Two distinct sequences of blue straggler stars in the globular cluster M 30

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Abstract
Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy’s history1. ‘Blue stragglers’ are stars massive enough2 that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions3 and stellar mergers resulting from direct collisions between two stars4. Recently the binary explanation was claimed to be dominant5. Here we report that there are two distinct parallel sequences of blue stragglers in M 30. 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 been enhanced6. We suggest that the two observed sequences are a consequence of cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.
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