A massive cloud of interstellar gas and dust composed mainly of molecules. The typical diameter is 100 l.y., and masses range from a few hundred thousand to 10 million solar masses. GMCs contain mostly hydrogen molecules (H2, 73% by mass), helium atoms (He, 25%), dust particles (1%), neutral atomic hydrogen (H I, less than 1%), and a rich cocktail of interstellar molecules (less than 0.1%). Our Galaxy contains over 3000 GMCs, the most massive of which lies near the radio source Sagittarius B2 at the galactic centre. They comprise half the mass of all interstellar matter, although they occupy less than 1% of its volume. The average gas density is a few thousand molecules per cubic centimetre. GMCs are found mainly in the spiral arms of disk galaxies, and they are the birthplace of stars. GMCs exist for some 30 million years, during which time only a small fraction of their mass is converted to new stars. The nearest GMC lies in Orion, and is associated with the Orion Nebula. See also astrochemistry; interstellar molecule.