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Orion nebula(M42; NGC 1976) One of the brightest emission nebulae in the sky, about 400 parsecs distant. It is just visible to the naked eye as a diffuse luminous patch, 1° across, in the center of Orion's Sword; M43 (NGC 1982) is a small northern part of M42, a dust lane separating them. The Orion nebula is a complex region of ionized hydrogen (an H II region) that is associated with the Orion molecular cloud (OMC-1), part of a system of giant molecular clouds in the Orion constellation. The Orion nebula is centered on a very dense stellar cluster – the Trapezium cluster – containing four hot young stars that excite and ionize the nebula, so producing both radio and optical radiation. Their total luminosity is about 3 × 105 times that of the Sun. The nebula is also a source of X-rays. The region is one of active star formation, containing T Tauri stars, maser sources, Herbig–Haro objects, the BN object, and the Kleinmann-Low nebula. The interface between the nebula and the adjacent molecular cloud is an example of a photodissociation region.
a very large gas-dust cloud, the closest nebula to our solar system.
Located about 300 parsecs from our solar system, the Orion nebula is visible in the constellation Orion on moonless winter nights as a pale, twinkling spot. It measures about 5.5 parsecs in diameter. At its center is a small cluster of stars, among which is the Trapezium, which comprises four physically linked, hot, bright stars. Ultraviolet light from these stars causes the gas in the nebula—consisting primarily of hydrogen—to glow. The dust in the Orion Nebula absorbs light, which is partially responsible for the nebula’s wispy appearance.