So here's my problem: The distance between two stars is constant(d = 4,3 * 1010m), and they have a common center of mass. Ms = mass of our star, ma = 0,82 * Ms and mb = 2,2 * Ms. What I'm supposed to do is calculate the period of orbit of both stars, which is the same for them both, since the distance between them stays constant. I knew to set the centripetal force equal to the gravitational force for star A as follows: (Gmamb)/(ra + rb)2 = ma * ((2pi*ra)/T)2/ra. I know how to solve this equation for T. However, I'm missing some required information. Looking at the answer sheet, it says that because the stars have a common center of mass, mara = mbrb => mara = mb(d-rb). I do not understand where this relationship comes from. At all. It's the only missing information I need to solve the problem. Is there a simple explanation for this relationship? The thing that bugs me is that we didn't go over this information in class. Is it really that simple, that I'm just supposed to know this? Even our book says nothing about this, at least not the chapter that is related to this problem. EDIT: There are chapters later on in the book related to torque and things along those lines, but this problem can't seriously expect me to know stuff we haven't covered yet. Because if it does, the authors of this book messed up when they were writing it.