The stars in globular clusters spend much of their time describing mutually crossing orbits. When two stars approach each other closely enough their mutual gravitational pull perturbs their orbits causing them either eventually to escape from the cluster or to plunge inwards to the centre. There are between ten thousand and a million stars in a globular cluster; these tend to agglomerate towards the centre, where the stellar density can reach thousands of stars per cubic parsec. Consequently, collisions between stars are fairly common and give rise to binary systems. These stellar systems comprise an old, highly evolved star with a young companion revolving around it at a relatively short distance, both moving as a single body inside the cluster, in the same way that the Earth and Moon form a system in orbit around the Sun. It is these binary systems that are responsible for the X-ray emission observed in globular clusters which, as Professor Ramón Canal of the University of Barcelona explains, are more abundant in globulars than in the rest of the Galaxy, a possible indication that the process of their formation is different inside globular clusters.

Why are certain X-ray sources more abundant in globular clusters? Could the primordial material from which the earliest globular clusters formed have been chemically enriched by nucleosynthesis in a previous generation of stars?

"X-ray binary sources associated with low-mass stars are more abundant in globular clusters than in the rest of the Galaxy. This seems to indicate that the process of their formation has been different there: the high star density within globular clusters would favour the capture by isolated neutron stars (arising from the gravitational collapse and subsequent explosion of massive stars) of low-mass main-sequence stars into close-proximity orbits; this in turn would make possible the accretion of matter from the outer layers of the companion on to the surface of the neutron star. Very short period (thousandths of a second) pulsars are also very common in globular clusters.This would be consistent with the notion that they are descended from a proportion of these low-mass X-ray binary sources: the accretion of matter from the companion, often already ‘evaporated’, is what would have accelerated the rotation of the pulsar.

"It is becoming more and more evident that the formation of stars preceded that of the galaxies. For their part, globular clusters could be the ‘fossils’ of the first structures to host star formation. There is also evidence of a wide range of ages and metallicities for the globular clusters of our Galaxy. It is possible, therefore, that even the most primitive clusters (whose metallicities, although very low, are nevertheless not zero) had already been enriched through the nucleosynthesis of an even earlier generation of stars."

"The globular clusters of our Galaxy are perhaps the most accessible window on to the history of the early Universe, despite the ambiguities that remain with respect to their role in galaxy formation. Their ages set the highest known lower limit to the age of the Universe, although it is expected that for a while yet there will still be an uncertainty of the order of a couple of thousand million years that effectively increase the ages of the globular clusters."

"Age estimates of the oldest globular clusters are being revised downwards, but there is conflict only when we assum a critical density Universe whose rate of expansion is gradually decreasing and tending asymptotically to zero. If, as observations of distant thermonuclear supernovae seem to suggest, the expansion is accelerating rather than braking (with a vacuum energy contribution equivalent to a non-zero cosmological constant), the age of the Universe could well be some 14500 million years and there would be no conflict."

"On the one hand, the possibility of forming pair of low-mass neutron stars from the gravitational collapse of white dwarfs (something that Evry Schatzman and I suggested almost 25 years ago) continues to intrigue me. On the other, we are re-evaluating, on the basis of stellar population synthesis, the various routes to arrive at the forming binary systems with neutron stars, both in globular clusters and in other regions of our Galaxy."