Globular clusters are spherical groupings of stars tightly bound by gravity. While these objects are usually 12 to 13 billion years old, a new study has found that clusters can appear to be vastly different ages, depending on the behavior of their member stars.
Astronomers have used the European Southern Observatory's (ESO) MPG/ESO 2.2-meter telescope and NASA's Hubble Space Telescope to measure the age of the stars found in various globular clusters. The research will be published tomorrow in the journal Nature.
“Although these clusters all formed billions of years ago,” said Francesco Ferraro, team leader of the research and an astronomer at the University of Bologna. “We wondered whether some might be aging faster or slower than others. 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.”
The blue star Ferraro refers to is known as a "blue stragler" and is formed when aging stars receive extra mass that allows them to shine brighter. These stars can form when one star pulls matter off of another, or as a result of stellar collisions.
Since globular clusters form quickly, their member stars all have roughly the same age. The bright, high mass stars in the clusters burn out quickly, leaving what should only be low-mass, dim stars. The blue stragglers, then, gave researchers a chance to study how different globular clusters age.
Astronomers mapped the location of blue stragglers in 21 different globular clusters. They found three different types of clusters. One group appeared young, with blue stragglers distributed throughout the clusters. Another group appeared old, with their blue stragglers having migrated into a clump near the center of the clusters. The third group was in-between the others, with blue stragglers near the center of clusters and other, further out ones still migrating inward.
“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 and an astronomer at the University of Bologna. “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.”