Mizar and Alcor

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. Their magnitudes are 2.2 and 3.9, and the pair can easily be seen without the aid of a telescope. Mizar and its fainter companion Alcor are actually a senary system consisting of two pairs of double stars, one of which is itself a double binary system. The six stars are gravitationally bound to each other. The traditional name Mizar derives from the Arabic المئزر or mi'zar, meaning 'apron; wrapper, cover'. Alcor was originally the Arabic سها or suhā/sohā, meaning either 'the forgotten' or 'neglected one'. The ancient Persians and the Bedouins used distinguishing Mizar and Alcor as a test of vision.

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 star, the pair together forming a sextuple system. Mizar was the first known binary star system, claimed to be discovered by Italian astronomer Giovanni Battista Riccioli in 1650. The whole system lies about 83 light-years away from the Sun, as measured by the Hipparcos astrometry satellite.

With normal eyesight Alcor appears at about 12 arcminutes from Mizar. Alcor is of magnitude 3.99 and spectral class A5V. It has a faint red dwarf companion separated by 1 arcsecond.

Mizar and Alcor's proper motions show they move together, along with most of the other stars of the Big Dipper except Alpha Ursae Majoris and Eta Ursae Majoris, as members of the Ursa Major moving group, a mostly dispersed group of stars sharing a common birth. Parallax measurements by Hipparcos satellite indicate that Alcor and Mizar are somewhat closer together than previously thought: 0.36±0.19 pc. The uncertainty is due to not knowing exactly how far away the stars are. If they are exactly the same distance from us then the distance between them is only 17800 AU (0.281 ly). As Mizar is a quadruple and Alcor is a binary, the combined system would consist of six stars, being the second-closest known six-star system after Castor. The systems have been assigned a 100% probability of being gravitationally bound, as of 2011^[update].

Between Mizar and Alcor, the 8th-magnitude star Sidus Ludovicianum is a distant background object.

Benedetto Castelli, one of Galileo's colleagues in the 17th century, observed Mizar through a telescope and realized that it was a binary system: Mizar A and Mizar B. Then, throughout the 19th century and the beginning of the 20th century, with the help of spectroscopy, scientists showed that Mizar A and B were both binary systems. In 1908, the Alcor-Mizar system was the first apparent five-star system ever discovered.

In 2009, Eric Mamajek and his colleagues from the University of Rochester, while searching for exoplanets, discovered that Alcor was also a binary system, making Alcor and Mizar a six-star system. The same conclusion was independently found by Ben Oppenheimer from the American Natural History Museum.

Mizar is known as Vasishtha, one of the Saptarishi, and Alcor as Arundhati, wife of Vasishtha, in Indian astronomy. As a married couple, they are considered to symbolize marriage and in some Hindu communities to this day priests conducting a wedding ceremony allude to or point out the asterism as a symbol of the closeness marriage brings to a couple.