Gamma Draconis
Gamma Draconis (γ Draconis, abbreviated Gamma Dra, γ Dra), formally named Eltanin /ɛlˈteɪnɪn/, is a star in the northern constellation of Draco. Contrary to its gamma-designation (historically third-ranked), it is the brightest object in Draco at magnitude 2.2, outshining Beta Draconis by nearly half a magnitude and Alpha Draconis by over a magnitude.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Draco |
| Right ascension | 17h 56m 36.36988s |
| Declination | +51° 29′ 20.0242″ |
| Apparent magnitude (V) | 2.23 |
| Characteristics | |
| Spectral type | K5 III |
| U−B color index | +1.87 |
| B−V color index | +1.53 |
| Astrometry | |
| 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 |
| Details | |
| Mass | 1.72 M☉ |
| Radius | 48.15 ± 1.09 R☉ |
| Luminosity | 471 ± 30 L☉ |
| Surface gravity (log g) | 1.55 cgs |
| Temperature | 3,930 K |
| Metallicity [Fe/H] | –0.14 dex |
| Rotational velocity (v sin i) | 6.0 km/s |
| Other designations | |
| Database references | |
| SIMBAD | data |
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 relative movement of the Earth. Bradley's discovery apparently confirmed Copernicus' theory that the Earth revolved around the Sun. 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 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 the night sky; finding Vega, Gamma Draconis is the red star just north-northwest of it.