Spiral Galaxies
[Key Words]
[General] [Types of Spiral Galaxies] [Designation]
General
Spiral galaxies are flat and disk shaped
galaxies. At the center of a spiral galaxy is a bulge of old
stars, known as the nucleus. The nucleus is surrounded by an
invisible cloud of dark matter. Spiral galaxies have large arms
that spiral outward. This spiral shape is caused by the rotation
of the galaxy around the nucleus. Spiral galaxies usually have
just one arm, although some have two or three arms. Our galaxy,
for example, has four arms.
 M100, a spiral galaxy
Types
of Spiral Galaxies
There are two main types of spiral galaxies, normal spirals
and barred spirals. Barred spirals have a thick band
of bright stars running across the middle of the galaxy. An arm
emerges from each end of the bar, each creating a semi-circle
around the galaxy. Normal spirals have arms that emerge right out
of the nucleus.
Seyfert galaxies are
very similar to spiral galaxies. They are violent and fast
moving. They have an extremely bright nucleus
and very faint arms. Seyfert galaxies greatly outshine other
galaxies. Some are almost as bright as quasars. They emit
radiation from all over the electromagnetic
spectrum. Because of this, some astronomers believe that the
nuclei are faint quasars. Another theory is that they are a stage
of development of all spiral galaxies. If this is the case, the
Milky Way may spend 10% of it's life as a seyfert galaxy.
The nucleus contains very hot
gasses such as hydrogen, ionized oxygen, nitrogen, neon, sulfur,
iron, and argon. These gasses have the potential to explode which
causes the nucleus to rotate much faster that the rest of the
galaxy.
Designation
Spiral galaxies are designated by type S. They are
sub-categorized from there into 3 groups, labeled a, b,
and c, noting the increasing spread between the arms.
Barred spirals are designated by type SB with sub-categorizes a,
b, and c as explained above. There are some
types of spiral galaxies that seem to have no arms but have a
disk-like shape. These galaxies are designated type S0
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Irregular Galaxies
[Key Words]
[General] [Types of Spiral Galaxies] [Designation]
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