  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cepheus |
| Right ascension | 21h 28m 39.59685s[1] |
| Declination | +70° 33′ 38.5747″[1] |
| Apparent magnitude (V) | 3.16 – 3.27[2] |
| Characteristics | |
| Spectral type | B1 IV[3] |
| U−Bcolor index | –0.95[4] |
| B−Vcolor index | –0.22[4] |
| Variable type | β Cep[2] |
| Astrometry | |
| Radial velocity (Rv) | –8.2[5] km/s |
| Proper motion (μ) | RA: +12.54[1]mas/yr Dec.: +8.39[1]mas/yr |
| Parallax (π) | 4.76±0.30 mas[1] |
| Distance | 690 ± 40 ly (210 ± 10 pc) |
| Absolute magnitude (MV) | –3.03[3] |
| Orbit[6] | |
| Primary | β Cep Aa |
| Companion | 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° |
| Periastronepoch (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[6] M☉ |
| Radius | 7.22±0.42[7] R☉ |
| Luminosity | 20,000+5,200 −4,100[8] L☉ |
| Surface gravity (log g) | 4.12±0.07[7] cgs |
| Temperature | 23,600±1,000[7] K |
| Metallicity [Fe/H] | –0.23[9] dex |
| Rotational velocity (v sin i) | 28[10] km/s |
| Age | 8.7[3] Myr |
| β Cep Ab | |
| Mass | 5.0[6] M☉ |
| Other designations | |
| Alfirk,β Cep,8 Cephei,AAVSO 2127+70,AG+70°738,BD+69°1173,CCDM J21287+7034A,FK5 809,GSC 04465-02643,HD 205021,HIP 106032,HR 8238,SAO 10057,WDS J21287+7034 | |
| Database references | |
| SIMBAD | data |
Beta Cephei (β Cephei, abbreviatedBeta Cep,β Cep) is atriple star system of the thirdmagnitude in theconstellation ofCepheus. Based onparallax measurements obtained during theHipparcos mission, it is approximately 690light-years distant from theSun. It is the prototype of theBeta Cephei variable stars.
It consists of abinary pair (designated Beta Cephei A) together with a third companion (B). The binary's two components are themselves designated Beta Cephei Aa (officially namedAlfirk/ˈælfərk/, the traditional name for the system)[11][12] and Ab.
β Cephei (Latinised toBeta Cephei) is the system'sBayer designation. The designations of the two constituents asBeta Cephei A andB, and those ofA's components -Beta Cephei Aa andAb - derive from the convention used by the Washington Multiplicity Catalog (WMC) formultiple star systems, and adopted by theInternational Astronomical Union (IAU).[13]
Beta Cephei bore the traditional nameAlfirk, derived from theArabic الفرقةal-firqah "the flock" (of sheep). WithAlpha Cephei (Alderamin) andEta Cephei (Alkidr), they wereAl Kawākib al Firḳ الكوكب الفرق "the stars of the flock" byUlug Beg.[14][15] In 2016, the IAU organized aWorking Group on Star Names (WGSN)[16] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entiremultiple systems.[17] It approved the nameAlfirk for the component Beta Cephei Aa on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[12]
Like the starEpsilon Draconis in theconstellation ofDraco, Beta Cephei is visible primarily in thenorthern hemisphere, given its extreme northerndeclination of 70 degrees and 34 minutes. It is nevertheless visible to most observers throughout the world reaching as far south as cities likeHarare inZimbabwe,Santa Cruz de la Sierra inBolivia or other settlements north ± 19° South latitude. It iscircumpolar throughout all ofEurope, northern Asia, and North American cities as far south asGuadalajara in west centralMexico. All other locations around the globe having alatitude greater than ± 20° North will notice that the star is always visible in the night sky. Because Beta Cephei is a faint third magnitude star, it may be difficult to identify in mostlight polluted cities, though in rural locations the star should be easily observable.
Beta Cephei is a visible star located within 5° of theprecessional path traced across thecelestial sphere by the Earth'sNorth pole. During the same periodIota Cephei will also be within 5° of the precessional path, on the other side so that both are in contention aspole stars, a title currently held by unambiguously byPolaris.
| Preceded by | Pole Star | Succeeded by |
|---|---|---|
| Errai | 5200AD to 7500AD with Iota Cephei | Alderamin |
Beta Cephei is atriple star comprising aspectroscopic binary with amagnitude 8 optical companion.[19] Itsmagnitude varies between +3.16 and +3.27 with a period of 0.19048 days.[2]
Beta Cephei Aa is ablue subgiant star with astellar classification of B1 IV. It has previously been classified with either amain sequence orgiant luminosity class.[20] This star has a radius that has been estimated at 7.2 R☉[7] and a mass of 7 M☉. Other sources have given higher masses on the order of 12 M☉.[3] Like most high-massB-class stars, Beta Cephei Aa is a relatively young star with an estimated age of just a few million years. Like the majority of giant stars, it rotates slowly on its axis with a rotational velocity of 7 deg/day, a speed which takes the star approximately 51 days to make one complete revolution.
Beta Cephei Ab is aBe star in an 81-year orbit with the giant primary.[6] It has been resolved using speckle interferometry at a distance of 0.25" in 1972.[21] With a mass of about 5 M☉, it is likely a B-class star with a classification of B6.[6]
B Cephei B is a magnitude 7.8 A2 main sequence star 13.6" distant.[3]
Beta Cephei pulsates regularly every 4 hours 34 minutes, producing a variation in its visual brightness of 0.11 magnitudes. It is the prototype of theBeta Cephei class of variables, hotmain sequence andgiant stars that pulsate analogously toCepheid variables but with the pulsations driven byiron opacity rather than byhelium.[2]
Fixed. A is a spectroscopic binary.
