The dark heart of an interstellar nebula

Dark nebulae (also known as absorption nebulae) are a type of nebula in the interstellar medium that can be observed as dark regions superimposed on emission/reflection nebulae (which emit visible light) or crowded fields of stars. Examples of dark nebulae are molecular clouds, characterized by high densitiy and low temperature values that favor the formation of small grains of interstellar dust inside them. Precisely because of the high density values and the dust grains, the light from astronomical objects placed behind these nebulae is strongly absorbed. The model is an artist view of a dark nebula (the dark region at the center of the scene) interwoven with a reflection nebula (a cloud of interstellar gas which reflects the light of nearby stars). A popular example of dark nebula is the Horsehead Nebula (also known as Barnard 33), a small dark nebula in the constellation Orion (see Figure below).