The word Galaxy derives from the Greek term galaxias, meaning "milky circle" for its appearance in the sky. A Galaxy, or nebula, is a large gravitational bound system of stars, interstellar gas, dust, and plasma within the universe. The average galaxy contains more than 100 billion solar masses and ranges in diameter from 1,500 to 300,000 light-years, 90% of which is actually composed of largely unknown substance called dark matter.

Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. Besides stars and planets, galaxies contain clusters of stars; atomic hydrogen gas; molecular hydrogen; complex molecules composed of hydrogen, nitrogen, carbon, and silicon, among others; and cosmic rays.

There are probably more than one hundred billion galaxies in the observable universe. When viewed or photographed with a large telescope, only the nearest galaxies exhibit individual stars. For most galaxies, only the combined light of all the stars is detected. Every galaxy is controlled and held together by one central gravitational force.

• Elliptical
• Spiral
• Irregular

Elliptical galaxies
Elliptical galaxies have an overall globular shape, with a bright nucleus surrounded by a luminous structure less disk. Such galaxies contain a population of old stars, usually with little apparent gas or dust, and few newly formed stars. Elliptical galaxies come in a vast range of sizes, from giant to dwarf. The stars in an elliptical galaxy are often very close together making the center look like one giant star.

Spiral galaxies
Spiral galaxies are flattened disk systems containing not only some old stars but also large populations of young stars, much gas and dust, and molecular clouds that are the birthplace of stars. The most beautiful type of galaxies are Spiral Galaxies. Their long twisting arms are areas where stars are being formed. Often the regions containing bright young stars and gas clouds are arranged in long spiral arms that can be observed to wind around the galaxy. Generally a halo of faint older stars surrounds the disk; a smaller nuclear bulge often exists, emitting two jets of energetic matter in opposite directions.

Irregulars galaxies
Irregulars galaxies also have large amounts of gas, dust, and young stars, but no arrangement of a spiral form. They are usually located near larger galaxies, and their appearance is probably the result of a tidal encounter with the more massive galaxy. Some extremely peculiar galaxies are located in close groups of two or three, and their tidal interactions have caused distortions of spiral arms, producing warped disks and long streamer tails.

The nearest galaxy is the Milky Way which is about 100,000 light years distant, while the nearest giant neighbor, the Andromeda Galaxy, also a spiral, is about 2.4 million light years away and appears about 5 times the size of the full moon. It is visible to the naked eye under a very dark sky.

Many galaxies radiate a large fraction of their energy in forms other than visible light. With the development of radio astronomy, many radio galaxies were discovered. Other galaxies radiate strongly in the infrared, ultraviolet, or X-ray parts of the spectrum.

Galaxies do move. They rotate and move through space. Galaxies rotate around their centers with the sections of the galaxy that are farther out from the galaxy's center rotating more slowly than the material closer to the center. Galaxies are also moving away from each other due to the expansion of the universe brought on by the Big Bang.

The Milky Way Galaxy which is about 80,000 to 120,000 light-years across and about 10,000 light-years thick.

The sun is about 26,000 light-years from the center of the Milky Way Galaxy. It takes the sun roughly 200-250 million years to orbit once around the Milky Way. To reach the center of the Milky Way Galaxy starting from the Earth, aim toward the constellation Sagittarius. The galaxy appears brightest in the direction of Sagittarius, towards the galactic center.

• Perseus Arm
• Sagittarius Arm
• Centaurus Arm
• Cygnus Arm

Our Solar System is in a minor arm called the Orion Spur. The central bulge and halo contains old stars and at least one black hole; younger stars are in the arms, along with dust and gas that form new stars.

It is extremely difficult to define the age at which the Milky Way formed, but the age of the oldest stars in the Galaxy is currently estimated to be about 13.6 billion years, which is nearly as old as the Universe itself.