Clusters?

Hi, can some one explain what a star cluster is. whether open or otherwise, and when does a group of stars become a cluster? I know that the Pleiedes are an open cluster, but why, and not a constellation?
Bill H

Hi Bill,

I'm including below a draft of an essay I wrote on this topic a while back. I cant find the final version offhand, so apologies for the lack of accurate references, lack of images, bad spelling, terrible grammar etc.

Open clusters are formed from clouds of gas in the plane of a galaxy, as pockets of gass condense they become stars. The pleiades is a good example where you can still see the nebulosity around the stars, the gas from which they all formed. Stars in open clusters are all relativly young.

Globular clusters on the other hand, lie throught the shpere of the galaxy, on their own orbits. They are almost exclusivly old stars groups, containing millions of stars, that are thought to have been formed at the same time as the milky way.

Globular clusters are fairly obvious as mothballs of stars, however open clusters can be harder to spot.

Constellations get named based on what someone once upon a time thought looked like a shape. The stars in a constellation are only viewd in 2 dimensions, so even though they may appear to be near to each other and the same brightness they may be several times further out than each other, and moving in different directions. One case where an open cluster is part of a constellation is Tarus, the Hyades are an open cluster, this can be determined by measuing their distance and velocity and composition to show that they are all about the same distance and moving as though they formed in the same area.


Cheers,
~Al

Open and Globular Clusters
Albert White

To the naked eye the Pleiades and M13, the globular cluster in Hercules, look very different. The first being a group of seven stars of similar brightness and colour, the latter a small smudge. With a telescope the smudge of M13 resolves into what appears to be a spherical ball of many stars, and the Pleiades becomes clearly a spread out group of only a relatively few stars. Both are what we call star clusters, one open and one globular, but both are very different. In this essay, I will contrast the two types of clusters, in terms of their appearance, composition, location, distance and size in our own and other galaxies.

Appearance

Open clusters vary from the Pleiades [Fig 1], a reasonably close together grouping of stars with some nebulosity around them detectable in photographs to clusters like the Hyades in Taurus in which the stars appear so far apart that its not clear that its a cluster at all. Globular clusters, for example M13 in Hercules [Fig 2] on the other hand are unmistakable in telescopes as a 'ball' of stars.

[Fig 1 IMAGE OF PLEIDAES]
[Fig 2 IMAGE OF M13]

Harlow Shapley devised a simple scheme to categorise the various types of open clusters we can see based on richness and concentration. R.J. Trumpler (1930) later expanded on this classification to a scheme which characterises the cluster based in its concentration, range in brightness and richness. The Pleiades in Fig 1 is classified as a 1-3-R open cluster using the Trumpler Scheme, indicating that it is a cluster well detached from the surrounding stars, has a large range in brightness and has more than 100 stars.

Globular clusters look very much alike, so a more complex and quantitate approach is required to classify them. Webbink (1985) gives parameters for 154 globular clusters in the Galaxy including metallically, magnitudes, luminosities, diameters and densities, with descriptions and definitions of the values given. Alcanio (1976) presents a simpler set of data consisting of 47 parameters including a richness index and spectral type. M13 shown above in fig 2 for example is one of the richest clusters, with a richness index of 0.70 and a spectral type of F5.

Closer inspection of the appearance of the clusters yields more information on their composition, their distance from us and their origins.

Cluster Composition
If we assume that all the stars in a cluster are of about equal distance from us we can plot them on a HR diagram using apparent magnitude instead of absolute Magnitude to get a relative HR diagram. We will later return to calculating the distances.

The resulting diagrams show a stark contrast between open and globular clusters.

[Fig 4 HR OF PLEIDAES]
[Fig 5 HR OF M13]

The open cluster diagram mainly has its stars on the Main sequence with only a few having left the main sequence to become red giants. Whereas the globular clusters have much fewer stars on the main sequence and also show horizontal branches.

From this we can tell that the open clusters are relatively young groups of stars where only a few have left the Main sequence. This give us an age of open clusters of only up to several hundred million years. Some clusters have stars that have yet to all reach the main sequence e.g. NGC 2264, indicating an age of about 2 million years ago[]. In the case of the Pleiades above all stars have reached the main sequence and some have started to leave it, this indicates an age of about 50 million years.

The globular clusters have a clear point where their stars break away from the main sequence to become red giants, this is called the Main Sequence Turnoff Point (MSTP). Since the position on the main sequence is dependent on Mass, we can calculate the Mass of stars that are currently leaving the main sequence. This turns out to be about 0.8 solar masse. Current stellar evolution models suggest that it take about 12billion years for a 0.8 solar mass star to leave the main sequence so the globular clusters must have formed very early in the universes history.

There are several other types of stars indicated on the globular HR diagrams that are not on the open diagram.

Some stars appear to linger on the main sequence. These are known as 'Blue stragglers'. In order for these stars to linger on the main sequence they must be getting additional mass. Current studies suggest that these stars are in binary systems and the other stars in the binary have seeded mass to the blue stragglers.

The horizontal branch stars in the HR diagrams of globular clusters represents the stage in a stars evolution after it experiences helium flash. These are among the larger stars in the cluster and have begun helium burning in their cores.

The most massive, and hence most evolved stars in globular clusters are on the asymptotic branch. These stars have burned all the helium in their cores and are on the final phase to becoming planetary nebulae, though few planetary nebulae are seen in globular clusters.

