Although the Struve Symposium was planned to discuss conditions in binary stellar systems, it is necessary to consider the observations of extended atmospheres in single stars in order to compare them with those of binary systems. The Sun, being the closest star, has the first chromosphere that was observed, and we can assume that the many features observed on the Sun will be much enhanced in giant stars with more extended atmospheres. Among the early-type stars, the hydrogen emission lines in the spectra of Be stars are known to vary and this suggests the atmospheres are unstable. Similar, even more pronounced effects, are seen in the spectra of shell stars such as 48 Librae. The supergiant stars have also been shown to have spectral lines that change, often quite rapidly, with time. Smolinski has observed in the spectra of some F and G-type supergiants much-broadened absorption lines that also vary with time. Wilson's systematic studies of the H and K emission lines in late-type stars have greatly extended our knowledge of stellar chromospheres, and the ionized magnesium lines at 2800 Å should give additional information as more extra-terrestrial observations are obtained. Numerous observations of the ζ Aur stars have been obtained near eclipse; ζ Aur and 31 Cyg seem to be similar systems and show similar phenomena – a rise in temperature outward from the photosphere and an extensive, variable calcium chromosphere, but a viable theory for these chromospheres has not yet been given. VV Cep and 32 Cyg probably are larger and more complex systems. The emission lines in the ultraviolet spectra of the M-type stars may be indicators of coronae in these cooler atmospheres. Most variable stars seem to have extended atmospheres and, when detailed studies are made, show changes in both light and spectrum. Shock-wave phenomena are undoubtedly important in the analysis of many of these stars, as also are fluorescent, selective-excitation processes. Flare stars, though generally dwarfs, are of great interest in studies of stellar atmospheres.
Extended atmospheres can be detected from observed gross deviations from local thermodynamic equilibrium, as in the study of emission lines, large velocity fields, and dilution effects. It is probable that the presence of a companion affects the atmosphere of the primary star in close double stars, but in stars like ζ Aur and 31 Cyg, the atmospheric effects produced near eclipse may be among the best clues for the interpretation of extended stellar atmospheres.