Stars are born from the interstellar medium, and during their lifetime return material to it. In the 1950's and 1960's the interstellar medium was studied intensively using the 21-cm HI line. More recently, the development of infrared and submillimeter astronomy have added new impetus to these studies. Models of the physical and chemical structure of the interstellar medium have become more and more sophisticated with the advent of ever faster computers. One area of active research in Leiden is that of molecular clouds.
Molecular clouds are relatively dense, cold clouds of gas, spread throughout the disk of our Galaxy.
They are the main sites of star formation. In an astronomical sense they are dense, but the gas concentrations are still so low that many chemical compounds that do not exist on Earth, do occur in these clouds. The study of the chemistry of these clouds is therefore a challenge both from an astronomical and chemical point of view. The Huygens building also houses the Astrophysics Laboratory, which was the first laboratory to simulate the chemical and physical processes that occur at extremely low temperatures on the surfaces of interstellar grains. Another area of interest is circumstellar matter.