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A galaxy is a massive, gravitationally bound system that consists of stars, an interstellar medium of gas and dust, and dark matter. Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with one trillion (1012) stars, all orbiting a common center of gravity. Galaxies can also contain a large number of multiple star systems and star clusters as well as various types of interstellar clouds. Historically, galaxies have been categorized according to their apparent shape. Interactions between nearby galaxies, which may ultimately result in a galaxy merger, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. There are probably more than a hundred billion (1011) galaxies in the observable universe. Most galaxies are a thousand to a hundred thousand parsecs in diameter and are usually separated from one another by distances on the order of millions of parsecs. Intergalactic space, the space between galaxies, is filled with a tenuous gas with an average density less than one atom per cubic metre. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies. These massive objects are believed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus. (more...)
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