Our neighbor, the Andromeda spiral galaxy.
Click on image for full size
Image provided by Jason Ware
Spiral galaxies may remind you of a pinwheel. They are rotating disks
of mostly hydrogen gas, dust and stars. Through a telescope or
binoculars, the bright nucleus of the galaxy may be visible but the
spiral arms are dimmer and difficult to see.
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 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. The halo
is also the home of dark
matter in spiral galaxies.
Spirals are subdivided based on the appearance of the arms and the
central region. Sa types have a large, bright central region and
tightly wound arms, while Sc types have a smaller central region and
loosely wound arms. Sb types are somewhere in between. Spiral
galaxies can also have bar-like structures through them. These
galaxies are classified as SB.
Galaxies like to live together in groups called clusters. There are
not many of spirals in a cluster usually, but they are more common
than ellipticals in the
regions between clusters.
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