A report on Double star, Mizar, Mizar and Alcor and Binary star

Mizar and Alcor are two stars forming a naked eye double in the handle of the Big Dipper (or Plough) asterism in the constellation of Ursa Major.

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.

Mizar is the second star from the end of the Big Dipper's handle, and Alcor its fainter companion.

It forms a well-known naked eye double star with the fainter star Alcor, and is itself a quadruple star system.

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 sure whether Mizar and Alcor are gravitationally bound.

The Mizar and Alcor system lies about 83 light-years away from the Sun, as measured by the Hipparcos astrometry satellite, and is part of the Ursa Major Moving Group.

Mizar, also designated Zeta Ursae Majoris (ζ Ursae Majoris, abbreviated Zeta UMa, ζ UMa), is itself a quadruple system and Alcor, also designated 80 Ursae Majoris (80 UMa), is a binary, the pair together forming a sextuple system.

Mizar is a visual double with a separation of 14.4 arcseconds, each of which is a spectroscopic binary.

Mizar, in Ursa Major, was observed to be double by Benedetto Castelli and Galileo.

The more general term double star is used for pairs of stars which are seen to be close together in the sky.

Early examples include Mizar and Acrux.

The Alcor–Mizar visual binary in Ursa Majoris also consists of six stars: four comprising Mizar and two comprising Alcor.