Massive, collisionally produced Blue Stragglers in the galactic globular cluster NGC 6397

A remarkable linear structure of Blue Straggler Stars (BSS) has been identified in Hubble Space Telescope (HST) Faint Object Camera images of the core of the Milky Way Globular Cluster NGC 6397. We have obtained and analyzed HST spectra of these BSS, establishing that they are massive - one BSS has a mass fully three times the most massive main sequence turn-off star in the cluster, while 4 other BSS have masses twice as massive as turn-off stars. BSS are thought to be formed by mergers of lower-mass stars, either in binary star systems, or by direct physical stellar collisions. BSS3 is too massive to have been formed by a binary merger - it must therefore have been formed by the collisional merger of at least three lower-mass stars. The existence of four other massive BSS also argues strongly for a collisional origin. While stellar collisions have long been thought to have a profound influence on globular cluster evolution, this is the first-ever direct confirmation of their existence.