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How do planetary nebulae show that heavy elements become more abundant toward some parts of the Milky Way?

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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 serve as indicators of the chemical composition of the Milky Way Galaxy because they reflect the original composition of the stars from which they formed. By studying these objects, astronomers have identified a clear heavy-element abundance gradient within the galaxy.1

Specifically, observations show that heavy-element abundances are higher toward the galactic center compared to local regions. For example, at a distance of 12,000 light-years inward toward the center of the Milky Way, heavy-element abundances in planetary nebulae are approximately 50 percent larger than those found locally. This gradient suggests that the production of heavy elements may have been more concentrated or efficient near the galactic center than in other parts of the galaxy. Because planetary nebulae are formed from the material ejected by evolved stars, their chemical makeup provides a record of the enrichment of the interstellar medium over time.23145