Alpha Ursae Majoris, formally named Dubhe, is, despite being designated "α", the second-brightest object in the constellation of Ursa Major.
Stellar system
Alpha Ursae Majoris forms part of the Big Dipper, and is the northern of the 'pointers', the two stars of Ursa Major which point towards Polaris, the North Star. α Ursae Majoris is about 123 light years from the Sun. It is a spectroscopic binary made up of the stars α Ursae Majoris A and α Ursae Majoris B. α Ursae Majoris A is the primary star, and it is a giant star that has evolved away from the main sequence after consuming the hydrogen at its core. The secondary star, α Ursae Majoris B, is a main sequence star that has a stellar classification of F0V. It orbits at a mean separation of about 23 astronomical units and completes an orbit every 44.4 years. There is another spectroscopic binary 8 arcminutes distant, a 7th magnitude pair showing an F8 spectral type. It is sometimes referred to as Alpha Ursae Majoris C, but is separately catalogued as HD 95638. α Ursae Majoris has been reported to vary in brightness by about a thousandth of a magnitude. Ten radial oscillation modes have been detected, with periods between 6.4 hours and 6.4 days. Although it is part of the constellation of Ursa Major, it is not part of the Ursa Major Moving Group of stars that have a common motion through space.
Nomenclature
α Ursae Majoris is the star system's Bayer designation. It bore the traditional names Dubhe and Ak.Dubhe derives from the Arabic for 'bear', dubb, from the phrase żahr ad-dubb al-akbar 'the back of the Greater Bear'. The rarer Ak means 'The Eye'. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Dubhe for the star α Ursae Majoris A. The Hindus refer to the star as Kratu, one of the Seven Rishis. In Chinese, 北斗 Běi Dǒu, meaning Northern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese name for Alpha Ursae Majoris itself is 北斗一 Běi Dǒu yī, and 天樞 Tiān Shū,.
