molecular cloud

A molecular cloud (or dense cloud) is a gas cloud of sufficient size and density and sufficiently low temperature that molecules form, particularly molecular hydrogen (aka HH or H₂). They are cooler than HI regions or HII regions because too high a temperature would dissociate the molecules. Stars form in molecular clouds, where the low temperature leads to the highest cloud densities.

Molecular clouds are typically detected and measured by observing the luminosity of carbon monoxide (CO), which is presumed present. The amount and density of H₂ is estimated using the CO to H₂ factor. The cloud's temperature and density are of interest with respect to star formation activity/potential and can be determined from the relative strength of various molecular spectral lines.

A cloud's diameter is typically in the 5 to 200 parsec range. The largest are termed giant molecular clouds (GMCs), a term generally used for those on the order of a thousand solar masses and above, and which can be as much as ten million solar masses

A magnetized molecular cloud is such a cloud with a (significant/detectable) magnetic field. If incoming electromagnetic radiation ionizes some of the cloud matter making it conductive, an external magnetic field can interact with the cloud, affecting its behavior, perhaps helping or hindering collapses leading to star formation. A cloud's magnetism can be inferred from the Zeeman effect, presumably observed through Zeeman-Doppler imaging.

Some terms used for molecular clouds, or dense portions of them, used in various contexts:

• GMC (very large).
• star-forming region.
• dense core.
• Bok globule.
• filament.
• nebula (could be a molecular cloud or other fuzzy body).
• specifically, a dark nebula (molecular clouds are opaque because they include a lot of dust).
• complex (e.g., molecular cloud complex) a group of clouds.

Some example molecular clouds are Taurus Molecular Cloud 1 and Serpens Cloud, and some complexes are the Taurus-Auriga Complex, the Rho Ophiuchi Cloud Complex, and the Orion Molecular Cloud Complex (which includes the Orion Nebula).