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Two distinct sequences of blue straggler stars in the globular cluster M30

Two distinct sequences of blue straggler stars in the globular cluster M30

Publication date: 24 December 2009

Authors: Ferraro, F.R. et al.

Journal: Nature
Volume: 462
Issue: 7276
Page: 1028-1031
Year: 2009

Copyright: Nature Publishing Group

Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy's history. 'Blue stragglers' are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently the binary explanation was claimed to be dominant. 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 enhanced. 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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