Two stars that appear close to one another on the sky. Such pairs may be divided into two classes: optical doubles, where the components are not gravitationally bound; and physical doubles, in which the stars are orbiting their common barycentre. The term ‘double star’ is often restricted to the former group, and the term binary star is used for the latter. In fact, optical doubles are relatively uncommon, and the majority of doubles are actually true binary systems.
Subjects: Astronomy and Astrophysics.