Open and Globular Clusters

Open and Globular Clusters – a comparison

Graeme Ing, HET603A, Swinburne Astronomy Online

Introduction

A star cluster is defined as a grouping of stars in a relatively small volume of space, bound together by mutual gravitational attraction. There are two broad categories of clusters: open and globular. This paper intends to contrast the two types and provide a comparison of their age, formation, structure and distribution throughout our galaxy.

The Pleiades Omega Centauri

It is visually apparent that the Pleiades cluster can be resolved into a very loose collection of individual stars, and because of this looseness, the Pleiades are a typical open cluster. Omega Centauri appears as a much tighter bound cluster, whose center resembles a dense spherical mass of stars. Because of their shape, we label such clusters as globulars.

Consider the major differences that this paper intends to investigate:

	Pleiades	Typical Open Cluster	Omega Centauri	Typical Globular cluster
Number of Stars	600+	10 to 10,000	1,000,000+	100,000 to 10⁶
Cluster diameter	5 parsecs	1 to 10 parsecs	46 parsecs	20 to 100 parsecs
Age	100 million years	< 1 billion years	12 billion years	10 to 20 billion years
Star separation	1 parsec	< 1 parsec	Down to 0.03 pc	< 0.3 parsecs
Total stellar mass	< 1,000 solar mass	10 to 10,000	5,000,000 solar mass	1 -5 million
Distribution		Plane of Milky Way		All directions

Sources: Evans, J. C.; Schombert; Evans, J.; WebNOAO; Janes

Cluster Distribution

At the turn of the 20^th century, it became clear to astronomers that the distribution of open clusters throughout the galaxy was very different to that of globular clusters. Globular clusters had been discovered in all directions of the sky, not only in the dense plane of the galaxy (the galactic disk) but also high above or below where few other objects were found; a spherical region labeled the galactic halo. In contrast, open clusters had been observed almost exclusively in the main disk of the galaxy; a fact that led to open clusters often being referred to as galactic clusters. (Belkora)

After considerable observation, Shapley determined that 50% of all globulars lay in the direction of Sagittarius, whilst less than 10% lay in the opposite direction. (Webseds2) Bohlin hypothesized that this asymmetric distribution (i.e. not centred on our Sun), implied that the centre of our galaxy was in the direction of Sagittarius. Shapley later provided evidence for Bohlin’s hypothesis and placed the Sun 20% of the galactic diameter from the centre, or 60,000 light years. (Belkora)

Whilst no correlation is apparent between cluster size and location, we find the youngest globulars in the galactic centre and disk, whilst older clusters are dispersed throughout the halo. (Fusi-Pecci & Clementini) These distant globulars orbit the galaxy at distances up to 100,000 parsecs in highly eccentric orbits of 100 million years or more that often penetrate the galactic disc.

Cluster Age

The Pleiades is considered a young cluster, perhaps 100 million years old, whereas Omega Centauri could be as old as 12 billion years. In general, we believe globular clusters to be some of the oldest objects in the universe, dated at 12 to 20 billion years old. (WebSeds1) In comparison, open clusters rarely reach a billion years, with a couple of hundred million being typical. How do we determine the age of a cluster?

One important property of all clusters is that its member stars form at approximately the same time, from the same molecular cloud of dust and gas. As a rule, the stars in a cluster are all of the same age; however, their mass will differ according to local densities within their host molecular cloud. Dense clouds allow high mass stars to form, which collapse quickly under gravity and begin fusion, in the form of hydrogen burning, much sooner than low mass stars. With fusion underway, these high mass stars are first to evolve on to the main sequence. (Figure 1) In as short as 30 million years, these stars exhaust their hydrogen supply and move off the main sequence to become giants. The point at which they depart the main sequence we label as the turnoff point. In contrast, lower mass stars may remain on the main sequence for several billion years (like our Sun). (Freedman & Kaufmann) As the cluster ages, more stars reach the turnoff point and become giant stars.

Figure 1 shows the Hertzsprung-Russell, or H-R diagram, of a typical mid-age cluster.

Figure 1 - Hertzsprung-Russell Diagram of a typical cluster

By analyzing individual stars at their turnoff point, we can determine their mass and hence how long they have been on the main sequence. Since we know that member stars of a cluster formed at the same time, this main-sequence lifetime marks the age of the cluster. In general, the fewer stars that remain on the main sequence, the older the cluster.

The binding of a cluster influences its age too. In an open cluster, the gravitational binding is weak due to its looseness. It is commonplace for member stars to escape the cluster, either by their random motion within the cluster, or after being disturbed by the shockwave from a nearby object such as a supernova. Whilst individual member stars may live for billions of years, a typical open cluster has dispersed within a few hundred million years. The Pleiades is likely to be unrecognizable as a cluster within 250 million years. (WebSeds3) In a globular cluster, the mutual gravitational attraction created by hundreds of thousands of member stars requires significantly greater effort to break apart, and the globular can remain bound for billions of years.

