A cloud of interstellar gas that glows by emitting light at specific wavelengths when its atoms are ionized and excited by nearby hot stars. The characteristic red color comes primarily from hydrogen alpha emission, while other elements contribute different colors to create the nebula's spectrum.
From Latin 'emissio' (sending out) and 'nebula' (cloud, mist), the term emerged in the early 20th century with the development of spectroscopy. Astronomers distinguished these actively glowing clouds from reflection nebulae that only scatter starlight.
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