Blue straggler stars are an astronomical mystery - they are bigger and age more slowly than stars born at the same time. It turns out these stars are created by two kinds of violent interstellar struggles.
Earlier this week, we told you about the vampiric ways of blue stragglers. Now Nature has published two papers on blue stragglers, which together prove that these bizarre stars are the result of interstellar violence and colonization.
According to Nature:
Blue straggler stars - hotter and more massive than would be expected for their apparent age - are found in stellar clusters, where all the stars are thought to have formed at the same time. Massive stars exhaust their nuclear fuel more quickly than their low-mass counterparts; it is therefore remarkable that the stragglers have not yet evolved into red giants, or the cooling stellar remnants known as white dwarfs. A likely explanation is that blue stragglers originate from normal stars that have undergone a recent increase in mass - either through stellar collision and merger, or by mass transfer between binary companions.
Now it seems that both mechanisms are at work. Francesco Ferraro and colleagues report the existence of two distinct populations of blue stragglers in the globular cluster M30, one redder than the other. They present evidence that the redder stars formed from mass transfer within binaries, whereas the bluer stars formed from stellar collisions. Meanwhile, Robert Mathieu and Aaron Geller studied blue stragglers in another cluster in our Galaxy, the open cluster NGC 188. They report that 76% of the blue stragglers in the cluster are in binary systems - a frequency three times that found among the normal stars. From this observation, and some unusual features of the binary orbits, the authors conclude that most or all of the blue stragglers in NGC 188 formed from multiple-star systems, and that both mass transfer and stellar collisions were involved.