BI Cygni

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
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]
Distance4,349+548
−440 ly
(1,334+168
−135 pc)[3]
Absolute magnitude (MV)−6.78[4]
Details
Mass17[5] M☉
Radius852+12
−9 – 908+12
−10[3] R☉
Luminosity89,300[5] L☉
Surface gravity (log g)-0.35[6] cgs
Temperature3,575[3][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
SIMBADdata

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,[4] 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 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.[3] 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]

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