Zeta Centauri

Zeta Centauri, also named Leepwal,^[11] is a binary star system in the southern constellation of Centaurus. 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] It is drifting further away with a radial velocity of +6.5 km/s.^[4]

Zeta Centauri

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]
Characteristics
Spectral type	B2.5 IV[3]
U−B color index	−0.91[2]
B−V color index	−0.22[2]
Astrometry
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 mas[1]
Distance	382 ± 6 ly (117 ± 2 pc)
Absolute magnitude (MV)	−2.79[5]
Orbit[6]
Period (P)	8.024 days
Semi-major axis (a)	0.0014"[7]
Eccentricity (e)	0.5
Periastron epoch (T)	2413719.321 JD
Argument of periastron (ω) (secondary)	290°
Details
Mass	7.8±0.1[3] M☉
Radius	5.80±0.53[8] R☉
Surface gravity (log g)	3.84±0.08[8] cgs
Temperature	23561±283[8] K
Rotational velocity (v sin i)	235[9] km/s
Age	39.8±5.7[3] Myr
Other designations
Leepwal, ζ Cen, CD−46 8949, CPD−46 6560, HD 121263, HIP 68002, HR 5231, SAO 224538[10]
Database references
SIMBAD	data

ζ Cen is a double-lined spectroscopic binary system,^[12] 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.^[6] The estimated angular separation of the pair is 1.4 mas.^[7]

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^[8] times the Sun's radius. This star is rotating rapidly with a projected rotational velocity of 235 km s⁻¹.^[9]

Nomenclature

Zeta Centauri, Latinized from ζ Centauri, is the star's Bayer designation. Along with other modern Bayer designations in Centaurus, it was assigned by Nicolas Louis de Lacaille in his 1756 star catalog; the star referred to as ζ Centauri in Bayer's Uranometria of 1603 is Acrux in the Southern Cross, then considered part of Centaurus.^[13]

This star has been referred to by the proper name Alnair /æˈnɛər/, from Arabic: نير بطن قنطورس, romanized: nayyir baṭan qanṭūris, lit. 'the bright (star) of the body of the centaur',^[14][15] though this was originally a name for Acrux (see above).^[16][13] Alnair is also the proper name of α Gruis and was officially approved by the IAU as the name of that star.^[11] This star has also been referred to as Baten Kentaurus, from the same Arabic source.^[17]

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.^[18] Consequently, the Chinese name for ζ Centauri itself is 庫樓一 (Kù Lóu yī, English: the First Star of Arsenal.)^[19] (See also ι Centauri.)

In Marshallese, this star is named Ļeepwal (pronounced leyepwal), the third of the ten sons of Lōktañūr (Capella).^[20] The IAU Working Group on Star Names approved the name Leepwal for this star on 18 July 2024 and it is now so entered in the IAU Catalog of Star Names.^[11]