  In open star clusters, where all members formed at about the same time, blue
straggler stars are typically observed to be brighter and bluer than
hydrogen-burning main-sequence stars, and therefore should already have evolved
into giant stars and stellar remnants. Correlations between blue straggler
frequency and cluster binary fraction, core mass, and radial position suggest
that mass transfer or mergers in binary stars dominates the production of blue
stragglers in open clusters. Analytic models, detailed observations, and
sophisticated N-body simulations, however, argue in favor of stellar
collisions. Here we report that the blue stragglers in long-period binaries in
the old (7 Gyr) open cluster NGC 188 have companions with masses of about half
a solar mass, with a surprisingly narrow mass distribution. This conclusively
rules out a collisional origin, as the collision hypothesis predicts a
companion-mass distribution with significantly higher masses. Mergers in
hierarchical triple stars are marginally permitted by the data, but the
observations do not favor this hypothesis. The data are closely consistent with
a mass transfer origin for the long-period blue straggler binaries in NGC 188,
in which the companions would be white dwarfs of about half a solar mass.
