OPEN STAR CLUSTERS

INTRODUCTION

Of all the types of stellar groups in the sky the most interesting are among the stellar collections that we know as star clusters. A typical star cluster may have anywhere between twenty and several million stars, which astronomers have divided into two distinct classes - the open star clusters and globular star clusters. [1] In this section, however, we will restrict our discussion the Open Star Clusters and their subgroup systems known as Associations.

As their name suggests, most open clusters appear as loose congregations of stars related only by their stars mutually shared gravitation. Of these groups, telescopically, several of the brightest are perhaps the most spectacular vistas of all the celestial objects available to amateur observers. Many are gloriously exquisite being available even to small or moderate sized telescopes. Some may contain anywhere between several multicoloured bright stars of varying magnitudes, while others are old reddish stars - appearing to contain just uninteresting collections of many faint stars. Serious amateurs are restricted to observing only a few faint variable stars and double stars. Some amateurs testing the powers of their apertures in resolving faint or difficult clusters, which have advantages in city skies. However, it is only the professional astronomers that can really achieve useful information by serious investigation, much pondering, to eventually discovering their true natures and evolutionary histories. We can summarise this as follows.

We do know that all open clusters are born among the nebulae - the stellar nurseries within the Milky Way. As such, all stars consequently do not form independently, but are generally created in vast numbers to form into star clusters. A number of these stars are collected together, forming either single, binary stars or multiple systems. Such hierarchical systems are important as they can determine and influence the long-term fate of the cluster. Open clusters are often considered quite stable gravitational systems but in truth are prone to suffer losses of stellar members. They are thought to eventually disintegration due to the internal forces influencing the dynamics of the cluster, the gravitational forces of the galaxy or by nearby stars. Astronomers are very interested into what happens once such clusters fall apart - which are termed OCR’s (Open Cluster Remnants). These are investigated to find clues about early star clusters and towards the early evolution of the Milky Way. Such consequences are among the current cutting-edge study and investigation by astronomers, who compare real telescopic observations with computer simulations. This is now the primary focus of many star cluster investigations.

The Sub-Clusters :
Associations and Moving Clusters

The common used terminology of the sub-classes of star clusters are;

As the name infers, associations are groups of stars that are loosely associated together. It was V.A. Ambarcumjan who discovered this class of object in 1952, and who eventually subdivided them into their modern designations. (1) It was only later that the astronomers W.W. Morgan and K.H.Schmidt called them stellar aggregates. (2) Unlike the open and globular clusters, the gravitational attraction between these stars is much weaker. Most associations are considered as gravitationally quite unstable objects that can only exist for periods up to ten million years or so before literally falling apart into individual star that just seemingly moving in the same direction through anagalactic space of our Milky Way. Most subtend anywhere between thirty and two hundred parsecs in diameter. Sometimes they can contain multiple stars within their own environs and are often associated with long chains of stars of similar spectral types. Associations divide in sub categories - the OB Associations, T Associations, C Associations and the Moving Clusters.

OB Associations

Any OB Association normally has anywhere between ten and a thousand stars. The majority are comprised of hot spectral class stars, usuallybeing either O and B. The OB Associations types are numerous and diverse. Some may have bright open star clusters as their nuclei - though this is not always the case, and rarely some are associated with bright nebulosity. Most OB types appear as seemingly an amorphous collection of stars, and thus are impossible to identify except by using of finder charts. Most of the stars attached to any group were found by spectroscopy, by determining their similar radial velocities, then sorting out those which appear to exceeded some particular range of criteria. For the nearest associations, some have been determined by measuring the star’s parallax shifts. Of these loose stellar groups we know more than one hundred and forty, with most typically lying near the galactic plane. Distances for OB Associations are mainly found between 500 and 4 000 parsecs from the Sun.

