6 Geminorum

6 Geminorum is a variable star in the zodiac constellation of Gemini, located roughly 5,800 light years away from the Sun. It has the variable star designation BU Geminorum; 6 Geminorum is the Flamsteed designation. At its brightest this reddish hued star is barely visible to the naked eye but is readily visible with binoculars, found southeast of M 35, just to the south of WY Geminorum. It is moving further from the Earth with a heliocentric radial velocity of +27 km/s.^[5] The star is a member of the Gemini OB1 association.^[12]

6 Geminorum

Location of 6 Geminorum (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Gemini
Right ascension	06h 12m 19.09884s[1]
Declination	+22° 54′ 30.6531″[1]
Apparent magnitude (V)	5.74 – 8.10[2]
Characteristics
Spectral type	M1-2 Ia-Iab[3]
U−B color index	+1.93[4]
B−V color index	+1.63[4]
Variable type	LC[2]
Astrometry
Radial velocity (Rv)	+27.16±0.42[5] km/s
Proper motion (μ)	RA: +0.144[1] mas/yr Dec.: −2.085[1] mas/yr
Parallax (π)	0.5642±0.0957 mas[1]
Distance	approx. 5,800 ly (approx. 1,800 pc)
Absolute magnitude (MV)	−6.32[6]
Details
Mass	~20[7] M☉
Radius	821+60 −27[8] R☉
Luminosity	86,000[9] L☉
Surface gravity (log g)	0.0[6] cgs
Temperature	3,789[10] K
Other designations
6 Gem, BU Gem, BD+22°1220, GC 7896, HD 42543, HIP 29450, HR 2197, SAO 78098, TYC 1877-1719-1, 2MASS J06121911+2254305[11]
Database references
SIMBAD	data

This is an evolved red supergiant with a stellar classification of M1-2 Ia-Iab.^[3] It is a semiregular variable star, ranging from visual magnitude +5.7 down to +7.5 over a period of 325 days. It has been given the sub-classification of Lc, which means "Irregular variable supergiants of late spectral types having amplitudes of about 1 mag. in V.O".^[13] The star has expanded to 821^[8] times the Sun's radius and is radiating 86,000^[9] times the luminosity of the Sun from its swollen photosphere at an effective temperature of 3,789 K.^[10]

• 
  A light curve for BU Geminorum, plotted from TESS data^[14]
• 
  Messier 35, with 6 Gem visible right at the bottom edge of the image