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## Homework Statement

A binary star system has a period of 90 days. It consists of two equally massed stars each with a mass of twice that of the sun, that rotate like a dumbbell about the center of mass at the midpoint between them. How far apart are these stars?

## Homework Equations

F = ma v = 2 pi r/T force of g = GMm/r^2 distance = 2r

## The Attempt at a Solution

I set GMm/r^2 = m v^2/r then solve for r and d is 2 r. My problem is that my solution sheet uses force of g = GM^2/(2r)^2 and then force = to M v^2/r I know this has something to do with the fact that we have two stars and therefore, I guess M times M but I am confused... Could someone please explain why the equations used in the solution sheet are the correct ones and why in turn mine are incorrect??? Thanks, Frostking