ABSTRACT
The space between the stars, called the interstellar medium, is composed primarily of H and He gases incorporating a small percentage of small micron-sized particles. Interstellar clouds constitute a few per cent of galactic mass and are enriched by material ejected from evolved dying stars. Diffuse interstellar clouds are characterized by low densities and temperatures of ∼100 K. Interstellar chemistry shapes the raw material for the formation of stars and planets. The gravitational collapse of an interstellar cloud led to the formation of the protosolar nebula approximately 4.6 billion years ago. In diffuse interstellar clouds, dust interacts with hot gas, UV radiation and cosmic rays, and evolves or gets destroyed in shocks and by sputtering. Strong differences in the dust component of dense and diffuse interstellar clouds exclude rapid cycling of cloud material.
