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 of these mechanisms are at work during the dramatic phase of the cluster core collapse. Two distinct parallel sequences of blue stragglers are present in M 30, a 'bluer' population arising from direct stellar collisions and a 'redder' one from the evolution of close binaries. Observations of the 21 blue stragglers in the old open cluster NGC 188 show that 16 (76%) are currently in binary systems, a frequency three times that found among normal solar-type main-sequence stars. Most of the NGC 188 blue stragglers are rotating faster than normal main-sequence stars of the same surface temperatures. In News & Views, Melvyn Davies reflects on what these two studies say about the origin of blue stragglers.