Beta Centauri

This article is about β Centauri and should not to be confused with Alpha Centauri B. For B Centauri, see B Centauri. For b Centauri, see HD 129116.

β Centauri

Location of β Centauri (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Centaurus
Right ascension	14h 03m 49.40535s
Declination	–60° 22′ 22.9266″
Apparent magnitude (V)	0.61
Astrometry
Radial velocity (Rv)	+5.9 km/s
Proper motion (μ)	RA: –33.27 mas/yr Dec.: –23.16 mas/yr
Parallax (π)	8.32 ± 0.50 mas
Distance	390 ± 20 ly (120 ± 7 pc)
Absolute magnitude (MV)	−4.53
Characteristics
U−B color index	–0.98
B−V color index	–0.23
β Cen A1
Spectral type	B1 III
Variable type	β Cep
β Cen A2
Spectral type	B1 III
Variable type	β Cep
β Cen B
Spectral type	B1V?
Orbit
Primary	β Cen A1
Companion	β 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) (secondary)	62.2°
Semi-amplitude (K1) (primary)	57.4 km/s
Semi-amplitude (K2) (secondary)	72.3 km/s
Orbit
Primary	β Cen A
Companion	β Cen B
Period (P)	288.267 yr
Semi-major axis (a)	0.870"
Details
Luminosity	41,700 L☉
Age	14.1 ± 0.6 Myr
β Cen A1
Mass	10.7 ± 0.1 M☉
Surface gravity (log g)	3.5 ± 0.4 cgs
Temperature	25,000 ± 2,000 K
β Cen A2
Mass	10.3 ± 0.1 M☉
Surface gravity (log g)	3.5 ± 0.4 cgs
Temperature	25,000 ± 2,000 K
β Cen B
Mass	4.61 M☉
Other designations
Agena, Hadar, HR 5267, HD 122451, CD−59°5365, LHS 51, SAO 252582, FK5 518 , HIP 68702, GC 18971, CCDM J14038-6022
Database references
SIMBAD	data

Beta Centauri (β Cen, β Centauri) is a trinary 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 the constellation Centaurus and the tenth brightest star in the night sky. It has the traditional names Hadar and Agena. The name Hadar comes from the Arabic حضار (the root's meaning is "to be present" or "on the ground" or "settled, civilized area"), while the name Agena may ultimately be derived from the Latin gena 'knees'. The Chinese name for the star is 马腹一 (Mandarin: mǎ fù yī, the First Star of the Horse's Abdomen).

Properties

The Beta Centauri system has three components: Beta Centauri A1, A2 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. According to parallax measurements from the astrometric Hipparcos satellite, the distance to this system is about 400 light-years (120 parsecs).

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 orbits the primary at 0.6 light years, or 100k AUs, with a period of roughly 1500 years. 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, (A1 and A2) with nearly identical mass that orbit each other over a period of 357 days with a large eccentricity of about 0.825. The pair are separated by a mean distance of roughly 4 astronomical units.

Both A1 and A2 apparently have a stellar classification of B1 III, 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.

Cultural significance

Beta Centauri is well known in the Southern Hemisphere as the inner of the two "Pointers" to 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 to effectively determine south.

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