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What do ancient planetary nebulae in M15 and the Milky Way halo reveal about early galactic gas?

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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.

Ancient planetary nebulae found in the globular cluster M15 and the halo of the Milky Way Galaxy provide critical insights into the chemical state of the early universe. These specific objects are notable for having a very low heavy-element content—approximately 50 times lower than normal—while maintaining a normal abundance of helium.1

These findings suggest that the primeval gas from which these ancient stars formed was characterized by a low concentration of heavy elements but contained an almost standard amount of helium. This supports the scientific understanding that the majority of helium in the galaxy originated from the big bang, the initial expansion of the universe. By studying these deviant nebulae, astronomers can better reconstruct the composition of the early interstellar medium before it was significantly enriched by subsequent generations of stars.1