A dark interstellar cloud that absorbs the light from bright objects behind it. Absorption nebulae range in size from small globules to large clouds visible to the unaided eye, such as the Coalsack in the southern Milky Way. Absorption nebulae contain both dust and gas, and the temperatures in them are low enough for simple molecules to form. Much of the knowledge about these nebulae comes from observations of infrared and radio radiation, which, unlike visible light, can pass through them. Strong evidence has been found that the initial stages of star formation take place in such dark nebulae. See also: molecular cloud.