The interstellar medium (ISM) comprises gases at different temperatures and densities, including ionized, atomic and molecular species, and dust particles¹. The neutral ISM is dominated by neutral hydrogen² and has ionization fractions of up to eight per cent³. The concentration of chemical elements heavier than helium (metallicity) spans orders of magnitudes in Galactic stars⁴, because they formed at different times. However, the gas in the vicinity of the Sun is assumed to be well mixed and to have a solar metallicity in traditional chemical evolution models⁵. The ISM chemical abundances can be accurately measured with ultraviolet absorption-line spectroscopy. However, the effects of dust depletion^6,7,8,9—which removes part of the metals from the observable gaseous phase and incorporates it into solid grains—have prevented, until recently, a deeper investigation of the ISM metallicity. Here we report the dust-corrected metallicity of the neutral ISM measured towards 25 stars in our Galaxy. We find large variations in metallicity over a factor of ten (with an average of 55 ± 7 per cent solar metallicity and a standard deviation of 0.28 dex), including many regions of low metallicity, down to about 17 per cent solar metallicity and possibly below. Pristine gas falling onto the Galactic disk in the form of high-velocity clouds can cause the observed chemical inhomogeneities on scales of tens of parsecs. Our results suggest that this low-metallicity accreting gas does not efficiently mix into the ISM, which may help us understand metallicity deviations in nearby coeval stars.

星际介质 (ISM) 包含不同温度和密度的气体，包括电离、原子和分子物质，以及尘埃颗粒¹。中性 ISM 以中性氢²为主，电离分数高达 8% ³。比氦重的化学元素（金属性）的浓度在银河恒星⁴中跨越了几个数量级，因为它们形成于不同的时间。然而，在传统的化学演化模型中，假设太阳附近的气体混合良好并具有太阳金属丰度⁵。ISM 化学丰度可以用紫外吸收线光谱法准确测量。然而，灰尘耗尽的影响^6,7,8,9——从可观察到的气相中去除部分金属并将其结合到固体颗粒中——直到最近，才阻止对 ISM 金属丰度进行更深入的研究。在这里，我们报告了对银河系中 25 颗恒星测量的中性 ISM 的尘埃校正金属丰度。我们发现金属丰度的巨大变化超过 10 倍（平均为 55 ± 7% 的太阳金属丰度，标准偏差为 0.28 dex），包括许多低金属丰度的区域，低至约 17% 的太阳金属丰度，甚至可能低于. 以高速云的形式落入银盘的原始气体会导致观察到的化学不均匀性达到数十秒差距。我们的结果表明，这种低金属吸积气体不能有效地混合到 ISM 中，