Observations of binary stars provide our information on the masses of stars. Binary star systems are held together by the gravitational force between the two stars. The larger the masses, the greater the gravitational force. The larger the gravitational force, the faster the stars must move in their orbits.
A useful limit is the case in which one star is much less massive, and moves in a circular orbit around the more massive star. Its motion is then described by the circular orbit equation which crops up all the time in astronomy. It is a relation between V, the speed at which the star moves, M the mass of the massive star, and R, the radius of the circular orbit. The circular orbit equation is
where 
is the gravitational constant, one of the
``Fundamental Physical Constants'', which in this case tells us how strong gravity is.