High resolution problems in astrometry and spectroscopy of binaries and their significance for the progress of stellar physics

The information that is obtained from the study of close and wide physical binaries (visual and astrometric binaries and photometric and spectroscopic binaries) is discussed. Topics, of direct astrophysical importance such as the study of the density, the internal chemical composition, and the spectrophotometric properties of the stars are examined. The observed parameters are compared with those computed from stellar evolutionary models which are based on a limited number of initial conditions, auxiliary parameters, and hypotheses concerning the physical processes. The data are important for an understanding of the stars, the evolution of the Galaxy, and the formation of the elements. Some problems related to the binary nature of solar-mass stars are considered.