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How does ultraviolet radiation make the gas in a planetary nebula glow?

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Planetary nebula Southern Ring Nebula as captured by the James Webb Space Telescope in the near-infrared (left) and the mid-infrared (right). The dimmer star at the center of this scene has been sending out rings of gas and dust for thousands of years in all directions. The Southern Ring Nebula is approximately 2,500 light-years away.

A planetary nebula glows because its central star, which is extremely hot, emits intense ultraviolet radiation that ionizes the surrounding shell of gas.12

This process works through the following steps:

Ionization: The high-energy ultraviolet photons from the central star strip electrons away from atoms within the gas, such as hydrogen and helium.31
Recombination: Once ionized, these atoms frequently recapture free electrons. As the electrons settle into lower energy levels within the atoms, they release energy in the form of visible light.31
Conversion: Essentially, the nebula acts as a converter, transforming the star's invisible ultraviolet energy into the visible radiation that makes the nebula appear luminous to observers.4

In addition to this recombination process, the gas also glows due to collisionally excited forbidden lines produced by various ions present in the nebula.15