Some people are in great shape at the age of 90, while others are decrepit before they’re 50. We know that how fast people age is only loosely linked to how old they actually are — and may have more to do with their lifestyle. A new study with the NASA/ESA Hubble Space Telescope reveals that the same is true of star clusters.
While the stars in a globular cluster are old and the clusters formed in the distant past, astronomers using the NASA/ESA Hubble Space Telescope and the MPG/ESO 2.2-metre telescope at the ESO La Silla Observatory have found that some of these clusters are still young at heart.
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 neighbour, 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.
Heavier stars sink towards the centre 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. Analysing 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 centre. 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 centre.
As a cluster’s heaviest stars sink towards the centre, the cluster eventually experiences a phenomenon called core collapse, where the centre 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.