Formation channels for blue straggler stars 

Melvyn Davies (Lund University, Sweden)
 

In this chapter we consider two formation channels for blue straggler stars: 1) the merger of two single stars 
via a collision, and 2) those produced via mass transfer within a binary.

We review how computer simulations show that stellar collisions are likely to lead to relatively little mass loss 
and are thus effective in producing a young population of more-massive stars. The number of blue straggler 
stars produced by collisions will tend to increase with cluster mass. We review how the current population of 
blue straggler stars produced from primordial binaries decreases with increasing cluster mass. This is 
because exchange encounters with third, single stars in the most massive clusters tend to reduce the 
fraction of binaries containing a primary close to the current turn-off mass. Rather, their primaries tend to be 
somewhat more massive and have evolved off the main sequence, filling their Roche lobes in the past, often 
converting their secondaries into blue straggler stars (but more than 1 Gyr or so ago and thus they are no 
longer visible today as blue straggler stars).