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A galaxy is an accumulation of billions or trillions of stars and gas that is held together by gravity. Galaxies range in diameter from 1,500 to 300,000 light-years. Most stars in the universe, including the Sun, are in galaxies. Nearly all of the stars visible in the night sky are within our own galaxy, which is called the Milky Way and is visible as a bright band in areas without much artificial light (such as in the countryside). Most of the objects that are visible outside of the Milky Way are other galaxies. The Italian astronomer Galileo Galilei discovered in 1610 that the Milky Way is composed of a vast number of faint stars. In 1755, the German philosopher Immanuel Kant, speculated that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces, analogous to the solar system but on a much larger scale. He also thought that some of the objects (nebulae) in the night sky might be separate galaxies. In 1845, the Irish astronomer Lord Rosse constructed a telescope sufficiently powerful to enable him to distinguish between elliptical and spiral galaxies and to discern individual points of light in some of them. However, it was not until the early 1920s that these objects were generally accepted as distant separate galaxies. This was the result the use of an advanced telescope used by Edwin Hubble, one of the leading astronomers of the twentieth century. It is now known that a vast number of galaxies exist. Current estimates are several hundred billion, but this figure includes only those galaxies that are within viewing range of the Hubble space telescope. And the actual number could be far greater. The word galaxy comes from the ancient Greek term kyklos galaktikos, which means milky circle because of the Milky Way's appearance in the night sky. Created December 4, 2006. |