Beta Cephei
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cepheus |
| Right ascension | 21h 28m 39.59685s |
| Declination | +70° 33′ 38.5747″ |
| Apparent magnitude (V) | 3.16 to 3.27 |
| Characteristics | |
| Spectral type | B1 IV |
| U−B color index | –0.95 |
| B−V color index | –0.22 |
| Variable type | β Cep |
| Astrometry | |
| Radial velocity (Rv) | −8.2 km/s |
| Proper motion (μ) | RA: +12.54 mas/yr Dec.: +8.39 mas/yr |
| Parallax (π) | 4.76±0.30 mas |
| Distance | 690 ± 40 ly (210 ± 10 pc) |
| Absolute magnitude (MV) | −3.03 |
| Orbit | |
| Primary | β Cep Aa |
| Name | β Cep Ab |
| Period (P) | 29616.54±1.36 d |
| Semi-major axis (a) | 206.96±0.53 mas |
| Eccentricity (e) | 0.7478±0.0005 |
| Inclination (i) | 88.80±0.07° |
| Longitude of the node (Ω) | 227.83±0.14° |
| Periastron epoch (T) | 2450944.5±24.2 |
| Argument of periastron (ω) (secondary) | 202.43±0.31° |
| Semi-amplitude (K1) (primary) | 9.63±0.26 km/s |
| Details | |
| β Cep Aa | |
| Mass | 7.4 M☉ |
| Radius | 7.22±0.42 R☉ |
| Luminosity | 20,000+5,200 −4,100 L☉ |
| Surface gravity (log g) | 4.12±0.07 cgs |
| Temperature | 23,600±1,000 K |
| Metallicity [Fe/H] | −0.23 dex |
| Rotational velocity (v sin i) | 28 km/s |
| Age | 8.7 Myr |
| β Cep Ab | |
| Mass | 5.0 M☉ |
| Other designations | |
| Alfirk, β Cep, 8 Cephei, AAVSO 2127+70, AG+70°738, BD+69°1173, FK5 809, HD 205021, HIP 106032, HR 8238, SAO 10057, CCDM J21287+7034A, WDS J21287+7034, GSC 04465-02643 | |
| Database references | |
| SIMBAD | data |
Beta Cephei is a triple star system in the northern constellation of Cepheus. Its name is a Bayer designation that is Latinized from β Cephei, and abbreviated Beta Cep or β Cep. This system is a visible to the naked eye as a point of light with a combined apparent visual magnitude that ranges from 3.16 to 3.27, making it a third magnitude star. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 690 light-years distant from the Sun. It is the prototype of the Beta Cephei variable stars.
It consists of a binary pair (designated Beta Cephei A) together with a third companion (B). The binary's two components are themselves designated Beta Cephei Aa (officially named Alfirk /ˈælfərk/, the traditional name for the system) and Ab.