star cluster

Group of related stars, usually held together by gravity. Members of a star cluster are thought to form together from one large cloud of gas in space. Open clusters such as the Pleiades contain from a dozen to many hundreds of young stars, loosely scattered over several light years. Globular clusters are larger and much more densely packed, containing perhaps 10,000–1,000,000 stars.

The more conspicuous clusters were originally catalogued with the nebulae, and are usually known by their Messier or New General Catalogue (NGC) numbers. A few clusters like the Pleiades, Hyades, and Praesepe are also known by their traditional names.

In the dense clusters, gravitation may hold the cluster stars together almost indefinitely, while in the less dense clusters the stars quickly dissipate.

Globular clusters About 150 globular clusters in our Galaxy are known, all similar in appearance. They are nearly spherically symmetrical, and contain an immense number of stars, which are concentrated towards the centre where they are so close together that it is impossible to see the individual stars even in the biggest telescopes. The brightest stars are red, and there are usually variable stars from which the distances of the clusters can be estimated. These distances range from 10,000 to 200,000 light years, and it is possible that some of the more distant ones are not members of our Galaxy. The diameters of the clusters range from 60 to 300 light years, with 10,000–1,000,000 stars in a cluster, and an absolute magnitude in the range 5–9. They are very old objects, possibly 10,000 million years old.

Open clusters The open clusters, of which the Pleiades, Hyades, and Praesepe are typical, are much smaller than the globulars; they contain fewer stars, which are more widely scattered, and show little concentration towards the centre. They are relatively young objects and are found in the disc of the Galaxy, along the Milky Way. About 1,000 of them have been identified, but there must be many more hidden by dust clouds or indistinguishable against the stellar background. The number of stars in an open cluster varies from a dozen to over a thousand, but on average they contain a few hundred stars scattered over an area less than 30 light years in diameter. The brightest stars are blue or red depending on the cluster's age. Some of the youngest contain bright blue supergiant stars.

Associations Associations can be regarded as very extended open clusters with few members. They must have been recently formed or they would already have dispersed. About 100 associations have been recognized in the spiral arms of the Galaxy, but there must be many more hidden from view. A typical association contains up to 50 stars scattered irregularly over an area from 100 to 600 light years in diameter. One of the best know associations is that centred on a group of stars in the middle of the Orion nebula.

As all the members of a star cluster are about the same distance away, the observed relation between their colours and apparent magnitudes is essentially that between their colours and absolute magnitudes, which can be compared directly with the Hertzsprung-Russell diagram to measure the cluster's distance. Differences in the shape of this relationship from cluster to cluster provide evidence for the way stars evolve, and also a method of determining a cluster's age.