# Double star

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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.[1][2] 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.[3] 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.[2] 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:

• optical doubles — unrelated stars which appear close together through chance alignment with Earth
• visual binaries — gravitationally-bound stars which are separately visible with a telescope
• non-visual binaries — stars whose binary status was deduced through more esoteric means such as occultation, spectroscopy, or anomalies in proper motion.

Conceptually, there is no difference between the latter two categories, and improvements in telescopes can shift previously non-visual binaries into the visual class, as happened with Polaris in 2006. Thus it is only our inability to observe the third group telescopically that makes the difference.

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### History

Mizar, in Ursa Major, was observed to be double by Giovanni Battista Riccioli in 1650[1][4] (and probably earlier by Benedetto Castelli and Galileo).[5] The identification of other doubles soon followed: Robert Hooke discovered one of the first double-star systems, Gamma Arietis, in 1664,[6] while the bright southern star Acrux, in the Southern Cross, was discovered to be double by Fontenay in 1685.[1] Since that time, the search has been carried out thoroughly and the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0.[7] At least 1 in 18 stars brighter than 9.0 magnitude in the northern half of the sky are known to be double stars visible with a 36-inch (910 mm) telescope.[8]