 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 has been found that younger population I stars have generally lower <unk> than older , population II stars . The latter have elliptical orbits that are inclined to the plane of the galaxy . A comparison of the <unk> of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds , and are referred to as stellar associations . 
