  Globular clusters contain many stars with surface abundance patterns
indicating contributions from hydrogen burning products, as seen in the
anti-correlated elemental abundances of e.g. sodium and oxygen, and magnesium
and aluminium. Multiple generations of stars can explain this phenomenon, with
the second generation forming from a mixture of pristine gas and ejecta from
the first generation. We show that massive binary stars may be a source of much
of the material that makes this second generation of stars. Mass transfer in
binaries is often non-conservative and the ejected matter moves slowly enough
that it can remain inside a globular cluster and remain available for
subsequent star formation. Recent studies show that there are more short-period
massive binaries than previously thought, hence also more stars that interact
and eject nuclear-processed material.
