I realize this doesn't say much, but understand that very little is known and what is known is mostly theory.

Spiral are of the disk class of galaxies (the other two being elliptical and irregular). The Milky Way and Andromeda are two examples of spiral galaxies.

All spirals have a relatively thin disk and a bulge, although the bulge greatly varies in size. The disk contains stars, gas and dust but the bulge is almost completely stars.

The disk in all spiral galaxies orbits the core, although in some it will rotate as one body, and in others it will rotate with all stars traveling at similar speeds.

Current theory suggests that the spiral is the result of gravitational pull from neighboring galaxies, which causes gas to compress in certain places. Once the gas reaches critical density, hot stars are created. The areas in which the new stars are created tend to be aligned along the spiral arms.

Spiral galaxies are classified (using the Hubble Scheme) as follows:
  • S - Normal spiral
  • SB - Barred spiral
  • Sa, SBa - Spiral with bulge
  • Sb, SBb - Spiral with bulges, pronounced arms
  • Sc, SBc - Spiral dominated by the arms
  • Sd, SBd - No core or distinct bulge
  • S0 - Spiral which has lost most of its matter and no longer shows any pattern in the disks

20040115: fixed grammar error