Bally, John AT&T Bell Laboratories, Holmdel, New Jersey.
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The brightest emission nebula in the sky, designated M42 in Messier's catalog. The Great Nebula in Orion consists of ionized hydrogen and other trace elements (Figs. 1 and 2). The nebula belongs to a category of objects known as H II regions (the Roman numeral II indicates that hydrogen is in the ionized state), which mark sites of recent massive star formation. Located in Orion's Sword at a distance of 460 parsecs or 1500 light-years (8.8 × 1015 mi or 1.4 × 1016 km), the Orion Nebula consists of dense plasma, ionized by the ultraviolet radiation of a group of hot stars less than 100,000 years old known as the Trapezium cluster (Fig. 3). The nebula covers an area slightly smaller than the full moon and is visible with the aid of binoculars or a small telescope. See also: Orion
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