Introduction

In this chapter several aspects of galactic structure and evolution are discussed. Some of the ideas introduced in Chap. 1 are used and the contents of this chapter will be needed in Chaps. 7–10.

In the study of stellar structure and evolution in Vol. II, we could begin with a series of physically justifiable assumptions, derive the relevant equations describing the stars, and integrate these equations to understand the structure and evolution of stars. Such an approach is impossible in the case of galaxies for several reasons. To begin with, we do not understand how galaxies have formed. (It is true that there are several uncertainties in the case of star formation as well but they refer to details rather than to the fundamental process itself.) Second, observational data related to the galaxies are by no means statistically as well determined and abundant as data related to the stars. The reason essentially has to do with the fact that galaxies are located farther away from us and thus are more difficult to observe with the same level of accuracy. Third, galaxies, being collisionless systems of stars, are intrinsically more complicated compared with stars – which are made of collisional gas – from the point of view of mathematical description.

Given these difficulties, it is better to divide the study of galactic structure and dynamics into several separate aspects and investigate each of them as though they are disconnected from each other.