Mass is the most fundamental property of a star. In fact, the amount of total mass in a star and the way that mass is distributed among the various chemical elements essentially determines all the other properties of the star, including its size, luminosity, and surface temperature. But unlike size (which can be found from a star's apparent angular diameter and distance), luminosity (which can be calculated from the star's apparent brightness and its distance), and surface temperature (which can be deduced from spectroscopy), there is no easy means of measuring the mass of most stars. Mass determination can be accomplished only indirectly, by observing separations between objects and their motions; motions are changed by their mutual gravitational attraction, which depends on both masses and distances. Thus, knowledge of two factors (motion and separation) yields the desired mass. The simplest circumstance is if two stars are in orbit about each other forming a binary star system, then Kepler's Third Law may be applied to obtain the stellar masses from the size and period of the orbit. Analysis of orbital motion even today has yielded accurate masses for only a limited number (less than 100) of stars.