Galaxies in the Universe An Introduction

The main feature of a spiral or S0 galaxy is its conspicuous extended stellar disk. Stars in the disk of a large spiral galaxy, like our Milky Way, follow nearly circular orbits with very little random motion. Ordered rotation accounts for almost all the energy of motion, with random speeds contributing less than ~5%: the disk is dynamically ‘cold’. In smaller galaxies, random motions are proportionally larger, but most of the disk's kinetic energy is still in rotation. Because the stars have little vertical motion perpendicular to the disk plane, the disk can be quite thin.

Spiral galaxies are distinguished from S0 systems by the multi-armed spiral pattern in the disk. The disks of spiral galaxies still retain some gas, whereas S0 systems have lost their disk gas, or converted it into stars. Both S0 and spiral galaxies can show a central linear bar; in Figure 1.11, the sequence of barred galaxies SB0, SBa, …, SBm runs parallel to the ‘unbarred’ sequence S0, Sa, … Apart from the bar and spiral arms, the stellar disks of large galaxies are usually fairly round; but many smaller systems are quite asymmetric.