Dark nebulae are clouds of gas and dust that are visible only because they block the light of stars and nebulae beyond them. They range in size from minute, more or less spherical Bok Globules, usually seen in photographs against the glow of emission nebulae, to the naked-eye clouds of the southern Coal Sack and the giant Rho Ophiuchi dark cloud, which affects 1000 square degrees – two percent – of the sky.

Composition

The clouds consist of a mixture of dusty particles and gas, the whole having a composition similar to the standard cosmic abundance of about 75 percent hydrogen and 23 percent helium with heavier elements making up the remainder. The dust itself comprises only about 0.1 percent of the mass of a cloud but it is believed to play an important role in the formation of molecules in space. The surface of the dust particles provides a surface to which atoms within the cloud can adhere and perhaps combine to form simple molecules (i.e. H2) or much more complicated compounds such as formaldehyde or even amino acids. These relatively fragile compounds survive because they are protected from energetic ultraviolet radiation from stars by the cloud itself. The interior of these clouds are thus very cold, typically only 10K, which allows them to gradually collapse under their own gravity, eventually to begin the process of star formation.


Horsehead Nebula


Pipe Nebula


IC2944


Barnard 68 Nebula


Snake in the Dark