Star Clusters

Abstract

A brief overview of the different types of stellar aggregation or clustering is followed by discussion of how knowledge of these clusters can be applied to interpret more general stellar evolution, and the lifespans of stars.

Keywords: stellar aggregation, stellar grouping, stellar cluster, stellar association, stellar evolution

Introduction

Stars seem to be naturally gregarious and often found moving through space together in small groups or larger clusters, probably because they were all formed out of the same initial gas cloud. There are two major types of cluster, globular clusters and open clusters, the latter also known as galactic clusters. There are looser aggregations known as associations, also.

The study of star clusters is very useful for many purposes but especially for obtaining observational information about the evolution of stars. It is impossible to follow the evolution of individual stars observationally – the time scales are far too long. However, because the size of a cluster is typically small compared to its distance from us, all the stars in a given cluster are also effectively at the same distance. This last fact is crucial, for it means that the relative brightnesses of the stars are well-known, even if the distance itself is poorly determined and the absolute luminosities are not known. We can therefore find the shape of the Hertzsprung-Russell Diagram for a cluster, without having to know the zero point of the magnitude scale.

Types of Stellar Aggregation

(The following text mostly after Smith 1995, pp. 211 to 217.)

Globular Clusters

These are centrally condensed and very regular in structure, most of them being perfectly spherical. An exception is the slightly flattened southern cluster Omega Centauri. They contain very many  stars, typically between 10⁵ and 10⁶, the brightest of which are red. A typical linear diameter is about 40 pc. About 150 are known in our Galaxy and the total galactic population is probably about 500. They are not confined to the galactic plane but form a roughly spherical distribution about the centre of the Galaxy. None shows any significant amounts off dust or gas. The chemical composition differs from that of the sun: globular cluster stars are metal-poor, i.e. the abundances of elements other than hydrogen and helium are much less than in the sun. However, the relative proportions of these heavier elements to one another are about the same as in the sun. The overall fraction of heavy elements is less than in the sun by a factor of between 10 and 1000.

Open (or Galactic) Clusters

Clusters of the second type used to be called ‘galactic clusters,’ because we see them inside the body of our galaxy, but now it is more common to refer to them as ‘open clusters’ because they are much looser and their stars more spread out on the sky than are those in globular clusters.  They are different to globulars in almost every possible way. They are loose structures containing anywhere from a few dozen to a few thousand stars, the brightest of which are mostly blue in colour. They are irregular in shape and there is a great range in size (1-20 pc) and in number of members. Some 700 to 800 are known in our Galaxy and the distribution is concentrated about the galactic plane. Many of them contain gas and dust. Some, such as the Praesepe cluster, are very sparse and difficult to detect against the normal stellar background. The chemical composition of the stars is similar to that of the sun.

Other Stellar Aggregations

Other groups of stars include associations, loose groupings of some 100 stars with a common space motion, and moving groups, stars whose only identifying feature is their common space motion. These extremely diffuse groups may contain a few associations and open clusters as tighter condensations within them, and may consist of stars which originally belonged to the tighter groupings but have gradually become detached under the tidal influence of the rest of the Galaxy.

Even open clusters can only survive disruption by tidal forces for some 2 x 10⁹ yr, about 10 to 20 percent of the age of the Galaxy, and the looser associations must be younger than about 10⁷ yr to have survived.

Only globular clusters, and a few of the more concentrated open clusters, are sufficiently tightly bound to have survived since the Galaxy was formed.

Applications

The study of star clusters is very useful for many purposes but especially for obtaining observational information about the evolution of stars. It is impossible to follow the evolution of individual stars observationally – the time scales are far too long, being thousands of years even through the fastest evolutionary stages, and more typically hundreds of millions of years. We simply observe stars with a range of properties, one of these being age. However, if we look at clusters we can disentangle the effects of age from the other variables, since all the stars in a particular cluster (of either type) were presumably formed together at the same time, out of the same material. They are therefore the same age and have the same chemical composition, varying from one another only in mass.

Because the size of a cluster is typically small compared to its distance from us, all the stars in a given cluster are also effectively at the same distance. This last fact is crucial, for it means that the relative brightnesses of the stars are well-known, even if the distance itself is poorly determined and the absolute luminosities are not known. We can therefore find the shape of the colour-magnitude diagram for a cluster, without having to know the zero point of the magnitude scale. The shapes of the colour-magnitude diagrams turn out to be very different for open and globular clusters.

Most of the stars in an open cluster lie in the narrow main sequence band, with only a few stars in the giant region to the upper right of the diagram. The most prominent stars are the blue stars at the upper end of the main sequence. The main sequence in a globular cluster is much less prominent and the main features are a well-populated giant branch and a horizontal branch which stretches out to the blue about half way up the joint brunch. The most prominent stars are the luminous cool supergiants at the tip of the red giant branch. Different globular clusters show very similar colour magnitude diagrams to one another, whereas the colour magnitude diagrams for galactic clusters show a considerable variation in the length of the main sequence and the proportion of giants.

References

Smith, Robert C. (1995): Observational Astrophysics.  Cambridge.

Van den Bergh, Sidney; Lafontaine, Andy 1984: Luminosity Function of the Integrated Magnitudes of Open Clusters. Astronomical Journal 89: 1822-1824.