How fast does gas expand from a planetary nebula?

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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 gas within a planetary nebula typically expands away from the central star at speeds ranging from 24 to 56 kilometers (15 to 35 miles) per second.1

In the early stages of a nebula's evolution, the expansion speed of the gas is often cited as approximately 30 kilometers (19 miles) per second. During this phase, the nebula may appear to expand faster than the individual gas atoms because the ionization zone is actively moving outward and consuming surrounding neutral material. Once the entire mass of gas is ionized, the expansion is driven entirely by the motion of the gas itself. Because the gravitational pull of the central star is negligible at the distance of the shell, this expansion continues until the nebula eventually dissipates into the surrounding interstellar gas.21