An artist's conception showing a blue straggler being created by mass transfer in a binary star system. The giant star, seen in the upper left of the illustration, has lost hold of its outer envelope. This material is pulled towards its partner, forming an accretion disk, and is eventually consumed by the "proto-blue straggler", seen in the lower right of the illustration. Soon the giant star will donate the remainder of its envelope, leaving only the half-solar-mass white dwarf core (shown peaking through the giant's tenuous envelope) as the companion to the blue straggler. Observations made by astronomers from Northwestern University and the University of Wisconsin - Madison show that the majority of the blue stragglers in the open star cluster named NGC 188 have half-solar-mass companions, closely consistent with an origin through mass transfer, in which the companions would be white dwarfs.

Illustration: Aaron M. Geller