could probably be called "star
cities". Galaxies are collections of stars, planets, moons and lots
of dust and gases. There are about 100-200 billion galaxies in our
universe. Each galaxy contains as many as 200 billion stars.
Scientists have identified three main types of galaxies in our
GALAXIES: These galaxies may be
round and flat (like a Frisbee), spherical (like a ball), or
elongated (like a cucumber). They account for 57% of all the galaxies
that we an see from Earth, making them the most common type of
galaxy. Elliptical galaxies do not contain very much dust and gas
(the building blocks of the universe) and the stars in an elliptical
galaxy are often older, more mature versions of stars.
GALAXIES: These galaxies all have a
thickened center, or nucleus, with arms that wrap spirally around the
nucleus. Galaxies of this type make up about 33% of all the galaxies
visible in the universe, making them the second most common type of
galaxy. We live in a spiral galaxy called the Milky Way. There is one
variation of the spiral galaxy called the BARRED SPIRAL GALAXY. These
barred spiral galaxies are essentially the same as the regular
spiral, except that they have a bar of dust and gas cutting across
the center of the nucleus and the spiral arms extend from this bar.
Barred spiral galaxies are much less common than regular spiral
GALAXIES: These galaxies
have no "typical" shape. They are not uniform and each looks
different from all of the others. The best way to describe these
galaxies is "cloudlike". Irregular galaxies account for only about
10% of the galaxies int he universe and are therefore the least
common galaxy that we've seen. Irregular galaxies contain large
amounts of dust and gas and have fairly young versions of stars. It
is for these reasons that some people consider the irregular galaxy
to be a very young and undeveloped form of a galaxy.
All galaxies vary greatly in size, but
in general, a galaxy is about 100,000 light years wide and from
10,000 to 15,000 light years thick.
Gravity causes galaxies to move in two
ways. They move through space and they rotate. Actually, a galaxy
moves much like a Frisbee does when you toss it!!