Introduction

Since gravity is a long-range attractive force, any star in a galaxy attracts all the other stars in the galaxy all the time. For simplicity, we can regard the stars as point particles. Then a galaxy or a star cluster can be regarded as a collection of particles in which all the particles are attracting each other through an inverse square law of force. The aim of stellar dynamics is to study the dynamics of such a system of self-gravitating particles. We, of course, know that there is also gas between the stars in a galaxy, which can add extra complications. However, it is generally believed that stellar dynamics holds the key to understanding the structure of galaxies or star clusters.

We have discussed our Galaxy in Chapter 6 and shall discuss external galaxies in Chapter 9. Although some galaxies are irregular in appearance, we shall see in §9.2 that most galaxies have very regular shapes. The fundamental question of stellar dynamics is: why do collections of self-gravitating mass particles tend to take certain particular configurations in preference to many other possible configurations? A fully satisfactory answer to this question is still not known. Hence the subject of galactic structure is on a much less firm footing compared to the subject of stellar structure. We know that the gravitational attraction of the stars has to be balanced by their motions, to ensure that the stars do not all fall towards the centre of the stellar system together due to their mutual gravitational attraction.