Beta Cephei

β Cephei
Location of β Cephei (circled)
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−B color index –0.95[4]
B−V color 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]
Distance690 ± 40 ly
(210 ± 10 pc)
Absolute magnitude (MV)–3.03[3]
Orbit[6]
Primaryβ Cep Aa
Companionβ 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
Mass7.4[6] M☉
Radius7.22±0.42[7] R☉
Luminosity20,000+5,200
−4,100[8] L☉
Surface gravity (log g)4.12±0.07[7] cgs
Temperature23,600±1,000[7] K
Metallicity [Fe/H]–0.23[9] dex
Rotational velocity (v sin i)28[10] km/s
Age8.7[3] Myr
β Cep Ab
Mass5.0[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
SIMBADdata

Beta Cephei (β Cephei, abbreviated Beta Cep, β Cep) is a triple star system of the third magnitude in the constellation of Cepheus. 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)[11][12] and Ab.

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