SYDNEY: A study of X-rays beamed from globular clusters of stars – among the oldest objects in the universe – shows that they are at a surprisingly early phase in their cosmic evolution, say researchers.

The study, by astrophysicist John Fregeau of Northwestern University in Chicago, US, was published in the Astrophysical Journal Letters.

Globular clusters, dense groups of thousands to millions of stars spread throughout the halo of matter around galaxies, range in age from nine to 13 billion years old. Since these clusters contain some of the oldest known stars, understanding how they formed may help explain how galaxies first evolved.

From adolescence to old age

The clusters evolve in three phases, adolescence, middle and old age. Their adolescent phase is characterised by the onset of a collapsing core. During middle age, binary (paired) star systems form within the centre of the cluster, and their interactions prevent further core collapse. These binaries can be detected by the X-rays given off as the paired stars exchange mass. In old age, some of the binaries are ejected and the core collapses inwards again.

Because of the high density of their cores, the conventional wisdom is that most clusters are at the middle or final stages of evolution.

However, the new study, made using NASA's Chandra X-ray Observatory, indicates that ten of the 13 galaxies observed are in fact still 'adolescents' at their earliest phase of evolution, while the remaining three are middle-aged.

"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. This would represent a major change in thinking about the current evolutionary status of globular clusters," said Fregeau.

His observations reveal that three of the clusters studied were rich in X-rays, particularly in their centre, indicating they were at the middle-stage of evolution. The number of X-ray sources in the centre of a cluster depends both on the formation of double stars and the encounter rate of double and single stars in the crowded core.

The remaining galaxies had fewer X-ray sources but higher encounter rates within their centres, suggesting these clusters are still in the process of core contraction (and therefore still 'adolescents'), the paper states.

"Since most GCs are still in the early phases of their evolution, their current properties are much more reflective of their properties at the time of formation. This means that the clusters we observe today actually tell us more about their formation than we initially thought," Fregeau said.

The finding also removes the need for various "exotic" theories, such as the presence of a black hole, to drive the clusters' evolution. Other theoretical research indicated that to reach the final phase of evolution some clusters would have to be older than the age of the Universe.

"If this [new] result holds up, we don't have to worry about the exotic scenarios any more," said Fregeau.

Intriguing result

Astrophysicist John Dickey, of the University of Tasmania, Australia, who studies the evolution of galaxies and clusters, said it's an interesting result.

For 30 to 40 years, astronomers have talked about globular clusters having a very condensed core, and therefore being in their final phase of evolution, he said.

"In space nothing pushes back against gravity. If there are a lot of stars zooming around in a globular cluster, most will lose their energy through collisions and drop down into the centre [of the cluster]. Binaries can suppress the condensation, but eventually, collisions will throw some binaries out and compress others towards the centre," said Dickey.

"This paper shows globular clusters have not progressed as far down this path as we would expect," he added.