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Revision as of 22:58, 9 June 2010
 Mizar and Alcor in constellation Ursa Major.
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With normal eyesight one can make out a faint binary system just to the east of Mizar, the second star from the end of the Big Dipper's handle, named Alcor or 80 Ursae Majoris. Alcor is of magnitude 3.99 and spectral class A5V. 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 was long believed they do not form a true binary star system, but simply a double star. However, in 2009, it was reported by astronomer Eric Mamajek and collaborators that Alcor actually is itself a binary, consisting of Alcor A and Alcor B, and that this binary system is most likely gravitationally bound to Mizar, bringing the full count of stars in this complex system to six.[1] Their study also demonstrated that the Alcor binary and Mizar quadruple are much closer together than previously thought - approximately 74,000 ± 39,000 Astronomical Units.[2]
The whole four-star system lies about 78 light-years away from Earth. 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.
History
In Japanese mythology, Alcor is known as the lifespan star or "jumyouboshi" （寿命星）as it was believed that one who could not see this star would pass away by year's end. Of incidental note, the popular Japanese manga, Fist of the North Star, used this legend as a model for its death omen star (死兆星), in which it was said that people who saw the star would die later in the year.
Other Names
Alcor is known as Arundhati in Indian astrological books. Mizar is Chickadee and Alcor is his cooking pot in the Mi'kmaq myth of the great bear and the seven hunters.
See also
References
External links
- Alcor article at Jim Kaler's Stars website
- Shichōsei on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
Template:Stars of Ursa Major Coordinates: 13h 25m 13.5s, +54° 55′ 31″