Astrophysics (Index)About

molecular cloud

(dense cloud)
(interstellar gas cloud that includes molecules)

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:

(hydrogen,star formation,cloud type,molecular hydrogen,gas)
Further reading:

Referenced by pages:
Atacama Pathfinder Experiment (APEX)
carbon (C)
cloud fragmentation
carbon monoxide (CO)
cold gas
cosmic dust
dense core mass function (DCMF)
dense core
evaporating gas globule (EGG)
star formation feedback
FIRE Simulations
galaxy cloud
gas flow
glycine (Gly)
giant molecular cloud (GMC)
The H2O Southern Galactic Plane Survey (HOPS)
HII region (HII)
HI region (HI)
HL Tau
hydrogen (H)
hydrogen deuteride (HD)
hydroxyl (OH)
interstellar astrophysics
infrared dark cloud (IRDC)
interstellar medium (ISM)
Lyman-Werner photon
Messier 100 (M100)
Whirlpool Galaxy (M51a)
Messier 74 (M74)
Mach number
main sequence fitting
molecular cloud turbulence
metallicity (Z)
open cluster (OC)
Orion Molecular Cloud Complex
Orion Nebula (M42)
polycyclic aromatic hydrocarbon (PAH)
PAH emissions
photodissociation region (PDR)
planet formation
position-position-velocity space (PPV)
quenched galaxy
rare designator prefixes
Rho Ophiuchi Cloud Complex
Serpens Cloud
star formation rate (SFR)
star-forming region (SFR)
shock wave
spectral line
star formation (SF)
stellar age determination
stellar association
submillimeter astronomy
surface density (Σ)
T association
Taurus-Auriga Complex (Tau-Aur Complex)
three dimensional model
Taurus Molecular Cloud 1 (TMC-1)
Toomre Q parameter (Q)
T-Tauri star (TTS)
velocity-metallicity relation
virial parameter
CO to H2 factor (Xco)