Washington, April 29 : A new study has suggested that globular clusters, which are some of the oldest objects in the Universe, might be surprisingly less mature in their development than previously thought.
Globular clusters, dense bunches of up to millions of stars found in all galaxies, are among the oldest known objects in the Universe, with most estimates of their ages ranging from 9 to 13 billions of years old.
As such they contain some of the first stars to form in a galaxy and understanding their evolution is critical to understanding the evolution of galaxies.
Conventional wisdom is that globular clusters pass through three phases of evolution or development of their structure, corresponding to adolescence, middle age, and old age. These "ages" refer to the evolutionary state of the cluster, not the physical ages of the individual stars.
In the adolescent phase, the stars near the center of the cluster collapse inward. Middle age refers to a phase when the interactions of double stars near the center of the cluster prevents it from further collapse. Finally, old age describes when binaries in the center of the cluster are disrupted or ejected, and the center of the cluster collapses inwards.
For years, it has been thought that most globular clusters are middle-aged with a few being toward the end of their evolution.
However, data from NASA's Chandra X-ray Observatory, along with theoretical work, suggest this may not be the case.
When single and double stars interact in the crowded centers of globular clusters, double stars can form that transfer mass and give off X-rays.
Since such double stars are expected to mostly be formed in the middle of a globular cluster's evolution and then lost in old age, the relative number of X-ray sources gives clues about the stage of evolution the cluster is in.
A new study by John Fregeau of Northwestern University of 13 globular clusters in the Milky Way shows that three of them have unusually large number of X-ray sources, or X-ray binaries, suggesting the clusters are middle-aged.
Previously, these globular clusters had been classified as being in old age because they had very tight concentrations of stars in their centers, another litmus test of age used by astronomers.
The implication is that most globular clusters, including the other ten studied by Fregeau, are not in the middle age of their evolution, as previously thought, but are actually in adolescence.
"It's remarkable that these objects, which are thought to be some of the oldest in the Universe, may really be very immature in their development," said Fregeau. "This would represent a major change in thinking about the current evolutionary status of globular clusters," he added.