Associations are normally listed by combining three-letter constellation abbreviation followed by the letters ‘OB’ and have attached an additional Roman numeral - based on the order of their discovery. This particular system was first introduced by the observer Markarjan - correctly called the Markarjan designations. Ie. Cassiopeia V, etc. Morgan, and later Schmidt, who also arranged these associations by a Roman numeral then followed by the constellation. Each group was also placed in order of galactic longitude within the constellation. Such designations caused needless problems to the IAU, and by 1962, Commission 37 ‘Designation of Astronomical Objects’ recommended that the system needed significant improvement. (3) The final system was suggested and entrusted to J. Ruprecht in 1966 at the IAU Commission 37. (4) Based on the nomenclature report, associations are now named as; Ie. CMa OB1, Sco OB4 etc. (5) and (6))

Some are also named by the closest bright star that lies near the association. Ie. Persei Association. (7),(8)

Some OB associations are truly huge. Some are designated by constellation to constellation like the so called Sco-Cru Association that extends from Scorpius to Centaurus - including stars in Cen, Cru, Cru, Cen and many other bright stars in Lupus, Ara and Scorpius. Another is less known is the Pup-CMa Association. (9)

T Associations

T Associations are similar to the OB types, except they consist of newly formed stars with their embryonic nebulosity. The stars are comprise of the cooler spectral types, equal or less than that of the Sun. Most of these stars are variables of either the T Tauri or RW Aurigae type. Fifty eight of these are known, each containing between 30 to more than +400 stars. The distances are typically between 100 and 800 parsecs.

C Associations

These are associations that contain a number of Cepheid variables and are thought to be due to heavy mass star production during the star formation process. First listings of these association types were made by Efremov (1978) (10), contain perhaps a total around thirty-five in all. Explanation of these C-Associations and how they relate to stellar evolution is at present little understood.

Moving Clusters

The weakest type of the gravitational attached associations is the moving cluster. These are apparently unrelated stars but have similar motions and velocities through space. Most familiar is the Ursa Major Group, which contains most of the bright stars in constellations of Ursa Major, Leo, CMa, Eri, Aur and CrB. This group was later suggested to have the Sun as one of its members, but this theory in modern times has been dismissed as unlikely. Many astronomers think the existence of moving clusters infers that the component stars may have been nurtured from similar associations, clusters and nebulae. Perhaps for the Sun this occurred about five billion years ago, but the evidence for this has long faded into history and may never be a proven fact.

More Significant Stellar Groups

Open Star Clusters

Galactic or Open Star Clusters are exquisitely beautiful in either small or medium sized telescopes, and as their name suggests, they are relatively easy to resolve in a telescope into individual stars. Each are gravitationally associated collections of stars, numbering between twenty and one thousand components.
Some open star clusters display very young stars giving the cluster a distinct blue colour, and the most commonly known examples include The Pleiades (M45) and The Hyades both in the zodiac constellation of Taurus, or the southern example known as the Southern Pleiades (IC 2602) in Carina. Some other clusters visually have some impressive variations in colour that may contain gems of blue, yellow and red stars, that includes the famous Jewel Box (NGC 4755) and NGC 3293 in Carina.

Such observed colour differences transpire because when the star cluster is formed, all the stars start on their evolutionary paths simultaneously. As the cluster ages, the larger stars are first start run out of fuel, and begin their individual evolution upon such energy crises and soon the star rapidly swells to becoming a red giant. The other cluster stars will then follow, each evolving in turn by order of decreasing mass. If any star cluster contains one or more red stars among their number, then they will be likely be the heaviest, and sometimes the oldest, of all the cluster stars. Most of these coloured clusters do appear rather attractive in telescopes. As mentioned, one of the best example for southern observers is the so-called Jewel Box (NGC 4755) quite near the bright star Mimosa (β Crucis) in the Southern Cross. This ‘A’ shaped asterism contains the full compliment of star colours and is truly magnificent.
(See Detailed Article on this site “The Jewel Box - NGC 4755”)

History of Open Clusters

Visual Discoveries

Open clusters of the famous asterisms of the Pleiades and the Hyades, have been known since Ancient times. For example, both these clusters are mentioned in Homer’s ‘The Iliad’ and also are mentioned in the Bible. In fact, the most quoted example is appears in Job 9,9;

“[He] who made the Bear and Orion, the Pleiades and the chambers of the south.” (11)

According to Richard Allen’s Star-Names and their Lore and Meanings, the first written record of the stars was from Chinese annals in 2 357 BC of the star Alcyone being only 3° from the then vernal equinox. (12) The next record then comes around 325 BC, Aristotle (384 BC-322 BC) in his four-volume De Caelo (‘Of the Heavens’) reported the open cluster M41 (NGC 2287) as “a cloudy spot”. C.E. Barns was the has claimed this as “... possibly the faintest object recorded in classical antiquity.”; M41 is likely the very first recording of a deep-sky object, though many others may have been known.

