Star in the constellation Cygnus
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! style="text-align: center; background-color: #FFFFC0;" colspan="2" | Observation dataEpoch J2000      Equinox J2000
! style="text-align:left" | Constellation
! style="text-align:left" | Right ascension
! style="text-align:left" | Declination
! style="text-align:left" | Apparent magnitude (V)
! style="background-color: #FFFFC0; text-align: center;" colspan="2"| Characteristics
! style="text-align:left" | Spectral type
! style="text-align:left" | B−V 
! style="text-align:left" | Variable type
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 | temperature=3,535
 | luminosity=150,000
 | age_myr = ;9
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.
It is considered a member of the stellar Cygnus OB1 association, and within it the open cluster Berkeley 87, which would place at a distance of of the Solar System; 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 .
BC Cygni was found to have a luminosity of 145,000 L☉ and an effective temperature of 2,858K in the year 1900, and a luminosity of 112,000 L☉ and a temperature of 3,614K in the year 2000. At its brightest and coolest has been calculated to be 1,553 R☉ compared to 856 R☉ at the hottest and faintest. It is one of largest stars known, and currently is times larger than the Sun. 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 per year.
Louisa Wells discovered that the star's brightness varied, based on the examination of 15 photographic plates. That discovery was announced in 1911. It was given its variable star designation, BC Cygni, in 1914. The brightness of BC Cyg varies from visual magnitude +9.0 and +10.8 with a period of 720 ± 40 days. Between around the year 1900 and 2000 appears to have increased its average brightness of 0.5 magnitudes.
References.
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