A galaxy is a collection of stars as well as dust and gas clouds. Although all these are several light years away, they are bound together by the force of gravitational attraction. A galaxy is a kind of a 'star system'. Our home galaxy, the Milky Way, has a diameter of about 120,000 light years. It contains several hundred billion stars, and there are billions of such galaxies. They often occur in groups, called galaxy clusters. The cluster to which the Milky Way belongs is called the 'Local Group'.
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