Stars form from clouds of molecular gas in interstellar space. Molecules are protected from radiative dissociation by the surface layers of the cloud, in which light is absorbed by particles of dust mixed with the gas. The absence of starlight in the center of a molecular cloud allows the gas to become very cold, which in turn lowers the pressure. Lacking the support of pressure against gravity, the densest regions at the cloud's center collapse and fragment to form stars. When stars finally form, their irradiance heats the cloud from within, causing it to chemically evolve and eventually to disperse. The entire process, from giant molecular cloud to star, is estimated to take around 1–10 million years. Although the overall picture of how stars form from molecular clouds is known at this simple level, the details of the process are just beginning to be understood.