Globular Clusters

Abstract

Globular clusters are characterised by high central stellar densities and tend to be extremely round. Some of their basic characteristics are described here, using primarily southern hemisphere examples.

Keywords: globular cluster, star formation, cluster evolution, luminosity function

Introduction

Globular clusters are gravitationally bound, spherical groupings of 10⁵ to 10⁶ stars, usually having a marked concentration toward the centre, which can measure up to several hundred light years in diameter. Like most astronomical objects, globular clusters exhibit a range of characteristics, but certain features are common to a majority of them.

Those known from the Milky Way occur in a halo 150,000 light years across, around the centre of the galaxy. These are believed to have formed early in the history of the Galaxy and contain some of the most ancient stars known; possibly 12 billion years or so old (cf. open clusters).

However, globular clusters are not confined to the Milky Way. It is believed that all bright galaxies, and many dwarfs, probably have globular cluster systems also.

Characteristics

Luminosity

Globular clusters demonstrate a Gaussian luminosity function, with a median around M_v = -7 to -8.

Fig. 1: Luminosity function for globular and open (galactic) clusters. (After van den Bergh and Lafontaine 1984.)

Composition

The mean metallicity increases with the luminosity of the parent galaxy.

Size Range and Distribution

For Galactic globulars it is found that the mean of the half light diameters (D_h) is related to Galactocentric distance (R) by the relation:

D_h ľ R^0.5

(1)

A similar increase of cluster diameter with distance from the centre of the parent galaxy has also been noted for NGC 5128 (Centaurus A) and the LMC (see van den Bergh 1993, p. 2). The Hubble type is different for each of these three galaxies (Sbc, S0p and Ir, respectively), suggesting that the property is independent of Hubble type. "Possible disruption of low-density globulars with large diameters near the centres of galaxies contributes to this trend. However, the absence of stable small-diameter clusters in the Galactic halo is, no doubt, a real effect" (ibid).

"One of the long-standing problems in modern astronomy is the curious division of Galactic globular clusters, the "Oosterhoff dichotomy," according to the properties of their RR Lyrae stars. Here, we find that most of the lowest metallicity ([Fe/H] < -2.0) clusters, which are essential to an understanding of this phenomenon, display a planar alignment in the outer halo. This alignment, combined with evidence from kinematics and stellar population, indicates a captured origin from a satellite galaxy. We show that, together with the horizontal-branch evolutionary effect, the factor producing the dichotomy could be a small time gap between the cluster-formation epochs in the Milky Way and the satellite. The results oppose the traditional view that the metal-poorest clusters represent the indigenous and oldest population of the Galaxy" (Yoon & Lee 2002, p. 532, Abstract).

Interpretation

Globular clusters in the LMC are seen to be systematically larger than those in the Galaxy. This, together with the dependence of cluster diameter on galactocentric distance, suggests that the most compact globulars formed in regions of highest density.

The densest regions of galaxies also have the shortest collapse timescales. Therefore, clusters with small diameters might, on average, be somewhat older than those with larger diameters.

Occurence/Examples

More than 100 globular clusters are known from our galaxy.

Without doubt the finest globular cluster to be seen from anywhere on earth is NGC 5139 (Omega Centauri) which is so prominent it was mistaken for a star on early star maps. It appears at magnitude 3.6 to the naked eye (about the brightness of Epsilon Crucis) and subtends about 20’ – roughly two thirds of a full moon. It lies at 5,000 pc (approximately 16,000 light years) distance. Under good conditions, even binoculars will begin to resolve the outer regions into stars.

Fig. 1: NGC 5139 – Click image for AAO photograph.

Second only to Omega Centauri is NGC 104 (47 Tucanae) which, like the former, was catalogued as a star on early maps. NGC 104 is estimated at 4,800 pc (about 15,500 light years) distance. Although readily visible in binoculars, an aperture of 100 mm or better is required to begin resolving stars.

NGC 6656 (M22) in Sagittarius is generally reckoned to be the most spectacular globular cluster visible from the northern hemisphere. M22 is elliptical and is one of the nearest globular clusters, lying at a distance of 3,100 pc (approximately 10,000 light years). It is barely visible to the naked eye but readily seen in binoculars. 75 mm apertures will begin to resolve stars and reveal the elliptical outline.

NGC 6205 (M13) in Hercules is another showpiece of the northern hemisphere sky, although it may be seen from New Zealand close to the northern horizon in July. It contains around 300,000 stars and is at least 100 light years in diameter. It lies at a distance of 7,200 pc (22,500 light years). M13 is visible to the naked eye and unmistakable in binoculars. Small telescopes (~75 mm) will begin to resolve individual stars.

References

Ashman, Keith M.; Zepf, Stephen E. 1998: Globular Cluster Systems. Cambridge University Press, 171 pp.

van den Bergh, Sidney 1993: Globular Clusters – A Provocative Introduction. In Djorgovski, S.G.; Meylan, G. (eds.) Structure and Dynamics of Globular Clusters. ASP Conference Series, vol. 50.

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

Yoon, Suk-Jin; Lee, Young-Wook 2002: An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way. Science 297: 532-533.

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