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.
Milky Way Inhomogeneity - V2
University of Zurich