Globular clusters: the ancient behemoths

by Dr Josef Borg

The M3 globular cluster in Canes Venatici. Photo: Josef Borg

This article appeared on the Times of Malta on May 23, 2021. It is being reproduced here with the consent of the author.

The beauty of a star-strewn sky greets the eyes of avid stargazers every night, anywhere on the globe where light pollution has not yet deprived us of this natural heritage. A look through a humble telescope reveals the even more numerous collection of stars pertaining to our home galaxy, as well as complex gaseous structures such as nebulae and whole other galaxies, outside our own Milky Way. Notwithstanding the massive distances between different stars, however, several stars do not spend their churning lifetime as solitary fusing bodies. On the contrary, several stars tend to group up – binary, trinary, quadruple and quintuple systems are well known. The famed Northern star, Polaris, is itself a quintuple system, actually composed of five gravitationally-bound stars.

Stars can however group up in significantly larger numbers. Such clusters of stars are formally known as star clusters within the astronomical community. These can be of two types – open star clusters and globular star clusters. The former type of star clusters tends to be the more well known, with two examples of such open star clusters visible to the naked eye. The Pleiades star cluster, also known as the Seven Sisters, and the Hyades star cluster, are both open star clusters easily visible from Malta in our winter skies. Open star clusters are typically short-lived structures, containing a smaller number of stars, with stars in such regions forming together from the same original nebula, eventually ousting the longer-lived smaller stars due to strong gravitational interactions until eventually only the larger, shorter-lived stars remain. For this reason, any open star clusters we see are necessarily ‘young’ – which in cosmic terms, means a few tens of millions of years – otherwise, all the massive stars in such clusters would have already died out, and the cluster would be no more.

Globular clusters, on the other hand, are significantly different. While open star clusters tend to have random star arrangements, globulars tend to have well- rounded, spherical structures with higher concentrations of stars towards their cores. Globular clusters are notoriously large – with some examples having upwards of a hundred thousand stars very tightly bound together and – once again – having originally most likely formed from the same nebula, even though multiple episodes of star formation have been proposed. The strong gravitational force keeping these structures together means that several smaller stars are not lost over time, as opposed to open star clusters. This means that these longer-lived stars outlive their larger counterparts in the cluster itself, happily fusing hydrogen for billions of years. As a result, globular clusters represent some of the oldest structures in our galaxy – at around 10 to 12 billion years old, these have been around since the early days of the Milky Way’s formation.
Famous examples of globular clusters include the Hercules globular cluster, some 25,000 light years away from earth and containing upwards of 300,000 stars, and the famous Omega Centauri globular cluster, some 17,000 light years from earth and a whopping 150 light years across, hosting some 10 million stars alone. The nature of globular clusters provides a unique opportunity for constraining estimates of the age of the universe as well as provide estimates for the presence of the galactic core.

Dr Josef Borg completed a PhD in Astronomy at the Institute of Space Sciences and Astronomy, University of Malta, and is currently a researcher at the Faculty of Health Sciences at the University of Malta.