The Molecular Clouds

A large amount of interstellar matter resides in molecular regions, largely concentrated in form of giant clouds of mass $10^4 - 10^8$ $M_{\odot}$ disposed along the spiral arms of the Galaxy. These clouds are concentrations of gas and dust with very high number densities (more than $10^3$ $cm^{-3}$) with respect to the average density of the Galaxy (approximately one atom per $cm^{3 }$). The presence of a high-density medium and dust makes these regions optically thick to the visible and ultraviolet radiation, allowing the existence of molecular Hydrogen that otherwise would not be possible. In our solar neighborhood, molecular clouds typically have masses of about $10^{5}$ $M_{\odot}$ and are about 50 pc in diameter. These entities are self-gravitating and are coupled to the stochastic pressure in the interstellar medium only at their surfaces. The molecular clouds have a relatively small scale thickness, about 120 pc, and they are the birthplace of both the massive and the low-mass stars.