Despite its “alpha” designation, the orange giant star Dubhe (Alpha Ursae Majoris) with an apparent magnitude of +1.79 is the second-most luminous star in the constellation Ursa Major. It is also the 36th most luminous star in the entire night sky, and while Dubhe is included in the constellation of Ursa Major, it is not a member of the Ursa Major Moving Group (aka Collinder 285), which is a small group of stars that are believed to be remnants of an open cluster that now share a common proper motion towards a point in the constellation Sagittarius.
Quick Facts
• Constellation: Ursa Major
• Coordinates: RA 11h 03m 43.67152s |Dec. +61° 45′ 03.7249″
• Distance: 123 light years
• Star Type: K0III + F0V
• Mass: (a UMa A) 4.25 sol |(a UMa B) 1.6 sol
• Radius: 30 sol
• Apparent Magnitude: +1.79
• Luminosity: (a UMa A) 316 sol
• Surface Temperature: (a UMa A) 4,660K
• Rotational Velocity: (a UMa A) 2.6 km/sec
• Other Designations: Dubhe, Ak, a Ursae Majoris, a UMa, Alpha UMa, 50 UMa, BD+62°1161, CCDM J11037+6145AB, FK5 417, GC 15185, HD 95689, HIP 54061, HR 4301, PPM 17705,
Visibility
In terms of visibility, Dubhe is a naked-eye star located in Ursa Major, the third biggest constellation in the night sky which observers from the northern hemisphere can see all year round. It is one of the pointer stars that is found in the bowl of the asterism known as the “Big Dipper” or “Plough” in the UK, and together with the star Merak, Dubhe forms the front edge of the Dipper’s bowl. Together they are referred to as the Pointer Stars as a line extended through both stars always point toward Polaris, the North Star. Refer to the image below for details on the location of Dubhe, and its relationship relative to Merak and Polaris.
Physical Properties
Located about 123 light years away, Dubhe is an orange, K-type giant that has exhausted the hydrogen fuel at its core, and has subsequently evolved into its giant phase. In terms of luminosity, Dubhe is a suspected variable, although its brightness varies by only around one thousandth of a magnitude. Moreover, the star’s observed variability ranges over ten officially recognized radial oscillations with periods ranging between 6.4 hours, and 6.4 days.
Dubhe is also a spectroscopic binary, with its companion being an F7-type main sequence star, situated about 23 astronomical units from the primary component. Spectroscopic studies show that the orbital period for this pair is around 44.4 years, in an orbit with a 0.4 eccentricity. There is also second spectroscopic companion located 8 minutes of arc away from the primary star, which is a 7th magnitude F8-type star listed as Alpha Ursae Majoris C (HD 95638). In addition to the primary pair, another binary system is located about 90,000 astronomical units away, which makes Dubhe a 5-star system.
History
The stars’ traditional name derives from the Arabic phrase meaning “[the] back of the Great Bear”, while some Native American culture see the bowl of the Big Dipper as a celestial bear being chased by hunters represented by the Bowl’s handle. Hindus, on the other hand, believe Ursa Major’s seven brightest stars represents the Seven Sages of Vedas and Hindu literature known as the seven rishis, while in China, Dubhe is part of a seven star asterism called “Bei Dou”, meaning “The Northern Dipper”. Due to its position in this asterism, Dubhe is known as “Bei Dou yi”, or “Tian Shu”, meaning “[the] First Star of Northern Dipper” and “Star of Celestial Pivot”, respectively.