|Designation|| ζ Ursae Majoris (Mizar)|
80 Ursae Majoris (Alcor)
|Right ascension|| 13h 23m 55.5s (Mizar)|
13h 25m 13.5s (Alcor)
|Declination|| +54o 55′ 31″ (Mizar)|
+54o 59' 17" (Alcor)
|Type of object||Binary star (Sextuplet)|
|Magnitude|| Apparent: 2.23 Absolute: 0.33 (Mizar)|
Apparent: 3.99 Absolute: 2.01 (Alcor)
|Distance from Earth|| 78.1 ± 1 ly (Mizar)|
81.1 ± 1.2 ly (Alcor)
|Radial velocity||−9 km/s|
|Proper motion|| RA: 121.23 mas/yr|
Dec.: −22.01 mas/yr
|Parallax||41.73 ± 0.61 mas|
Mizar, also known as 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 later as a spectroscopic binary. Today the system is considered a Sextuplet of stars.
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.
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.
Mizar itself is a system made out of six stars, grouped into three 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.
For years some astronomers argued that the Mizar and Alcor were not a binary pair, but individual stars. Although the two star shared the same proper motion as part of the Ursa Major Moving Group (which includes most of the stars of the Big Dipper), it was thought because of their apparent distance from each other of three light years, that the two stars were only an optical binary and not gravitationally bound. Very recent discoveries in December 2009 by astronomers at the University of Rochester, however, reveal that Alcor and Mizar are gravitationally bound as members of a single system. They also discovered that Alcor itself is a binary star.
The Star System
As of December 2009, there are six known stars in the Mizar sextuplet system, grouped in three binary pairs.
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.
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.
Alcor itself is a binary pair. The primary star is a white dwarf star of spectral class A5 V, 12 times as visually luminous as our own Sun, although it puts out considerably more ultraviolet light. The surface temperature is measured at 8000 K. In the night sky, Alcor has an apparent magnitude of 3.99, making it readily visible to the unaided eye, although much more dim then Mizar's magnitude of 2.27. In December 2009, it was discovered Alcor has a companion known as Alcor B.
The apparent distance of Alcor from the other stars of the Mizar system caused astronomers to question if the two stars were actually gravitationally bound. Measurements using the Hipparcos satellite revealed that Alcor is 81.1 light years and Mizar is 78.1 light years from our Sun respectively, at such a distance, the influence of other stars would pull them apart. However the margin of error allows the stars to be as close as 0.7 light years. Some astronomers argue the error in the distances may be greater and the two stars are actually the same distance from our Sun. If this is true, they would be only 0.27 light years apart, close enough for a mutual orbit of 750,000 years.