  We present an analytic model for blue straggler formation in globular
clusters. We assume that blue stragglers are formed only through stellar
collisions and binary star evolution, and compare our predictions to observed
blue straggler numbers taken from the catalogue of Leigh, Sills & Knigge
(2011). We can summarize our key results as follows: (1) Binary star evolution
consistently dominates blue straggler production in all our best-fitting
models. (2) In order to account for the observed sub-linear dependence of blue
straggler numbers on the core masses (Knigge, Leigh & Sills 2009), the core
binary fraction must be inversely proportional to the total cluster luminosity
and should always exceed at least a few percent. (3) In at least some clusters,
blue straggler formation must be enhanced by dynamical encounters (either via
direct collisions or by stimulating mass-transfer to occur by altering the
distribution of binary orbital parameters) relative to what is expected by
assuming a simple population of binaries evolving in isolation. (4) The
agreement between the predictions of our model and the observations can be
improved by including blue stragglers that form outside the core but later
migrate in due to dynamical friction. (5) Longer blue straggler lifetimes are
preferred in models that include blue stragglers formed outside the core since
this increases the fraction that will have sufficient time to migrate in via
dynamical friction.
