Thuban
Thuban (/ˈθjuːbæn/), with Bayer designation Alpha Draconis or α Draconis, is a binary star system in the northern constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star from the 4th to 2nd millennium BC.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Draco |
| Right ascension | 14h 04m 23.3500s |
| Declination | 64° 22′ 33.062″ |
| Apparent magnitude (V) | 3.67 |
| Characteristics | |
| Spectral type | A0IV + A1V |
| U−B color index | −0.08 |
| B−V color index | −0.049±0.005 |
| Variable type | suspected Maia |
| Astrometry | |
| Radial velocity (Rv) | −13.0 km/s |
| Proper motion (μ) | RA: −56.34 mas/yr Dec.: 17.21 mas/yr |
| Parallax (π) | 10.76 ± 0.17 mas |
| Distance | 303 ± 5 ly (93 ± 1 pc) |
| Absolute magnitude (MV) | −1.20 |
| Orbit | |
| Period (P) | 51.417350 d |
| Semi-major axis (a) | 5.52 mas |
| Eccentricity (e) | 0.43 |
| Inclination (i) | 85.4° |
| Longitude of the node (Ω) | 252.6° |
| Periastron epoch (T) | JD 2453498.8 |
| Argument of periastron (ω) (secondary) | 22.4° |
| Semi-amplitude (K1) (primary) | 48.512 km/s |
| Semi-amplitude (K2) (secondary) | 63.58 km/s |
| Details | |
| α Draconis A Alpha Draconis A | |
| Mass | 3.186 M☉ |
| Radius | 4.932 R☉ |
| Luminosity | 240 L☉ |
| Surface gravity (log g) | 3.555 cgs |
| Temperature | 10225 K |
| Rotational velocity (v sin i) | 25.4 km/s |
| Age | 280 Myr |
| α Draconis B Alpha Draconis B | |
| Mass | 2.431 M☉ |
| Radius | 2.326 R☉ |
| Luminosity | 47.5 L☉ |
| Surface gravity (log g) | 4.090 cgs |
| Temperature | 9930 K |
| Rotational velocity (v sin i) | 168 km/s |
| Age | 345 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Johann Bayer gave Thuban the designation Alpha and placed it as the only member of his secundae magnitude class in Draco, although its current apparent magnitude of 3.65 means it is 3.7 times fainter than the brightest star in the constellation, Gamma Draconis (Eltanin), which Bayer placed in his tertiae magnitude class although its current apparent magnitude is 2.24.
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