Complex sentence: As astrometry became more accurate, cluster stars were found to share a common proper motion through space, while spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars born at the same time and bound together as a group.
Split into simpler sentences: As astrometry became more accurate, cluster stars were found to share a common proper motion through space. Spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars born at the same time and bound together as a group.