Introduction

A plasma is a gas in which at least some atoms have been broken into positively charged ions and negatively charged electrons. Most of the matter in the Universe exists in the plasma state. The gases inside stars are ionized because of the high temperature, as can be shown easily with the help of the Saha equation (2.29). We have seen in §6.6.4 that HII regions in the interstellar medium are fully ionized due to energetic photons from very hot stars. Even the HI regions are partially ionized, with some free electrons present in them. Our aim in this chapter is to give an introduction to some dynamical principles as well as some radiation processes involving plasmas, which are of great relevance to astrophysics.

The reader may wonder why this introductory chapter on plasma astrophysics is put exactly in this place of the book. We could, of course, introduce the subject much earlier. However, since we shall illustrate the dynamical principles by applications to stars and the interstellar medium, I felt that a prior acquaintance with these systems will put you in a better position to appreciate the relevance of plasma processes in astrophysics. There is also some justification for introducing this subject before a discussion of extragalactic astronomy. In Chapter 9 we shall discuss some extragalactic systems suchas active galaxies in which plasma processes are extremely important. So it will be helpful to have some knowledge of plasma astrophysics before we launch into a study of extragalactic astronomy.