The new study shows that some of the blue stragglers have instead been rejuvenated by a sort of cosmic facelift, courtesy of cosmic collisions. These stellar encounters 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 both be about twice the typical mass of individual stars in the cluster.
Our observations demonstrate that blue stragglers formed by collisions have slightly different properties from those formed by vampirism. This provides a direct demonstration that the two formation scenarios are valid and that they are both operating simultaneously in this cluster, says team member Giacomo Beccari from ESA.
Using data from the now-retired Wide Field Planetary Camera 2 aboard Hubble, astronomers found that these straggling stars are much more concentrated towards the centre of the cluster than the average star. This indicates that blue stragglers are more massive than the average star in this cluster, says Ferraro. More massive stars tend to sink deep into the cluster the way a billiard ball would sink in a bucket of honey.
The central regions of high density globular clusters are crowded neighbourhoods where interactions between stars are nearly inevitable. Researchers conjecture that one or two billion years ago, Messier 30 underwent a major core collapse that started to throw stars towards the centre of the cluster, leading to a rapid increase in the density of stars. This event significantly increased the number of collisions among stars, and favoured the formation of one of the families of blue stragglers. On the other hand, the increase of stellar crowding due to the collapse of the core also perturbed the twin systems, encouraging the vampirism phenomenon and thus forming the other family of blue stragglers. Almost ten percent of galactic globular clusters have experienced core collapse, but this is the first time that we see the effect of the core collapse imprinted on a stellar population, says Barbara Lanzoni, University of Bologna.
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