A guide to GLOBULAR CLUSTERS.


What are they?

Globular Clusters are symmetrical systems of up to a million stars formed about 13 to 15 billion years ago, and as such they are the oldest surviving stellar subsystems in galaxies. As of 1993, there were about 125 of these known in our own galaxy, most of them in a spherical halo surrounding the wheel-like shape formed by the spriral arms.


What do they look like?

A typical globular cluster is a nearly symmetrical system of stars, with the highest concentration of stars near its own center. In these central regions of the cluster, the stars are so closely packed that often it becomes difficult to distinguish individual points of light from each other.

This photograph, taken by the Digital Sky Survey, is of the M2 globular cluster, which is located in the halo of the M31 cluster, in the constellation of Aquarius. As can be seen clearly, the center of the cluster contains a higher density of stars than the outer layers of the cluster.


General information on Globular Clusters.

The average star density in a Globular Cluster is about 0.4 stars per cubic parsec. In the dense center of the cluster, the star density can increase from 100 to 1000 per cubic parsec. However, even in the center of clusters, there is still plently of space between the stars. "Solid centers" as shown in photographic images, are not due to the higher number of stars, but are because of finite resolution in the telescopes used to obtain the image, and distortion of light due to the Earth's atmosphere. The Hubble Space Telescope, which orbits above Earth's atomosphere, will therefore be able to take more accurate images of globular clusters, with less interference.

It is the largest stars in a cluster that are the first ones to use up the hydrogen in their core and evolve off the main sequence, to become Red Giants. As time goes on, the stars of successively lower mass leave the main cluster, making it seem to fade away like a dying flame. By now, the only stars still remaining in the main cluster have masses comparable to that of the Sun or less.

In some clusters, such as M67, however, the giant stars are brighter than the brightest main stars, and must have become brighter during their evolution.

To show that a lot of data can be obtained from just one cluster, here is another image of the M2 cluster taken by IRAS (Infra Red Astronomy Satellite). This image gives us information regarding the chemical makeup of a cluster, the red areas indicate carbon monoxide, and the smaller, yellow areas indicate hydrogen.

The average linear diameters of globular clusters range from 20 to 100 parsecs or more. In a relatively nearby globular cluster, more than 30,000 stars have been counted. In that particular cluster, there are almost certainly many more stars unobservable due to their weak light. The combined light from all these stars usually gives a typical cluster an absolute magnitude with a range of -5 to -10, or 104 to 106 times the luminosity of the sun.

One of the brightest known clusters in our Galaxy is w Centauri, and the brightest known cluster in the galaxy of Andromeda is the M2 cluster.


Using Colour-Magnitude graphs.

A way of analysing globular clusters is to use Colour-Magnitude diagrams. A colour-magnitude diagram is a plot of the apparent magnitudes of the stars in a cluster against their colour indices. Globular clusters nearly all have very similar colour-magnitude diagrams. The one below is a standard colour-magnitude graph, plotting the stars' colour index by their luminoscity.

This graph shows the appearance of the colour-magnitude diagram for a typical globular cluster of known distance, for which the apparent magnitudes have been converted to absolute magnitudes. The region from a to b is the main sequence. The main sequence would probably extend farther down than a if the cluster were near enough for us to observe its fainter stars. Above point b, however, the main sequence appears to terminate; with most globular clusters, this point occurs at about absolute magnitude (Mv = +3.5). From b to c there is a sequence of stars that are yellow and red giants. The brightest and reddest of them at Mv = -3 are brighter than typical red giants in the solar neighbourhood. A third sequence of stars extends from d to f; this is the horizontal branch of the H-R diagram for a globular cluster. The stars on the horizontal branch have already been through the red giant phase of evolution.

If a cluster had been recently formed, say within the past three million years, then it would produce a colour-magnitude diagram similar to the hypothetical one below, developed by R. Kippenhahan in Munich. This graph plots the stars' luminoscity against their surface temperature in Kelvin. It was later discovered that there are indeed real star clusters that fit this description.


Here are a number of facts regarding Globular Clusters.

Number known in Galaxy 125
Location in Galaxy Halo and nuclear bulge
Diameter(pc) 20-100
Mass(solar masses) 104-105
Number of stars 104-105
Colour of brightest stars Red
Integrated luminosity of cluster(Ls) 104-106
Density of stars(Ms/pc3) 0.5-1000
An example of one 47 Tucanae


Where they are located.

