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Mizar(mÿ -zar) (ζ UMa) A white main-sequence dwarf that is one of the brighter stars of the Plough. It forms an optical double with the 4th-magnitude Alcor with a separation of 12′. But Mizar itself is also a double, a fact discovered by Riccioli in 1650. The two components of Mizar, A (the brighter) and B, are currently 14″ apart. It is thought that they complete one orbit about each other in 5000 years. Both components are spectroscopic binaries (see spectroscopic binary). Mizar A was the first spectroscopic binary to be identified (E.C. Pickering, 1889) and has a period of 20.5 days; Mizar B has a period of 361 days. All the stars in the Mizar system are main-sequence dwarfs. The brighter components are of spectral type A2, while the fainter ones are of A5 or A7. mv : 2.27; Mv : 0.07; spectral type: A2 V; distance: 23 pc.
also ζ Ursae Majoris, a star located at a distance of 27 parsecs from the sun and having a visual magnitude of 2.0 and a luminosity 74 times the solar luminosity. Mizar is actually a system of three stars. Displacement of the spectral lines of the components of Mizar was first discovered in 1889; these lines arise as a consequence of the orbital motion of the separate components about a common center of gravity.