Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||06h 22m 57.62686s|
|Declination||+22° 30′ 48.8979″|
|Apparent magnitude (V)||2.857|
|Spectral type||M3 III|
|U−B color index||+1.924|
|B−V color index||+1.643|
|Radial velocity (Rv)||+54.38 ± 0.24 km/s|
|Proper motion (μ)||RA: +56.39 mas/yr
Dec.: –110.03 mas/yr
|Parallax (π)||14.08 ± 0.71 mas|
|Distance||230 ± 10 ly
(71 ± 4 pc)
|Surface gravity (log g)||1.50 cgs|
|Rotational velocity (v sin i)||8.4 km/s|
Mu Geminorum (μ Gem, μ Geminorum) is the Bayer designation for a star in the northern constellation of Gemini. It has the traditional name Tejat Posterior, which means back foot, because it is the foot of Castor, one of the Gemini twins. This name was formerly applied to an asterism consisting of this star, along with γ Gem (Alhena), ν Gem, η Gem (Tejat Prior), and ξ Gem (Alzirr). The names Calx (Latin, meaning heel), Pish Pai (from the Persian Pīshpāy, پیشپای, meaning foreleg), and Nuhatai (from Arabic Al Nuḥātai, the dual form of Al Nuḥāt, "a Camel's Hump") have also been applied to Mu Geminorum.
In Chinese, 井宿 (Jǐng Su), meaning Well (asterism), refers to an asterism consisting of μ Geminorum, γ Geminorum, ν Geminorum, ξ Geminorum, ε Geminorum, 36 Geminorum, ζ Geminorum and λ Geminorum. Consequently, μ Geminorum itself is known as 井宿一 (Jǐng Su yī, English: the First Star of Well.)
Mu Geminorum has an average apparent visual magnitude of 2.9, which makes it the fourth brightest member of Gemini. From parallax measurements made during the Hipparcos mission, the distance to this star is roughly 230 light-years (71 parsecs). Its visual magnitude is diminished by 0.07 as a result of extinction from intervening gas and dust.
This star is a slow irregular variable of type LB. Its brightness varies between magnitude +2.75 and +3.02 over a 72-day period, along with a 2,000 day period of long term variation. It is a red giant at a stellar classification of M3 III, with a surface temperature of 3,773 K, meaning it is brighter, yet cooler than our Sun. The star is currently on the asymptotic giant branch and is generating energy through the nuclear fusion of hydrogen and helium along concentric shells surrounding an inert core of carbon and oxygen.
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