Cocoon Nebula

IC 5146 sits at the eastern end of a long dark filament in Cygnus, and it is three kinds of nebula at once: hydrogen glowing red where ultraviolet light ionises it, dust scattering starlight blue around its edges, and cold opaque dust blocking whatever lies behind. The single bright star at its centre, BD+46°3474, is a young B-type star that both lit the cavity and hollowed it out — the cocoon is a bubble blown into the end of the cloud by the star it made. Trailing west is Barnard 168, the same structure where nothing is illuminating it; the star count thins sharply along that lane, and it is dust doing the thinning rather than an absence of stars. The embedded cluster Collinder 470 is barely a million years old, and far-infrared mapping of the filament by Herschel made this one of the standard case studies for star formation proceeding along threads of cold gas rather than in isolated clumps. The distance has moved with better data: the long-quoted figure is around 3,300 light-years, while Gaia parallaxes of the cluster members put it nearer 2,600.