Eta Ursae Majoris

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Eta 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
Distance 103.9 ± 0.8 ly
(31.9 ± 0.2 pc)
Mass 6.1 ± 0.1[5] M
Radius 3.4[6] R
Luminosity 1,350[7] L
Surface gravity (log g) 4.27[8] cgs
Temperature 16,823 ± 177[6] K
Rotational velocity (v sin i) 150[9] km/s
Age 10.0 ± 2.6[5] Myr
Other designations
Alkaid, Benetnash, Benetnasch,[10] Elkeid, η Ursae Majoris, η UMa, Eta UMa, 85 Ursae Majoris, 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 (Eta UMa, η Ursae Majoris, η UMa) is a star in the constellation Ursa Major. It has the traditional names Alkaid (or Elkeid from the Arabic القايد القائد), and Benetnash (Benetnasch). Alkaid is the most eastern (leftmost) star in the Big Dipper (Plough) asterism. However, unlike most stars of the Big Dipper, it is not a member of the Ursa Major moving group.[11] With an apparent visual magnitude of +1.84,[12] it is the third brightest star in the constellation and one of the brightest stars in the night sky.

This is a 10-million-year-old[5] 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.[13] It has six[5] times the mass of the Sun and 3.4[6] times the Sun's radius. Eta Ursae Majoris is radiating around 1,350[7] times as much luminosity as the Sun from its outer atmosphere at an effective temperature of about 16,823 K,[6] giving it the blue-white hue of a B-type star.[14] This star is an X-ray emitter with a luminosity of 9.3 × 1028 erg s−1.[7]

In culture[edit]

  • The name 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.
  • It is known as 北斗七 (the Seventh Star of the Northern Dipper) or 瑤光 (the Star of Twinkling Brilliance) in Chinese.
  • This star as Marīci, one of the Seven Rishis.[15]
  • 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 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 Hagunsei/Mukokseong.[16]
  • In the Nintendo DS game Devil Survivor 2, Benetnasch appears as the last of the Septentriones, a series of strange beings that invade Japan throughout the game. The other six are also named after stars in the Big Dipper (Dubhe, Merak, Phecda, Megrez, Alioth, Mizar).
  • USS Alkaid (AK-114) was a United States Navy Crater class cargo ship named after the star.


  1. ^ a b c d e van Leeuwen, Floor (November 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  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 (Astronomisches Rechen-Institut Heidelberg) 35 (35): 1, Bibcode:1999VeARI..35....1W 
  5. ^ 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 
  6. ^ a b c d 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 
  7. ^ a b c 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 
  8. ^ Niemczura, E. (June 2003), "Metallicities of the SPB stars from the IUE ultraviolet spectra", Astronomy and Astrophysics 404 (2): 689–700, Bibcode:2003A&A...404..689N, doi:10.1051/0004-6361:20030546 
  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: 139. Bibcode:1936PASP...48..139R. doi:10.1086/124681. 
  11. ^ Motz, Lloyd; Nathanson, Carol (1988). The Constellations: An Enthusiast's Guide To The Night Sky. Doubleday. p. 39. ISBN 978-0385176002. 
  12. ^ 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 
  13. ^ 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 
  14. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16 
  15. ^ 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. 
  16. ^ Littleton, C. Scott (2005), Gods, goddesses, and mythology 11, New York: Marshall Cavendish Corporation, p. 1338, ISBN 9780761475590 

External links[edit]

  • [1] at Jim Kaler's Stars website