Beta Centauri

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β Centauri
Centaurus constellation map.svg
Red circle.svg
Location of β Centauri (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Centaurus
Right ascension 14h 03m 49.40535s[1]
Declination –60° 22′ 22.9266″[1]
Apparent magnitude (V) 0.61[2]
U−B color index –0.98[2]
B−V color index –0.23[2]
β Cen A1
Spectral type B1 III[3]
Variable type β Cep[4]
β Cen A2
Spectral type B1 III[3]
Variable type β Cep[4]
β Cen B
Spectral type B1V?[3]
Radial velocity (Rv)+5.9[5] km/s
Proper motion (μ) RA: –33.27[1] mas/yr
Dec.: –23.16[1] mas/yr
Parallax (π)8.32 ± 0.50[1] mas
Distance390 ± 20 ly
(120 ± 7 pc)
Absolute magnitude (MV)−4.53[6]
Primaryβ Cen A1
Companionβ Cen A2
Period (P)356.94 days
Semi-major axis (a)0.0253″
Eccentricity (e)0.825
Inclination (i)67.4°
Periastron epoch (T)2451600.08
Argument of periastron (ω)
Semi-amplitude (K1)
57.4 km/s
Semi-amplitude (K2)
72.3 km/s
Primaryβ Cen A
Companionβ Cen B
Period (P)288.267 yr
Semi-major axis (a)0.870″
Luminosity41,700[8] L
Age14.1 ± 0.6[3] Myr
β Cen A1
Mass10.7 ± 0.1[3] M
Surface gravity (log g)3.5 ± 0.4[3] cgs
Temperature25,000 ± 2,000[3] K
β Cen A2
Mass10.3 ± 0.1[3] M
Surface gravity (log g)3.5 ± 0.4[3] cgs
Temperature25,000 ± 2,000[3] K
β Cen B
Mass4.61[7] M
Other designations
Agena, Hadar,[9] HR 5267, HD 122451, CD−59°5365, LHS 51, SAO 252582, FK5 518 , HIP 68702, GC 18971, CCDM J14038-6022[10]
Database references

Beta Centauri (β Centauri, abbreviated Beta Cen, β Cen), also named Agena and Hadar,[11] is a triple star system in the southern constellation of Centaurus. The system's combined apparent visual magnitude of 0.61 makes it the second-brightest star in Centaurus and one of the brightest stars in the night sky. According to parallax measurements from the astrometric Hipparcos satellite,[12][13] the distance to this system is about 390 ± 20 light-years (120 ± 6 parsecs).[1]

Beta Centauri is well known in the Southern Hemisphere as the inner of the two "Pointers" to the asterism known as the Southern Cross. A line made from the other pointer, Alpha Centauri, through Beta Centauri leads to within a few degrees of Gacrux, the star at the top of the cross. Using Gacrux, a navigator can draw a line with Acrux at the bottom to effectively determine South.[14]


β Centauri (Latinised to Beta Centauri) is the star's Bayer designation.

It bore the traditional names Hadar and Agena. Hadar comes from the Arabic حضار (the root's meaning is "to be present" or "on the ground" or "settled, civilized area"[15]), while the name Agena may ultimately be derived from the Latin genua, meaning "knees". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN approved the name Hadar for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[11]

The Chinese name for the star is 马腹一 (Mandarin: mǎ fù yī, "the First Star of the Horse's Abdomen").[17]

The indigenous Boorong people of what is now northwestern Victoria named it as Bermbermgle (together with Alpha Centauri),[18] two brothers who were noted for their courage and destructiveness, and who spear and kill Tchingal, "The Emu" (Coalsack Nebula).[19] The two brothers in Wotjobaluk people is Bram-bram-bult.[18]


February 2012 image of α and β Centauri

The Beta Centauri system has three components: Beta Centauri Aa, Ab, and B. All the spectral lines detected are consistent with a B1 type star, with only the line profiles varying, so it is thought that all three stars have the same spectral type.

In 1935, Joan Voûte identified Beta Centauri B, giving it the identifier VOU 31. The companion is separated from the primary by 1.3 seconds of arc, and has remained so since the discovery, although the position angle has changed six degrees since. Beta Centauri B is a B1 dwarf with an apparent magnitude of 4.

In 1967, Beta Centauri's observed periodic variation in radial velocity suggested that Beta Centauri A is a double-lined spectroscopic binary. This was confirmed in 1999. The primary consists of a pair of stars, (Aa and Ab) of similar mass that orbit each other over a period of 357 days with a large eccentricity of about 0.8245.[20]

The pair were calculated to be separated by a mean distance of roughly 4 astronomical units (based on a distance to the system of 161 parsecs) in 2005.[21]

Both Aa and Ab apparently have a stellar classification of B1 III,[21] with the luminosity class of III indicating giant stars that are evolving away from the main sequence. They are both Beta Cephei variable stars with multiple pulsation periods of just a few hours. The full range of variability has not been identified, but is no more than a few hundredths of a magnitude.[21][22]

