At the bend of the Big Dipper's handle is the striking pair of Zeta and 80 Ursae Majoris, or Mizar and Alcor. They are nearly as well-known as the Dipper itself. The name Mizar derives from the Arabic word "mi'zar", meaning the "groin" or "girdle" of the celestial Bear, and was mistakenly drawn from Merak, in the Dipper's bowl. The name Alcor was stolen from that for the star Alioth, at the end of the Dipper's handle. Both Alcor and Alioth come from an Arabic word that means the "black horse."
Together, Mizar and Alcor were termed the "horse and rider" by the Arabs. Mizar is known as Vasistha, and Alcor as Arundhati, in Indian astrological books.
Components
At magnitude 2.06 and 4.01, Mizar and Alcor are a splendid binocular pair, separated by 11.8' (a fifth of a degree); an 8th-magnitude field star lies between them. The ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, though even people with rather poor eyesight can pass it.
Even without Alcor, Mizar takes its place in the celestial hall of fame as the first double star to be discovered by telescope (by Riccioli in 1650), the first star to be photographed (by Bond in 1857), and the first spectroscopic binary detected (by Pickering in 1889).
Low power in a small telescope will separate Mizar and Alcor at opposite ends of the field of view, and reveal Mizar's magnitude 3.95 companion, which is 14" away. More remarkably, each of these two components is also double - but their duplicity can only be detected spectroscopically, not visually. The brighter component (ζ UMa A) is a very close pair only 7 or 8 thousandths of an arcsecond apart, with orbital period of 20.5 days; the fainter (ζ UMa B) is a pair with a period of about half a year. Mizar is thus actually a quartet of stars. The two pairs are at least 500 AU apart, and take at least 5000 years to orbit each other.
Alcor appears to be moving together through space along with Mizar. But is it really a physical companion? We are still not sure. Precision parallax measurements by the Hipparcos satellite show Mizar to be 78 light years away, and Alcor at 81 light years. A separation of three light years would make a gravitational pairing unlikely. However, the measurement uncertainties allow a separation as close as 0.7 light years, and the uncertainty may be even greater than listed. Analysis of Mizar's orbit suggests that Mizar might actually be farther than Alcor! If Mizar and Alcor are actually at the same distance, their separation is only 0.27 light years - close enough that they could truly orbit each other, though with a period of 750,000 years.
Properties
All of Mizar's components are main-sequence, hydrogen-fusing stars like our Sun, but of white class A (the brighter both A2 V, the fainter probably A5 V or A7 V), with temperatures between 7500 K and 9000 K, and luminosities from 10 to 30 suns. The orbit of the brighter pair shows its components to have masses of 2.5 suns; the masses of the fainter pair are estimated at around 1.6 suns. The stars have odd chemical abundances as a result of slow rotation, which allows for quiet atmospheres and chemical separation. The brighter pair is rich in silicon and strontium, whereas the fainter are "metallic line stars" deficient in aluminum and calcium, but rich in silicon, cerium and samarium.
Alcor is also a white class A5 V star, with a temperature of 8000 K, a luminosity of 12 suns, and a mass around 1.6 suns. While the components of Mizar are slow rotators with peculiar chemical compositions as a result of elemental separation, Alcor is a rapid spinner, rotating at least 218 km/sec. As a result, its atmosphere is stirred, and its composition normal. For a time Alcor was thought to be double, but it now appears to be single. It is, however, slightly variable.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]