Molecular Cloud

A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H₂).

Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H₂ is CO (carbon monoxide). The ratio between CO luminosity and H₂ mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.