Eta Ursae Majoris

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η Ursae Majoris
Eta Ursae Majoris is located in 100x100
Eta Ursae Majoris

Alkaid in the Big Dipper of Ursa Major.
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Major
Right ascension 13h 47m 32.43776s[1]
Declination +49° 18′ 47.7602″[1]
Apparent magnitude (V) +1.86[2]
Spectral type B3 V[3]
U−B color index –0.68[2]
B−V color index –0.19[2]
Radial velocity (Rv)–10.9[4] km/s
Proper motion (μ) RA: –121.17[1] mas/yr
Dec.: –14.91[1] mas/yr
Parallax (π)31.38 ± 0.24[1] mas
Distance103.9 ± 0.8 ly
(31.9 ± 0.2 pc)
Absolute magnitude (MV)−0.67[5]
Mass6.1 ± 0.1[6] M
Radius3.4[7] R
Luminosity594 ± 31[8] L
Surface gravity (log g)3.78[8] cgs
Temperature15,540 ± 1157[8] K
Rotational velocity (v sin i)150[9] km/s
Age10.0 ± 2.6[6] Myr
Other designations
Alkaid, Benetnash, Benetnasch,[10] Elkeid, η UMa, 85 UMa, BD+50°2027, FK5 509, GC 18643, HD 120315, HIP 67301, HR 5191, PPM 53742, SAO 44752.
Database references

Coordinates: Sky map 13h 47m 32.4s, +49° 18′ 47.8″Eta Ursae Majoris (Latinised from η Ursae Majoris, abbreviated Eta UMa, η UMa), formally named Alkaid /ælˈkd/,[11][12] is a star in the constellation of Ursa Major. It is the most eastern (leftmost) star in the Big Dipper (or Plough) asterism. However, unlike most stars of the Big Dipper, it is not a member of the Ursa Major moving group.[13] With an apparent visual magnitude of +1.84,[14] it is the third-brightest star in the constellation and one of the brightest stars in the night sky.

Physical properties[edit]

Eta Ursae Majoris is a 10-million-year-old[6] B-type main sequence star with a stellar classification of B3 V.[3] Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[15] It has six[6] times the mass; 3.4[7] times the radius, and is radiating around 594[8] times as much energy as the Sun. Its outer atmosphere has an effective temperature of about 15,540 K,[8] giving it the blue-white hue of a B-type star.[16] This star is an X-ray emitter with a luminosity of 9.3 × 1028 erg s−1.[17]

Eta Ursae Majoris was listed as a standard star for the spectral type B3 V. It has broadened absorption lines due to its rapid rotation, which is common in stars of this type. However, the lines are very slightly distorted and variable, which may be caused by some emission from a weak disk of material produced by the rapid rotation.[18]

Eta Ursae Majoris is a relatively nearby and bright star and has been examined closely, but no exoplanets or companion stars have been discovered.[19][20]


η Ursae Majoris (Latinised to Eta Ursae Majoris) is the star's Bayer designation.

It bore the traditional names Alkaid (or Elkeid from the Arabic القايد القائد) and Benetnasch /ˈbɛnɛtnæʃ/.[11] Alkaid derives from the Arabic phrase meaning "The leader of the daughters of the bier" (قائد بنات نعش qā'id bināt naʿsh). The daughters of the bier, i.e. the mourning maidens, are the three stars of the handle of the Big Dipper, Alkaid, Mizar, and Alioth; while the four stars of the bowl, Megrez, Phecda, Merak, and Dubhe, are the bier. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[21] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[22] included a table of the first two batches of names approved by the WGSN; which included Alkaid for this star.

It is known as Běidǒuqī (北斗七 - the Seventh Star of the Northern Dipper) or Yáoguāng (瑤光 - the Star of Twinkling Brilliance) in Chinese.

The Hindus knew this star as Marīci, one of the Seven Rishis.[23]

In Japan and Korea, Alkaid is known as Hagunsei and Mukokseong respectively ("the military breaking star" or "most corner star"). Both meanings come from ancient China's influence in both countries.

In culture[edit]

USS Alkaid (AK-114) was a United States Navy Crater class cargo ship named after the star.