Claudius Ptolemy (c.90 AD-168 AD) in c.138 AD found the next open cluster being M7 (NGC 6475) in the tail of Scorpius, who noted in ‘Almagest’ “...a lucid spot”. Another is the Coma Berenices Star Cluster (13) (Mel 111) surrounding the star 15 Coma Berenices. Another three clusters ‘nebulosities’ were listed by Ptolemy - though it seems possibly that they were really discovered by the acknowledged ‘Father of Astronomy’ Hipparchus. (fl.160 BC to 125 BC)

Brightest of the unresolved naked-eye clusters is known as the Praesepe or The Beehive, later catalogued it as M44 (NGC 2632) by Messier, who Ptolemy described as; “Centre of the cloud-shaped convolutions in the breast [of Cancer] called Praesepe.” The second (and third) was the famous far northern ‘Double Cluster’ (NGC 869 and NGC 854) in Perseus. Here Ptolemy describes; “At the tip of the right hand [in Perseus] and it is misty.’”

Until the end of the 16th Century all these early found star clusters were classed just as unresolved nebulae. Each appeared as naked-eye clouds of opalesencence, haze or lucid spots scattered occasionally along the Milky Way or among the constellations.

New Discoveries in the 17th Century

The advent of the telescope as first gazed upon by Galileo Galilei (1564-1642) found that some, if not all, of the Milky Way and the unresolved nebulae were actually made of many many faint stars invisible to the naked-eye. All though he may have never seen explicitly saw any open clusters the implications was clear that most, if not all, of these nebulosities were composed of stars. Such views were not disproved until the discovery of the deep-sky class of objects - the planetary nebulae - that William Herschel discovered were of some ‘stellar fluid’ surrounding a central star. This notion was finally concluded in 1864 when Higgins discovered that many of the nebulae were clouds of hot and luminous incandescent gas.

First to examine the nature of the star clusters was the recently discovered records of the Sicilian Giovanni Batiska Horierna (1597-1660). He discovered and examined a few new objects, including the southern open clusters Canis Major’s M41 (NGC 2287), M47 (NGC 2422/2478) and NGC 2451 both in Puppis, and the Tau Canis Majoris Cluster (NGC 2362). He also examined other bright clusters such as Praesepe (M44), the Double Clusters of Perseus (NGC 869 and NGC 884), M6 and M7. Hodienna examined these nebulosities in some detail, categorising them into three principal types (13a).

Edmond Halley (1656-1742) in 1677 first listed M6 (NGC 6124) and M7 (NGC 6475), by describing each in his observations notes as;

“...nebula in the third joint.” (No.20 ) &

“Nebula between the tail of Scorpius and the bow of Sagittarius.” (Object No.29)

These observations were obtained during an expedition to the island of St.Helena in the South Atlantic Ocean, which produced his ‘Catalogue of Southern Stars’. Nicolas Louis Abbe de Lacaillé (1713-1762) next found M6 in 1752 then M7 in 1755, but its fame was listing both in the Messier catalogue on the 23 May 1764.(14) Philippe Loys de Cheseaux (1781-1751) was the first to identify M7’s stellar nature. Flammarion described its shape as being like; “...three starry avenues leading to a large square.”

Lacaillé during his southern sky survey found many new nebulae that were hitherto unknown. This included the second most prominent globular star cluster, 47 Tucanae (NGC 104) and the amed Tarantula Nebula (NGC 2070) in the Large Magellanic Cloud (LMC). Among the open clusters he discovered no less than nineteen or twenty of them. This included the best clusters in the south like; ‘The Jewel Box’ (NGC 4755), Scorpius’ NGC 6231, ‘The Omicron Velorum Cluster’ (IC 2395); The Southern Pleiades (IC 2602), ‘The Football Cluster’ (NGC 3532) and NGC 2516 in Carina, and Centaurus’, NGC 3766.