Globular clusters revolve about the nucleus of a galaxy on orbits of high eccentricity and high inclination to the galactic plane. About a third of globular clusters are concentrated around the galactic center. A typical cluster has a period of revolution around the order of 108 years. A cluster spends most of its time far from the center of a galaxy, and so most of them can, and have been discovered in the spaces between galaxies.

This image of the M2 cluster, clearly shows the cluster (mainly red), moving around the Galaxy (mainly yellow) in a very large orbit around the nuclear bulge of the Galaxy. Again, the red represents carbon monoxide, and the yellow represents hydrogen.


How they hold themselves together.

To ensure the stability of an isolated cluster, the average speed of its individual stars must not exceed the escape velocity from the cluster. If this occurred, the stars would escape into space, and the cluster would dissipate. If the stellar velocities are low enough to satisfy this condition, then the cluster is gravitationally bound, i.e. the force of gravity is strong enough to keep the member stars from escaping.

Due to clusters moving in various orbits in the Galaxy, they are bound together with gravitational forces that are stronger than the disrupting forces exerted on it by the Galaxy or other nearby stars, and this results in an added condition for the stability of a cluster. Another factor in the stability of clusters is size-the smaller and more compact the cluster, the greater its own gravitational binding force compared with the disrupting forces, and the more chance it has to survive to old age.

Because globular clusters are highly compact systems, they are consequently very stable, and so most globular clusters will probably maintain their identity almost indefinitely.

But even these clusters lose some stars, especially if they have a slow mass. This is due to there always being a few stars in a cluster that move faster than the cluster's average speed.

When a star escapes, it carries with it energy, removing this energy from the cluster as a whole. This eventually results in the cluster developing a tightly bound core surrounded by a rarefied halo of stars-much like the first image of the M2 cluster, and similar to the following image of the cluster NGC 5927.

In the dense core of a cluster, the stars in it occasionally collide, and some of the debris eventually coalesces. Predictions indicate that this dynamical evolution could lead to the development of a large Black Hole at the cluster's center.At the same time, a few stars in the outer parts of the cluster would continue to escape. The escape rate and dynamical evolution for the rich globular clusters are so slow that the clusters can easily survive for many billions of years, remaining mostly unchanged.


Analysis of certain Globular Clusters.

These following images will be used to analyse the chemical content of various globular clusters. The three types of image used will be; ordinary photo images, infra-red images (IRAS) taken, unless otherwise stated, at a frequency of 100 microns, and finally, nH images, which all show the cluster in relation to the Galaxy.

The clusters presented here are to be investigated by a team of people, including Professor A. Evans, S. Eyres, and M.E.L. Hopwood from Keele University. They will be saerching for molecular gas in these metal-rich globular clusters, with the first three to be searched in November 1997, and the rest in March 1998. Until then, here is some information on those particular clusters.


47 Tucanae

This partial photo image of 47 Tucanae, shows that it looks like a typical globuar cluster, with a central, dense concentration of stars, and the less dense, darker surrounding layers, held in place by the cluster's gravity.

This IRAS image of 47 Tucanae shows a little hydrogen to the middle-left of the image, and even smaller trace amounts of carbon monoxide next to the top left corner.

This nH image of 47 Tucanae shows it to contain a lot of carbon monoxide, in contrast to the yellow streak of hydrogen below it, representing part of the Galaxy.

By combining the results of all three images, it can be seen that 47 Tucanae is a large cluster that contains carbon monoxide, but not in overwhelming amounts, and only has a trace of hydrogen present.


NGC 1261

This photo image of NGC 1261 shows at least three distinct different layers of stars; the dense center, the less dense middle-outer layer, and the outer layer, where the stars have lots more space between them, and where the gravitaional pull on them from the center is weakest.

This IRAS image of NGC 1261 shows there to be large amounts of hydrogren in the surrounding space, and slightly less in the actual center of the cluster itself.

This nH image of NCG 1261 shows there to be large amounts of carbon monoxide, especially in the outer layers. Compared with the previous example (47 Tucanae), this cluster is relatively closer to the Galaxy.

Using the results from all three images, it can be seen that NGC 1261 is one of the smaller clusters that contains reasonably large amounts of both hydrogren and carbon monoxide.


M2

Again, this photo image of the M2 cluster shows a highly packed center, with the density of stars gradually decreasing, as the distance from the center increases. The M2 cluster is the brightest known cluster in the galaxy of Andromeda.

