Look up to the night sky on a clear night and you may notice a “star” that looks smudged or fuzzy around the edges. Pick up a pair of good binoculars or look through an astronomical telescope and that single star suddenly becomes a bright core of thousands or even millions of stars, surrounded by more stars that are less densley packed together around its edges. What you are looking at is a globular cluster.
Due to the extreme gravitational density at the centre of the globular cluster a sphere of stars is created within its influence. Globular is an adaptation of the Latin word – globulus, which translates into English as “small sphere”. The reason for the bright core is due to the extremely high concentration of stars packed together at the centre. As the gravitational field weakens, so does the number, or density, of the stars that are encompassed within the sphere. It is this less densely packed area of stars that give the effect of the “star” being smudged to the naked eye.
Globular clusters are actually fairly common. In our galaxy alone, the Milky Way, there are around 180 globular clusters, the actual number remains fluid as more are discovered and there are predictions that there could be up to 50 more still to be found. Every galaxy that has been observed and investigated by astronomers has so far been found to contain globular clusters. It is unknown what has caused the formation of these marvellous structures, but it is known that they contain some of the oldest stars that we know of.
Abraham Ihle in 1665 discovered the first globular cluster, M22, but it was not until 1764 with improvements in telescope design that Charles Messier identified individual stars within the M4 cluster.
It has been found that the make up of globular clusters is in the order of hundreds of thousands, to millions of stars that are all roughly of the same stellar age showing they were all created around the same time. The age of the stars within these clusters can be assessed by their composition of low-metal compositions, that is they are low in carbon, nitrogen, neon, oxygen and the elements other than hydrogen and helium. Our sun, which is a young star in stellar age, shows much higher levels of metallicty within its structure than those within globular clusters.