Alpha Tucanae

Alpha Tucanae (α Tuc, α Tucanae), also named Lang-Exster,^[10] is a binary star system in the southern circumpolar constellation of Tucana. With an apparent visual magnitude of 2.86,^[2] it can be seen with the naked eye from the southern hemisphere. Using parallax measurements, the distance to this system can be estimated as 184 light-years (56 parsecs). A cool star with a surface temperature of 4300 K, it is 424 times as luminous as the sun and 37 times its diameter. It is 2.5 to 3 times as massive. It is unclear what stage of evolution the star is in.^[7]

α Tucanae

Location of α Tucanae (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Tucana
Right ascension	22h 18m 30.11244s[1]
Declination	−60° 15′ 34.6664″[1]
Apparent magnitude (V)	2.86[2]
Characteristics
Spectral type	K3 III[3]
U−B color index	+1.54[2]
B−V color index	+1.39[2]
Astrometry
Radial velocity (Rv)	+45.8[4] km/s
Proper motion (μ)	RA: −77.000[1] mas/yr Dec.: −32.823[1] mas/yr
Parallax (π)	17.7324±0.3290 mas[1]
Distance	184 ± 3 ly (56 ± 1 pc)
Absolute magnitude (MV)	−1.05[5]
Absolute bolometric magnitude (Mbol)	−1.97[5]
Orbit[6]
Period (P)	4197.7 days
Eccentricity (e)	0.39
Periastron epoch (T)	18666.4
Argument of periastron (ω) (secondary)	48.5°
Semi-amplitude (K1) (primary)	7.2 km/s
Details
Mass	2.5 - 3[7] M☉
Radius	37.3[8] R☉
Luminosity	424[7] L☉
Temperature	4310[8] K
Other designations
Lang-Exster, CPD−60°7561, FK5 841, HD 211416, HIP 110130, HR 8502, SAO 255193[9]
Database references
SIMBAD	data

This is a spectroscopic binary, which means that the two stars have not been individually resolved using a telescope, but the presence of the companion has been inferred from measuring changes in the spectrum of the primary. The orbital period of the binary system is 4197.7 days (11.5 years).^[6] The primary component has a stellar classification of K3 III,^[3] which indicates it is a giant star that has exhausted the supply of hydrogen at its core and evolved away from the main sequence. It has the characteristic orange hue of a K-type star.

The IAU Working Group on Star Names approved the name Lang-Exster for this star system on 19 September 2024 and it is now so entered in the IAU Catalog of Star Names. "Lang" is a Malay and Indonesian word meaning hornbill, and "Exster" is a Dutch word meaning magpie; both were historically used as names for the constellation Tucana. Since this star is a binary system it is given a double name, making it the second star system with multiple IAU-named components, after Alpha Centauri.^[10]