Abell 13 (known as the Onion ring nebula) is a large and highly evolved planetary nebula located 3900 light years from Earth in the constellation of Orion. It has a reddish color and is very faint. Abell 13 has a circular outer ring of Ha (Hydrogen-alpha). The nebula was discovered in 1955 by an American astronomer named George Ogden Abell on the photo plates of the Palomar Observatory Sky Survey (POSS).
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