text: Apart from the Sun and stars which act as gravitational lenses , this can be done only in binary and multiple star systems , making the binary stars an important class of stars . In the case of a visual binary star , after the orbit and the stellar parallax of the system has been determined , the combined mass of the two stars may be obtained by a direct application of the Keplerian harmonic law .
simplified: A binary star is two stars which orbit around each other .