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An interstellar self-luminous gas cloud that radiates visible light due to ionisation. The most common source of ionisation is high-energy photons emitted from a nearby hot star. There are several different types of emission nebulae, which include HII star formation regions. Another type is a planetary nebula where a dying star has thrown off its outer layers, which are then illuminated by ionisation from the left over hot core of the star.

M42 "The Orion Nebula" is a naked eye HII star formation region. Bright planetary nebulae include M27 "The Dumbbell Nebula" and M57 "The Ring Nebula".

M42 The Great Orion Nebula (credit:- NASA, ESA, M. Robberto, Space Telescope Science Institute)

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