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ASTROPHYSICS
Observations of star clusters in the Milky Way show that collisions between stars as well as mass flow within binary systems can explain how the peculiar family of blue straggler stars came to be born.
What is the origin of the population of oddly massive stars that lurk in the cores of old star clusters? These stars have become known as blue stragglers because they tend to be bluer and younger than most of the clusters' stars. Their existence in star clusters is at odds with a simple picture of stellar evolution, which demands that they should have exhausted their nuclear fuel and evolved long ago to become cooling, Earth-sized stellar remnants known as white dwarfs. This fate awaits our own Sun, around five billion years from now. In this issue, Ferraro et al.1 (page 1028) and Mathieu and Geller2 (page 1032) present observations of star clusters that may provide important clues to the origin of blue stragglers.
The study of blue stragglers has a long history. By imaging a star cluster and plotting the brightness versus colour for the individual stars, in what is known as a colour-magnitude diagram, one sees that the population of blue stragglers lies in an extension bluewards of the regular main-sequence band of stars on which most stars are located; main-sequence stars are powered by the fusion of hydrogen into helium in their cores. Blue stragglers were found in this way...