In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e. a binary system of stars in mutual orbit, gravitationally bound to each other, or because it is an optical double, a chance alignment of two stars in the sky that lie at different distances. Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters. Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs. If the relative motion of a pair determines a curved arc of an orbit, or if the relative motion is small compared to the common proper motion of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical. Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars. There are three types of paired stars: \n* Optical Doubles are unrelated stars that appear close together through chance alignment with Earth. \n* Visual Doubles are stars whose binary status was deduced through more esoteric means, such as occultation (eclipsing binaries), spectroscopy (spectroscopic binaries), or anomalies in proper motion. (astrometric binaries). \n* Visual Binaries are gravitationally-bound stars that are separately visible with a telescope. Conceptually, there is no difference between the two visual categories, and improvements in telescopes can shift previously double stars into visual binaries, as happened with Polaris in 2006. Thus it is only the inability to telescopically observe or detect orbital motion in the visual double stars that differentiates these two groups.

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