If the force of gravity, G*M*m/d^2, is exactly equal to the centripetal force, m*v^2/d, keeping the orbiting objects moving in a circle within a galaxy of stars, how much mass, in total, must exist inside the orbit of a star that orbits three times further away from the center of the galaxy to keep it moving at the same linear speed as another star closer to the center as they both circle the core of the galaxy?