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 Dra, γ Draconis) is the Bayer designation for a star in the northern constellation of Draco. It has the traditional name Etamin, Eltanin or Ettanin and the Flamsteed designation 33 Draconis. Despite its gamma designation, it is actually the brightest star in Draco at magnitude 2.4, outshining Rastaban (Beta Draconis) by nearly half a magnitude. Its proximity to the zenith point directly overhead of London has earned it the name "Zenith Star." As for other places, it is relatively easy to locate in the night sky. If one finds Vega, Eltanin is the red star just north-northwest of it.
Eltanin lies around 154.3 light-years (47.3 parsecs) away, as determined by parallax measurements from the Hipparcos astrometry satellite. In 1728, while unsuccessfully attempting to measure the parallax of this star, 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.
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. It may have a physical companion. If so, the two are separated by about 1000 AU. The luminosity of this object suggests it is a red dwarf star. In 1.5 million years, Eltanin will pass within 28 light years of Earth. At this point (assuming 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.
The name Etamin comes from the Arabic التنين At-Tinnin The great serpent. The name "Rastaban" was formerly used for Eltanin, and the two terms share an Arabic root meaning "serpent" or "dragon." This star, along with β Dra (Rastaban), μ Dra (Erakis), ν Dra (Kuma) and ξ Dra (Grumium) were Al ʽAwāïd, "the Mother Camels", which was later known as the Quinque Dromedarii.
In Chinese, 天棓 (Tiān Bàng), meaning Celestial Flail, refers to an asterism consisting of γ Draconis, ξ Draconis, ν Draconis, β Draconis and ι Herculis. Consequently, γ Draconis itself is known as 天棓四 (Tiān Bàng sì, English: the Fourth Star of Celestial Flail.)
- van Leeuwen, F (November 2007). "Hipparcos, the New Reduction". Astronomy and Astrophysics (VizieR; Centre de Données astronomiques de Strasbourg) 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. Retrieved 2010-11-21.
- Oja, T. (August 1991). "UBV photometry of stars whose positions are accurately known. VI". Astronomy and Astrophysics Supplement Series 89 (2): 415–419. Bibcode:1991A&AS...89..415O.
- Morgan, W. W.; Keenan, P. C. (1973), "Spectral Classification", Annual Review of Astronomy and Astrophysics 11 (1): 29, Bibcode:1973ARA&A..11...29M, doi:10.1146/annurev.aa.11.090173.000333
- Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209
- Piau, L.; et al. (February 2011), "Surface convection and red-giant radius measurements", Astronomy and Astrophysics 526: A100, arXiv:1010.3649, Bibcode:2011A&A...526A.100P, doi:10.1051/0004-6361/201014442
- Dehaes, S.; et al. (September 2011), "Structure of the outer layers of cool standard stars", Astronomy & Astrophysics 533: A107, Bibcode:2011A&A...533A.107D, doi:10.1051/0004-6361/200912442
- McWilliam, Andrew (December 1990). "High-resolution spectroscopic survey of 671 GK giants". Astrophysical Journal Supplement Series (PDF) 74: 1075–1128. Bibcode:1990ApJS...74.1075M. doi:10.1086/191527.
- "SIMBAD query result: NAME ETAMIN -- Star in double system". Centre de Données astronomiques de Strasbourg. Retrieved 2010-11-21.
- Kaler, James B. "ELTANIN (Gamma Draconis)". Stars. University of Illinois. Retrieved 2010-11-21.
- "γ Dra (Eltanin)". Wikisky.org. Retrieved 2010-11-22.
- Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics 323: L49–L52, Bibcode:1997A&A...323L..49P
- 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
- 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
- "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
- Ayres, Thomas R.; Brown, Alexander; Harper, Graham M. (November 2006), "The Coronae of γ Draconis", The Astrophysical Journal 651 (2): 1126–1129, Bibcode:2006ApJ...651.1126A, doi:10.1086/507763
- Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York, NY: Dover Publications Inc, p. 207, ISBN 0-486-21079-0, retrieved 2010-12-12
- (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.
- (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.