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Why do planetary nebulae often appear as rings or shells?

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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.

Planetary nebulae often appear as rings or shells because they consist of gas that was ejected from a central star during a brief period of instability. Before this ejection, the star exists as a red giant, losing mass through a slowly expanding stellar wind.12

As the star evolves, its hot core is exposed, and the resulting ultraviolet radiation ionizes the surrounding gas. This ionized material forms a shell that expands outward from the star at speeds typically ranging from 24 to 56 km (15 to 35 miles) per second. Because these nebulae possess relatively regular inner and outer boundaries, they frequently take on the appearance of a ring or shell when observed. Additionally, some nebulae feature faint outer rings, which are remnants of material ejected by the star at an even earlier stage of its evolution.21