Mizar (Zeta Ursae Majoris) is a star in the constellation Ursa Major and is the second star from the end of the Big Dipper's handle. The name comes from the Arabic 'mi'zar', meaning a waistband or girdle. Mizar's apparent magnitude is 2.23 and its spectral class is A1 V. It lies about 78 light-years away from us.

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 A5 V. 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.

In Star Trek, this system was home to a Federation population in the 23rd century. Mizar II is the homeworld of the Mizarians, in the Mizar system. Due to the Mizarians' pacifist traditions, the planet was conquered six times in a matter of three centuries. Kova Tholl, a Mizarian, was one of the captives abducted with Captain Jean-Luc Piccard when he was replaced on the Enterprise by an alien clone (TNG: "Allegiance")

The fourth brightest star in the constellation Ursa Major and the middle star in the handle of the Big Dipper. It forms a well known naked-eye double with fourth magnitude Alcor (80 Ursae Majoris), 0.2° to the northeast.

Mizar is also famous as the first known true binary, in which a pair of stars that orbit each other, were seen telescopically, by Giovanni Riccoli in 1650. The components, Mizar A and B, lie 14" apart (at least 500 AU) and take some 5,000 years to complete one mutual circuit. Spectroscopic observations reveal that each of these components is itself a binary. The Mizar quartet moves through space at a similar speed and direction to Alcor, but measurements by the Hipparcos satellite suggest that Mizar and Alcor are separated by 2-3 light-years – too far for the systems to be physically associated.