Blue stragglers photographed in the core of the globular cluster 47 Tucanae by the Hubble Space Telescope.
A blue straggler is a hot, bright star in a globular cluster or, occasionally, an open cluster, that lies close to the cluster's extrapolated main sequence but a few magnitudes above its turnoff point. Blue stragglers first came to the attention of astronomers in 1953. Various theories have been put forward to explain why such stars have not already evolved to become red giants. The most favored of these involve mass transfer from, or coalescence with, a binary companion. Blue stragglers may also arise from the collision of stars in the crowded interiors of globular clusters.