摘要： Our understanding of the Sun, other stars, and celestial objects in general is almost entirely based on observations and analysis of electromagnetic radiation in a broad wavelength range, from g-rays up to the radio. This radiation travels to us through the vast cosmic space, more than 8 min from the Sun and billions of years from distant universes. What we detect by our instruments is the result of a complex interaction between the photons and matter inside the objects under study, but also other interactions along the long path to us. To understand these interactions, we have to solve a coupled, generally nonlinear problem of how radiation is transported through such media and how the media, mostly various kinds of plasmas, are affected by penetrating radiation. In this way, we can model the spectrum emergent from various objects; this gives us a deep insight into the structure and dynamics of, e.g., solar and stellar atmospheres. We call this the spectral diagnostics. However, the actual state of an atmosphere may strongly depend on the spectral distribution of the internal radiation field via complex atomic processes, leading, for example, to heating or cooling of the plasma by radiation, along with other processes such as the heat conduction, wave dissipation, or magnetic heating. The whole complexity of these interactions is governed by a coupled system of (magneto)-hydrodynamical equations and equations describing the transport of radiation. This is called radiation hydrodynamics (RHD) and we will briefly describe it at the end of this chapter. On the other hand, considering only the radiative transfer in a prescribed model atmosphere, we deal with a “classical” astrophysical problem that was thoroughly studied during the past century, and to a great extent, the basic concepts were developed by solar physicists. This is usually called the theory of radiative transfer, which was regarded as a synonym of solar or stellar atmospheric physics (e.g., Physik der Sternatmospha¨ren by Unso¨ld, 1938). Among the atmospheres of various stars, the solar atmosphere has the unique privilege of providing us with insight into different structural patterns and their dynamical behavior. This then serves as a guide for our understanding of various phenomena on other stars, namely late-type cool stars. One of the most prominent stellar astrophysicists, D. Mihalas, who was engaged in both solar and stellar atmospheric research, once said, “My views have been colored by my long period of residence at the High Altitude Observatory in Boulder, where I was confronted daily by the ghastly reality of the Sun as seen at high time, spatial, and spectral resolution in a wide range of spectral bands. There is probably no more sobering an experience for a stellar atmospheres modeller than a detailed inspection of solar data!” (Mihalas, 1991). The theory of stellar atmospheres is summarized in an excellent textbook by Hubeny and Mihalas (2015).