A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.
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. They are :
1 ) Spiral Galaxy
2 ) Elliptical Galaxy
3 ) Irregular Galaxy
Spiral Galaxies are the most beautiful. Their long twisting arms are areas where stars are being formed. Like ripples in a pond, the spiral are seen in this kind of galaxy are circling waves. These waves cause new stars to form. Some of the new stars created in the wave are very large. Because of their size, these large stars glow brighter causing the nearby dust clouds to grow brightly. Spirals that we see are the glowing clouds illuminated by large, hot stars.
The large, bright stars created in the waves don't live very long. Their large size make 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.
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 within them were created long ago. Although they are usually smaller, they can also be large. Most have only a few thousand stars, but some can have billions. The stars in an elliptical galaxy are often very close, making the center look like one giant star. If the Earth were inside an elliptical galaxy, it would would have been bright both day and night.
Irregular galaxies are simply all the galaxies that are neither spiral nor 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 an accident which changed them such as collision with another galaxy.
Many other irregular galaxies probably were never spiral or elliptical; they simply didn't evolve that way.
The Spacestellar Creations