With the possible exception of blue stragglers, none of these star types are seen in open clusters as open clusters a simply too young to have these kinds of stars.

Stellar Compositions

Using spectroscopy we can examine the properties of the stars within the clusters. Again this shows a clear contrast between the globular and open clusters.

The globular clusters show strong absorption lines for Hydrogen and Helium, but very little trace of heavier elements are present. These are classed as population II stars.

Open clusters on the other hand are higher metallically stars showing the presence of many heavier elements, these are classed as population I stars.

Heavy elements can only be created in large quantities in supernova explosions, and open clusters must be formed around the diffuse nebulae resulting from such clouds. The Pleiades for example still shows some of the nebulosity in photographs. This shows that the earliest open clusters can only have formed after the first of the Population II stars had gone supernova, and again shows that open clusters are relatively young. The clusters with higher metallicities may be composed of material that has been recycled through supernovae several times. The lack of metals in the globular clusters also confirms their age as very anchent stars which must have formed before there was much metal around, ie before supernova activity had started among the early massive stars.

Location, distribution, and size.

The most noticeable point about the distribution of the open and globular clusters in the galaxy is their location.
The open clusters are almost all confined to the spiral arms in the plane of the Milky Way, and there are over 1100[1] catalogued open clusters. As they reside on the galactic plane it is probable that many more are obscured by dust, and the galactic bulge, and there are estimates that there may be as many as 100,000 Milky Way open clusters.

Globular clusters by contrast occur thorough the sky, with the greatest concentration towards the Sagittarius region of the sky containing the galactic bulge( of the 150 known clusters in our Galaxy, 91.3% are located in the hemisphere of Sagittarius). Shapley (1917) used this observation to show that the solar system lies “about half way from the centre to the edge of the galaxy”.

The distance to the closest clusters can be determined using parallax. For stars in clusters further away we rely on measurement of the Period of Cephid variables and of the shorter period RR Lyrae variables which are fairly common in globular clusters. The distance to the more distant open clusters cannot be determined by parallax, and they typically do not have any variable stars to calculate distance from. Percival et. Al (2003) however have shown that good agreement can be found for Main Sequence fitting and the Hipparcos parallax distance for four relatively close open clusters when the the mismatch due to “the clusters' colours which are inconsistent with their spectroscopic metallically” is taken into account.

Measurements of the radial velocities of globular clusters tell us that many move in highly elliptic orbits about the centre of the Galaxy. Their orbits have been shown to form a roughly spherical halo around the Galaxy, which can extend to 100,000 light years, significantly bigger than the diameter of the galactic plane. Their orbits also show that they do not all follow the same rotation as the galactic plane. Some orbits also imply that the clusters must pass through the galactic plane on their orbits (Shapley 1917). Within the globular clusters the stars orbit about a common centre of mass, this can allow some stars to reach high enough velocities to escape from the cluster which causes the cluster to shrink and as a consequence stars get more kinetic energy and more are able to escape[3]. This process is known as core collapse (Henon 1960's). Core collapse and tidal forces acting on globular clusters when they move through the plane of the galaxy are believed to be responsible for the entire population of stars in the galactic halo[3].

The stars within an open cluster are generally all moving in the same direction with the same velocity. In some cases such as the Ursa Major moving cluster this is about the only way to define them as an open cluster. The more closely packed open clusters will experience more gravitational interaction between the member stars, but the cluster still travels as a unit.

Origins and the future

I have shown that globular clusters formed early in the universe. There is no evidence to suggest that globular clusters are still forming in our galaxy so we must assume that there were special circumstances early in the Milky Ways history to facilitate the creation of these clusters. The details of the origins of globular clusters remains a mystery, though they have been detected in other galaxies (e.g. S&T Oct 03 deep field pic from hubble of M31 limb) so they are not unique to the milky way. The future of globular clusters is hard to predict as they continually cause surprises for astronomers, for example a gas giant planet has recently been discovered in the globular cluster M4. It seems likely however that over the coming billions of years the globular clusters will continue to erode though core collapse and passing through the galactic plane and it is unlikely that any new clusters will be formed.

The origins of open clusters is fairly well understood as I have explained above. Open clusters continue to be formed and give us key information into the process of star formation and evolution from the diffuse nebulae from which they originate.



References:

Alcaino, G., Basic Data For Galactic Globular Clusters, 1977, Publications of the Astronomical Society of the Pacific, 491-503

Djorgovski, S. The Dynamic Lives of Globular Clusters, October 1998, Sky and Telescope, 38-43

Freedman, R. A., Kaufmann W. J. III, Universe 6th Ed.

Percival, S., Salaras, M., Kilkenny D., The Open Cluster Distance Scale, 2003, Astronomy and Astrophysics manuscript.

Shapley, H. Globular Clusters and the structure of the Galactic System, 1917, Publications of the Astronomical society of the Pacific, 42-54

Webbink, R. F., Structure Parameters of galactic globular clusters, 1985, International Astronomical Union, 541-577.

Many thanks for a detailed and very enlightening reply Al. Much appreciated.
Bill H

The pleiades is a good example where you can still see the nebulosity around the stars, the gas from which they all formed.

It is now known that the nebulosity surrounding the Pleiades is separate from the cluster. The cluster is merely passing through that cloud and illuminates it. There are countless other clouds in the galaxy. This one is just lucky enough to have a cluster passing by to illuminate it.