top of page
Globular Cluster M13 by Pete Carson

Taken on 8-9th April 2005 from his back garden in Leigh on Sea, Essex, UK
Televue TVNP101 100mm APO refractor 540mm fl.
Sbig ST4 autoguider. Starlight Xpress HX916 CCD camera. IDAS LPR filter with True Tech RGB colour filters. RGB image, R15mins 1x1, G15mins 1x1, B15mins 1x1 all 3min sub exposures.

You can see more of Pete Carson's photographs on his website by going to our Links page.

Visually, globular clusters must be amongst the most amazing objects in the night sky, especially when viewed through large aperture telescopes such as the 16 inch to 20 inch scopes that are becoming increasingly common in amateur astronomy.

They are quite unlike the other kind of star clusters, namely open clusters, from which they differ in almost every way. Open clusters, often also known as galactic clusters, are generally speaking no more than a few tens of light years across. In contrast, globular clusters are typically 200 to 300 light years across. Open clusters may contain anything from a few dozen stars to perhaps a thousand or so, but globular clusters usually contain some hundreds of thousands of stars, going up to a million or more in some cases.

Their locations relative to their host galaxies are also markedly different. Open clusters are a common feature of large spiral galaxies where they are to be found mainly within the spiral arms. They are also found in some irregular galaxies such as the Magellanic Clouds, but in any event they are located pretty much within the boundaries of their host galaxy. On the other hand, globular clusters are found around their hosts, which can be either spirals or ellipticals, on orbits that can have radii far greater than the radii of the of the host galaxies they orbit. In the case of spiral galaxies, open clusters orbit the centres of such galaxies in or close to the galactic plain, but globular clusters are found on orbits with random orientations. A consequence of this is that in the case of a globular cluster on an orbit with a radius that is smaller than the radius of its host, the cluster must pass through the galactic disc twice on each orbit of the galactic centre.

Whereas open cluster stars form from the galactic pool of material within their galaxy, globular clusters formed from a much earlier universal pool of material and so have an existence all of their own that is somewhat independent of their host galaxy. The material from which open cluster stars are made is material that has been progressively more and more enriched with “metals” over the life of the galaxy in which they form. (Beginners should note that as far as astronomers are concerned, all the elements except hydrogen and helium are metals, with outrageous disregard for their properties or places in the Periodic Table). Globular cluster stars, however, are formed from material dating from very early times in the life of the universe, when very little enrichment had taken place. So it is that globular cluster stars are said to have a very low metalicity.

A consequence of their different locations is that the stars within open clusters only interact with one another very weakly gravitationally. Notwithstanding the fact that many stars in open clusters will be gravitationally bound to one another as double, triple or even quadruple star systems, such systems and all the solitary stars within an open cluster are not bound together strongly enough to stop the cluster as a whole from dispersing within a few hundred million years. In contrast, the stars within globular clusters are sufficiently strongly bound by the cluster’s significant gravitational attraction that globular clusters hang together more or less indefinitely.

bottom of page