Referat Galaxy, The gal`-uhk-see The giant spiral assemblage of several billion stars that is home to the Sun and its family of planets, including Earth, is only one of the billions of star systems known to exist in the universe (see EXTRAGALACTIC SYSTEMS). Because it is our star system, however, it is usually called the Galaxy. The Milky Way, another name for it, is the portion visible to the naked eye. In the 20th century, astronomers determined that the Galaxy is a disk-shaped object, far larger than most of the galaxies in its neighborhood (see LOCAL GROUP OF GALAXIES). Its visible disk is about 100,000 LIGHT YEARS wide but only about 2,000 light-years thick. A halo of other materials, including star clusters, surrounds the disk. The total mass of the Galaxy can be measured by studying the motions of individual stars and clouds of hydrogen gas in different parts of the galaxy and by applying CELESTIAL MECHANICS to calculate a total mass that will account for the observed motions. The mass can also be determined from the motions of the Galaxy`s small satellite galaxies, especially the nearby dwarf elliptical galaxies, and globular clusters. Recent computations by both methods agree that the Galaxy`s mass is possibly 1,000 to 2,000 billion times the mass of the Sun. As the Sun`s mass is about average for a star in the Galaxy, the total number of stars must also be of this order. Most of these stars are invisible from the Earth, however, because the solar system lies in the dense ...
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