Cambridge Encyclopedia » Cambridge Encyclopedia Vol. 53

nebula - Astrophysics of nebulae

stars gas produced light

A cloud of gas and dust in space, appearing either light or dark. Some nebulae are areas where stars form; others are produced by the death of stars. Star-forming nebulae include the large and bright Orion Nebula, the gas of which is excited to luminescence by the ultraviolet light from the stars within it. Dark nebulae have no illuminating stars; examples are the Coalsack and the Horsehead Nebula. Shells of gas thrown off by old stars are known as planetary nebulae, such as the Helix Nebula. Other nebulae are produced by supernova explosions, such as the Crab Nebula.

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