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I read that ultraviolet light is the cause of HII regions, but this light is invisible. So why are these objects the color red?

David Siegel, Gloucester City, New Jersey
Lagoon-Nebula
The Lagoon Nebula (M8) is a beautiful example of a specific type of emission nebula called an HII region. It appears red because it emits radiation at the Hydrogen-alpha wavelength, which appears red to human vision.
Gerald Rhemann

Often, one of those unbound electrons will meet an electron-free hydrogen atom (which is just a proton).

This process emits a photon with a wavelength of 656.3 nanometers, which is firmly in the red part of the visible spectrum.

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