Mizar (? UMa / ? Ursae Majoris) is a star in the constellationUrsa 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. In Chinese it is ??????).
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. Astronomer Sir Patrick Moore has suggested that this in fact refers to another star which lies visually between Mizar and Alcor. Mizar and Alcor lie three light-years apart, and though their proper motions show they move together (they are both members of the Ursa Major Moving Group), it is not believed they form a true binary star system, but simply an optical binary.
More components of the Mizar system were discovered with the advent of the telescope and spectroscopy; a fine, easily-split visual target, Mizar was the first telescopic binary discovered—most probably by Benedetto Castelli who in 1617 asked Galileo Galilei to observe it. Galileo then produced a detailed record of the double star. Later, around 1650, Riccioli wrote of Mizar appearing as a double. The secondary star, Mizar B, 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 was 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 was later found to be a spectroscopic binary as well. In 1996 the components of the Mizar A binary system were imaged in extremely high resolution using the Navy Prototype Optical Interferometer.
The whole four-star system lies about 78 light-years away from us. 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.
Other Names
Mizar is known as Vasistha and Alcor is known as Arundhati in Indian astrological books.
Encyclopedia
top
