Determination of Masses in Binaries

The determination of masses in binary systems generally uses Kepler's 3rd Law,

where P is the orbital period, m₁ and m₂ are the respective masses, and R = d₁ + d₂, and the "seesaw equation" for the center of mass:

From the first of these equations, if the period P and the average separation R are known, we can solve for the total mass M = m₁ + m₂ of the binary system. Then, if we know enough about the orbits to determine the distances d₁ and d₂ separately, the second equation can be used to determine the individual masses m₁ and m₂.