Wednesday, December 10, 2008

A galaxy is one of billions of systems in the universe, including stars, nebulae, star clusters, globular clusters, and interstellar matter, held together by gravitational bound. The solar system is in a galaxy called the Milky Way. Astronomers estimate that there are more than 100 billion galaxies scattered throughout the visible universe. Millions of them have been photographed through telescopes from astrophysical observatories. The most distant galaxies ever photographed are as far as 10 billion to 13 billion light-years away.

Galaxies have been categorized according to their visual morphology. A common shape is the elliptical galaxy, that has an ellipse-shaped profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular forms are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Galaxies have diameters of 3,260 to 326,000 light years. The Milky Way has a diameter of about 100,000 light years with the solar system lying about 25,000 ligh years from the center of the galaxy.

The word galaxy derives from the Greek and means “milky.” Aside from the the Milky Way, there are only three other galaxies which are visible to the naked eye. In the northern hemisphere, people can see Andromeda Galaxy, that is about 2 million light years away. In the southern hemisphere, the Large Magellanic Cloud and the Small Magellanic Cloud can be seen.

Galaxies are unevenly scattered throughout the universe. Some stand alone with no close neighbor. Others occur in pairs, with each orbiting the other. But most galaxies are found in groups called clusters. A cluster may contain from a few dozen to several thousand galaxies. It may have a diameter as large as 10 million light-years. Clusters of galaxies, in turn, are grouped in larger structures called superclusters.