Beta Cephei

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Alfirk, β Cep
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 39.5971s[1]
Declination +70° 33′ 38.578″[1]
Apparent magnitude (V) 3.1438[2]
Spectral type B0.5IIIs[1]
U−B color index –0.95[3]
B−V color index –0.22[3]
Variable type Beta Cephei variable
Radial velocity (Rv) –8.2[1] km/s
Proper motion (μ) RA: +12.54[2] mas/yr
Dec.: +8.39[2] mas/yr
Parallax (π) 4.76 ± 0.30[2] mas
Distance 690 ± 40 ly
(210 ± 10 pc)
Absolute magnitude (MV) –3.468[citation needed]
Mass 12[4] M
Radius 9.0[5] R
Luminosity 36,900[6] L
Surface gravity (log g) 3.80[7] cgs
Temperature 26,700[4] K
Metallicity [Fe/H] –0.23[8] dex
Rotational velocity (v sin i) 28[9] km/s
Other designations
Alfirk, 8 Cep, 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 Aa,Ab.[1]
Database references

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

Beta Cephei (β Cep, β Cephei) is a third magnitude star in the constellation Cepheus. It has the traditional name Alfirk (Arabic الفرقة al-firqah), meaning "The Flock" (referring to a flock of sheep) This star, along with α Cep (Alderamin) and η Cep (Alkidr), were Al Kawākib al Firḳ (الكوكب الفرق), meaning "the Stars of The Flock" by Ulug Beg.[10][11] Beta Cephei is the prototype of the Beta Cephei variable stars.


Like the star Epsilon Draconis in the constellation of Draco, Alfirk is visible primarily in the northern hemisphere, given its extreme northern declination of 70 degrees and 34 minutes. The star 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. The star 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.


Beta Cephei is a triple star comprising a spectroscopic binary with a magnitude 8 optical companion.[12] Its magnitude varies between +3.15 and +3.21 with a period of 0.1904844 days.

Beta Cephei A is a blue giant star with a stellar classification listed in the SIMBAD astronomical database as B2IIIev.[1] The suffix "ev" comes from the Yerkes classification system and stands for "Spectral emission that exhibits variability". This giant star has a radius that has been estimated at 9 R and a mass of 12 M. Like most high-​mass B-​class stars, β Cep is a relatively young star with an estimated age of around 50 million years old. Like the majority of giant stars, Beta Cephei A 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.

In 2007, Floor van Leeuwen and his team recalibrated the star's apparent magnitude at 3.1438 and updated the star's parallax to 4.76 ± 0.30 mas yielding a distance of 210.1 parsecs (pc) or approximately 685 light years (ly) from Earth.[2] The previous estimate from the Hipparcos mission had placed the star at about 595 ly. Given a surface temperature of 26,700 Kelvin, theoretical calculations yield a total luminosity for the star of about 36,900 L.[note 1]

See also[edit]


  1. ^ The formula for the bolometric luminosity of Beta Cephei is as follows:
    where... A = Alfirk, L = Luminosity, R = Radius and T = Temperature.
    Note: Luminosity calculations are highly dependent on size and distance variables.


  1. ^ a b c d e f "V* bet Cep -- Variable Star of beta Cep type". SIMBAD Astronomical Object Database. Centre de Données astronomiques de Strasbourg. Retrieved 2010-12-26. 
  2. ^ a b c d e van Leeuwen, F. (November 2007). "Hipparcos, the New Reduction". Astronomy and Astrophysics. Centre de Données astronomiques de Strasbourg. 474 (2): 653–664. arXiv:0708.1752free to read. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  3. ^ 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. 
  4. ^ a b Kaler, James B. "ALFIRK (Beta Cephei)". Stars. University of Illinois. Retrieved 2010-12-26. 
  5. ^ Stebbins, J.; Kron, G. E. (September 1954). "Six-color photometry of stars. VIII. The pulsating star Beta Cephei". The Astrophysical Journal. GCPD. 120: 189. Bibcode:1954ApJ...120..189S. doi:10.1086/145902. 
  6. ^ See Notes Section for calculations
  7. ^ Lefever, K.; Puls, J.; Morel, T.; Aerts, C.; Decin, L.; Briquet, M. (June 2010). "Spectroscopic determination of the fundamental parameters of 66 B-type stars in the field-of-view of the CoRoT satellite". Astronomy and Astrophysics. 515: A74. arXiv:0910.2851free to read. Bibcode:2010A&A...515A..74L. doi:10.1051/0004-6361/200911956. 
  8. ^ 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/0611618free to read. Bibcode:2007MNRAS.374..664C. doi:10.1111/j.1365-2966.2006.11196.x. 
  9. ^ Hoffleit (1991). "Bright Star Catalogue". VizieR (5th revised ed.). Centre de Données astronomiques de Strasbourg. Retrieved 2010-12-26. 
  10. ^ 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. 
  11. ^ Davis, George R. (1944). "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names". Popular Astronomy. 52: 8. Bibcode:1944PA.....52....8D. 
  12. ^ Norton, Arthur P. (1973). Norton's Star Atlas. p. 118. ISBN 0-85248-900-5. Fixed. A is a spectroscopic binary. 

External links[edit]