Introduction

Spiral structure is the frosting on the cake of galactic rotation. Like frosting, it is very alluring and has been greatly admired. Self-gravitating stellar disks are very lively objects; many processes contribute to the observed spiral patterns in galaxies. Our knowledge of this subject, greatly extended in the last two decades, still changes quickly. This section just describes some of the basic gravitational principles and questions common to many discussions.

What are the forces that drive spiral structure? To answer ‘differential rotation’ is true, but not much more informative than the ancient physicians who replied that morphine produces sleep because it contains a ‘dormative principle’. Spiral instabilities resulting from differential rotation are familiar in both magnetic and fluid systems, with or without gravitation.

The patterns of rotating magnetic field lines are so suggestive that for many years they were popular as an explanation for spiral structure, either directly or through their influence on star formation. The decline of magnetic explanations occurred when more accurate measurements of the galactic magnetic field strength showed it was significantly less than expected. Even today these measurements are uncertain because the topology of the field is obscure and we are not really sure how to average it over large distances.