BI Cygni

BI Cygni (BI Cyg, IRC +40408, BD+36 4025) is a red supergiant in the constellation Cygnus. It is an irregular variable star with a maximum brightness of magnitude 8.4 and a minimum of magnitude 9.9. It is considered a member of the Cygnus OB1 stellar association,^[3] its distance is around 1,300 parsecs (4,200 ly) of the Solar System. It is less than a degree south of another variable red supergiant, BC Cygni.

A visual band light curve for BI Cygni, plotted from INTEGRAL OMC data^[7]

BI Cygni

BI Cygni is the small very red dot right on the left edge of this image. The bright star at the centre is γ Cygni and north is to the right. Credit: Erik Larsen
Observation data Epoch J2000      Equinox J2000
Constellation	Cygnus
Right ascension	20h 21m 21.8869s[1]
Declination	+36° 55′ 55.729″[1]
Apparent magnitude (V)	8.4 – 9.9[2]
Characteristics
Evolutionary stage	Red supergiant[3]
Spectral type	M4 Iab[2]
Variable type	Lc[2]
Astrometry
Radial velocity (Rv)	−7.48±0.42[1] km/s
Proper motion (μ)	RA: −2.751[1] mas/yr Dec.: −5.459[1] mas/yr
Parallax (π)	0.3541±0.0377 mas[1]
Distance	4,349+548 −440 ly (1,334+168 −135 pc)[4]
Absolute magnitude (MV)	−6.78[3]
Details
Mass	17[5] M☉
Radius	852+12 −9 – 908+12 −10[4] R☉
Luminosity	89,300[5] L☉
Surface gravity (log g)	-0.35[6] cgs
Temperature	3,575[4][5] K
Metallicity [Fe/H]	+0.40[6] dex
Age	>12[5] Myr
Other designations
RAFGL 2559, BI Cyg, BD+36 4025, WDS J20214+3656, TIC 13249363, TYC 2684-522-1, GSC 02684-00522, IRAS 20194+3646, 2MASS J20212192+3655555
Database references
SIMBAD	data

BI Cyg is a slow irregular variable star classified as type Lc, an irregular supergiant. Its brightness changes between extremes of magnitude 8.4 and 9.9.^[2] Frequency analysis of its light curve shows no significant periods.^[8]

BI Cyg is one of the largest known stars with a radius around 850 R_☉, measured by its angular diameter by the CHARA array.^[4] It is about 90,000 times more luminous that the Sun and has a cool effective temperature of 3,535 K.^[5] Its mass is estimated at 17 solar masses, and it took 12 million years to enter the red supergiant phase.^[5]

See also

• RW Cygni
• KY Cygni
• NML Cygni