Zeta Centauri

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Zeta Centauri
Diagram showing star positions and boundaries of the Centaurus constellation and its surroundings
Cercle rouge 100%.svg

Location of ζ Centauri (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Centaurus
Right ascension 13h 55m 32.38565s[1]
Declination −47° 17′ 18.1482″[1]
Apparent magnitude (V) +2.55[2]
Spectral type B2.5 IV[3]
U−B color index −0.91[2]
B−V color index −0.22[2]
Radial velocity (Rv) +6.5[4] km/s
Proper motion (μ) RA: −57.37[1] mas/yr
Dec.: −44.55[1] mas/yr
Parallax (π) 8.54 ± 0.13[1] mas
Distance 382 ± 6 ly
(117 ± 2 pc)
Period (P) 8.024 days
Semi-major axis (a) 0.0014"[6]
Eccentricity (e) 0.5
Periastron epoch (T) 2413719.321 JD
Argument of periastron (ω)
Mass 7.8 ± 0.1[3] M
Radius 5.80 ± 0.53[7] R
Surface gravity (log g) 3.84 ± 0.08[7] cgs
Temperature 23,561 ± 283[7] K
Rotational velocity (v sin i) 235[8] km/s
Age 39.8 ± 5.7[3] Myr
Other designations
Alnair, CD−46 8949, CP−46 6560, HD 121263, HIP 68002, HR 5231, SAO 224538.[9]
Database references

Zeta Centauri (ζ Centauri, ζ Cen) is a binary star system in the southern constellation of Centaurus. It has the proper name Alnair, from the scientific-Arabic[10] Nayyir Badan Qanṭūris (نير بطن قنطورس), meaning "The Bright (Star) of the Body of the Centaur".[11] With a combined apparent visual magnitude of +2.55,[2] it is one of the brighter members of the constellation. This system is close enough to the Earth that its distance can be measured directly using the parallax technique. This yields a value of roughly 382 light-years (117 parsecs), with a 1.6% margin of error.[1]

In Chinese, 庫樓 (Kù Lóu), meaning Arsenal, refers to an asterism consisting of ζ Centauri, η Centauri, θ Centauri, 2 Centauri, HD 117440, ξ1 Centauri, γ Centauri, τ Centauri, D Centauri and σ Centauri.[12] Consequently, ζ Centauri itself is known as 庫樓一 (Kù Lóu yī, English: the First Star of Arsenal.)[13]

ζ Cen is a double-lined spectroscopic binary system,[14] which indicates that the orbital motion was detected by shifts in the absorption lines of their combined spectra caused by the Doppler effect. The two stars orbit each other over a period of slightly more than eight days with an orbital eccentricity of about 0.5.[5] The estimated angular separation of the pair is 1.4 mas.[6]

At an estimated age of 40 million years,[3] the primary component of this system appears to be in the subgiant stage of its evolution with a stellar classification of B2.5 IV.[3] It is a large star with nearly 8[3] times the mass of the Sun and close to 6[7] times the Sun's radius. This star is rotating rapidly with a projected rotational velocity of 235 km s−1.[8]


  1. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752free to read, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ a b c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99), Bibcode:1966CoLPL...4...99J 
  3. ^ a b c d e f Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883free to read, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x 
  4. ^ Wilson, R. E. (1953), General Catalogue of Stellar Radial Velocities, Carnegie Institute of Washington D.C., Bibcode:1953GCRV..C......0W 
  5. ^ a b Maury, A. C. (1922), "The Orbit of the Spectroscopic Binary ζ Centauri", Harvard College Observatory Circular, 233: 1–4, Bibcode:1922HarCi.233....1M 
  6. ^ a b Halbwachs, J. L. (April 1981), "List of Estimated Angular Separations of Spectroscopic Binaries", Astronomy and Astrophysics Supplement, 44: 47, Bibcode:1981A&AS...44...47H 
  7. ^ a b c d Fitzpatrick, E. L.; Massa, D. (March 2005), "Determining the Physical Properties of the B Stars. II. Calibration of Synthetic Photometry", The Astronomical Journal, 129 (3): 1642–1662, arXiv:astro-ph/0412542free to read, Bibcode:2005AJ....129.1642F, doi:10.1086/427855 
  8. ^ a b Bernacca, P. L.; Perinotto, M. (1970), "A catalogue of stellar rotational velocities", Contributi Osservatorio Astronomico di Padova in Asiago, 239 (1), Bibcode:1970CoAsi.239....1B 
  9. ^ "zet Cen -- Spectroscopic binary", SIMBAD Astronomical Object Database, Centre de Données astronomiques de Strasbourg, retrieved 2012-03-01 
  10. ^ Kunitzsch, P.; Smart, T. (2006), A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd revised ed.), Cambridge, MA: Sky Pub, p. 4, ISBN 978-1-931559-44-7 
  11. ^ ibid., p.28.
  12. ^ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  13. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 25 日
  14. ^ van Rensbergen, W.; De Loore, C.; Jansen, K. (February 2006), "Evolution of interacting binaries with a B type primary at birth", Astronomy and Astrophysics, 446 (3): 1071–1079, Bibcode:2006A&A...446.1071V, doi:10.1051/0004-6361:20053543