2025NatAs...9.1692G

Elements heavier than hydrogen and helium, collectively termed metals, were created inside stars and dispersed through space at the final stages of stellar evolution. The relative amounts of different isotopes (variants of the same element with different masses) in stellar atmospheres provide clues about how our galaxy evolved chemically over billions of years. M dwarfs are small, cool, long-lived stars that comprise three-quarters of all stars in our galaxy. Their spectra exhibit rich fingerprints of their composition, making them potential tracers of chemical evolution. Here we measure rare carbon and oxygen isotopes in 32 nearby M dwarfs spanning a range of metallicities using high-resolution infrared spectroscopy. We find that stars with higher metal content have lower ¹²C/¹³C ratios, indicating they formed from material progressively enriched in ¹³C over time. This pattern is consistent with models where novae eruptions contributed significant amounts of ¹³C to the interstellar medium over the past few billion years. Our measurements of the ¹⁶O/¹⁸O ratio align with theoretical predictions and indicate that metal-rich stars attain significantly lower ¹⁶O/¹⁸O ratios than the Sun. These results establish M dwarfs as tracers of chemical evolution throughout cosmic history.