The questions are about binary star system.
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?
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.