A cloud of interstellar gas and dust composed mainly of molecules. The smallest known molecular clouds contain less than 1 solar mass of gas, whereas giant molecular clouds have masses of up to 10⁷ solar masses. Most of the molecular gas is very cold, with typical temperatures of around 20 K. Molecular clouds can be detected optically as dark nebulae. Infrared telescopes detect the small amount of heat emitted by the dust. Radio telescopes detect numerous spectral lines from the interstellar molecules in such clouds.