This IRAS image of the M2 cluster shows that there is a substansial portion of carbon monoxide in the cluster, with trace amounts of hydrogen on its outskirts, with the rest ordinary space.

This nH image of the M2 cluster shows it to contain quite a lot of carbon monoxide, with fewer amounts in the cluster's center.

By using the various images, it can be seen that the M2 cluster is quite large, and contains a large amount of carbon monoxide, with only trace amounts of hydrogen.


w Centauri

This photo image shows the brightest globular cluster in our own Galaxy, w Centauri. As with most of globular clusters, it follows the standard pattern, with a dense center and less dense outer layers. However, this cluster overall, will probably contain more stars than the usual average number of stars found in a cluster. This is because the cluster is one of the brightest there is.

This IRAS image of w Centauri, taken at a frequency of 60 micons, shows there to be hydrogen in the cluster, and greater amounts of it in the surrounding space.

However, this IRAS image of w Centauri shows there to be large amounts of carbon monoxide, with no trace of hydrogen showing up on the image.

This nH image of w Centauri shows the large amounts of carbon monoxide present in the cluster, and shows the cluster to be quite large in size.

Using all the images together, it can be seen that w Centauri is a large cluster, containing large amounts of both hydrogen and carbon monoxide.


NGC 4833

This photo image of NGC 4833 shows it to be a relatively quite small cluster, with its center appearing only slightly more dense than the surrounding layers of stars. Its small size will be due to the cluster's center not having enough gravity to hold on to many of its stars, and so, they would have left the cluster.

This IRAS image of NGC 4833 shows that there appears to be roughly equal amounts of hydrogen (at the top of the image), and carbon monoxide (at the bottom of the image).

This nH image of NGC 4833, shows it to contain carbon monoxide in certain areas. In the image, there appears to be more carbon monoxide towards the top of the picture, considerably less in the center of the cluster, and gradually less towards the bottom of the cluster.

Overall, using all the data in the images, it can be seen that NGC 4833 is a very small cluster containing roughly equal amounts of both carbon monoxide and hydrogen.


NGC 5634

This photo image of NGC 5634 shows it to appear much like a typical cluster, with again a dense center surrounded by layers of gradually decreasing star density. It appears to be quite small, but this does not appear to have affected the cluster in a serious way, instead, all it is now, is a miniture version of an average-sized cluster.

This IRAS image of NGC 5634 shows only that there are small amounts of both carbon monoxide and hydrogen in the cluster, the rest being unremarkable space.

This nH image of NGC 5634 shows there to be scattered amounts of hydrogen present in the cluster, with more hydrogen on the outside of the cluster, and less hydrogen on the inside of the cluster.

Using all three images, it can be seen that NGC 5634 is a small cluster containing little hydrogen or carbon monoxide.


NGC 5927

This photo image of NGC 5927 looks very similar to the photo image of NGC 4833, showing a small cluster with a center star density not that much higher from the rest of the cluster. The difference with this one compared with NGC 4833, is that this cluster's center is slightly more dense, and so, compared with NGC 4833, looks slightly more orderly.

Like the IRAS image of NGC 4833, this IRAS image of NGC 5927 shows there to be roughly equal amounts of both hydrogen, with a biase towards the amount of hydrogen.

This nH image of NGC 5927 shows it to contain a lot of carbon monoxide, both in the center and outer layers of the cluster.

Using all the images together, it can be seen that NGC 5927 is a very small cluster containing large amounts of both hydrgen and carbon monoxide.


NGC 2808

This photo image of NGC 2808 shows, like the other clusters, a dense center. However, the surrounding stars are not arranged in an orderly fashion, as in the previous globular clusters. Instead, stars just seem to be de-localised, and only just held in place by the cluster's gravity.

This IRAS image of NGC 2808 shows there to be a greater amount of carbon monoxide present, than hydrogen.

Like the IRAS image, this nH image of NGC 2808 shows the cluster to contain a lot of carbon monoxide.

By using all the images together, it can be seen that NGC 2808 is a generally de-localised cluster containing large amounts of carbon monoxide, and fewer amounts of hydrogen.


By combining the general results from all of the images comapred together, the following conclusions can be reached.

Written by Jon Talpur at Keele University, 1997.


Links

For more information on Globular Clusters, try these sites:

The Crowded Heart Globular Cluster Page

The Globular Star Clusters Page


Biblography

"Exploration Of The Universe" by George O. Abell, David Morrison & Sidney C. Wolff