The aging of star clusters is linked more with their lifestyle than with how old they actually are, according to a new NASA/ESA Hubble Space Telescope study coauthored by Steinn Sigurdsson, professor of astronomy and astrophysics at Penn State. "Our observations of star clusters have shown us that, although they all formed over ten billion years ago, some of them are still young at heart," Sigurdsson said. "We now can see how fast the clusters are racing toward their final collapse. It is as if each cluster has its own internal clock, some of which are ticking slower than others." Sigurdsson is a Penn State theorist working in collaboration with the European Research Council's Cosmic-Lab project. The study is published in the current issue of the journal Nature.
Globular clusters are spherical collections of stars, tightly bound to each other by their mutual gravity. The roughly 150 globular clusters in the Milky Way contain many of our galaxy's oldest stars. These 12-to-13 billion-year-old relics of the early universe are nearly as old as the Big Bang. "Although these clusters all formed billions of years ago, we wondered whether some clusters might be aging faster or slower than others," said Francesco Ferraro of the University of Bologna in Italy, the leader of the team that made the discovery. "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 their aging."
Star clusters form in a short period of time, so 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, the clusters should contain only low-mass stars within them. But Sigurdsson and his colleagues discovered that, in certain circumstances, stars can be given a new burst of life. "Stars can receive extra fuel that bulks them up and substantially brightens them if one star pulls matter off a neighbor, if two neighboring stars merge together, or if two stars collide," Sigurdsson said.
These reinvigorated stars have a large mass and high brightness. They are called blue stragglers because they are blue in color and their evolution lags behind that of their neighbors. Blue stragglers are the only stars that combine high mass and high brightness within clusters.
Heavier stars sink like sediment toward the center of a cluster as the cluster ages. The high-mass blue stragglers are strongly affected by this process, and their brightness makes them relatively easy for astronomers 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 the Hubble Space Telescope, the European Southern Observatory's MPG/ESO 2.2-meter telescope, the Canada-France-Hawaii telescope, and the Subaru Telescope of the National Astronomical Observatory of Japan. Hubble provided high-resolution imagery of the crowded centers of 20 of the clusters, while images from ground-based telescopes 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 study reveals big differences in the speed of evolution from cluster to cluster," said Barbara Lanzoni at 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 into the center, it eventually experiences a phenomenon called core collapse, where the center of the cluster bunches together extremely densely. The processes leading toward core collapse are rather well understood, and revolve around the number, density and speed of movement of the stars. However, the rate at which they happen was not known until now. "This study provides the first evidence, based totally on data from observations, of how quickly different globular clusters age," Sigurdsson said.