On the origin of blue stragglers

Blue stragglers are a peculiar class of stars which lie on the main sequence of the Hertzsprung-Russell diagram in a region that is bluer and more luminous that the main sequence turn-off point.  Theories of stellar evolution tell us that the most massive blue stars will burn brightest and therefore exhaust their supply of hydrogen more quickly than lower mass stars. Once their core hydrogen supply is exhausted, they should move off the main sequence and migrate up the giant branch.  However, the luminosity and temperature of blue stragglers puts blue-ward of main sequence turn-off point, which puzzled astronomers since their discovery. They must be normal main sequence stars that have recently increased in mass.  But how can main sequence stars increase in mass?

Main sequence turn-off & blue stragglers

HR diagram showing main sequence turn-off and location of blue stragglers. (From Tout, 2011, Nature, 478, 331)

A hint to their formation comes from the fact that blue stragglers are only found in stellar clusters.  In evolved clusters they easily stand out amongst the mostly red old stellar population. Since stars in clusters must all form at about the same time, this suggests that bright blue stragglers formed much later then the other cluster stars.  In dense globular cluster systems the distance between stars is small and so stellar collisions are relatively common. This lead astronomers to hypothesis that blue stragglers might form as a result of the merger of hierarchical triple systems or via direct collisions between two old red stars, producing a higher mass and hence bluer and brighter star.

In this week’s edition of the journal Nature, Aaron Geller and Robert Mathieu present new results about the mass transfer origin for blue stragglers.  They studied the old (7 Gyr) open cluster NGC 188, whose blue stragglers have a very high long-period binary fraction:  75%.  Analysis and modelling of the Doppler shift of the blue stragglers showed that their companions all have masses of about half a solar mass. They argue that this rules out the collisional origin of blue stragglers and likely the merger scenario as well.  Their data supports a mass transfer origin – at least for the long-period blue straggler binaries in NGC 188 – where a blue straggler forms when a main sequence stars accretes mass from its evolved binary companion.  To prove their hypothesis, Geller & Mathieu aim to detect the flux of the white dwarf companion, which will allow them to distinguish between the binary mass transfer model and merger hypothesis.

For more details, see

[Post by Francesco Pignatale & Sarah Maddison]

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