Introduction

There will be no attempt in this book to describe the detailed physical properties of the interstellar medium (gas, dust, cosmic rays, magnetic field) in our own and other galaxies; that would require a whole book to itself. It is, however, necessary and desirable to discuss some of its properties for several reasons. In the first place I believe that galaxies were initially composed of gas alone, so that the process of galactic evolution is largely a process of conversion of gas into stars with the subsequent evolution of the stars. In the second place, if even five per cent of the visible mass of a galaxy is in the form of gas at the present time as we have seen to be true for our Galaxy, it is not really possible to discuss the structure of the galaxy whilst ignoring the gas. The gas content of galaxies will be discussed in this and the following two chapters.

Here I discuss the present structure of the gas disk in our Galaxy, with some comments about other galaxies and with some remarks about the possible past behaviour of the disk. In Chapter 7 I shall discuss the chemical evolution of galaxies, about which information is obtained by a study of the chemical composition of stars of different ages and of the present composition of the interstellar gas.