Beta Draconis
Beta Draconis, a name Latinized from β Draconis, is a binary star system and the third-brightest star in the northern circumpolar constellation of Draco. The two components are designated Beta Draconis A (officially named Rastaban /ˈræstəbæn/, the traditional name of the system) and B respectively. With a combined apparent visual magnitude of 2.79, it is bright enough to be easily seen with the naked eye. Based upon parallax measurements from the Hipparcos astrometry satellite, it lies at a distance of about 380 light-years (120 parsecs) from the Sun. The system is drifting closer with a radial velocity of −21 km/s.
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Draco |
| Right ascension | 17h 30m 25.96170s |
| Declination | +52° 18′ 04.9993″ |
| Apparent magnitude (V) | 2.79 |
| Characteristics | |
| Spectral type | G2Ib-IIa |
| U−B color index | +0.954±0.007 |
| B−V color index | +0.98 |
| R−I color index | +0.48 |
| Astrometry | |
| Radial velocity (Rv) | −21.00±0.23 km/s |
| Proper motion (μ) | RA: −15.89 mas/yr Dec.: +12.28 mas/yr |
| Parallax (π) | 8.58 ± 0.10 mas |
| Distance | 380 ± 4 ly (117 ± 1 pc) |
| Absolute magnitude (MV) | –2.457 |
| Details | |
| Mass | 6.0±0.2 M☉ |
| Radius | 40 R☉ |
| Luminosity | 996 L☉ |
| Surface gravity (log g) | 1.86±0.04 cgs |
| Temperature | 5,160±150 K |
| Metallicity [Fe/H] | 0.02±0.10 dex |
| Rotational velocity (v sin i) | 10.7 km/s |
| Age | 65 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
The binary system consists of a bright giant orbited by a dwarf companion once every four millennia or so. The companion is about 11 magnitudes fainter than the primary star, and the two are separated by 4.2″.
The spectrum of the primary, Beta Draconis A, matches a stellar classification of G2Ib-IIa, showing mixed features of a bright giant and a supergiant star, and is listed as a standard star for that spectral class. It is about 65 million years old and is currently undergoing its first convective dredge-up. Compared to the Sun, Beta Draconis A is an enormous star with six times the mass and roughly 40 times the radius. At this size, it is emitting about 950 times the luminosity of the Sun from its outer envelope at an effective temperature of 5,160 K, giving it the yellow hue of a G-type star. The star has a particularly strong chromospheric emission that is generating X-ray and far-UV radiation. There is a detectable magnetic field with a longitudinal field strength of −1.16±0.25 G.
Beta Draconis lies on or near the cepheid instability strip, yet only appears to be a microvariable with a range of about 1/100th of a magnitude. It was confirmed as a variable star with a range of about 1/100th of a magnitude by Gabriel Cristian Neagu using data from the TESS and Hipparcos missions. The variability was reported to the AAVSO (American Association of Variable Star Observers), in the Variable Star Index.