If we are very lucky, the stars lie in a flat plane, and pass in front
of each other once or possibly twice each orbit (an eclipsing binary):
If we are able to determine the size of the orbit, we can
solve directly for the exact masses of the two stars using Kepler's
3rd Law. Here, m and M are the masses (in Sun Masses)
of the two stars, a is the average distance between the stars
(in A.U.), and P is the period (in years) of their orbit.
Binary stars with masses 2 Mo and 3 Mo, have
a period of 3 years. What is the average distance between the
two stars?