Eta Ursae Majoris
Epoch J2000 Equinox J2000
|Right ascension||13h 47m 32.43776s|
|Declination||+49° 18′ 47.7602″|
|Apparent magnitude (V)||+1.84|
|Spectral type||B3 V|
|U−B color index||–0.68|
|B−V color index||–0.18|
|Radial velocity (Rv)||–10.9 km/s|
|Proper motion (μ)||RA: –121.17 mas/yr
Dec.: –14.91 mas/yr
|Parallax (π)||31.38 ± 0.24 mas|
|Distance||103.9 ± 0.8 ly
(31.9 ± 0.2 pc)
|Mass||6.1 ± 0.1 M☉|
|Surface gravity (log g)||4.27 cgs|
|Temperature||16,823 ± 177 K|
|Rotational velocity (v sin i)||150 km/s|
|Age||10.0 ± 2.6 Myr|
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) 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. With an apparent visual magnitude of +1.84, 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 B-type main sequence star with a stellar classification of B3 V. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It has six times the mass of the Sun and 3.4 times the Sun's radius. Eta Ursae Majoris is radiating around 1,350 times as much luminosity as the Sun from its outer atmosphere at an effective temperature of about 16,823 K, giving it the blue-white hue of a B-type star. This star is an X-ray emitter with a luminosity of 9.3 × 1028 erg s–1.
- 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 搖光 (the Star of Twinkling Brilliance) in Chinese.
- This star as Marīci, one of the Seven Rishis.
- In Japan, Alkaid in known as Hagunsei ("the military breaking star" or "most corner star"). Both meanings come from ancient China's influence in Japan. 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.
- 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.
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- 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
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- The Colour of Stars, Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
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-  at Jim Kaler's Stars website