Beta Gruis

"Tiaki" redirects here. For the Japanese proper name, see Chiaki.

Beta Gruis (β Gruis, abbreviated Beta Gru, β Gru), formally named Tiaki /tiˈɑːki/,^[13] is the second brightest star in the southern constellation of Grus. It was once considered the rear star in the tail of the constellation of the (Southern) Fish, Piscis Austrinus: it, with Alpha, Delta, Theta, Iota, and Lambda Gruis, belonged to Piscis Austrinus in medieval Arabic astronomy.^[14]

Beta Gruis

Location of β Gruis (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Grus
Right ascension	22h 42m 40.05027s[1]
Declination	−46° 53′ 04.4752″[1]
Apparent magnitude (V)	2.146[2] (2.0 - 2.3[3])
Characteristics
Evolutionary stage	AGB[4]
Spectral type	M4.5 III[5]
U−B color index	+1.757[2]
B−V color index	+1.620[2]
Variable type	SRb[3]
Astrometry
Radial velocity (Rv)	+1.6[6] km/s
Proper motion (μ)	RA: +135.16[1] mas/yr Dec.: −4.38[1] mas/yr
Parallax (π)	18.43±0.42 mas[1]
Distance	177 ± 4 ly (54 ± 1 pc)
Absolute magnitude (MV)	−1.61±0.052[7]
Details
Mass	2.4[8] M☉
Radius	154[9] R☉
Luminosity	3,221±242[9] L☉
Surface gravity (log g)	0.4[10] cgs
Temperature	3,508±125[9] K
Metallicity [Fe/H]	0.0[10] dex
Age	450[11] Myr
Other designations
Tiaki, Beta Gru, CD−47 14308, FK5 856, HR 8636, HD 214952, HIP 112122, SAO 231258.[12]
Database references
SIMBAD	data

Nomenclature

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

It bore the traditional Tuamotuan name of Tiaki.^[15] In 2016, the IAU organized a Working Group on Star Names (WGSN)^[16] to catalog and standardize proper names for stars. The WGSN approved the name Tiaki for this star on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.^[13]

In Chinese, 鶴 (Hè), meaning Crane, refers to an asterism consisting of Beta Gruis, Alpha Gruis, Epsilon Gruis, Eta Gruis, Delta Tucanae, Zeta Gruis, Iota Gruis, Theta Gruis, Delta² Gruis and Mu¹ Gruis.^[17] Consequently, Beta Gruis itself is known as 鶴二 (Hè èr, English: Second Star of the Crane).^[18] The Chinese name gave rise to another English name, Ke.^[19]

Properties

A visual band light curve for Beta Gruis, plotted from data published by Otero and Moon (2006).^[3] The inset plot shows the points plotted in red with an expanded scale.

Beta Gruis is a red giant star^[3] on the asymptotic giant branch^[4] with an estimated mass of about 2.4 times that of the Sun^[20] and a surface temperature of approximately 3,500 K, just over half the surface temperature of the Sun.^[9] This low temperature accounts for the dull red color of an M-type star. The total luminosity is about 3,200 times that of the Sun, and it has 150 times the Sun's radius.^[9]

It is one of the brightest stars at infrared and near-infrared wavelenghts. At the K band, it is the fifth-brightest star in the night sky.^[21]

Alan William James Cousins announced that Beta Gruis is a variable star in 1952.^[22] Beta Gruis is a semiregular variable (SRb) star that varies in magnitude by about 0.4. It varies between intervals when it displays regular changes with a 37-day periodicity and times when it undergoes slow irregular variability.^[3]