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 can be assumed to be cool as compared to HI regions or HII regions because too high a temperature would dissociate the molecules. Stars are assumed to formed in molecular clouds, where the low temperature leads to the highest cloud density.

Molecular clouds are typically detected and measured by observing the luminosity of carbon monoxide, which is assumed to be 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.

The clouds' diameter is typically in the 5 to 200 parsec range. The largest are termed giant molecular clouds, 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 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 a collapse 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 object).
• complex, group of clouds, e.g., Taurus-Auriga Complex.