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The "Ring Nebula", M57. A planetary nebula in the constellation Lyra. The result of an old, pulsating star throwing off its outer shell of atmosphere and leaving behind a hot, white dwarf. The w.d. is the source of radiation which ionizes the gas. Oxygen emission lines dominate, making these objects greenish.
Canon Dig Reb + 12" + focal reducer. 30 sec, ISO 800.