1. The problem statement, all variables and given/known data The questions are about binary star system. 2. Relevant equations Why the two stars in a binary star system are always diametrically opposite positions? Why the two stars have common period? Why the inner star is more massive? 3. The attempt at a solution For the period question, I'm thinking that they have same angular velocity, but I know that I should use force to explain why, but I don't know how to explain using gravitational force. For the mass question, I think it's because of conservation of momentum, but don't know if it is right or how to explain.