The Interstellar Medium (ISM)

Abstract

Our ideas on the interstellar medium (ISM) have changed radically during the past two decades with the rapid advancement of our knowledge of space astrophysics. Originally the interstellar medium was thought to be a simple, quiescent medium; but now it is well known that the ISM is a complex and dynamical medium and plays a key role in the cycle of birth to death of stars. Out of this medium new stars are born and it is being continuously replenished by the mass loss from stars. The ISM is a dynamic medium which is continuously being blown by stellar winds, supernova explosions and the expanding envelopes of supernova remnants. The interstellar medium has four main constituents: (a) Gas in all phases,-molecular, atomic and ionized gases, (b) Dust particles, which obscures several regions from visible and UV radiations, (c) High energy cosmic ray particles (which were discussed in detail in Chapter 7), and (d) the Galactic magnetic field. Here we shall discuss the basic features of the first two aspects.