Introduction

For many observational astronomers who study the properties of binary stars, the ultimate goal of their work is to make direct determinations of the masses, radii, shapes, temperatures, and luminosities of the component stars, often referred to as the astrophysical parameters. The term absolute dimensions has been used to indicate that analyses of the radial-velocity curves and light curves for binaries really do provide descriptions of the stars in SI units, regardless of the distances of the binaries from us. As noted in Chapter 1, because the luminosities of the stars in binaries are determined directly, they act, potentially, as excellent standard candles for distance determinations amongst nearby galaxies. Much effort has been devoted to finding ways of ensuring that such data are free from systematic errors and have the smallest possible random errors, so that direct comparisons can be made between these empirical results and the predictions from stellar-structure and stellar-evolution theories applied to binary stars. The main theme underlying Chapters 3–5 in this text has been to demonstrate the ways in which systematic errors can be overcome, and random errors minimized, by making use of spectroscopy and photometry at the best spectral and temporal resolution consistent with the observational task at hand. This chapter will summarize the progress that has been achieved in these directions amongst the different subclasses of binary stars.