I realize this doesn't say much, but understand that very little is known and
known is mostly theory
Spiral are of the disk class of galaxies (the other two being elliptical and
irregular). The Milky Way
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
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
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
Spiral galaxies are classified (using the Hubble
) 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