(interstellar gas cloud including molecules)
A molecular cloud (or dense cloud) is
a gas cloud of sufficient size and density that
molecules form, particularly molecular hydrogen
(aka HH or H2).
They can be assumed to be cool as compared HI region or HII region,
as too high a temperature would dissociate all the molecules.
Stars are typically formed in molecular clouds, where
the lower temperature leads to a higher density.
They are typically detected by observing the
luminosity of carbon monoxide which is assumed
to be present in molecular clouds and applying the CO to H2 factor.
Their 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.
Their diameter is typically in the 5 to 200 parsec range.
The larger end includes giant molecular clouds which are those
on the order of a thousand solar masses and above,
and can be as much as ten million solar masses.
A magnetized molecular cloud is 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.
There are a number of terms used for particular types of molecular
clouds or dense portions of them:
(hydrogen,star formation,cloud type,molecular hydrogen,gas)
Atacama Pathfinder Experiment (APEX)
carbon monoxide (CO)
dense core mass function (DCMF)
star formation feedback
giant molecular cloud (GMC)
The H2O Southern Galactic Plane Survey (HOPS)
HII region (HII)
HI region (HI)
infrared dark cloud (IRDC)
interstellar medium (ISM)
Messier 100 (M100)
Whirlpool Galaxy (M51a)
Messier 74 (M74)
main sequence fitting
molecular cloud turbulence
Orion Molecular Cloud Complex
Orion Nebula (M42)
polycyclic aromatic hydrocarbon (PAH)
photodissociation region (PDR)
position-position-velocity space (PPV)
rare designator prefixes
Rho Ophiuchi Cloud Complex
star formation rate (SFR)
star-forming region (SFR)
star formation (SF)
stellar age determination
surface density (Σ)
three dimensional model
Taurus Molecular Cloud 1 (TMC-1)
Toomre Q parameter (Q)
T-Tauri star (TTS)
CO to H2 factor (Xco)