Globular clusters are spherical clusters of stars that typically contain a few hundreds of thousands of stars, or even as many as a million. They are not galaxies, but are associated with galaxies in that they orbit around the galactic plane in a spherical halo.
About 150 globular clusters have been detected around our own galaxy, at distances of 60,000 light years (or more) from the galactic plane. A few can be detected with the naked eye, looking like faint, fuzzy stars. One of the best-known has been given the designation M13 and is visible on summer evenings in the constellation of Hercules. Seen through binoculars it has the appearance of a mothball, and it is only when seen through a large telescope that its true nature is revealed. The best images have come from the Hubble Space Telescope which has shown it to be a whirling mass of tightly packed stars.
However, although the density of stars towards the centre of a globular cluster is up to 100 times greater than in our own region of space, that does not mean that the stars are constantly crashing into each other. If you fired a gun at a point near a cluster’s centre, the chance of hitting a star would be less than 10 to the power of 11 (i.e. 1 with 11 zeroes following, which is 100 billion) to one.
Seen another way, if you lived on a planet that circled a star at the heart of a globular cluster, the other stars would appear as points of light just as neighbouring stars do to us on Planet Earth. They would be light months away as opposed to light years, and there would be many more of them, but they would be points of light nonetheless. The night sky would always have a glow to it, similar to that of faint moonlight on Earth, because of the thousands of stars in the sky that would make it extremely difficult to see any objects that did not belong to the cluster. A Hubble telescope in orbit round our fictional planet would have a much harder task in aiding our understanding of the Universe than the real one does!
Observations of stars in globular clusters show that they are very old, and that the clusters would have formed between 12 and 20 billion years ago, which predates the age of the Milky Way (our own galaxy). Their orbits tend to be highly elliptical and not in the same plane as the galaxy. Such an orbit is therefore similar to that of a comet within our own Solar System, and might take as much as 100 million years to complete.
Star ages are determined by calculating their mass, luminosity and temperature, which in turn shows what stage they have reached in their life cycle. Depending on their original mass, stars go through different evolutionary sequences, but a typical process for a star of similar mass to our Sun is for it to turn into a red giant when the hydrogen at its core has been exhausted. Before this point, a Sun-like star is said to be in the “main sequence” of stars which are stable and have plenty of fuel left to continue the thermonuclear reactions that supply their energy output.
At a later stage in a red giant, gravitational collapse may provide the conditions for the helium at the core to ignite, which is termed the “helium flash” stage. A star that has reached this stage will be both smaller and hotter at the surface than a red giant.
Globular clusters are known to be old because they contain no relatively high-mass main sequence stars, these having evolved into red giants. The main sequence has become shorter because many stars have reached the “turnoff point”. There will also be post-helium flash stars, many of which will have started with relatively low mass, and some “blue stragglers” which are stars that are hotter than main sequence stars but have not turned into red giants, possibly because they are gaining hydrogen fuel from a neighbouring star.
Another characteristic that typifies members of globular clusters is that they tend to be “metal poor”. This means that their spectra do not reveal the presence of heavy elements which are found within stars such as our Sun. This is believed to be because such elements were created by the supernova explosions of massive ancient stars, the matter from which then condensed to form later star generations. The stars within globular clusters were formed from material that was available when there was no supernova detritus around.
Globular clusters have, in the past, been very difficult to study because of the problem of distinguishing individual stars. However, tools such as Hubble have made this much easier, with the result that astronomers and astrophysicists are now able to learn much more about how the Universe evolved.
Abell, G. (et al) Exploration of the Universe. 5th ed. Saunders, 1987
Kaufmann, W. Universe. 2nd ed. Freeman, 1987