Blue Straggler Stars:
"thin-looking fat" stars
hidden in a see of slimmer sisters
In 1953, astronomer Allan Sandage found a puzzling new population of stars which seemed to go against the rules of stellar evolution in globular clusters. Sandage detected hot (blue) stars in the globular cluster Messier 3 (M3), suggesting masses larger than that of their sisters and significantly younger ages (in fact, more massive stars evolve more rapidly). He dubbed them "stragglers" because they looked like they were trailing or left behind by other blue stars that, long ago, evolved to the red giant stage. The presence of these massive and (apparently) young stars in M3 was quite strange, since star formation essentially stopped 13 billion years ago in globular clusters, and astronomers therefore expect to find only old stars.
The formation of Blue Stragglers
After almost 60 years of investigation, Blue Stragglers are now thought to arise in "twin systems" where two stars form a tight binary. In such a pair, the less massive star would act as a "vampire", siphoning fresh hydrogen from its companion and increasing its own mass. Thanks to the new fuel supply, such a star also heats up, growing bluer and hotter - behaving like stars that are earlier in their evolution and thus appearing rejuvenated. Another possibility are nearly head-on stellar collisions, in which the stars might actually merge and mix their nuclear fuel. The newly formed star (a Blue Straggler) is more massive than its sisters and, with the fires of nuclear fusion "re-stoked", it also appears hotter, more luminous and younger.
This illustration shows the two ways that Blue Stragglers - or "rejuvenated stars" - in globular clusters form.
The upper illustration shows the collision model, where two low-mass stars in a overcrowded environment experience a head-on collision, combining their fuel and mass to form a new, single, hot (hence blue) and seemingly young star.
The lower illustration depicts the "vampire" model, consisting of a pair of stars where the lower-mass object drains its heavier companion of hydrogen, that fuels its rebirth.
Why do collisions and vampirism rejuvenate stars?
The life-stage of a star is essentially driven by the amount of unspent fuel (hydrogen) remaining in its centre. Adding fresh hydrogen from the outside, either by a collision or by transferring mass from a binary companion in the stellar core, prolongs the life of the star and makes it look more youthful (blueness and brightness being the attributes of younger stars).