Cygnus OB2-12

Cygnus OB2 #12 is an extremely bright blue hypergiant with an absolute bolometric magnitude (all electromagnetic radiation) of −10.9, among the most luminous stars known in the galaxy. This makes the star nearly two million times more luminous than the sun, although less than half the estimates when the star was first discovered.

It is a member of the Cyg OB2 Association, a cluster of young massive stars about 5000 light years away in Cygnus, and resides in a region of the Galaxy from which visible light is heavily absorbed by interstellar dust when viewed from the Earth. The dust causes the star to be strongly reddened despite being an intrinsically hot and blue star, hence it has been extensively studied in the infra-red. Were it not for the dust extinction, the star would have a visual magnitude about 1.5, nearly as bright as Deneb (Alpha Cygni), but because of the dust the observed visual magnitude is 11.4 so that it requires binoculars or a small telescope to be seen.

Cygnus OB2 #12 is a candidate Luminous Blue Variable star or LBV. Its position in the HR diagram, luminosity, and spectrum all classify it as an LBV, but it has not been observed to vary in brightness although the spectral type apparently has varied since its discovery.