An illustration of the concept of proper motion. Proper motion is the actual motion of a star across the sky. Two or more stars have "common proper motion" if they move together through space. If a companion candidate is a background star, it does not move together with the central star. Therefore, their relative configuration changes over time. In contrast, when the companion candidate is a true companion, these two objects have the same proper motion and move together. Thus their relative configuration does not change over time. Credit: NAOJ