BC Cygni

BC Cygni (BC Cyg, HIP 100404, BD + 37 3903) is a red supergiant and pulsating variable star of spectral type M3.5Ia in the constellation Cygnus.

BC Cygni

BC Cygni is visible as a red star (circled). 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 38.55s[1]
Declination	+37° 31′ 58.9″[1]
Apparent magnitude (V)	9.0 - 10.8[2]
Characteristics
Spectral type	M3.5 Ia[3] (M2 - M5[4])
B−V color index	+3.13 - +3.21[4]
Variable type	SRc[3]
Astrometry
Radial velocity (Rv)	−20.97[1] km/s
Proper motion (μ)	RA: −3.710 mas/yr[1] Dec.: −6.307 mas/yr[1]
Parallax (π)	0.5760±0.0500 mas[1]
Distance	5,418+470 −430 ly (1,662+144 −132 pc)[5]
Absolute magnitude (MV)	−7.71[6]
Details
Mass	19[4][7] M☉
Radius	1,187[1] R☉
Luminosity	150,000[7] 204,000+159,000 −56,000[8] L☉
Temperature	3,535[7] K
Age	>9[7][a] Myr
Other designations
BC Cyg, HIP 100404, HV 3339, BD+37°3903, IRAS 20197+3722, 2MASS J20213855+3731589
Database references
SIMBAD	data

It is considered a member of the stellar Cygnus OB1 association, and within it the open cluster Berkeley 87,^[9] which would place at a distance of 1,673 parsecs (5,000 ly) of the Solar System;^[10] it is less than a degree north of another variable red supergiant, BI Cygni. According to its Gaia Data Release 3 parallax, it is at about 1,700 pc.^[1]

BC Cygni was found to have a luminosity of 145,000 L_☉ and an effective temperature of 2,858 K in the year 1900, and a luminosity of 112,000  L_☉ and a temperature of 3,614 K in the year 2000. It is one of largest stars known, at its brightest and coolest calculated to be 1,553 R_☉ compared to 856 R_☉ at its hottest and faintest.^[11] If it were in the place of the Sun, its photosphere would engulf the entire inner Solar System and reach close to the orbit of Jupiter. With a mass of about 19 M_☉, it is estimated that the stellar mass loss, as dust, as the atomic and molecular gas could not be evaluators is 3.2×10⁻⁹ M_☉ per year.^[12]

A visual band light curve for BC Cygni, from AAVSO data^[13]

Louisa Wells discovered that the star's brightness varied, based on the examination of 15 photographic plates. That discovery was announced in 1911.^[14] It was given its variable star designation, BC Cygni, in 1914.^[15] The brightness of BC Cyg varies from visual magnitude +9.0 and +10.8 with a period of 720 ± 40 days.^[2] Between around the year 1900 and 2000 appears to have increased its average brightness of 0.5 magnitudes.^[4]

See also

• NML Cygni
• KY Cygni
• RW Cygni

Notes

1. This is only the age at which the star enters the red supergiant phase.