The interstellar medium (ISM) is the matter (gas 99%, dust 1%) distributed inthe space between the stars. Hydrogen (H) is the most abundant element of theISM, and its physical/chemical state defines the ISM phases: the ionised phase,in which H is atomic ionised, the neutral phase, in which H is neutral atomic,and the molecular phase, in which H is neutral molecular (H2). Star formationis the se…