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Why are helium atoms sometimes doubly ionized in planetary nebulae?

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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.

In planetary nebulae, helium atoms are sometimes doubly ionized primarily due to the extremely high temperature of the central star, or nucleus.1

While the exciting stars of H II regions typically have temperatures below 60,000 K, the nuclei of planetary nebulae can reach temperatures as high as 150,000 K to 200,000 K. This intense heat generates a significant amount of ultraviolet radiation with high-energy photons. Specifically, the double ionization of helium requires photons with energies exceeding 54 electron volts. Because the central star's spectrum shifts dramatically toward these higher energies as its temperature increases, it provides the necessary radiation to strip two electrons from helium atoms, a process that does not occur to the same extent in cooler H II regions.12