Epoch J2000 Equinox J2000
|Right ascension||21h 28m 39.5971s|
|Declination||+70° 33′ 38.578″|
|Apparent magnitude (V)||3.1438|
|U−B color index||–0.95|
|B−V color index||–0.22|
|Variable type||Beta Cephei variable|
|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)||–3.468|
|Surface gravity (log g)||3.80 cgs|
|Metallicity [Fe/H]||–0.23 dex|
|Rotational velocity (v sin i)||28 km/s|
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. 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 A is a blue giant star with a stellar classification listed in the SIMBAD astronomical database as B2IIIev. 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 9R☉ and a mass of 12M☉. 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 28 km/s, 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. 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 36,900L☉.[note 1]
- The formula for the bolometric luminosity of Beta Cephei is as follows:
- where... A = Alfirk, L = Luminosity, R = Radius and T = Temperature.
- V* bet Cep -- Variable Star of beta Cep type, SIMBAD Astronomical Object Database (Centre de Données astronomiques de Strasbourg), retrieved 2010-12-26
- van Leeuwen, F. (November 2007), Hipparcos, the New Reduction, Astronomy and Astrophysics (Centre de Données astronomiques de Strasbourg) 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, retrieved 2010-12-26
- Mermilliod, J.-C. (1986), Compilation of Eggen's UBV data, transformed to UBV (unpublished), Catalogue of Eggen's UBV data. (SIMBAD), Bibcode:1986EgUBV........0M
- Kaler, James B., ALFIRK (Beta Cephei), Stars (University of Illinois), retrieved 2010-12-26
- Stebbins, J.; Kron, G. E. (September 1954), Six-color photometry of stars. VIII. The pulsating star Beta Cephei, The Astrophysical Journal (PDF) (GCPD) 120: 189, Bibcode:1954ApJ...120..189S, doi:10.1086/145902
- See Notes Section for calculations
- Lefever, K. et al. (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:0910.2851, Bibcode:2010A&A...515A..74L, doi:10.1051/0004-6361/200911956
- Cenarro, A. J. et al. (2007), Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters, Monthly Notices of the Royal Astronomical Society (PDF) 374 (2): 664–690, arXiv:astro-ph/0611618, Bibcode:2007MNRAS.374..664C, doi:10.1111/j.1365-2966.2006.11196.x
- Hoffleit (1991), Bright Star Catalogue, VizieR (5th revised ed.) (Centre de Données astronomiques de Strasbourg), retrieved 2010-12-26
- 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.
- Davis Jr., G. A., "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names,"Popular Astronomy, Vol. LII, No. 3, Oct. 1944, p. 16.
- Norton, Arthur P. (1973). Norton's Star Atlas. p. 118. ISBN 0-85248-900-5. "Fixed. A is a spectroscopic binary."
- Jim Kaler's Stars, University of Illinois:ALFIRK (Beta Cephei)
- AAVSO Variable Star of the Month, Winter 2005: The Beta Cephei Stars and Their Relatives
- An Atlas of the Universe: Multiple Star Orbits