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Gum nebulaAn immense emission nebula that dominates the Milky Way in the southern constellations Puppis and Vela. It is an expanding shell of ionized gas some 35° in diameter. It is thought to be one of the closest supernova remnants to the Solar System; the center is estimated to be about 400 parsecs distant but the outer edge closest to us may be as little as 100 parsecs away. It has been suggested that the Gum nebula is a fossil Strömgren sphere produced by a supernova that exploded maybe up to a million years ago and gave off sufficient ultraviolet radiation to ionize interstellar gas out to an immense distance. Several very hot stars within the shell are now ionizing the gas, thus preventing it from returning to a neutral state. The nebula takes its name from the Australian astronomer Colin Gum (1924–1960), who first identified and photographed it in 1952. The Vela pulsar and its supernova remnant lie in the same direction as the Gum nebula.
Gum Nebula[′gəm ′neb·yə·lə]
A giant nebula about 250 parsecs (5 × 1015 miles or 8 × 1015 kilometers) in diameter, with its near edge about 300 parsecs (6 × 1015 miles or 9 × 1015 kilometers) distant, which is both an old supernova remnant and an H II region.