Mizar (ζ UMa, ζ Ursae Majoris) is a quadruple system of two binary stars in the constellation Ursa Major and is the second star from the end of the Big Dipper’s handle. Its apparent magnitude is 2.23 and its spectral class is A1V. Mizar’s name comes from the Arabic مئزر mīzar, meaning a waistband or girdle.)
With normal eyesight one can make out a faint companion just to the east, named Alcor or 80 Ursae Majoris. Alcor is of magnitude 3.99 and spectral class A5V.
Mizar and Alcor together are sometimes called the “Horse and Rider,” and the ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. Arabic literature says that only those with the sharpest eyesight can see the companion of Mizar. Astronomer Sir Patrick Moore has suggested that this in fact refers to another star which lies visually between Mizar and Alcor. The name the Arabs used for Alcor was سها (suha), meaning either the ‘forgotten’ or ‘neglected’ one.
As of 2007, the best estimates of Mizar and Alcor’s respective distances place them 1.1 light-years apart, and though their proper motions show they move together (they are both members of the Ursa Major Moving Group), it was long believed that they did not form a true binary star system, but simply a double star. However, in 2009, it was independently reported by two groups of astronomers (Eric Mamajek et al, and Zimmerman et al) that Alcor actually is itself a binary, consisting of Alcor A and Alcor B, and that this binary system is most likely gravitationally bound to Mizar, bringing the full count of stars in this complex system to six. These studies also demonstrated that the Alcor binary and Mizar quadruple are somewhat closer together than previously thought: approximately 74,000 ± 39,000 astronomical units or 0.5-1.5 light years.
The whole six-star system lies about 83 light-years away from Earth. The components are all members of the Ursa Major moving group, a mostly dispersed group of stars sharing a common birth, as determined by proper motion. The other stars of the Big Dipper, except Dubhe and Alkaid, belong to this group as well.