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]
Characteristics
Spectral type B0.5IIIs[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☉
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
SIMBAD data

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.

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 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.

Properties

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

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. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ 3.0 3.1 Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ 4.0 4.1 Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ See Notes Section for calculations
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ 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.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

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