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Mizar is a star in the constellation Ursa Major, lying just at the corner of the Big Dipper's handle. The name comes from the Arabic Mi'Zar, meaning a waistband or girdle; the Bayer designation of the star is Zeta Ursae Majoris. Mizar has apparent magnitude 2.40 and spectral class A1 V.

With good eyesight one can make out a faint companion just to the east, called Alcor or 80 Ursae Majoris. The two are often called the horse and rider, and the ability to see the second is a traditional test of eyesight. Alcor has magnitude 4.02 and spectral class A5 V. The two stars lie more than a quarter of a light year apart but proper motions show they actually do form a binary star system, not an optical binary as previously thought.

More components were discovered with the advents of the telescope and spectroscopy. Mizar was the first telescopic binary discovered, by Riccioli in 1650. The secondary star has magnitude 4.0 and spectral class A7, and comes within 380 AU of the primary; the two take thousands of years to revolve around each other. Mizar A then became the first spectroscopic binary to be discovered, by Pickering in 1889. The two components are both about 35 times as bright as the sun, and revolve around each other in about 20 days. Mizar B and Alcor were both later found to be spectroscopic doubles as well.

The whole six star system lies about 78 light-years away from us. The components are all members of the Ursa Major moving cluster, a mostly dispersed group of stars sharing a common birth, as determined by proper motion. The other stars of the Big Dipper, except the end two, belong to this group as well.

For epoch 2000, Mizar has right ascension 13'24" and declination +54.9.

Mizar is also the name of a language for writing strictly formalized mathematical definitions and proofs, for a computer program to check these proofs, and for a library containing definitions and theorems. See Mizar system.