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), also 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 Rastaban (Beta Draconis) by nearly half a magnitude. In 1.5 million years, Eltanin 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 (and thus Leipzig, Düsseldorf, Astana and Calgary) which occurs near midnight (24:00h) at London which led to its vaunting in these places as the "zenith star" or "London zenith star".[n 1] From other locations it has a nearby bright, well-known star in Lyra in night sky; finding Vega, Eltanin is the red star just north-northwest of it.
Eltanin 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, 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 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, Gamma Draconis itself is known as 天棓四 (Tiān Bàng sì, English: the Fourth Star of Celestial Flail.)
Notes and references
- 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, arXiv:0905.1240, 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.
- "IAU Catalog of Star Names". Retrieved 28 July 2016.
- Kaler, James B. "ELTANIN (Gamma Draconis)". Stars. University of Illinois. Retrieved 2010-11-21.
- 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, archived from the original on 2012-03-10, 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
- "γ Dra (Eltanin)". Wikisky.org. Retrieved 2010-11-22.
- IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
- 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
- (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.
- (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived January 30, 2011, at the Wayback Machine., Hong Kong Space Museum. Accessed on line November 23, 2010.
- A distance 24 km south of London, similar as to Leipzig and Düsseldorf but north of Astana and Calgary