Globular star clusters are believed to be amongst the oldest objects in the universe, some with ages exceeding 13 billion years. They can be usually found in the company of other galaxies as satellites, containing several million stars packed into a very small volume of space. Because they contain some of the first stars in the universe, globular clusters are extremely important for cosmologists.
The problem is that according to a recent study conducted by John Frageau, from the Northwestern University, these objects are not so old as previously thought. "For many years, globular clusters have been used as wonderful natural laboratories to study the evolution and interaction of stars. So, it's exciting to discover something that may be new and fundamental about the way they evolve", says Fregeau.
Astronomers classify globular star clusters according to age and there are three categories - adolescent, middle aged and old. Most of them belong to the second and the third class. Yet, new observations conducted with NASA's Chandra X-ray Space Observatory show that the estimations related to the age of globular clusters are in fact inaccurate.
For example, the high concentration of mass in the center of the cluster is expected to determine interactions between single and double stars. During this time mass exchange takes place and a great deal of X-ray radiation is emitted into space. Although, such events usually take place during the middle age of the cluster, Chandra showed that globular clusters previously thought to be very old have very high numbers of X-ray sources inside them, suggesting that stellar interactions are still occurring.
Fregeau found three such globular star clusters in our galaxy alone, out of a population of thirteen studied objects, all of which had been previously catalogued as old. What is even more baffling is that the other ten star clusters are in their adolescence, although they had previously been classified as middle age objects.
"It's remarkable that these objects, which are thought to be some of the oldest in the universe, may really be very immature. This would represent a major change in thinking about the current evolutionary status of globular clusters", writes Fregeau.
If the theory is indeed correct, then high stellar concentrations in the central regions of globular clusters may suggest a middle age, rather than an old age. Alternatively, some researchers have even suggested that the universe would end its life long before globular star clusters even reach old age. This study does not only provide us with a better understanding of the stellar interactions taking place inside globular star clusters, but it also diminishes the need for black holes in order to keep them from reaching old age.
"Some exotic scenarios, including some of my own, have been invoked to try to make sense of the observations and save the old theory. If this result holds up, we won't have to worry about the exotic scenarios any more", says Fregeau.
Oldest Objects in the Universe, Not So Old After All
Globular star clusters seem younger than previously thought
Comparison between the centers of two globular star clusters inside the Milky Way, NGC 6397 and NGC 6121. X-ray brightness shows the evolutionary stage of the two clusters. NGC 6397 presents a surplus of X-ray sources.