Introduction

This chapter discusses the physical features of the material that exists between the stars in our galaxy. It draws heavily from Vol. I, Chaps. 6–9.

Overview

We have seen in Chap. 3 that stars form out of clouds of gas in the galaxy. This process of star formation from the protostellar cloud is never totally efficient and will certainly lead to the existence of a residual, ambient medium around the stars. We also saw that the there is transfer of material from the stars to the surrounding region; stellar winds of high-mass stars, ejection of the outer mantle in the formation of planetary nebulas, and supernova explosions are three processes that lead to such a mass transfer. These phenomena couple the stars directly with the medium around them. This medium is generically called the interstellar medium (ISM).

The physics of the ISM is extremely complex because the medium is very inhomogeneous and is made of regions with fairly diverse physical conditions. We shall first provide a general overview and a description of the ISM and then take up specific topics for discussion.

The composition of our galaxy is made of stars that provide a mass of approximately (1010−1011) M⊙ and the ISM that provides a mass of ~109M⊙. Both stars and the ISM are distributed predominantly on the disk of the galaxy, with a typical radius of 10 kpc and a thickness of 250 pc.