Glorious GLOBULARS
From fuzzy blobs to star cities, Paul Money takes us on a tour of 15 of the best globular clusters to observe in this season’s night skies
Fuzzy blobs – that’s often what new stargazers viewing a globular cluster think when they spot their first one. It’s true that these concentrations of ancient stars, ranging in number from tens of thousands to millions, can be a bit lacking in their visual impact with smaller instruments, not quite resembling the stunning images you’ll regularly see online or in our Gallery pages. Yet, with care, a little more aperture, and yes, by capturing the objects with an astrophoto, a globular cluster’s own ‘personality’ – the detail that makes them interesting and enjoyable to view – shines through.
All about globulars
You could argue that the first globular cluster seen was Omega Centauri, listed by Johann Bayer in his Uranometria in 1603. But that very designation belies the fact it was visible to the naked eye and considered a star. The first globular defined as a ‘nebula’ was accidentally discovered by the German amateur astronomer Abraham Ihle in 1665, one we now know as M22 in Sagittarius. More were quickly discovered, but their nature was still not properly understood. They were still described as nebulous patches or round spots of cloud until 1764, when Charles Messier resolved stars in M4.
William Herschel coined the term ‘globular cluster’ in 1786, when he found he could resolve the objects into clouds of stars. Herschel’s revelation may stem not just from his visual acuity but from the growing apertures of the instruments at his disposal, mainly reflectors he produced himself.
Globulars form a halo around the Milky Way, centred on one side of the sky, close to the Sagittarius–Scorpius border. Observing them, Harlow
Shapley used a type of variable star known as RR Lyrae stars to estimate their distances in 1914. Then, noting the clusters’ distribution in 1918, he established a startling and vastly larger size for the Milky Way than was commonly accepted, along with the conclusion that the galactic centre lay in Sagittarius.
Today, globular clusters have been observed around all types of galaxies. Their origin is still up for debate, though one theory is that they formed in areas where the interstellar medium was particularly concentrated at an early epoch of the Universe. Sometimes even older than the galaxy they surround, their stars are typically ‘metal poor’, or lacking elements heavier than hydrogen and helium. To date, it is thought that globulars are not environments suitable for planetary formation, due to the lack of these elemental building blocks and the density of stars within them.
Despite their reputation as mere fuzzy blobs, globular clusters can range from tight, condensed cores to more loosely confined spheres of stars. They can be slightly flattened, have outer halos of fainter stars, curved trails of stars, coloured stars and dark features, which can be seen visually and photographically.
Globular clusters are enchanting objects if examined carefully with larger apertures. So come with us as we explore 15 of the most rewarding of these ‘star cities’ to observe in early autumn skies.