Two distinct sequences of blue straggler stars in the globular cluster

Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy's history 1. 'Blue stragglers' are stars sufficiently massive 2 that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions 3 , and stellar mergers induced by direct collisions between two stars 4. Recently, the binary explanation was claimed to be dominant 5. Here we report on the discovery of two distinct parallel sequences of blue stragglers in M30. This globular cluster is thought to have undergone 'core collapse', during which both the collision rate and the mass transfer activity in binary systems would have