Role of Open or Galactic Clusters

Open Star Clusters have held the prominent rôle in the development of stellar astronomy. Studies have particularly revealed much about the type and nature of stars, having made it possible for astronomers to understand stellar evolution — from their first nebula bearing creation to their demise. This information has also been used to divulge the Milky Way structure. (19) Based on current evolution theory, stars are born in nebulae not in ones and twos, but in many hundred or thousands, forming into open galactic clusters. Most of these individual stars within star clusters as either single stars or as close orbiting binaries.

Astronomers can measure all the changes in the motion of individual stars within the open clusters, and then distinguish if the stars are actual members, or are merely field stars that happen to lie in the line of sight. These common proper motion (cpm) studies can only be made with photographic surveys. Attaching membership of individual stars to the star cluster cannot be directly understood unless we can measure the individual stars orbital paths within the cluster. Determining such motion will disclose true membership, but unveiling all true stellar motions can only be ascertained using the many photographic surveys of the heavens over the last one hundred years or so. (20) Photographic or CCD imaging can also reveal the physical or angular size of the cluster and the approximate number of stars. Most star cluster members are found to move at an average velocity of about 5 km.s-1. Most of the time it is relatively easy to tell stellar membership, as the measured space velocity will often appear quite different from the average velocity of the cluster. Additional data from this technique can be determined by observing the Doppler shifts seen in stellar spectrum via spectroscopy.

Most important of all for astronomers is the production of the Colour-Magnitude Diagram — simple graph plotting measures of star colours versus visual magnitude. From this plotted diagram, as first realised in the 1920s, the stellar ages of cluster stars, and therefore, the clusters age can be calculated. Furthermore, we can extend this data to include the deduction of the range and individual stellar masses, sizes, luminosities and densities.

Open Cluster Classifications

Development of the classification and descriptions of star clusters has had a very convoluted history. One of the first studies of clusters were undertaken in Harvard University by astronomers Shapley and Melotte in the years from 1915.

Shapley / Melotte Classification

Shapley and Melotte set up an useful rough initial two-dimensional array of classification. One parameter relates to the apparent number of stars and the compactness of the cluster. The second parameter was dependant on the colour (and later spectral classes) among the cluster members.

Initial classifications sets the physical divisions between open clusters and globular clusters, which was were later formally defined in 1927. This proved to be useful in other important studies later on. The first classification for open clusters used the lower case letters a to g.

(a) Associations containing Field Irregularities

This category applies to associations. Using random stellar counts, it becomes obvious which stars are real field irregularities. This is achieved by either visual scanning photographic plates or by statistical analysis. Most of the field irregularities vary in their populations; from several scattered stellar members to vast congregations of stars. Most of these have never been catalogued, nor probably never will. The recognition of the group is significant to stellar distributions.

(b) Distinction only by a Star Count. Considered Very Very Loose

This category falling into the wide-spread moving clusters, such as the Ursa Major Group and the peculiar stars of high proper motion or parallel velocities. Most of these have been discovered through common proper motion studies. The b class gradually merges into the c class. An intermediate of this class with c I.e. The Orion Nebula Cluster fits into this group.

(c) Very Loose and Irregular

These types are generally large and very scattered. Typical examples of this class include the Pleiades and the Hyades. Others include the large clusters around h and χ Persei, IC 4665 in Ophiuchus, Mel 111 in Coma Berenices. Other transitional clusters with b include the southern examples of IC 2391 in Vela and the Southern Pleiades; IC 2602 in Carina.

(d) Loose and Poor in Number

This cluster type has very small number of stars and appear loose and ill-defined at their edges. They include clusters: M21 and M34. Southern examples include NGC 3572 in Carina, and NGC 3293 and NGC 2670 in Vela.

(e) Intermediate Rich and Concentrated

These clusters are far more concentrated and compact, and like the subdivisions of f and g, are obvious in a telescope. Examples include M38 in Auriga and NGC 3114 in Carina.

(f) Fairly Rich and Concentrated

This group is similarly as compact as e, but contain more stars. Examples include M37 in Auriga and NGC 3532 in Carina.

(g) Considerably Rich and Concentrated

This group is similarly as compact as f, but contain even more stars. Examples include NGC 2477 and the Jewel Box (NGC 4755) in Crux.

The Shapley / Melotte Classification

This classification that was devised by Shapley for the globular star clusters lies beyond class g. Surprisingly, some open clusters appear more compact than the most dispersed of the globular clusters. In practice open clusters are distributed between the and groups. They are roughly distributed as c 8.2%, d 34.0%, e 26.8%, f 18.9% and g 12.0%

Shapleys second parameter* is based on the colour of the cluster. It can roughly be divided in groups of either; The Blue or Pleiades cluster types and the Yellow or Hyades types. (This colour classification was quickly discontinued soon after its conception because it did not relate to ages or evolution sequences.)

This system is now found only in the older observational books or catalogues and has fallen out of favour because of its obvious astrophysical limitations. This is due to this classification being solely based on size and distribution, becoming highly dependent on the population type of the stars and cluster distance. Yet it is still useful for amateur observational astronomers because it really describes the cluster as you see it.

