2025Sci...387.1209Z

Interstellar dust grains cause extinction (absorption and scattering) of light from background astronomical sources. The spectral shape of the extinction curve depends on the dust composition. We used low-resolution optical spectra to measure the extinction curve of 130 million stars. By inverting these data, we mapped the extinction curve parameter R(V) within the Milky Way in three dimensions and within the Magellanic Clouds in two dimensions. These maps provide improved extinction corrections for astronomical observations. We find that R(V) varies with extinction, consistent with dust grains growing by accretion in low-extinction regions and by coagulation in higher-extinction regions. Star-forming regions have high R(V) values, indicating either preferential destruction of small dust grains or additional supply of large dust grains in those regions.