What happens when all of a planetary nebula’s gas becomes ionized?

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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.

When a planetary nebula reaches the stage where its entire mass of gas has become ionized, some of the ultraviolet radiation emitted by the central star begins to escape into space.1

At this point, the expansion of the nebula is driven entirely by the motion of the gas itself, rather than by the outward progression of an ionization front. Most observed planetary nebulae are currently in this middle stage of evolution. Eventually, as the central star becomes less luminous, it will no longer provide sufficient ultraviolet radiation to maintain the ionization of the gas, causing the outer regions of the nebula to become neutral and invisible before the material ultimately mingles with the general interstellar gas.1