In the planetary nebula NGC 7293 (Helix Nebula), what specific atomic emission is often responsible for observed blue-green tones?
Answer
Doubly ionized oxygen atoms
The mechanism causing blue light in emission nebulae, particularly planetary nebulae like the Helix Nebula (NGC 7293), contrasts strongly with reflection nebulae. In NGC 7293, the prominent blue or blue-green coloration frequently originates from the light emitted by doubly ionized oxygen atoms. When oxygen atoms lose two electrons and then recapture one, they emit photons at specific wavelengths in the blue-green spectrum. This emission is a direct result of the high-energy environment created by the dying central star ejecting gas, rather than light being scattered by external dust.
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