Gravitational potential energy of a system of two particles of masses $$m_1$$ and $$m_2$$, separated by distance $$r$$, is given by $$U=-\frac{Gm_1m_2}{r}$$. Consider two stars, each of mass $$M$$, initially separated by distance $$d$$ and at rest with respect to each other. The stars start moving towards each other under their mutual gravitational attraction. The stars can be treated as point objects and motion is assumed non-relativistic. As measured from the laboratory frame, the speed of each star when they are at a distance $$d/2$$ apart from each other is