Galaxies 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 universe.  Gravity, the universal force of attraction holds the various parts of each galaxy in place, and causes the galaxies to slowly rotate on an imaginary axis, much like a planet, but many, many times larger.  It takes our own galaxy, the Milky Way about 250 million years to spin just once!  Gravity is also responsible for holding each moon in orbit around its planet, and each planet in orbit around its sun. Even the stars are held in place within larger groups called "star clusters".  In every case, GRAVITY is the force that binds it all together. 

ELLIPTICAL 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.

SPIRAL 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. Sprial Galaxies have less dust and gas than Elliptical Galaxies, and the stars in these galaxies tend to be middle-aged, about like our sun.

Irregular 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!!

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