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Galaxy, aggregation of stars, dust, and gas. The earth's galaxy is average sized: It contains approximately 100 billion stars, a beam of light would take approximately 100,000 years to cross from 1 side to the other, and it is shaped like a disk, with arms spiraling out from the bright nucleus. At the edge of the galaxy is the bright band of stars called the Milky Way. The sun, a star, lies about two-thirds of the way to the edge of the galaxy and takes about 250 million years to circle the galaxy once. The galaxy, often referred to simply as the Milky Way, has 2 small companion galaxies, called the Magellanic Clouds after the explorer Ferdinand Magellan. Other galaxies may be round or elliptical in shape, with no arms. Others are S-shaped. The nearest large galaxy, the Andromeda Nebula, is named after the constellation of Andromeda, inside whose borders it appears. The term nebula describes its cloudlike appearance. When the Andromeda Nebula was named, it was thought to be a cloud of gas in the earth's galaxy. In 1923 the astronomer Edwin Hubble found that it was a separate galaxy over a million light years away. Later studies showed that the 2 galaxies are near-twins, both in size and appearance. Galaxies tend to congregate in groups, linked by gravity, suggesting a common origin; they may have condensed out of 1 giant cloud of gas. The Milky Way is in a group including the Andromeda Nebula and about 20 other galaxies. Perhaps a billion galaxies are visible in the telescopes operating today.

Galaxies emit radio waves. The strongest emitters, known as radio galaxies, seem to have suffered internal explosions. There are often radio-emitting areas on either side of a galaxy. Quasars, star-like objects that emit strong radio waves, may be galaxies in the making. A quasar quiets down as it gets older, becoming a radio galaxy. Seyfert galaxies, discovered by Carl Seyfert, are galaxies with quasar-like characteristics and may be another stage of a galaxy's growth.

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