1. The problem statement, all variables and given/known data A binary star consists of two stars that are orbiting a common centre. The only force acting on the stars is the gravitational force of attraction in a direction along the line joining the stars. a) Explain carefully why the total momentum of the binary is constant. b) Explain why the two stars are always in a diametrically opposite position. c) Hence explain why the two stars have a common period of rotation and why the inner star is the more massive of the two. 2. Relevant equations No equations needed. 3. The attempt at a solution a) The total momentum p = m1 x v1 + m2 x v2. Since there are no external forces acting on the binary system, momentum is by definition conserved. --> Does that also imply that the momentum is constant? Because I think that the momentum could vary during one complete revolution and still be conserved right? b) They must be in a diametrically opposite position in order for their velocity vectors to have opposite directions. If the were not diametrically opposite, the net momentum would not be constant. c) I have no clue here..