Galaxies are systems or clusters of gas,
dust, and stars which are held together by gravitational force; a galaxy
contains anywhere from 100,000,000 to 3,000,000,000 stars. It is among the
oldest large scale objects in the Universe. They are
located throughout the Universe, and they vary greatly in size and shape. Astronomers
classify galaxies according to their shape and appearance. A
galaxy may be found alone in space, but most are in grouped in formations
called clusters. Clusters of galaxies range in size from a few dozen members
to several thousand.
Galaxies are measured in light years. A light year is 9.5 trillion km, and is the distance light travels in one year. Galaxies vary in size from a few thousand to a million light years in diameter.
Spiral galaxies, which number about half of all galaxies, are shaped like a disk, resembling a pinwheel with arms which spiral outward as it rotates. Spiral galaxies contain middle-aged stars along with clouds of dust and gas. The Milky Way Galaxy is a spiral galaxy, and is only one of the uncountable galaxies. Elliptical galaxies contain older stars and very little dust and gas. They range in their shape from round to flattened, oval-shaped pancakes. Irregular galaxies have an undefined shape, and are full of young stars, gas, and dust.
It wasn't until this century that astronomers understood the nature of the galaxy that surrounds us. All of the stars that you see with just your eyes on any given night belong to the Milky Way Galaxy. Through an amateur astronomer's telescope, most galaxies look like small smudges of light aginst a black background. Even the view through a large telescope doesn't show the galaxies as they often look like in pictures taken in space. The Hubble Space Telescope takes spectacular photographs and collects valuable information about even the most distant galaxies, forcing astronomers to change their ideas of galaxy formation. Scientists believe that all of the galaxies are moving away from each other; galaxies that are the farthest from the Earth are travelling the fastest of any galaxies.
Bibliography and Additional Sources of Information
Classifying Galaxies www.smv.org/hastings/galaxy.htm