My research activity revolves around the InterStellar Medium (ISM). The ISM is the complex intertwining of gas phases that fills the volume of a galaxy, between its stars. As an illustration, Fig. 1 shows an image of the Orion nebula. We can clearly see stars mixed with swirls of dusty gas. Stars are born out of the gravitational collapse of interstellar clouds. At the end of their lifetime, stars return a significant fraction of their matter to the ISM, whether exploding as supernovae or gently expelling their envelope as planetary nebulae. This is why we talk about an interstellar lifecycle, as stars deplete and replenish the interstellar matter reservoir. Each successive generation of stars also enriches the ISM in heavy elements (C, N, O, Fe, etc.), changing its thermodynamic and radiative properties. This cycle is however not endless. Young galaxies (irregular galaxies) are gas-rich, whereas old galaxies (elliptical galaxies) are almost totally devoid of interstellar gas, because most of it is locked in old stars.