Aa is 12.02 ± 0.13 times as massive as the Sun, while Ab is 10.58 ± 0.18 times as massive.[20] The radius of β Centauri as a single star was calculated to be 8.6 D and its luminosity 41,700 L.[8]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 van Leeuwen (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. arXiv:0708.1752. Bibcode 2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. 2.0 2.1 2.2 Hoffleit, Dorrit; Jaschek, Carlos (1991). "The Bright star catalogue". New Haven, Conn.: Yale University Observatory, 5th rev.ed.. Bibcode
  3. 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 Ausseloos, M.; Aerts, C.; Lefever, K.; Davis, J.; Harmanec, P. (August 2006). "High-precision elements of double-lined spectroscopic binaries from combined interferometry and spectroscopy. Application to the β Cephei star β Centauri". Astronomy and Astrophysics 455 (1): 259–269. arXiv:astro-ph/0605220. Bibcode 2006A&A...455..259A. doi:10.1051/0004-6361:20064829.
  4. 4.0 4.1 Ausseloos, M.; Aerts, C.; Uytterhoeven, K.; Schrijvers, C.; Waelkens, C.; Cuypers, J. (2002). "Beta Centauri: An eccentric binary with two beta Cep-type components". Astronomy and Astrophysics 384: 209. Bibcode 2002A&A...384..209A. doi:10.1051/0004-6361:20020004.
  5. Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". Determination of Radial Velocities and their Applications (University of Toronto: International Astronomical Union) 30: 57. Bibcode 1967IAUS...30...57E.
  6. Davis, J.; Mendez, A.; Seneta, E. B.; Tango, W. J.; Booth, A. J.; O'Byrne, J. W.; Thorvaldson, E. D.; Ausseloos, M. et al. (2005). "Orbital parameters, masses and distance to β Centauri determined with the Sydney University Stellar Interferometer and high-resolution spectroscopy". Monthly Notices of the Royal Astronomical Society 356 (4): 1362. arXiv:astro-ph/0411054. Bibcode 2005MNRAS.356.1362D. doi:10.1111/j.1365-2966.2004.08571.x.
  7. 7.0 7.1 Tokovinin, A. A. (1999). "VizieR Online Data Catalog: Multiple star catalogue (MSC) (Tokovinin 1997-1999)". VizieR On-line Data Catalog: J/A+AS/124/75. Originally published in: 1997A&AS..124...75T 412: 40075. Bibcode 1999yCat..41240075T.
  8. 8.0 8.1 Prinja, Raman K. (1989). "Ultraviolet observations of stellar winds in Be and 'normal' B non-supergiant stars". Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711) 241 (4): 721. Bibcode 1989MNRAS.241..721P. doi:10.1093/mnras/241.4.721.
  9. Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York: Dover Publications Inc, p. 154, ISBN 0-486-21079-0,*.html, retrieved 2010-12-12
  10. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  11. 11.0 11.1 Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  12. Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J. (July 1997). "The Hipparcos Catalogue". Astronomy and Astrophysics 323: L49–L52. Bibcode 1997A&A...323L..49P.
  13. Perryman, Michael (2010), "The Making of History's Greatest Star Map", The Making of History's Greatest Star Map:, Astronomers’ Universe (Heidelberg: Springer-Verlag), Bibcode, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
  14. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  15. Hans Wehr, A Dictionary of Modern Written Arabic, 1961
  16. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  17. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 29 日
  18. 18.0 18.1 Hamacher, Duane W.; Frew, David J. (2010). "An Aboriginal Australian Record of the Great Eruption of Eta Carinae". Journal of Astronomical History & Heritage 13 (3): 220–34. arXiv:1010.4610. Bibcode 2010JAHH...13..220H.
  19. Stanbridge, WM (1857). "On the Astronomy and Mythology of the Aboriginies of Victoria". Transactions Philosophical Institute Victoria 2: 137–140. Archived from the original on 2 June 2013.
  20. 20.0 20.1 Pigulski, A.; Cugier, H.; Popowicz, A.; Kuschnig, R.; Moffat, A. F. J.; Rucinski, S. M.; Schwarzenberg-Czerny, A.; Weiss, W. W. et al. (2016). "Massive pulsating stars observed by BRITE-Constellation. I. The triple system β Centauri (Agena)". Astronomy and Astrophysics 588: 17. arXiv:1602.02806. Bibcode 2016A&A...588A..55P. doi:10.1051/0004-6361/201527872. A55.
  21. 21.0 21.1 21.2 Raassen, A. J. J.; Cassinelli, J. P.; Miller, N. A.; Mewe, R.; Tepedelenlioǧlu, E. (July 2006), "XMM-Newton observations of β Centauri (B1 III): The temperature structure in the hot plasma and the photosphere-wind connection", Astronomy and Astrophysics 437 (2): 599–609, Bibcode 2005A&A...437..599R, doi:10.1051/0004-6361:20052650
  22. Sterken, Christiaan; Jerzykiewicz, Mikolaj (1993). "Beta Cephei stars from a photometric point of view". Space Science Reviews 62 (1–2): 95–171. Bibcode 1993SSRv...62...95S. doi:10.1007/bf00208707. ISSN 0038-6308.

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