A vast assemblage of molecular gas with a mass of approximately 103 to 107 times the mass of the Sun is called a giant molecular cloud (GMC).
GMCs are around 15 to 600 light-years in diameter (5 to 200 parsecs). Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great.
Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun.
The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps.
GMCs are so large that “local” ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt.
The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in