# Astronomy:Beta Centauri

Short description: Star system in the constellation of Centaurus
Observation dataEquinox J2000.0]] (ICRS) Constellation Location of β Centauri (circled) Centaurus 14h 03m 49.40535s[1] −60° 22′ 22.9266″[1] 0.61[2] –0.98[2] –0.23[2] B1 III[3] β Cep/SPB[4] B1 III[3] B1V?[3] Radial velocity (Rv) +9.59+0.23−0.21[4] 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 Distance 390 ± 20 ly (120 ± 7 pc) Absolute magnitude (MV) −4.9±0.2[5] Primary β Cen Aa Companion β Cen Ab Period (P) 356.915±0.015 d(0.97720±0.00004 yr) Semi-major axis (a) 0.02515+0.09−0.08″ Eccentricity (e) 0.8245±0.006 Inclination (i) 67.68±0.12° Longitude of the node (Ω) 108.80+0.14−0.15° Periastron epoch (T) 2452000.15202 Argument of periastron (ω)(secondary) 60.87+0.26−0.25° Semi-amplitude (K1)(primary) 62.9 km/s Semi-amplitude (K2)(secondary) 72.35 km/s Primary β Cen A Companion β Cen B Period (P) 288.267 yr Semi-major axis (a) 0.870″ Luminosity 66,100[5] L☉ Age 14.1±0.6[3] Myr Mass 12.02±0.13[4] M☉ Radius 9[7] R☉ Luminosity 31,600+18,500−11,700[5] L☉ Surface gravity (log g) 3.55±0.11[5] cgs Temperature 25,000±2,000[5] K Rotational velocity (v sin i) 190±20[8] km/s Mass 10.58±0.18[4] M☉ Radius 9[7] R☉ Luminosity 25,100+14,700−9,300[5] L☉ Surface gravity (log g) 3.55±0.11[5] cgs Temperature 23,000±2,000[5] K Rotational velocity (v sin i) 75±15[8] km/s Mass 4.61[6] M☉ Agena, Hadar[9], CD−59°5365, FK5 518, GC 18971, HD 122451, HIP 68702, HR 5267, SAO 252582, CCDM J14038-6022, LHS 51[10] SIMBAD data

Beta Centauri is a triple star system in the southern constellation of Centaurus. It is officially called Hadar (/ˈhdɑːr/). The Bayer designation of Beta Centauri is Latinised from β Centauri, and abbreviated Beta Cen or β Cen. The system's combined apparent visual magnitude of 0.61 makes it the second-brightest object in Centaurus and one of the brightest stars in the night sky. According to parallax measurements from the astrometric Hipparcos satellite, the distance to this system is about 390 light-years (120 parsecs).

## Nomenclature

β Centauri (Latinised to Beta Centauri) is the star system'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"[11]), while the name Agena /əˈnə/ is thought to be derived from the Latin genua, meaning "knees", from the star's position on the left knee of the centaur depicted in the constellation Centaurus. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[12] to catalog and standardize proper names for stars. The WGSN approved the name Hadar for the star β Centauri Aa on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[13]

The Chinese name for the star is 马腹一 (Mandarin: mǎ fù yī, "the First Star of the Horse's Abdomen").[14]

The Boorong people indigenous to what is now northwestern Victoria, Australia named it Bermbermgle (together with α Centauri),[15] two brothers who were noted for their courage and destructiveness, and who spear and kill Tchingal, "The Emu" (Coalsack Nebula).[16] The Wotjobaluk people name the two brothers Bram-bram-bult.[15]

## Visibility

Beta Centauri is one of the brightest stars in the sky at magnitude 0.61. Its brightness varies by a few hundredths of a magnitude, too small to be noticeable to the naked eye.[17] Because of its spectral type and the detection of pulsations, the Aa component has been classified as a β Cephei variable.[18]

Beta Centauri is well known in the Southern Hemisphere as the inner of the two "Pointers" to the constellation Crux, popularly 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 north end of the cross. Using Gacrux, a navigator can draw a line with Acrux at the south end to effectively determine south.[19]

## Stellar system

The Beta Centauri system is made up of three stars: Beta Centauri Aa, Beta Centauri Ab, and Beta Centauri 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 variation in radial velocity suggested that Beta Centauri A is a binary star.[20][21] This was confirmed in 1999.[21] It consists of a pair of stars, β Centauri Aa and β Centauri Ab, of similar mass that orbit each other over a period of 357 days with a large eccentricity of about 0.8245.[4]

