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
Sources:
http://www.seds.org/messier/spir.html
http://www-astronomy.mps.ohio-state.edu/~depoy/courses/lecture.notes/spirals.html
20040115: fixed grammar error