hey i've been having trouble understanding a part in this question. i'll just map out the basics to the question: 2 stars with a velocity of 220km/s are orbiting at equal distance. the masses of the stars are equal. period = 14.4 days it asks to find the mass of each star. i understand that u can use kepler's law to get the answer. GMm/r2 = mv2/r -which is what i used to come up with my answer seeing that i found the mass relative to a center mass. but according to the solution it's suppose to be: GMm/(2r)2 = mv2/r my question is, why is r denoted 2r in order to arrive at a correct answer?