* Newton's Law of Gravitation: We can directly measure the mass of stars by observing their gravitational influence on other objects. Binary stars, by definition, have a companion star, allowing us to observe this gravitational interaction.

* Kepler's Laws of Planetary Motion: By observing the orbital period and separation of the binary stars, we can apply Kepler's laws to determine the combined mass of the system.

* Direct Measurement: For single stars, we can estimate their mass based on their luminosity and spectral type, but this is less precise than the method used for binaries.

In summary: Binary stars provide a natural laboratory for measuring mass through their gravitational interaction, making the process more direct and accurate than measuring the mass of single stars.