Mizar
Mizar, also called Zeta Ursae Majoris, is a star in the constellation of Ursa Major, as well as the second star from the end of the handle of the asterism known as the Big Dipper. Located 78 light years away, it is one of the most famous stars in the sky. Mizar was the first star to be discovered both as a telescopic binary and as a spectroscopic binary.
Mizar in Culture and History
Originally known as Mirak, the current name is from the Arabic ميزر mīzar, meaning girdle or waist-cloth. Other forms of the name used are Mizat and Mirza. The name Mizar itself was originally a mistake, drawn from the star Merek, or Beta Ursae Majoris, but has become the accepted name over time. The star was also called Anāḳ al Banāt, meaning "the Necks of the Maidens. In Indian astrology, the star was called Vashishṭha, one of the seven sages.[1]
Much of the historical fame for the star comes from its coupling with the nearby star Alcor, or 80 Ursae Majoris. Like Mizar, Alcor is visible in the night sky to the unaided eye, and together the two stars are the asterism known as the Horse and Rider. It is said in Arabic literature that only those with the best sight can see Alcor along side Mizar, and the ability to resolve both stars has long been a measure of good eyesight. Alcor does have an apparent magnitude of 3.99, making it readily visible to the unaided eye in dark skies, although much more dim then Mizar's magnitude of 2.27. Although the two stars share the same proper motion, the two stars are actually three light years apart. Most likely then they are an optical binary, and are not part of the same system.
Mizar itself is a system made out of four stars, grouped into two binaries. In 1617 Benedetto Castelli first discovered the secondary star, Mizar B, while observing the Mizar system. Castelli asked Galileo Galilei to observed the star further, who did and who produced a detailed account of the two stars. Later in 1889, it was discovered that Mizar A itself was a binary star when he found the companion using a spectrometer, making the binary pair the first to be discovered using this method. Later in 1908, spectral analysis lead to the further discovery of Mizar B also being a spectroscopic binary.[2]
The Star System
Both the primary star of the Mizar A binary and its companion are white main sequence dwarf of spectral type A2 V. Both stars have a mass 2.5 times that of our own Sun with a surface temperature of around 9000 K, making them virtual twins. The pair's orbit is highly elliptical, with the distance between the two stars ranging from 16 to 54 million kilometers. Their mutual orbit, which would fit inside Mercury's orbit of our Sun, takes 20.5 days to complete.[2]
The Mizar B binary pair are also white, main sequence dwarfs. The two stars together have an apparent magnitude of 3.95, normally allowing them to be seen in the night sky, if they were not so close to Mizar A. Their spectral class is A5-7V, making them slightly cooler and fainter then Mizar A. Each of this fainter pair is around 1.6 times the mass of our Sun with a surface temperature of 7500 K. The Mizar B pair is separated from the Mizar A pair by and orbit of 340 AU, equivalent to eight times the distance of Pluto from the Sun.[3][4]