How do planetary nebulae form around dying red giant stars?

Planetary nebulae form during the late evolutionary stages of low-mass stars, such as those with masses comparable to the Sun. Before the nebula is ejected, the star exists as a red giant, a phase characterized by instability in the helium-burning shell.123

The formation process involves several key steps:

• Mass Loss: The red giant experiences a rapid loss of its outer atmosphere through a slowly expanding stellar wind. This ejection is driven by the outward force of radiation acting on the star's outer layers, often triggered by variations in nuclear luminosity.31
• Exposure of the Core: As the overlying atmosphere is shed, the star's hot, dense core is exposed. This core eventually becomes a white dwarf star.342
• Ionization: The hot central star emits intense ultraviolet radiation, which ionizes the previously ejected gas. This ionized shell expands outward at speeds typically ranging from 24 to 56 km per second.35
• Dissipation: Eventually, the central star cools and can no longer provide enough radiation to keep the gas ionized. The nebula becomes neutral, invisible, and eventually mingles with the interstellar medium.36