When both rates of movement are known , the space velocity of the star relative to the Sun or the galaxy can be computed . Among nearby stars , it have been found that younger population I star have generally lower velocities than older , population II stars . The latter have elliptical orbits that are inclined to the plane of the galaxy . A comparison of the kinematics of nearby stars have allowed astronomers to trace their origin to common points in giant molecular clouds , and are referred to as stellar associations .