Eta Centauri

Not to be confused with H Centauri.

Eta Centauri, Latinized from η Centauri, is a star in the southern constellation of Centaurus. It has an apparent visual magnitude of +2.35^[2] and is located at a distance of around 306 light-years (94 parsecs).^[1]

A light curve for Eta Centauri, plotted from TESS data^[10]

Eta Centauri

Location of η Centauri (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Centaurus
Right ascension	14h 35m 30.42416s[1]
Declination	−42° 09′ 28.1708″[1]
Apparent magnitude (V)	+2.35[2] (2.30 - 2.41[3])
Characteristics
Spectral type	B1.5 Vne[4]
U−B color index	−0.862[2]
B−V color index	−0.215[2]
Variable type	GCAS[3] + LERI
Astrometry
Radial velocity (Rv)	−0.2[5] km/s
Proper motion (μ)	RA: −34.73[1] mas/yr Dec.: −32.72[1] mas/yr
Parallax (π)	10.67±0.21 mas[1]
Distance	306 ± 6 ly (94 ± 2 pc)
Absolute magnitude (MV)	−2.53[6]
Details
Mass	12.0±0.3[7] M☉
Radius	6.10±0.12[8] R☉
Luminosity	8,700[4] L☉
Surface gravity (log g)	3.95±0.04[8] cgs
Temperature	25,700[4] K
Rotational velocity (v sin i)	330[4] km/s
Age	5.6±1.0[7] Myr
Other designations
η Cen, CD−41°8917, CPD−41°6839, FK5 537, HD 127972, HIP 71352, HR 5440, SAO 225044.[9]
Database references
SIMBAD	data

The stellar classification of this star is B1.5 Vne,^[4] indicating that it is a B-type main sequence star. The 'n' suffix means that the absorption lines are broadened from rapid rotation and the 'e' that it shows emission lines in its spectrum. It has a projected rotational velocity of 330 km s^−1[4] and completes a full rotation in less than a day.

As a Be star,^[11] it has variable emissions in its hydrogen spectral lines. This emission can be modelled by a decretion disk of gas that has been ejected from the star by its rapid rotation and now follows a near-Keplerian orbit around the central body.^[12] Its brightness is also slightly variable, and it is classified as a Gamma Cassiopeiae variable star with multiple periods of variability.^[3] The International Variable Star Index lists Eta Centauri as both a Gamma Cassiopeiae variable and a Lambda Eridani variable with variations caused by its rotation and pulsations.^[13]

Eta Centauri has about 12 times the mass of the Sun,^[7] placing it above the dividing line between stars that evolve into white dwarfs and those that turn into supernovae. It is radiating 8,700^[4] times the luminosity of the Sun from its outer atmosphere at an effective temperature of 25,700 K.^[4] At this temperature, the star glows with the blue-white hue common to B-type stars.^[14] Eta Centauri is a proper motion member of the Upper Centaurus–Lupus sub-group in the Scorpius–Centaurus OB association, the nearest such co-moving association of massive stars to the Sun.^[15]

In traditional Chinese astronomy, Eta Centauri was known as 庫樓二^[16] (meaning: the Second (Star) of Koo Low).^[17]