WHY do the universe's oldest star clusters tend to be roughly the same size? The answer is grizzly.
Globular star clusters are ancient, spherical blobs of stars. Most of these clusters are a few hundred thousand times the mass of our sun. The scarcity of much bigger clusters is no surprise as there must be an upper limit - but why are there so few small ones?
Using computer modelling, Diederik Kruijssen of the Max Planck Institute for Astrophysics in Garching, Germany, and his colleagues simulated the merger of two small galaxies. This process is thought to have been especially common in the early universe, when small structures were busy snowballing into larger ones.
The collision compressed gas into dense knots, sparking star formation and creating many new globular clusters. At the same time it seeded destruction, as the gravity of these knots disrupted clusters - both existing and new - passing nearby. Medium and large clusters survived, bound by their own strong gravity, but the smaller ones were ripped apart (arxiv.org/abs/1112.1065).
All big galaxies probably formed in such mergers so this might be why globular clusters fall into such a narrow size range.