 Molecular cloud, a molecular cloud, sometimes called the stellar nursery. If star formation does occur in within as a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen H2. This is in contrast to other arias of the interstellar medium that contain predominantly ionized gas. Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is carbon monoxide code. 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. Within molecular clouds are regions with higher density, where lots of dust and gas cores reside, called clumps. These clumps are the beginning of star formation, if gravity can overcome the high density and force the dust and gas to collapse.