Beta Cephei

Alfirk, β Cep

Location of β Cephei (center right).
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]
Characteristics
Spectral type B2IIIev [1]
U−B color index -0.95 [3]
B−V color index -0.22 [3]
Variable type Beta Cephei variable
Astrometry
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 [2]
Details
Mass 12 [4] M☉
Radius 9.0 [5] R☉
Luminosity 36,900 [6] L☉
Temperature 26,700 [4] K
Metallicity -0.23 Fe/H [7]
Rotational velocity (v sin i) 28[8] km/s
Age ? years
Other designations
Alfirk, 8 Cep, HR 8238, HD 205021, AG+70° 738, BD+69° 1173, FK5 809, GSC 04465-02643, HIP 106032, SAO 10057, CCDM J21287+7034A, WDS J21287+7034Aa,Ab, AAVSO 2127+70
Database references
SIMBAD data

Coordinates: 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.[9][10] Beta Cephei is the prototype of the Beta Cephei variable stars.

Contents

Visibility

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 on the west coast of 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.

Properties

Beta Cephei is a triple star comprising a spectroscopic binary with a magnitude 8 optical companion.[11] 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 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.[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 36,900L☉.[note 1]

See also

Notes

  1. ^ The formula for the bolometric luminosity of Beta Cephei is as follows:
    \frac{L_{\rm A}}{L_{\odot}} = {\left ( \frac{R_{\rm A}}{R_{\odot}} \right )}^2 {\left ( \frac{T_{\rm A}}{T_{\odot}} \right )}^4 where... A = Alfirk, L = Luminosity, R = Radius and T = Temperature.
    Therefore:
    \frac{L_{\rm A}}{L_{\odot}} = {\left ( {\frac{9}{1}} \right )}^2 {\left ( {\frac{26,700}{5,778}} \right )}^4 = 36,933 L_{\odot}
    Note: Luminosity calculations are highly dependent on size and distance variables.

References

  1. ^ a b c d e "SIMBAD query result: V* bet Cep -- Variable Star of beta Cep type". Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=bet+cep&submit=SIMBAD+search. Retrieved 2010-12-26. 
  2. ^ a b c d e f van Leeuwen, F (November 2007). "Hipparcos, the New Reduction". Astronomy and Astrophysics (Centre de Données astronomiques de Strasbourg) 474 (2): 653–664. Bibcode 2007A&A...474..653V. doi:10.1051/0004-6361:20078357. http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HIP%20106032. Retrieved 2010-12-26. 
  3. ^ a b Mermilliod, J.-C. (1986). "Compilation of Eggen's UBV data, transformed to UBV (unpublished)". Catalogue of Eggen's UBV data.. Bibcode 1986EgUBV........0M. 
  4. ^ a b Professor James B. (Jim) Kaler. "ALFIRK (Beta Cephei)". University of Illinois. http://stars.astro.illinois.edu/sow/alfirk.html. Retrieved 2010-12-26. 
  5. ^ Stebbins, J.; Kron, G. E. (September 1954). "Six-color photometry of stars. VIII. The pulsating star Beta Cephei" (PDF). The Astrophysical Journal 120: pp. 189. Bibcode 1954ApJ...120..189S. doi:10.1086/145902. 
  6. ^ See Notes Section for calculations
  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" (PDF). 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. 
  8. ^ "Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991)". VizieR. Centre de Données astronomiques de Strasbourg. http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HR%208238. Retrieved 2010-12-26. 
  9. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 157. ISBN 0486210790. http://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Cepheus*.html. Retrieved 2010-12-12. 
  10. ^ 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.
  11. ^ Norton, Arthur P. (1973). Norton's Star Atlas. p. 118. ISBN 0-85248-900-5. "Fixed. A is a spectroscopic binary." 

External links

Stars of Cepheus
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