In Chinese fortune-telling, north is believed to be a very unlucky direction. Northwest is even worse. Hunters and soldiers traditionally did not point guns and weapons in the direction of this star.[24]

See also[edit]


  1. ^ a b c d e van Leeuwen, Floor (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752v1, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600 Note: see VizieR catalogue I/311.
  2. ^ a b c Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
  3. ^ a b Morgan, W. W.; Keenan, P. C. (1973), "Spectral Classification", Annual Review of Astronomy and Astrophysics, 11: 29, Bibcode:1973ARA&A..11...29M, doi:10.1146/annurev.aa.11.090173.000333
  4. ^ Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, 35 (35): 1, Bibcode:1999VeARI..35....1W
  5. ^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  6. ^ a b c d Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x, S2CID 118629873
  7. ^ a b Underhill, A. B.; et al. (November 1979), "Effective temperatures, angular diameters, distances and linear radii for 160 O and B stars", Monthly Notices of the Royal Astronomical Society, 189 (3): 601–605, Bibcode:1979MNRAS.189..601U, doi:10.1093/mnras/189.3.601
  8. ^ a b c d e Baines, E.; Schmitt, H. R.; Zavala, R. T.; Hutter, D.; van Belle, G. T. (2017), "Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer", The Astronomical Journal, 155 (1): 30, arXiv:1712.08109, Bibcode:2018AJ....155...30B, doi:10.3847/1538-3881/aa9d8b, S2CID 119427037
  9. ^ Abt, Helmut A.; Levato, Hugo; Grosso, Monica (July 2002), "Rotational Velocities of B Stars", The Astrophysical Journal, 573 (1): 359–365, Bibcode:2002ApJ...573..359A, doi:10.1086/340590
  10. ^ Rumrill, H. B. (1936). "Star Name Pronunciation". Publications of the Astronomical Society of the Pacific. 48 (283): 139. Bibcode:1936PASP...48..139R. doi:10.1086/124681.
  11. ^ a b Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  12. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
  13. ^ Motz, Lloyd; Nathanson, Carol (1988). The Constellations: An Enthusiast's Guide To The Night Sky. Doubleday. p. 39. ISBN 978-0385176002.
  14. ^ Crawford, D. L.; Barnes, J. V.; Golson, J. C. (1971), "Four-color, H-beta, and UBV photometry for bright B-type stars in the northern hemisphere", The Astronomical Journal, 76: 1058, Bibcode:1971AJ.....76.1058C, doi:10.1086/111220
  15. ^ Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society, 25: 1319, Bibcode:1993AAS...183.1710G, retrieved 2012-02-04
  16. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
  17. ^ Cassinelli, J. P.; et al. (February 1994), "X-ray emission from near-main-sequence B stars", Astrophysical Journal, Part 1, 421 (2): 705–717, Bibcode:1994ApJ...421..705C, doi:10.1086/173683
  18. ^ Bopp, B. W; Caplinger, J. R; Crosby, N. A; Mahoney, D. F; O'Brien, S. P; Dempsey, R. C (1989). "Is the MK Standard eta UMa an Incipient be Star?". Information Bulletin on Variable Stars. 3347: 1. Bibcode:1989IBVS.3347....1B.
  19. ^ Howard, Andrew W; Fulton, Benjamin J (2016). "Limits on Planetary Companions from Doppler Surveys of Nearby Stars". Publications of the Astronomical Society of the Pacific. 128 (969): 114401. arXiv:1606.03134. Bibcode:2016PASP..128k4401H. doi:10.1088/1538-3873/128/969/114401. S2CID 118503912.
  20. ^ Roberts, Lewis C; Turner, Nils H; Ten Brummelaar, Theo A (2007). "Adaptive Optics Photometry and Astrometry of Binary Stars. II. A Multiplicity Survey of B Stars". The Astronomical Journal. 133 (2): 545. Bibcode:2007AJ....133..545R. doi:10.1086/510335.
  21. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  22. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  23. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 438. ISBN 978-0-486-21079-7. Retrieved 2012-09-04.
  24. ^ Littleton, C. Scott (2005), Gods, goddesses, and mythology, vol. 11, New York: Marshall Cavendish Corporation, p. 1338, ISBN 9780761475590

External links[edit]

  • Alkaid at Jim Kaler's Stars website