molecular cloud

A cloud of interstellar matter in which the gas is predominantly in molecular form. There are two distinct types, both found close to the plane of the Galaxy, within the Milky Way. These are termed small molecular clouds and giant molecular clouds (GMCs). The small clouds are typically a few light years in diameter, with 1,000 to 10,000 molecules per cubic centimetre and temperatures of around 10-20 K. They may contain even colder condensed "cores", with densities ten or a hundred times greater. These small clouds contain mostly molecular hydrogen (H2). They are very cold because there is no radiation from stars within to heat them. Giant molecular clouds are made up primarily of molecular hydrogen and carbon monoxide (CO), but they also contain many other interstellar molecules. They are the most massive entities within our Galaxy, containing up to ten million solar masses, and are typically 150 to 250 light years across. The density is as high as ten million molecules per cubic centimetre. Infrared emission from such clouds is evidence that they are regions of star formation. GMCs are nearly always found to be associated with clusters of hot, massive, young stars. Luminous clouds of ionized hydrogen (H II regions) may be produced by such stars formed near the edges of a GMC. In the Orion Nebula, a notable example, a GMC lies behind the optically visible nebula. Another is associated with the Omega Nebula (M17). A GMC containing between three and five million solar masses of material is located near the galactic centre, in front of the radio source Sagittarius B2. It contains many of the known types of interstellar molecule. Up to 4,000 GMCs are thought to exist in the Galaxy.