Globular clusters are spherical collections of stars bound to each other by their mutual gravity. They are old, relics of the early years of the Universe, with ages of typically 12-13 billion years, and we know of roughly 150 globular clusters in the Milky Way.

Yet, like many humans, these clusters are still young at heart. Some are aging faster than others and that discovery has led to a way to measure the rate of aging. 

Star clusters form in a short period of time, meaning that all the stars within them tend to have roughly the same age. Because bright, high-mass stars burn up their fuel quite quickly, and globular clusters are very old, there should only be low-mass stars still shining within them.

This, however, turns out not to be the case: in certain circumstances, stars can be given a new burst of life, receiving extra fuel that bulks them up and substantially brightens them. This can happen if one star pulls matter off a neighbor, or if they collide. The re-invigorated stars are called blue stragglers and their high mass and brightness are properties that lie at the heart of this study.

NGC 6388, a dynamically middle-aged globular cluster in the Milky Way. While the cluster formed in the distant past (like all globular clusters, it is over ten billion years old), a study of the distribution of bright blue stars within the cluster shows that it has aged at a moderate speed, and its heaviest stars are in the process of migrating to the center. A new study using Hubble data has discovered that globular clusters of the same age can have dramatically different distributions of blue straggler stars within them, suggesting that clusters can age at substantially different rates. Credit: NASA, ESA, F. Ferraro - University of Bologna

Heavier stars sink towards the center of a cluster as the cluster ages, in a process similar to sedimentation. Blue stragglers' high masses mean they are strongly affected by this process, while their brightness makes them relatively easy to observe.

To better understand cluster aging, the team mapped the location of blue straggler stars in 21 globular clusters, as seen in images from Hubble and the MPG/ESO 2.2-meter telescope at the ESO La Silla Observatory, among other observatories . Hubble provided high resolution imagery of the crowded centers of 20 of the clusters, while the ground-based imagery gave a wider view of their less busy outer regions.

Analyzing the observational data, the team found that a few clusters appeared young, with blue straggler stars distributed throughout, while a larger group appeared old, with the blue stragglers clumped in the center. A third group was in the process of aging, with the stars closest to the core migrating inwards first, then stars ever further out progressively sinking towards the center.

"Since these clusters all formed at roughly the same time, this reveals big differences in the speed of evolution from cluster to cluster," said Barbara Lanzoni of the University of Bologna, a co-author of the study. "In the case of fast-aging clusters, we think that the sedimentation process can be complete within a few hundred million years, while for the slowest it would take several times the current age of the Universe."

As a cluster's heaviest stars sink towards the center, the cluster eventually experiences a phenomenon called core collapse, where the center of the cluster bunches together extremely densely.

The processes leading towards core collapse are quite well understood, and revolve around the number, density and speed of movement of the stars. However, the rate at which they happened was not known until now. This study provides the first empirical way of investigating these different rates of aging.

Published in Nature.