Double star

In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes.

This occurs because the pair either forms a binary star (i.e. a binary system of stars in mutual orbit, gravitationally bound to each other) or is an optical double, a chance line-of-sight alignment of two stars at different distances from the observer.^[1][2] Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters. The only (possible) case of "binary star" whose two components are separately visible to the naked eye is the case of Mizar and Alcor (though actually a multiple-star system), but it is not known for certain whether Mizar and Alcor are gravitationally bound.^[3]

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.^[4] 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.

The following are three types of paired stars:

• Optical doubles are unrelated stars that appear close together through chance alignment with Earth.
• Visual binaries are gravitationally bound stars that are separately visible with a telescope.
• Non-visual binaries 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).

Improvements in telescopes can shift previously non-visual binaries into visual binaries, as happened with Polaris A in 2006.^[5] It is only the inability to telescopically observe two separate stars that distinguishes non-visual and visual binaries.