After hydrogen and helium, oxygen, carbon, and nitrogen—hereinafter, the CNO elements—are the most abundant species in the universe. They are observed in all kinds of astrophysical environments, from the smallest to the largest scales, and are at the basis of all known forms of life, hence, the constituents of any biomarker. As such, their study proves crucial in several areas of contemporary astrophysics, extending to astrobiology. In this review, I will summarize current knowledge about CNO element evolution in galaxies, starting from our home, the Milky Way. After a brief recap of CNO synthesis in stars, I will present the comparison between chemical evolution model predictions and observations of CNO isotopic abundances and abundance ratios in stars and in the gaseous matter. Such a comparison permits to constrain the modes and time scales of the assembly of galaxies and their stellar populations, as well as stellar evolution and nucleosynthesis theories. I will stress that chemical evolution models must be carefully calibrated against the wealth of abundance data available for the Milky Way before they can be applied to the interpretation of observational datasets for other systems. In this vein, I will also discuss the usefulness of some key CNO isotopic ratios as probes of the prevailing, galaxy-wide stellar initial mass function in galaxies where more direct estimates from the starlight are unfeasible.

继氢和氦之后，氧、碳和氮（以下简称 CNO 元素）是宇宙中最丰富的物种。它们可以在各种天体物理环境中观察到，从最小到最大的尺度，是所有已知生命形式的基础，因此是任何生物标志物的组成部分。因此，他们的研究在当代天体物理学的几个领域（延伸到天体生物学）中被证明至关重要。在这篇综述中，我将总结当前关于星系中 CNO 元素演化的知识，从我们的家园银河系开始。在简要回顾恒星中的 CNO 合成之后，我将介绍化学演化模型预测与恒星和气态物质中 CNO 同位素丰度和丰度比的观测之间的比较。这种比较可以限制星系及其恒星群的聚集模式和时间尺度，以及恒星演化和核合成理论。我要强调的是，化学演化模型必须根据银河系可用的大量丰度数据仔细校准，然后才能应用于其他系统的观测数据集的解释。在这方面，我还将讨论一些关键的 CNO 同位素比作为对星系中普遍存在的、全星系恒星初始质量函数的探针的有用性，在这些星系中，从星光进行更直接的估计是不可行的。