Star spectraStar spectra reflect the physical conditions in the thin layer in which a star becomes virtually “transparent” in the optical spectral range. This layer is usually referred to as the photospherePhotosphere and is, so to speak, the visible part of a star’s atmosphere. The entire stellar atmosphere, which also includes outer parts that are completely transparent in the optical spectral range (in the case of the sun, for example, the chromosphere and corona, and in certain stars also ejected shells and denser parts of “stellar winds”), can best be imagined as a transition region between the quasi-transparent plasma of the star’s interior and the interstellar medium.