Star Clusters Can Look Younger Than They Are
December 19, 2012

Star Clusters Reveal The Secret Of Aging Well

Lee Rannals for - Your Universe Online

Just as your lifestyle can cause you to show your age, the same is true for star clusters, according to a study published in the journal Nature. Astronomers using the Hubble Space Telescope have found that some globular clusters are still young at heart, despite being relics of the early Universe.

Globular clusters are spherical collections of stars that are held tightly together by their mutual gravitational attraction.

“Although these clusters all formed billions of years ago, we wondered whether some might be aging faster or slower than others," Francesco Ferraro, a professor at the University of Bologna and the leader of the team that made the discovery, said in a press release.

"By studying the distribution of a type of blue star that exists in the clusters, we found that some clusters had indeed evolved much faster over their lifetimes, and we developed a way to measure the rate of aging.”

Star clusters form in a short period of time, so all the stars within them tend to have about the same age. These stars burn up their fuel quickly, and globular clusters are very old, so there should only be low-mass stars still shining within them. However, this doesn't seem to be the case under certain circumstances.

In some situations, stars can be given a new burst of life by receiving extra fuel that helps to bulk them up and brighten them. This happens if one star pulls matter away from a neighbor or if they collide.

To get a better understanding of cluster aging, the team mapped out the location of so-called blue straggler stars in 21 globular clusters. 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 outer regions.

While analyzing the observational data, the team found that a few clusters appeared to be young, with blue straggler stars distributed throughout. A third group was in the process of aging, with the stars closest to the core migrating inwards first, then stars even further out 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, 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 the cluster's heaviest stars sink towards the middle, the cluster eventually experiences a phenomenon known as core collapse, in which the center of the cluster bunches together.

The processes leading towards core collapse are well understood, and revolve around the number, density and speed of movement of the stars. The rate at which they occur, however, was unknown until now. This study has provided the first way of investigating these different rates of aging.