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Why does a planetary nebula eventually blend into interstellar gas?

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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 eventually blends into the interstellar medium because it is an expanding shell of gas that continues to move outward from its central star.12

The process occurs in several stages:
* Ionization: Initially, the hot central star emits ultraviolet radiation that ionizes the expelled gas, making the nebula visible.32
* Diminishing Radiation: As the central star evolves, it eventually becomes less luminous and can no longer provide enough ultraviolet radiation to keep the expanding, increasingly dilute gas ionized.3
* Neutralization and Dispersion: Once the gas is no longer ionized, the outer regions of the nebula become neutral and invisible. Because the gravitational pull of the central star is too weak to halt the shell's progress, the gas continues to expand until it eventually mingles with the general interstellar gas.32

This transition marks the end of the nebula's life as a distinct, luminous object, though the material itself becomes a component of the surrounding interstellar environment.31