A bright red nebula is called an HII region when it contains A) molecular hydrogen gas, H₂. B) hot hydrogen gas thermally glowing. C) hydrogen atoms which have been ionized by ultraviolet radiation. D) hydrogen which is generating 21-cm radio waves.

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An HII region is characterized by ionized hydrogen atoms, which occur when ultraviolet radiation from nearby hot stars ionizes the hydrogen gas, causing it to emit light, often appearing bright red.

A Bright Red Nebula Is Called an HII Region When