= Molecular cloud
{wiki}
A molecular cloud is a cold region of the <interstellar medium> in which <hydrogen> is predominantly <molecular hydrogen>. Shielding against dissociating <ultraviolet radiation> helps molecules survive, and <radiative cooling> allows dense regions to develop. <Gravitational collapse> within these clouds produces <stars>, although <turbulence>, <magnetic fields> and <stellar feedback> affect which regions collapse. Cold <molecular hydrogen> is difficult to detect directly, so observers often map <carbon monoxide> <spectral lines> or <interstellar dust> emission as indirect tracers. Converting either signal into cloud <mass> requires assumptions about abundance, excitation or dust properties; a bright tracer is not itself a direct weighing of the cloud.<Star Formation in the Milky Way and Nearby Galaxies by Robert C. Kennicutt Jr. and Neal J. Evans II>[Gas and molecular-cloud diagnostics and the relation of molecular gas to star formation]{ref}
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