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.[22]

Both Aa and Ab apparently have a stellar classification of B1 III,[22] with the luminosity class of III indicating giant stars that are evolving away from the main sequence. Component Aa rotates much more rapidly than Ab, causing its spectral lines to be broader, and so the two components can be distinguished in the spectrum. Component Ab, the slow-rotating star, has a strong magnetic field although no detected abundance peculiarities in its spectrum. Multiple pulsations modes have been detected in component Aa, some of which correspond to brightness variations, so this star is considered to be variable. The detected pulsation modes correspond to those for both β Cephei variables and slowly pulsating B stars. Similar pulsations have not been detected in component Ab, but it is possible that it is also a variable star.[4]

Aa is 12.02 ± 0.13 times as massive as the Sun, while Ab is 10.58 ± 0.18 times as massive.[4]

## References

1. van Leeuwen (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. doi:10.1051/0004-6361:20078357. Bibcode2007A&A...474..653V.
2. Hoffleit, Dorrit; Jaschek, Carlos (1991). "The Bright star catalogue". New Haven, Conn.: Yale University Observatory, 5th Rev.ed.. Bibcode1991bsc..book.....H.
3. 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. doi:10.1051/0004-6361:20064829. Bibcode2006A&A...455..259A.
4. 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. doi:10.1051/0004-6361/201527872. A55. Bibcode2016A&A...588A..55P.
5. Shultz, M. E.; Wade, G. A.; Rivinius, Th; Alecian, E.; Neiner, C.; Petit, V.; Wisniewski, J. P.; MiMeS Collaboration et al. (2019). "The magnetic early B-type Stars II: Stellar atmospheric parameters in the era of Gaia". Monthly Notices of the Royal Astronomical Society 485 (2): 1508. doi:10.1093/mnras/stz416. Bibcode2019MNRAS.485.1508S.
6. Tokovinin, A. A. (1997). "MSC - a catalogue of physical multiple stars". Astronomy & Astrophysics Supplement Series 124: 75. doi:10.1051/aas:1997181. Bibcode1997A&AS..124...75T.
7. Kaler, James B.. "HADAR (Beta Centauri)". Stars. University of Illinois.
8. Alecian, E.; Kochukhov, O.; Neiner, C.; Wade, G. A.; De Batz, B.; Henrichs, H.; Grunhut, J. H.; Bouret, J.-C. et al. (2011). "First HARPSpol discoveries of magnetic fields in massive stars". Astronomy & Astrophysics 536: L6. doi:10.1051/0004-6361/201118354. Bibcode2011A&A...536L...6A.
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. Retrieved 2010-12-12.
10. "V* bet Cen -- Variable Star of beta Cep type". SIMBAD. Centre de Données astronomiques de Strasbourg.
11. Hans Wehr (1979). A Dictionary of Modern Written Arabic. Otto Harrassowitz Verlag. ISBN 978-3-447-02002-2.
12. 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. Bibcode2010JAHH...13..220H.
13. Stanbridge, WM (1857). "On the Astronomy and Mythology of the Aboriginies of Victoria". Transactions Philosophical Institute Victoria 2: 137–140.
14. Lefèvre, L.; Marchenko, S. V.; Moffat, A. F. J.; Acker, A. (2009). "A systematic study of variability among OB-stars based on HIPPARCOS photometry". Astronomy and Astrophysics 507 (2): 1141. doi:10.1051/0004-6361/200912304. Bibcode2009A&A...507.1141L.
15. Samus, N. N. et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S 1: B/gcvs. Bibcode2009yCat....102025S.
16. Kyselka, Will; Lanterman, Ray E. (1976). North Star to Southern Cross. University of Hawaii Press. 59. ISBN 0-8248-0419-8. Bibcode1976nsts.book.....K.
17. Breger, M. (May 1967). "A Spectroscopic Study of Two Southern B-Type Variables". Monthly Notices of the Royal Astronomical Society 136 (1): 51–59. doi:10.1093/mnras/136.1.51.
18. Robertson, J.G.; Bedding, T.G.; Aerts, C.; Waelkens, C.; Marson, R.G.; Barton, J.R. (January 1999). "Interferometry and spectroscopy of β Cen: a β Cephei star in a binary system". Monthly Notices of the Royal Astronomical Society 302 (1): 245–252. doi:10.1046/j.1365-8711.1999.02068.x. Bibcode1999MNRAS.302..245R.
19. 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. doi:10.1051/0004-6361:20052650. Bibcode2005A&A...437..599R.