The elements carbon, nitrogen, and oxygen (CNO) are involved in nuclear fusion reactions in the cores of stars, which affect the relative abundances of their isotopes. As a star ages, changing conditions can cause convection to bring material from the core to the surface, modifying the isotope ratios that are visible to astronomers. Abai et al. measured CNO isotope ratios in a sample of evolved objects known as asymptotic giant branch (AGB) stars and compared them with stellar evolution simulations. The observed oxygen isotope ratios are similar to those found in some grains present in meteorites; perhaps those grains originally formed in material ejected from AGB stars.