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

Two shades of blue straggler'Blue stragglers' are massive stars in clusters — where all the stars are pretty much the same age — that at their luminosity, should have evolved to become giants and white dwarfs. They are thought to be normal main-sequence stars that have gained mass through either transfer between binary companions or direct collision and merger between two stars. A study of the highly crowded star cluster M 30 suggests that both …