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.

β Cephei
Cepheus constellation map.svg
Cercle rouge 100%.svg
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]
Spectral type B1 IV[3]
U−B color index –0.95[4]
B−V color index –0.22[4]
Variable type β Cep[2]
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]
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)
202.43 ± 0.31°
Semi-amplitude (K1)
9.63 ± 0.26 km/s
β Cep Aa
Mass7.4[6] M
Radius5.6[3] R
Luminosity15,100[3] L
Surface gravity (log g)4.05[3] cgs
Temperature27,000[3] K
Metallicity [Fe/H]–0.23[7] dex
Rotational velocity (v sin i)28[8] 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

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)[9][10] and Ab.


β Cephei (Latinised to Beta Cephei) is the system's Bayer designation. The designations of the two constituents as Beta Cephei A and B, and those of A's components - Beta Cephei Aa and Ab - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[11]

Beta Cephei bore the traditional name Alfirk, derived from the Arabic الفرقة al-firqah "the flock" (of sheep). With Alpha Cephei (Alderamin) and Eta Cephei (Alkidr), they were Al Kawākib al Firḳ الكوكب الفرق "the stars of the flock" by Ulug Beg.[12][13] In 2016, the IAU organized a Working Group on Star Names (WGSN)[14] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[15] It approved the name Alfirk for the component Beta Cephei Aa on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[10]


Like the star Epsilon Draconis in the constellation of Draco, Beta Cephei is visible primarily in the northern hemisphere, given its extreme northern declination of 70 degrees and 34 minutes. It is nevertheless visible to most observers throughout the world reaching as far south as cities like Harare in Zimbabwe, Santa Cruz de la Sierra in Bolivia or other settlements north ± 19° South latitude. It is circumpolar throughout all of Europe, northern Asia, and North American cities as far south as Guadalajara in west central Mexico. All other locations around the globe having a latitude 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 most light polluted cities, though in rural locations the star should be easily observable.

Pole StarEdit

Beta Cephei is a visible star located within 5° of the precessional path traced across the celestial sphere by the Earth's North pole. During the same period Iota Cephei will also be within 5° of the precessional path, on the other side so that both are in contention as pole stars, a title currently held by unambiguously by Polaris.

Preceded by Pole Star Succeeded by
Errai 5200AD to 7500AD
Iota Cephei


Beta Cephei is a triple star comprising a spectroscopic binary with a magnitude 8 optical companion.[16] Its magnitude varies between +3.16 and +3.27 with a period of 0.19048 days.[2]

Beta Cephei Aa is a blue subgiant star with a stellar classification of B1 IV. It has previously been classified with either a main sequence or giant luminosity class.[17] This star has a radius that has been estimated at 6 R[3] and a mass of 7 M. Other sources have given higher masses on the order of 12 M.[3] Like most high-mass B-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 a Be 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.[18] 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 the Beta Cephei class of variables, hot main sequence and giant stars that pulsate analogously to Cepheid variables but with the pulsations driven by iron opacity rather than by helium.[2]


  1. ^ a b c d e van Leeuwen, F. (November 2007). "Hipparcos, the New Reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. ^ a b c d Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  3. ^ a b c d e f g h i j Nieva, María-Fernanda; Przybilla, Norbert (2014). "Fundamental properties of nearby single early B-type stars". Astronomy & Astrophysics. 566: A7. arXiv:1412.1418. Bibcode:2014A&A...566A...7N. doi:10.1051/0004-6361/201423373. S2CID 119227033.
  4. ^ a b Mermilliod, J.-C. (1986). "Compilation of Eggen's UBV data, transformed to UBV (unpublished)". Catalogue of Eggen's UBV Data. SIMBAD. Bibcode:1986EgUBV........0M.
  5. ^ Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Washington. Bibcode:1953GCRV..C......0W.
  6. ^ a b c d e Hutter, D. J.; Tycner, C.; Zavala, R. T.; Benson, J. A.; Hummel, C. A.; Zirm, H. (2021). "Surveying the Bright Stars by Optical Interferometry. III. A Magnitude-limited Multiplicity Survey of Classical Be Stars". The Astrophysical Journal Supplement Series. 257 (2): 69. arXiv:2109.06839. Bibcode:2021ApJS..257...69H. doi:10.3847/1538-4365/ac23cb. S2CID 237503492.
  7. ^ Cenarro, A. J.; Peletier, R. F.; Sánchez-Blázquez, P.; Selam, S. O.; Toloba; et al. (2007). "Medium-resolution Isaac Newton Telescope library of empirical spectra – II. The stellar atmospheric parameters". Monthly Notices of the Royal Astronomical Society. 374 (2): 664–690. arXiv:astro-ph/0611618. Bibcode:2007MNRAS.374..664C. doi:10.1111/j.1365-2966.2006.11196.x. S2CID 119428437.
  8. ^ Hoffleit (1991). "Bright Star Catalogue". VizieR (5th revised ed.). Centre de Données astronomiques de Strasbourg. Retrieved 2010-12-26.
  9. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  10. ^ a b "Naming Stars". Retrieved 18 June 2018.
  11. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  12. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 157. ISBN 0-486-21079-0. Retrieved 2010-12-12.
  13. ^ Davis, George R. (1944). "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names". Popular Astronomy. 52: 8. Bibcode:1944PA.....52....8D.
  14. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  15. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  16. ^ Norton, Arthur P. (1973). Norton's Star Atlas. p. 118. ISBN 0-85248-900-5. Fixed. A is a spectroscopic binary.
  17. ^ Skiff, B. A. (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009–2016)". VizieR On-line Data Catalog: B/Mk. Originally Published in: Lowell Observatory (October 2014). 1. Bibcode:2014yCat....1.2023S.
  18. ^ Gezari, D. Y.; Labeyrie, A.; Stachnik, R. V. (1972). "Speckle Interferometry: Diffraction-Limited Measurements of Nine Stars with the 200-INCH Telescope". Astrophysical Journal. 173: L1. Bibcode:1972ApJ...173L...1G. doi:10.1086/180906.

External linksEdit

Coordinates:   21h 28m 39.60s, +70° 33′ 39.0″