The diatomic molecule carbon monosulfide, CS, is widespread in interstellar and circumstellar molecular clouds, in our Milky Way and in external galaxies. Its emission at millimeter wavelengths is frequently used to estimate the density in these regions, since these pure rotational transitions require densities greater than about 105 molecules per cubic centimeter to be excited by collisions. Various isotopic variants have been observed astronomically, including those with 13C, 34S, and 33S, as well as the most abundant 12C32S. CS is also observed in cometary comae.
Interstellar CS was first detected by radio astronomers in 1971.