Epoch J2000 Equinox J2000
|Right ascension||17h 56m 36.36988s|
|Declination||+51° 29′ 20.0242″|
|Apparent magnitude (V)||2.23|
|Spectral type||K5 III|
|U−B color index||+1.87|
|B−V color index||+1.53|
|Radial velocity (Rv)||–28.19 ± 0.36 km/s|
|Proper motion (μ)|| RA: –8.48 mas/yr |
Dec.: –22.79 mas/yr
|Parallax (π)||21.14 ± 0.10 mas|
|Distance||154.3 ± 0.7 ly |
(47.3 ± 0.2 pc)
|Absolute magnitude (MV)||−1.93 ± 0.07|
|Radius||48.15 ± 1.09 R☉|
|Luminosity||471 ± 30 L☉|
|Surface gravity (log g)||1.55 cgs|
|Metallicity [Fe/H]||–0.14 dex|
|Rotational velocity (v sin i)||6.0 km/s|
Gamma Draconis (γ Draconis, abbreviated Gamma Dra, γ Dra), formally named Eltanin //, is a star in the northern constellation of Draco. Contrary to its gamma-designation (historically third-ranked), it is the brightest star in Draco at magnitude 2.4, outshining Beta Draconis by nearly half a magnitude and Alpha Draconis by over a magnitude.
Gamma Draconis is at a distance of 154.3 light-years (47.3 parsecs) from the Sun, as determined by parallax measurements from the Hipparcos astrometry satellite. In 1728, while unsuccessfully attempting to measure the parallax of this star, the English astronomer James Bradley discovered the aberration of light resulting from the movement of the Earth. Bradley's discovery confirmed Copernicus' theory that the Earth revolved around the Sun.
In 1.5 million years, Gamma Draconis will pass within 28 light years of Earth. For a period, if its current absolute magnitude does not change, it will be the brightest star in the night sky, nearly as bright as Sirius is at present. It is by far the brightest star having a zenith above a point near London which led to its vaunting in these places as the "zenith star". From other locations it has a nearby bright, well-known star in Lyra in night sky; finding Vega, Gamma Draconis is the red star just north-northwest of it.
Gamma Draconis is an evolved giant star with a stellar classification of K5 III. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It has 72% more mass than the Sun and it has expanded to around 48 times the Sun's girth. It is radiating about 471 times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 3,930 K. This is cooler than the Sun, giving this star the orange-hued glow of a K-type star.
Gamma Draconis has six companions listed in double star catalogues. All were discovered by the American astronomer Sherburne Wesley Burnham. The closest may be physically associated and would be separated by about 1,000 AU. The luminosity of this object suggests it is a red dwarf star. The others are all much more distant stars unrelated to Gamma Draconis.
It bore the traditional name Eltanin (or Etamin, Ettanin) derived from the Arabic التنين At-Tinnin 'The great serpent'. The name Rastaban was formerly used for Gamma Draconis, and the two terms share an Arabic root meaning "serpent" or "dragon". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Eltanin for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.
In Chinese, 天棓 (Tiān Bàng), meaning Celestial Flail, refers to an asterism consisting of Gamma Draconis, Xi Draconis, Nu Draconis, Beta Draconis and Iota Herculis. Consequently, the Chinese name for Gamma Draconis itself is 天棓四 (Tiān Bàng sì, English: the Fourth Star of Celestial Flail.)
Notes and references
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- Perryman, Michael (2010), "The Making of History's Greatest Star Map", The Making of History's Greatest Star Map, Astronomers’ Universe, Heidelberg: Springer-Verlag, Bibcode:2010mhgs.book.....P, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
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- Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051.
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- (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
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