Globular Clusters - Galactic Fossils

Tuesday 20th April, 2021

Our speaker (on Zoom) was Callum Potter who is the Director of the British Astronomical Society's Deep Sky Section who spoke to us about "Globular Clusters — galactic fossils", also known as "globulars". These are dense collections of stars that due to their mutual gravitational attraction form somewhat spherical shapes. The density of stars increases towards their centre and they are present in nearly all galaxies. In our own galaxy, the Milky Way, they are mostly found in the furthest regions, forming a halo around the galactic centre.

Callum then talked about the discovery history of these objects, which began in 1665 when the German amateur astronomer, Johann Abraham Ihle, discovered the first known globular cluster, M22, on 26th August 1665 while he was observing Saturn in Sagittarius. M22 is slightly more elliptical in shape compared with other globulars. It is one of the brightest globulars in our galaxy at magnitude +5.1 as it is one of the nearest ones, being a "mere" 10,000 light years from us. Interestingly, it is only one of four globulars that is known to contain a planetary nebula — an expanding shell of gas, usually thrown off by a red giant star as it ages. M22 also contains two black holes that were detected by the Very Large Array telescopes from New Mexico.

Twelve years later, in 1677, the English astronomer Edmond Halley, realised that what was thought to be a star, omega Centauri, was in fact a globular cluster. It is the largest globular in the Milky Way, containing roughly 10 million stars, and is thought to actually be the core remnant of a dwarf galaxy that, through interactions with the Milky Way, has lost some of its outermost stars.

Then, in 1702, M5 was found by the German astronomers, Gottfried and Maria Kirch when they were observing comet C/1702 H1. This was followed by: the discovery of M13 (named the "Great Globular Cluster") in 1714, M71 in 1745, and then three more in 1746 — M4, M15 and M2. Over the years many more globulars have been found and there are thought to be around 150 in our galaxy but there are probably many more still yet to be discovered. In fact, the European Space Agency's Gaia satellite is currently charting a three-dimensional map of our galaxy and has already discovered another eight globulars.

Despite being studied for some time, the origins of these ancient star clusters are still not understood. It was thought that they consisted of old stars with similar chemical composition, all about 12 billion years old, but modern observations show that they contain groups of stars with a variety of ages, usually falling into two distinct age groups. Also, due to the stars' close proximity, they often have close interactions or collisions leading to the formation of some exotic classes of stars such as rare blue stragglers, millisecond pulsars and low-mass X-ray binaries.

Blue stragglers stand out as they are much bluer and hotter than their older yellow neighbours. It is proposed that these stars have stolen material from a nearby companion star or even collided, and the resultant merger heats them up and so they look blue. Millisecond pulsars are also thought to have stolen mass from a companion and, as this matter falls on the pulsar it speeds up, ending up rotating in less that 10 milliseconds. X-ray binary stars are stars that emit copious amounts of X-rays due to, again, material falling on to them, but they are very dim in visible light.

Callum ended his talk by suggesting the best way to visually observe globular clusters using a telescope. He did not recommend using a narrowband filter with an eyepiece, which is sometimes used to enhance the visibility of deep sky objects. Instead he stressed the importance of choosing a dark site away from light pollution as well as giving your eyes time to adapt to the darkness. This can often take over 20 minutes once you are at a dark sky location. He also said that choosing an eyepiece to give a high magnification would help.