Cluster Structure

Spectroscopy has revealed that stars in open clusters have a much higher concentration of heavy elements than globular clusters. These younger stars are known as Population I, whereas the older, Population II stars in globular clusters are predominantly light elements such as Hydrogen and Helium. (Janes) Globular clusters that lie within the galactic disk have a higher concentration of heavier elements than those in the halo. Approximately 20% of all globulars are richer in heavy elements in this way. (WebSeds2; WebSTSI)

The largest globulars are home to a million or more stars, tightly packed under mutual gravitational attraction into a dense spherical ball. In comparison, open clusters are tiny - the most massive open cluster is barely the size of the least massive globular. (Janes) A typical open cluster may measure 5 parsecs in diameter, yet the larger globulars can span 50 parsecs or more. The centre of a globular cluster is so dense that there can be several thousand stars in a single cubic light year (l.y.). In the core of Omega Centauri, stars are as close as 0.1 l.y., compared to the 3 or 4 l.y. separations within the Pleiades. (Fusi-Pecci et al) Very little gas and dust remains in a globular, whereas open clusters are frequently found near to, or within emission nebulae.

Typically, open clusters like the Pleiades contain hot and luminous blue stars. Many smaller clusters are labeled OB associations due to this dominance of O and B type stars. These massive stars age quickly and few OB’s live to a billion years; instead they eventually self destruct, creating shock waves powerful enough to disrupt the loosely bound open cluster and disperse its stars. This helps explain the lack of longevity of open clusters. (WebAS) By contrast, globulars are home to less massive yellow and orange stars that age slowly and are capable of remaining stable for 10 billion years or more.

Cluster Formation

With the knowledge of how open and globular clusters differ in age, composition and distribution, we can hypothesize regarding the formation of both types.

The formation of open clusters is easier to understand. Considering the age of the universe and that opens have a maximum lifetime of around a billion years, it is clear that the process of their formation is ongoing and frequent. This explains why we can plot considerably more open clusters (1200) than globulars (147), and tens of thousands more open clusters may lie undetected within the denser regions of the galaxy. (Janes) Their structure mirrors the typical stellar nursery – a series of protostars condensing under gravity out of a molecular cloud. Most such stellar nurseries will form stars whose random motions carry them to their individual destinies, but sometimes the stars will become bound under mutual gravitational attraction and form an open cluster. As long as it remains undisturbed from violent events such as shock waves and supernovae, the cluster integrity will remain. This state is referred to as dynamically relaxed. (Evans, J.) This model of formation explains why open clusters are found in the galactic disk where extensive gas and dust deposits condense into molecular clouds. This material has been nucleosynthesized in stellar cores multiple times over billions of years, so that although the bulk of a molecular cloud is hydrogen, there are many heavier elements too. Because of this, open clusters are born from Population I stars.

Globular clusters are considerably larger than opens, and for them to follow the same formation model would require enormous and dense molecular clouds. The Hubble Space Telescope has observed globulars at a variety of ages in the Smaller Magellanic Cloud, demonstrating that globulars are still being formed within huge molecular clouds. (WebHubble) Such huge clouds are extremely rare, which might explain the small number of globulars. Younger globulars are concentrated near the bulge of our galaxy, where material is denser and more suitable to the birth of such huge clusters. (WebUniverseReview) In the first few billion years of the universe, when we believe globulars were formed, larger, denser clouds of material would have been more predominant, allowing large globulars to form. Most of this early matter would be devoid of the heavier elements that are the byproduct of stellar fusion, explaining why globulars are comprised of Population II stars. (WebSeds2)

An alternative formation model proposes that globulars are the core, or bulge, from dwarf galaxies. After colliding with our galaxy, the outer layers of stars of the dwarf galaxy could have been stripped away. The remaining galactic core would be a dense spherical cluster of stars, remarkably similar to the appearance of a globular cluster. (Janes) Such a galactic collision might place the globular cluster into the erratic halo orbits that we observe. Over time, even those globulars formed within the galactic disk could have been ejected out into the halo by angular momentum due to their high density. Each pass of a halo globular through the galactic disk is likely to strip stars from the globular and disturb its gravitational equilibrium. Due to this attrition, most of the globulars we see today could be broken apart within the next ten billion years. (Fusi-Pecci et al)

Conclusion

Although open clusters differ greatly from their larger globular cousins, through analysis of their age, composition and distribution we can theorize with high certainty about their origins and evolution.

Star clusters are useful laboratories for astronomers. Within a cluster, the stars are similar in age, internal composition and location, and differ only in mass. A star cluster allows us to study stellar evolution by considering a single variable, leading to a highly accurate analysis. (Evans, J.) Historically, the analysis of cluster distribution allowed astronomers to evolve their theories of “island universes” and begin to understand the galaxy’s shape and size. (Belkora) In the last 50 years, studying the age and distribution of globular clusters inside and outside our galaxy has allowed us to develop models of galactic evolution. Clusters have played an important role in our understanding of the universe within the last hundred years.

References

Fig 1: Hertzsprung-Russell Diagram of a typical cluster

Schombert, J. Dept. of Physics, University of Oregon.

http://zebu.uoregon.edu/~js/ast122/lectures/lec15.html

Belkora, L. 2003, Minding the Heavens

Evans, J. C. 1995, Physics & Astronomy Department, George Mason University

http://www.physics.gmu.edu/classinfo/astr103/CourseNotes/ECText/ch14_txt.htm#14.4.

Evans, J. 2004, Valparaiso University Department of Physics and Astronomy

http://physics.valpo.edu/courses/a252/reports/Star_Clusters.pdf

Freedman, R. & Kaufmann, W. 2001, Universe 6^th edition, pp. 486-489

Fusi-Pecci, F. & Clementini, G. 2000, Globular Clusters, Encyclopedia of Astronomy & Astrophysics, Institute of Physics

Janes, K. 2000, Star Clusters, Encyclopedia of Astronomy & Astrophysics, Institute of Physics