The Local Group

There are billions of galaxies in our Universe. Most of these are clumped together in small groups. Our own galaxy which is called The Milky Way Galaxy lies within a group of galaxies that we call The Local Group.

The Local Group consists of about 30 galaxies. The three largest are The Andromeda Galaxy, The Milky Way Galaxy, and Triangulum.

Irregular Galaxies

Irregular Galaxies are simply all the galaxies which are not spiral or elliptical. They can look like anything and have many different characteristics.
Many irregular galaxies probably used to be spiral, or elliptical until they had some kind of accident which changed them such as crashing with another galaxy.
Many other irregular galaxies probably were never spiral or elliptical; they simply didn't evolve that way.

Elliptical Galaxies

The stars found in Elliptical Galaxies are often very old. This is because elliptical galaxies don't actively create new stars.  The only stars found with in them were created along time ago.
Although they are usually smaller, this type of galaxy can be large. Most have only a few thousand stars, but some can have billions of stars. 
The stars in an elliptical galaxy are often very close together making the center look like one giant star.  If the Earth were inside an elliptical galaxy it would be bright both day and night.

Spiral Galaxies

The most beautiful type of galaxies are Spiral Galaxies. Their long twisting arms are areas where stars are being formed.

Where do the spirals come from?
Like ripples in a pond, the spiral arms seen in this kind of galaxy are circling waves.  These waves cause new stars to form. That's right, they are like star farmers, planting star seeds where ever they go. 

What causes the waves to glow?
Some of the new stars created in the wave are very large. Because of their size these large stars glow brighter than their smaller cousins, causing the nearby dust clouds to glow brightly. Thus any area near one of these waves glows like a fluorescent light.
In other words you can't actually see the waves, the spirals that we see are the glowing clouds illuminated by large, hot stars. As the waves move on the clouds behind them dim down, no longer glowing until another wave passes through.

Why doesn't the whole galaxy shine brightly?
The large bright stars created in the waves don't live very long. Their large size makes them burn all their fuel quickly. Usually they die before they ever leave the wave. Only the smaller stars which do not glow brightly survive to leave the waves they formed in.

Three Kinds of Galaxies

There are billions of Galaxies in the Universe. Some are very small with only a few million stars. While others could have as many as 400 billion stars, or even more. There are three kinds of Galaxies, Spiral, Elliptical, and Irregular. The only difference between the three is what shape they are. 

Galaxies of the Universe

Life Cycle of a Star