  Blue stragglers in globular clusters are abnormally massive stars that should
have evolved off the stellar main sequence long ago. There are two known
processes that can create these objects: direct stellar collisions and binary
evolution. However, the relative importance of these processes has remained
unclear. In particular, the total number of blue stragglers found in a given
cluster does not seem to correlate with the predicted collision rate, providing
indirect support for the binary-evolution model. Yet the radial distributions
of blue stragglers in many clusters are bimodal, with a dominant central peak:
this has been interpreted as an indication that collisions do dominate blue
straggler production, at least in the high-density cluster cores. Here we
report that there is a clear, but sublinear, correlation between the number of
blue stragglers found in a cluster core and the total stellar mass contained
within it. From this we conclude that most blue stragglers, even those found in
cluster cores, come from binary systems. The parent binaries, however, may
themselves have been affected by dynamical encounters. This may be the key to
reconciling all of the seemingly conflicting results found to date.
