  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Draco |
| Right ascension | 17h 56m 36.36988s[1] |
| Declination | +51° 29′ 20.0242″[1] |
| Apparent magnitude (V) | 2.23[2] |
| Characteristics | |
| Spectral type | K5 III[3] |
| U−Bcolor index | +1.87[2] |
| B−Vcolor index | +1.52[4] |
| Astrometry | |
| Radial velocity (Rv) | 28.19±0.36[5] km/s |
| Proper motion (μ) | RA: −8.48[1]mas/yr Dec.: −22.79[1]mas/yr |
| Parallax (π) | 21.14±0.10 mas[1] |
| Distance | 154.3 ± 0.7 ly (47.3 ± 0.2 pc) |
| Absolute magnitude (MV) | −1.93 ± 0.07[6] |
| Details[4] | |
| Mass | 2.14±0.16 M☉ |
| Radius | 51.8±0.26[7] R☉ |
| Luminosity | 598±21[7] L☉ |
| Surface gravity (log g) | 1.33±0.04[8] cgs |
| Temperature | 3,964±34[7] K |
| Metallicity[Fe/H] | +0.11±0.05 dex |
| Rotational velocity (v sin i) | 4.5±0.05 km/s |
| Age | 1.3±0.25 Gyr |
| Other designations | |
| Eltanin,γ Dra,33 Dra,BD+51°2282,FK5 676,HD 164058,HIP 87833,HR 6705,SAO 30653 | |
| Database references | |
| SIMBAD | data |
Gamma Draconis, formally namedEltanin/ɛlˈteɪnɪn/,[9][10] is astar in the northernconstellation ofDraco. Contrary to itsgamma-designation (historically third-ranked), it is the brightest object in Draco at magnitude 2.2,[1] outshiningBeta Draconis by nearly half a magnitude andAlpha Draconis by over a magnitude.
Gamma Draconis is at a distance of 154.3light-years (47.3parsecs) from theSun,[1] as determined byparallax measurements from theHipparcos astrometry satellite.[11][12] In 1728, while unsuccessfully attempting to measure the star's parallax,James Bradley discovered theaberration of light resulting from the relative movement of theEarth. Bradley's discovery confirmedCopernicus'theory that the Earth revolved around the Sun.[13] It is drifting closer to the Solar System with a radial velocity of about –28 km/s.
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 asSirius is at present.[13][14] Nearby this red star to the south-southeast isVega, a bright, well-known star in Lyra.
γ Draconis (Latinised toGamma Draconis, abbreviatedGamma Dra,γ Dra) is the star'sBayer designation.
It bore the traditional nameEltanin[13] (historically also seen with other spellings likeEtamin) derived from theArabic التنينAt-Tinnin 'the great serpent'. The nameRastaban was formerly used for Gamma Draconis, and the two terms share an Arabic root meaning "serpent" or "dragon".[15] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[16] to catalogue and standardize proper names for stars. The WGSN approved the nameEltanin for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[10]
In traditional Arabic astronomy, Gamma Draconis, along withBeta Draconis,Mu Draconis,Nu Draconis, andXi Draconis wereAl ʽAwāïd, theMother Camels, which was later known as theQuinque Dromedarii.[15]
InChinese,天棓 (Tiān Bàng), meaningCelestial Flail, refers to anasterism consisting of Gamma Draconis, Xi Draconis, Nu Draconis, Beta Draconis andIota Herculis.[17] Consequently, theChinese name for Gamma Draconis itself is天棓四 (Tiān Bàng sì, English:the Fourth Star of Celestial Flail.)[18]
Gamma Draconis lies near thezenith ofGreenwich,London, and so has been known as the "zenith star" in that region.[15] This made it an object of interest for 17th- and 18th-century English astronomers, includingRobert Hooke, to attempt to measure itsparallax.[19] In 1728, while unsuccessfully attempting to measure the star's parallax,James Bradley discovered theaberration of light resulting from the relative movement of theEarth. Bradley's discovery was thought to confirmCopernicus'theory that the Earth revolved around the Sun.[13] Due to its historical significance,George Airy in 1873 called it "the birth-star of modern astronomy".[20]
Gamma Draconis is anevolvedgiant star with astellar classification of K5 III.[3] Since 1943, thespectrum of this star has served as one of the stable anchor points by which other stars are classified.[21] It has about two times more mass than the Sun[4] and it has expanded to around 50 times its size. It is radiating about 600 times as much luminosity as the Sun from itsouter atmosphere at aneffective temperature of 3,964 K.[7] This is cooler than the Sun, giving this star the orange-hued glow of aK-type star.[22]
Gamma Draconis has six companions listed in double star catalogues. All were discovered by the American astronomerSherburne Wesley Burnham.[23] The closest may be physically associated and would be separated by about1,000 AU. The luminosity of this object suggests it is ared dwarf star.[24] The others are all much more distant stars unrelated to Gamma Draconis.[25]
The star displays periodicradial velocity variations that had been suspected to be caused by an orbiting planet, but later turned out to be caused by intrinsic stellar variability.[4] Similar variability may exist in other giant stars, such as42 Draconis andAldebaran, resulting in false positive planet detections.[26][27]
USSEtamin was aUnited States NavyCrater class cargo ship named after the star.
Ursula K. Le Guin's novelPlanet of Exile is set on the third planet orbiting Gamma Draconis.[28]
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