What physical process causes Emission Nebulae to emit their own visible light?

Answer

The cloud's gas is ionized by nearby hot, young stars, causing emission as electrons recombine.

Emission Nebulae glow because the gas within them, often hydrogen, is ionized by intense radiation from nearby hot, young stars, and light is emitted when electrons subsequently recombine with the atoms.

What physical process causes Emission Nebulae to emit their own visible light?
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