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How does helium burning inside a red giant help produce carbon-rich 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.

Helium burning inside a red giant contributes to the formation of carbon-rich planetary nebulae through the triple alpha process.

As a star evolves into a red giant, its core contracts and heats up until it reaches approximately 120 million K, allowing it to fuse helium into heavier elements. In this process, two helium nuclei ( $^{4}$ He) briefly form an unstable beryllium nucleus ( $^{8}$ Be), which then captures a third helium nucleus to create carbon-12 ( $^{12}$ C).1

This nuclear processing enriches the star's interior with carbon. As the star nears the end of its life, it experiences significant mass loss, ejecting its outer layers—which have been enriched by these internal nuclear reactions—into space. This expelled material forms the expanding shell known as a planetary nebula. Because the ejected gas is heavily influenced by the star's prior nuclear processing, some resulting nebulae exhibit a high carbon-to-oxygen ratio, distinguishing them as carbon-rich.213