BC Cygni (abbreviated to BC Cyg), also known as BD+37°3903, is a red supergiant or hypergiant pulsating variable star of spectral type M3.5Ia in the constellation Cygnus.
It is considered a member of the Cygnus OB1 stellar 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 and an effective temperature of 2,858K in the year 1900, and a luminosity of and a temperature of 3,614K in the year 2000. It is one of largest stars known, at its brightest and coolest calculated to be compared to at its hottest and faintest. 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 , 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.
See also NML Cygni KY Cygni RW Cygni
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