellar atmospheres. Many key results have been obtained and new numerical methods developed by solar physicists; they serve as a guide for stellar astrophysics. We start with the basics of radiation transport and demonstrate the classical ideas of the nonequilibrium line formation using a two-level atom example. Then we summarize the equations used to construct standard plane-parallel models of atmospheres in hydrostatic and radiative equilibrium and show examples of models with different levels of complexity. Although such models are well-suited for modeling photospheres, solar and stellar chromospheres require even more advanced approaches. The simplest is represented by so-called semiempirical models; we show some classical examples for the Sun and cool stars. A specific class of problems concerns isolated atmospheric structures illuminated by solar or stellar disk radiation. Based on various atmospheric models, we discuss how model spectra are synthesized and spectral lines are actually formed. Finally, we briefly introduce the topic of radiation hydrodynamics as the most sophisticated tool for solar and stellar atmospheric modeling.