A dark nebula, also known as an absorption nebula, is a type of nebula formed from gas and dust and visible because it blocks out and absorbs light from stars or bright nebulae behind. Dark nebulae may also contain stars, but these are obscured by the nebula. Well known examples of dark nebulae include the Horsehead nebula and the Coalsack nebula
Dark nebulae come in a range of sizes; the smallest are only a couple of light years across, containing only a couple of thousand solar masses of material whereas the largest can be over 600 light years across, some containing more than a million solar masses of interstellar material. The largest dark nebulae are called giant molecular clouds. Typical densities within clouds are 100-300 molecules per cubic centimetre.
Although they are called dark nebulae, they are only dark at visible wavelengths, in the region of 400-700 nm. The temperature inside dark nebula is often in the range 10-100 kelvin, meaning hydrogen within the nebula exists as H2 molecules. At this lower temperature, dust within the nebula emits strongly at longer infrared wavelengths.
In the outer layers of the nebula, interstellar ultraviolet radiation causes hydrogen in the cloud to exist as neutral atoms. These layers absorb this radiation, preventing it from reaching further into the cloud. This allows molecules to form deeper within the nebula, and over 70 different chemical species have been found within such nebulae. The extreme conditions of low temperatures and pressures produce unusual chemistry. As an example, both hydroisocyanic acid, HNC, and hydrocyanic acid, HCN, have been found in the same cloud in equal quantities, whereas on Earth hydroisocyanic acid would break down an reform into hydrocyanic acid.