Our neighbor, the Andromeda spiral galaxy.
Click on image for full size
Image provided by Jason Ware
Spiral Galaxies
Spiral galaxies may remind you of pinwheels turning slowly as though
in some intergalactic breeze. They are rotating disks of gas, dust and
stars. Through a telescope or binoculars, the bright nucleus of the
galaxy may be visible but the spiral arms which are more diffuse can
be difficult to discern.
Spiral galaxies are complex objects and
have several components: a disk, a bulge, and a halo. The disk
contains gas, dust, and young stars in its spiral arms. The dense
bulge or nucleus in the center of the disk contains mostly old stars
and no gas or dust. The halo is the home of a very few scattered
stars and globular
clusters. While mostly empty of visible matter, the halo is also
the realm of dark matter in spiral galaxies.
Spirals are further subdivided based on the appearance of the arms
and the nucleus. Sa types have large nuclei and tightly wound arms,
while Sc types have small nuclei and sprawling arms. Sb types are
somewhere in between. Spiral galaxies can also have bar-like
structures through them. These galaxies are classified as SB, and are
further subdivided a-c in the same way as regular spirals.
Spiral galaxies don't come in a dwarf variety, and there are not
usually many of them in clusters compared to the number of ellipticals. But they are
more common than ellipticals in the regions between clusters.
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