Astronomers have found that interstellar gas in the Milky Way often has low amounts of chemical elements. The believe this may due to cosmic gas that falls into the Milky Way in the form of high-velocity clouds with low numbers of heavy elements in them. This material provides 'fuel' for the formation of new stars but does not mix well with the interstellar medium, and can cause chemical inhomogeneities on scales of tens of parsecs. The inset shows a close-up of the solar neighbourhood, showing the inhomogeneity in the interstellar medium caused by this infalling gas.