Blue Stragglers:

“baby” stars in old stellar clusters

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 young blue stars in the globular cluster Messier 3, and subsequently in other globular clusters. 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 (apparently) young stars in a  globular cluster was quite strange since  star formation essentially stopped in globular clusters 13 billion years ago, so astronomers expect to find only old stars.

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 more massive companion star. With the new fuel supply the smaller star heats up, growing bluer and hotter – behaving like stars that are earlier in their evolution. Another possibility are stellar encounters which are nearly head-on collisions, in which the stars might actually merge, mixing their nuclear fuel and “re-stoking” the fires of nuclear fusion. Merged stars and binary systems would be about twice the mass of the cluster's individual stars.

The diagram shows the properties (temperatures and luminosities) of the two distinct populations of blue stragglers discovered in the cluster Messier 30. This is the first time that two distinct  populations of “rejuvenated” stars have been found in the same cluster. The luminosities are in Solar unity and the temperatures are in Kelvin degrees.

The formation of blue stragglers

This illustration demonstrates 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 an overcrowded environment experience a head-on collision, combining their fuel and mass and to form a single young, hot star.

The lower illustration depicts the “vampire” model consisting of a pair of stars that undergo a transformation, with the lower-mass star draining its larger-mass companion of hydrogen that fuels its rebirth.

The two  blue straggler families discovered in Messier 30

The core of Messier 30, where the two blue straggler families have been discovered. The two arrows indicate two stragglers “rejuvenated”  by different processes: the blue one by collisions and the red one by “vampirism” .