1. The problem statement, all variables and given/known data Plaskett’s binary system consists of two stars that revolve in a circular orbit about a center of mass midway between them. This statement implies that the masses of the two stars are equal. Assume the orbital speed of each star is 220 km/s and the orbital period of each is 14.4 days. Find the mass M of each star. 2. Relevant equations 3. The attempt at a solution Kelper's 3rd Law Mass of the planet = (4(pi)^2(r)^3)/GT^2 T = 1244160 r = 8.72 x 10^10 G = 6.672 x 10^-11 Mass = 2.54 x 10^32, but the answer is 1.26 x 10^32 which is exactly half of my answer why should I further divide my answer by 2? Is it because there are two planets in the system? But if I use the same equation to calculate the Sun's mass, I just need to substitute all those unknown into the equation without the need to consider the mass of the Earth. So what's wrong? Thanks in advance!