Trumpler System

Originating in 1930, the so-called Trumpler System was devised by Shapley and Melotte to superseded the older classes. (21) It is vastly superior than all the earlier systems only because it gives an abbreviated indication of the general nature of open clusters and usefully describes details than just stellar membership. This four part structure in the system is as follows;

1. Concentration

I Detached with strong central condensation
II Detached with little central condensation
III Detached with no central condensation
IV Not well attached, but displays a strong field concentration

2. Numerals

1, 2 or 3

The luminosity function or lmf follows the numerals 1, 2 and 3, and shows the luminosity of an open cluster compared to the nearby non-cluster regions. lmf is based on the mean luminosity of all the clusters stars. Therefore the lmf decreases in magnitude compared to the surrounding fields. For an actual cluster, this shows that lmf increases with the brighter of the cluster stars, followed by some decrease to the fainter ones.

3. Star Numbers

p : Poor > 50 stars
m : Medium > 50 to 100 stars
r : Rich < 100 stars

Star clusters can be easily classified by their population of stars that are contained within them. These simple star counts have been adopted since the end of the 17th Century, probably initially starting with Sir William Herschel. Star counts in the modern adopted divisions started from the 1930s with Harlow Shapley, and have changed little since this time. Lately, the 5th Lund Open Cluster Data files suggests an extension to this stellar richness scale. This adds two additional categories, bring very poor (vp) and very rich (vr), and although not said, these are clusters that are either vp less than twenty (20) stars or vr if they contain more the 250 stars. I have not seen this classification beyond the 6th OCD Catalogue, though observationally it gives a better picture of the visual open cluster.

4. Nebulosity

The n at the end of the classification indicates nebulosity associated with the open cluster.

NOTE: This does not differentiate between clusters having bright or dark nebulosity. I.e. NGC 3293 and NGC 3247 (Both in Carina).

Examples of Classification

M44 in Cancer is listed as Type d in the
Shapley classification or II 2 m - p
NGC 4755 / The Jewel Box / κ Crucis
is either g or type I 3 r -
NGC 3572 in Carina I 2 m n -
M23 in Sagittarius III 1 m -
NGC 6871 in Cygnus IV 3 p -

Cluster Parameters


Observationally, we may measure the apparent magnitude of all star clusters in terms of the magnitude of the brightest star, yet in some listings the 5th brightest star. Often some of the apparent magnitudes are listed, instead, into one integrated magnitude, where all the starlight is combined into one magnitude.

We also find that the fainter star clusters are typically quoted as photographic magnitudes. Each of these we have discovered by the photographic process as general higher concentrations in the star field and by star counts. More recent cluster studies have produced additional studies into the brightness of open clusters. Another means, as attempted by Brian Skiff in 1983, started to measure the magnitude of the cluster by using the available photometry. This then calculates the entire visual magnitude of the cluster, whose results are made by observing the brighter components determining the total visual magnitude. From this value, the general apparent magnitude of the component stars can be made. However, these quoted magnitudes are often misnomers because it does not reflect the true spread of the magnitudes nor the number of star that the cluster maintains. The authority for stellar counts and brightness is obtained in the Catalogue of OSC Data by G.Lyngå in 1983.

Other Useful Data

The following information is normally quoted in data on open clusters;

Total magnitude of the cluster is usually identified by its blue magnitude. This is usually obtained by photometry using either the B filter or the photographic plate — the latter that has been exposed through the B filter. Visual magnitudes are considered as unreliable because of the gross effects of interstellar absorption at longer wavelengths. Excesses in colour can be found by using global observation of all the members of the cluster.

Spectral characteristics of the cluster is either based on the hottest member of the cluster, while the magnitude is sometimes based on the fifth brightest star. This is used because clusters tend to have one or a few very bright stars followed by large numbers of gradually fainter stars. (22) Care should be taken when writing stellar visual magnitudes for any selected cluster because sometimes the source may not be specifying similar magnitude types. For visual observers, often use the fifth brightest magnitude star, or even the integrated magnitude, is probably the more realistic when estimating visibility and telescopic appearances.

Ages are calculated by the turn-off point of the main sequence using its colour magnitude diagram or the HR diagram.

The purity or metal content is measured, as with the globular star clusters, using the logarithmic ratio of iron to hydrogen or [Fe/H]. Typically this is found by using narrow band photometry in either the visible or in the infra-red.

Open Cluster Tables

I have included some Tables of the best observed clusters for small telescopes. These that have written text that I eventually hope to post soon in a further update of these pages. The new list in the link below has more bright cluster that can be seen by amateurs using small to moderate telescopes.

These observing lists are divided as three (3) separate Tables, being as follows;

A) Some Bright Open Clusters for Small Telescopes (OSC: Part 5)
1) Fifty Best Southern Open Star Clusters (OSC: Part 8)
2) Fifty Best Northern Open Star Clusters
3) Best One Hundred Open Star Clusters

References and Endnotes

19. In 1944, it also revealed Baades discovery of the difference in the galaxy of Population I and Population II stars. These two stellar types also became the fundamental basis for differences between open and globular clusters.
20. It was from these surveys that very loose star clusters and associations were discovered.
21. Published in the Lick Observatory Bulletin XIV. (14), 154 (1930)
22.The prime example for this type of cluster is NGC 2362 in Canis Major. It contains a single bright star τ CMa (Tau Canis Majoris), the red star VY CMa within the great multitude fainter stars, which are about 4 to 8 magnitudes faint than VY CMa itself.


Last Update : 19th April 2017

Southern Astronomical Delights © (2017)

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