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Why does the outer gas of an old planetary nebula become invisible?

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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.

The outer gas of a planetary nebula becomes invisible because the central star eventually loses its luminosity and can no longer provide the necessary ultraviolet radiation to keep the gas ionized.1

In the earlier stages of a planetary nebula's evolution, the hot central star emits intense ultraviolet radiation that ionizes the surrounding shell of gas, making it visible. However, as the central star continues to evolve, it eventually becomes less luminous. Once the star can no longer produce enough ultraviolet photons to maintain the ionization of the dilute gas, the atoms in the outer regions of the nebula recombine, returning to a neutral state. Because these neutral regions no longer emit the light associated with ionized gas, they become invisible and eventually disperse into the general interstellar gas.1