81 Geminorum

81 Geminorum is a binary star^[8] system in the northern constellation of Gemini. It has the Bayer designation g Geminorum, while 81 Geminorum is its Flamsteed designation. This system is visible to the naked eye as a faint, orange-hued point of light with an apparent visual magnitude of 4.89.^[2] The pair are located approximately 360 light years away from the Sun, based on parallax,^[1] and are moving further away with a radial velocity of +83 km/s, having come to within an estimated 164 light-years of the Earth nearly a million years ago.^[2] 81 Geminorum lies close enough to the ecliptic to undergo lunar occultations.^[4][9]

81 Geminorum

Location of 81 Geminorum (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Gemini
Right ascension	07h 46m 07.45014s[1]
Declination	+18° 30′ 36.0217″[1]
Apparent magnitude (V)	4.89[2]
Characteristics
Spectral type	K4 III[3]
B−V color index	1.425±0.034[2]
Astrometry
Radial velocity (Rv)	+83.13±0.08[2] km/s
Proper motion (μ)	RA: −79.687[1] mas/yr Dec.: −53.551[1] mas/yr
Parallax (π)	9.1500±0.2963 mas[1]
Distance	360 ± 10 ly (109 ± 4 pc)
Absolute magnitude (MV)	−0.19[2]
Orbit[4]
Period (P)	1,519.7±1.7 d
Semi-major axis (a)	≥ 142±3 Gm
Eccentricity (e)	0.325±0.015
Periastron epoch (T)	41,584±11 MJD
Argument of periastron (ω) (secondary)	73±3°
Semi-amplitude (K1) (primary)	7.21±0.13 km/s
Details
81 Gem A
Mass	1.22[5] M☉
Radius	33.7+2.0 −1.7[1] R☉
Luminosity	287.3±10.5[1] L☉
Surface gravity (log g)	1.94[6] cgs
Temperature	4,095+109 −115[1] K
Metallicity [Fe/H]	−0.18±0.06[2] dex
Age	6.32[5] Gyr
Other designations
g Gem, 81 Gem, BD+18°1733, FK5 1200, GC 10456, HD 62721, HIP 37908, HR 3003, SAO 97221, WDS J07461+1831AB[7]
Database references
SIMBAD	data

The variable velocity of this system was first suspected at the Dominion Astrophysical Observatory in 1921, then confirmed by the Lick Observatory in 1922. It is a single-lined spectroscopic binary with an orbital period of 4.2 years and an eccentricity of 0.325.^[4] The visible component is an aging giant star with a stellar classification of K4 III,^[3] having exhausted the supply of hydrogen at its core then expanded to 34^[1] times the Sun's radius. It is over six^[5] billion years old with 1.22^[5] times the mass of the Sun. This is a candidate alpha-enhanced star that displays a significant overabundance of silicon.^[10] The star is radiating around 287^[1] times the Sun's luminosity from its bloated photosphere at an effective temperature of 4,095 K.^[1]