BD+40°4210 is
heavily reddened and extinguished by the
interstellar dust of the
Milky Way and little studied. It has turned out to be one of the most luminous stars of the Cygnus OB2 association, with an
absolute magnitude of −7.66 and a
bolometric luminosity more than 600,000 times that of the Sun. It has been assigned a B1III spectral classification, but with peculiarities including unusually shallow lines and broad emission. Despite the giant luminosity class, the luminosity appears to be extremely high, placing it on or near the
S Doradus instability strip occupied by quiescent luminous blue variables. Its brightness varies by less than 0.1 magnitudes on a timescale around 100 days. BD+40°4210 is located at a projected 4.8
parsecs from the candidate luminous blue variable
G79.29+0.46 and probably at a similar distance since both are assumed members of Cygnus OB2; however, unlike the latter which is surrounded by an extensive ring-shaped
nebulosity, no nebula has been found around this star. ==References==