Molecular clouds typically have interstellar medium densities of 10 to 30 cm, and constitute approximately 50% of the total interstellar gas in a galaxy. Most of the gas is found in a molecular state. The visual boundaries of a molecular cloud is not where the cloud effectively ends, but where molecular gas changes to atomic gas in a fast transition, forming “envelopes” of mass, giving the impression of an edge to the cloud structure. The structure itself is generally irregular and filamentary.