Molecular cloud

A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is 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 areas 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 (CO). The ratio between CO luminosity and H₂mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.

Occurrence

Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of the medium, comprising roughly half of the total gas mass interior to the Sun's galactic orbit. The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs (11,000 and 24,000 light-years) from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center). Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region.