Why are some planetary nebulae rich in carbon?

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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 are chemically enriched by elements produced through nuclear processing within their central stars. Some planetary nebulae are carbon-rich because they contain twice as much carbon as oxygen, a result of the complex nuclear processing of material expelled from the evolved central star.1

These regions of material are often strongly enriched in heavy elements and can be deficient in hydrogen. This enrichment reflects the internal evolution of the star, which, prior to the ejection of the nebula, exists as a red giant experiencing significant mass loss. The specific chemical composition of a planetary nebula serves as an indicator of the nuclear history and the original composition of the star that gave rise to it.12