Dubhe, designated Alpha Ursae Majoris (α UMa), is a multiple star system in the constellation Ursa Major, notable for being one of the two pointer stars in the Big Dipper asterism that guide observers to Polaris, the North Star.[1][2] The system is located approximately 123 light-years from Earth and has a combined apparent visual magnitude of 1.79, making it the 33rd-brightest star in the night sky and the second-brightest in its constellation after Alioth.[1][2]Positioned at right ascension 11h 03m 43.7s and declination +61° 45' 04" (J2000 epoch), Dubhe lies in the northern celestial hemisphere and is circumpolar for observers above latitude 28°N, remaining visible year-round from mid-northern latitudes.[1] It exhibits a proper motion of -134.11 mas/year in right ascension and -34.70 mas/year in declination, indicating gradual movement across the sky relative to background stars.[1] As part of the Big Dipper's bowl, Dubhe forms the upper edge opposite the handle, contributing to the asterism's role in navigation and timekeeping in various cultures.[3]The primary component, α UMa A, is an evolved orange giant of spectral type K0 III with an effective temperature of about 4,650 K, a mass of roughly 3.7 solar masses, and a radius 26.85 times that of the Sun.[2] Its companion, α UMa B, is a white main-sequence star of spectral type A2 V or A5 V, with a mass around 2.5 solar masses, separated by an orbital semi-major axis of 0.98 arcseconds (current separation ~0.8 arcseconds), resolvable with moderate to large telescopes.[1][2][4] An additional faint F-type companion, α UMa C (magnitude 7.15), orbits the pair at a wider separation of about 381 arcseconds, forming a hierarchical quadruple system, though its gravitational binding to the main binary is uncertain.[2][5] Dubhe A shows spectroscopic variability suggestive of pulsations, and the system is not associated with the nearby Ursa Major Moving Group.[2]The name "Dubhe" derives from the Arabic phrase dhubba or dubb, meaning "bear," specifically referring to the "back of the Greater Bear" in reference to its position in Ursa Major.[6] This nomenclature reflects the constellation's ancient depiction as a bear in Greek mythology, linked to the nymph Callisto transformed by Zeus, and it has been used in navigation since antiquity.[2] In Chinese astronomy, Dubhe is known as Tiān Shū (天樞), the "Celestial Pivot," the first star of the Northern Dipper asterism.[6]
System Overview
Location and Observational Characteristics
Dubhe is positioned in the northern celestial hemisphere within the constellation Ursa Major, with equatorial coordinates of right ascension 11ʰ 03ᵐ 43.67ˢ and declination +61° 45′ 03.7″ (epoch J2000).[7] Its apparent visual magnitude of 1.79 renders it the 33rd brightest star in the night sky, readily visible to the naked eye under clear conditions.[8] The star exhibits an orange hue, characteristic of its primary component's K0III spectral classification as a late-type giant.[9]Given its declination exceeding 60°, Dubhe is circumpolar for northern latitudes above approximately 30° N, circling the celestial pole without setting and remaining observable year-round from such locations.[8] This visibility contributes to its prominence in northern skies, where it serves briefly as one of the pointer stars aligning toward Polaris.[2]Dubhe's proper motion across the sky measures -134.11 milliarcseconds per year in right ascension and -34.70 milliarcseconds per year in declination, indicating gradual southward and westward drift relative to distant background stars.[7] Additionally, its radial velocity is -9.4 km/s, showing the system approaches the Solar System along the line of sight.[7]
Role in the Constellation Ursa Major
Dubhe holds the designation Alpha Ursae Majoris, the standard Bayer designation for the brightest star in the constellation Ursa Major, even though it is actually the second-brightest star in the constellation after Alioth (Epsilon Ursae Majoris), which has an apparent magnitude of 1.77.[10][11] This anomaly arises from the historical method employed by