Astronomer's observations of binaries have been pivotal in our understanding of the masses of the stars.
Binaries consist of several sub-types:
Single-lined spectroscopic binaries have characteristic emission or absorption lines that enable astronomers to characterise their orbits using the mass function. In these systems the spectrum is dominated by one of the two stars. Spectroscopic binary systems are usually detected due to the movement of the emission and absorption lines in the observed spectrum, caused by the Doppler effect as the stars move in their orbit.
Eclipsing binaries suffer changes in their total luminosity due to the blockage of our line of sight to one or both of the stars. This allows deductions to be made about their orbital inclination, which must be nearly edge-on for the eclipse to occur. When combined with radial velocity curves and the mass function powerful constraints on the masses of the stellar components can be obtained.