Alpha Ursae Majoris

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Alpha Ursae Majoris
Ursa Major constellation map.svg
Red circle.svg

Location of α Ursae majoris (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Major
Right ascension 11h 03m 43.67152s[1]
Declination +61° 45′ 03.7249″[1]
Apparent magnitude (V) 1.79[2]
Spectral type K0III[3] + F0V[4]
U−B color index +0.93[2]
B−V color index +1.07[2]
Variable type Pulsating[5]
Radial velocity (Rv) –9[6] km/s
Proper motion (μ) RA: –134.11[1] mas/yr
Dec.: –34.70[1] mas/yr
Parallax (π) 26.54 ± 0.48[1] mas
Distance 123 ± 2 ly
(37.7 ± 0.7 pc)
Absolute magnitude (MV) –1.10 ± 0.04[7]
Companion α UMa B
Period (P) 44.4 yr
Semi-major axis (a) 0.603"
Eccentricity (e) 0.4
Inclination (i) 180°
Longitude of the node (Ω) 2000°
Periastron epoch (T) 1958.000
α UMa A
Mass 4.25[3] M
Luminosity 316[3] L
Surface gravity (log g) 2.46[8] cgs
Temperature 4,660[8] K
Metallicity [Fe/H] –0.20 ± 0.07[8] dex
Rotational velocity (v sin i) 2.6 ± 1.0[7] km/s
α UMa B
Mass ~1.6[3] M
Other designations
Dubhe, Ak, α Ursae Majoris, α UMa, Alpha UMa, 50 UMa, BD+62°1161, CCDM J11037+6145AB, FK5 417, GC 15185, HD 95689, HIP 54061, HR 4301, PPM 17705, SAO 15384, WDS J11037+6145AB[9]
Database references

Alpha Ursae Majoris (Alpha UMa, α Ursae Majoris, α UMa) is the second-brightest star in the constellation of Ursa Major (despite its Bayer designation of "alpha"). It has the traditional name Dubhe, and a rarer name Ak.


Alpha Ursae Majoris forms part of the Big Dipper (also known as the Plough or the Great Bear), and is the northern of the 'pointers' (or 'guards'), the two stars of Ursa Major which point towards Polaris, the North Star. Dubhe is about 123 light years away and is a giant star that has evolved away from the main sequence after consuming the hydrogen at its core. It is a spectroscopic binary with a main sequence companion α UMa B that has a stellar classification of F0V. The companion star orbits at a mean separation of about 23 astronomical units (AU) and completes an orbit every 44.4 years.[4] The yellow giant primary star shows rapid tiny pulsations at many frequencies, its brightness changing by less than a hundredth of a magnitude.[5]

There is another spectroscopic binary 8 arc minutes 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.[4]

Although it is part of the constellation of Ursa Major, it does not form part of the Ursa Major Moving Group of stars that have a common motion through space.[10]

Name and etymology[edit]

  • The traditional name Dubhe comes from the Arabic for "bear", dubb, from the phrase ظهر الدب الاكبر żahr ad-dubb al-akbar "the back of the Greater Bear". The other traditional name Ak was meaning The Eye.[11]
  • This star as Kratu, one of the Seven Rishis.[11]
  • In Chinese, 北斗 Běi Dǒu, meaning Northern Dipper, refers to an asterism consisting of α Ursae Majoris, β Ursae Majoris, γ Ursae Majoris, δ Ursae Majoris, ε Ursae Majoris, ζ Ursae Majoris and η Ursae Majoris. Consequently, α Ursae Majoris itself is known as 北斗一 Běi Dǒu yī, (English: the First Star of Northern Dipper) and 天樞 Tiān Shū, (English: Star of Celestial Pivot).[12]

In culture[edit]

Dubhe is the official star of the State of Utah. USS Dubhe (ID-2562) was a ship in the United States navy. The Danish National Home Guard Navy ship MHV 806 is named Dubhe.


Dubhe A & B
Dubhe "A" and faint companion binaries


  1. ^ a b c d e van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ a b c Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory 4 (99), Bibcode:1966CoLPL...4...99J 
  3. ^ a b c d Guenther, D. B.; Demarque, P.; Buzasi, D.; Catanzarite, J.; Laher, R.; Conrow, T.; Kreidl, T. (2000). "Evolutionary Model and Oscillation Frequencies for α Ursae Majoris: A Comparison with Observations". The Astrophysical Journal 530 (1): L45–L48. Bibcode:2000ApJ...530L..45G. doi:10.1086/312473. PMID 10642202. 
  4. ^ a b c d Tokovinin, A. A. (1997). "MSC - a catalogue of physical multiple stars". A & A Supplement series 124: 75. Bibcode:1997A&AS..124...75T. doi:10.1051/aas:1997181. 
  5. ^ a b Buzasi, D.; Catanzarite, J.; Laher, R.; Conrow, T.; Shupe, D.; Gautier, T. N.; Kreidl, T.; Everett, D. (2000). "The Detection of Multimodal Oscillations on α Ursae Majoris". The Astrophysical Journal 532 (2): L133. Bibcode:2000ApJ...532L.133B. doi:10.1086/312573. PMID 10715242. 
  6. ^ Wilson, Ralph Elmer (1953), General catalogue of stellar radial velocities, Carnegie Institution of Washington, Bibcode:1953QB901.W495..... 
  7. ^ a b Carney, Bruce W.; et al. (March 2008), "Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars", The Astronomical Journal 135 (3): 892–906, arXiv:0711.4984, Bibcode:2008AJ....135..892C, doi:10.1088/0004-6256/135/3/892 
  8. ^ a b c McWilliam, Andrew (December 1990), "High-resolution spectroscopic survey of 671 GK giants. I - Stellar atmosphere parameters and abundances", Astrophysical Journal Supplement Series 74: 1075–1128, Bibcode:1990ApJS...74.1075M, doi:10.1086/191527 
  9. ^ "DUBHE -- Spectroscopic binary", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2011-12-23 
  10. ^ Motz, Lloyd; Nathanson, Carol (1988). The Constellations: An Enthusiast's Guide To The Night Sky. Doubleday. p. 39. ISBN 978-0385176002. 
  11. ^ a b Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 438. ISBN 0-486-21079-0. Retrieved 2012-09-04. 
  12. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日