In September 1681, Gottfried Kirch (1639-1710) telescopically discovered M11 or ‘Wild Duck Cluster’ but described it as nebulous. Its true stellar nature was discovered Rev. William Durham (1657-1735) in 1733, claiming M11 as an open cluster. In 1690, nine years after Kirch, NGC 2244 around 12 Monoceros was found by John Flamsteed (1646-1719), followed in the same year by the nebula M8 and its cluster NGC 6530 in Sagittarius. He also, for the first time, fully resolved M41 in 1702 in Canis Major.

Ending this period was Charles Messier (1730-1817) who found many new open clusters. Twenty-five are listed in the first 101 objects, and one extra star cluster (M103) added to it by fellow French observer Pierre François André Méchain (1744-1804). (15),(16) Messier discovered thirteen these, Méchain only two, and the remainder distributed between various observers. (17)

Telescopically, many of these clusters are easy to resolve. Most lie in the region near our sector of the galaxy. Upon closer examination most are easily resolved into individual stars - so the true nature of clusters were therefore discovered after the invention of the telescope. Small telescopes easily resolve such naked eye clusters as; The Praesepe (M44) in Cancer, the Butterfly Cluster; M6 (NGC 6405) and M7 (NGC 6475) in the tail of Scorpio. Other visual southern clusters include ; the Southern Pleiades (IC 2602) and NGC 2516, both in Carina and the group that surrounding the star Omicron (ο) Ceti (IC 2391). (14a)

Discoveries in the 18th and 19th Centuries

As telescopes improved, these open clusters were found to contain many more stars. Initally, these clusters were discovered as nebulous hazes which when viewed with optical aid were resolved into multitudes of stars. Galileo was one of the first to discover these, but throughout the 17th Century, discovery of several more of these clusters occurred sporatically. After Messier, most of the northern clusters were discovered by Sir William Herschel in the late 1700’s during his ‘star-gauging’. observations to determine the structure of the Milky Way.

The first person to recognise the physical relationship of the stars in the open clusters were physically related was the Reverend John Michell in 1767. He calculated that the statistical probability of even just one group of stars like the open star cluster Pleiades being just chance alignment was a very small in 496,000. This appeared in the Royal Society’s Philosophical Transactions, 57, 234-264. (1767)

The earliest southern open clusters were first catalogued by Abbe Lacaillé between the years 1751 and 1752, when he was systematically positioning the bright southern stars. In Australia during the late 1820’s, Dunlop (Δ) found several of these in his famous 629 deep-sky object catalogue, mant of which found during the observations of the Brisbane Paramatta Star Catalogue (PSC). Less than ten years later, even more southern ones were found by Sir John Herschel between 1834 and 1838.

Discovery of southern and northern clusters remained static for almost fifty years until the introduction of Dreyer’s New General Catalogue (NGC) in 1888. Modern catalogues (Dias (2002)) recognise a total of 501 N.G.C. clusters (31%). In the early 1900’s, many addition wide clusters were discovered with the advent of astrophotograph, the first group of twenty-eight (28) became incorporated in the Index Catalogue (or IC). These include; IC 2602 (Southern Pleiades), IC 2395 (Omicron Velorum Cluster), IC 4665 (Ophiuchus) and IC 2944 (West of the Cross in Centaurus).

Discoveries and Catalogues of the 20th Century

Perhaps the greatest increase of the open cluster discoveries began roughly after the mid-1920’s. Many were new all-sky catalogues or those found by looking for cluster in more specific hemispheres or areas. Such clusters are simply catalogued by the observer’s surname followed by the catalogue number. Many are listed in order of increasing Right Ascension, but several are placed just randomly as they were found. Some common examples include the following;

NOTES:

* vdB-Ha van den Bergh and Hagen.
** fff-nNNN ESO field number plate : n object type SC for cluster NNN is the no. object in field
*** Loden published five papers but all are sequential in catalogue.

By the 1970’s there were over 25 000 open clusters exist in the Milky Way whose estiimated numbers have slowly risen to about 40 000 (1994). Some recent sources now claim in the order of 80 000 (2004). As a general subgroup, there are recognised about 1 600 (2002) well-established open star cluster that have been adequately investigated - all being available to amateur apertures.

Catalogues of Open Star Clusters

One of the first general catalogues on open star clusters was originally published in 1930 by the American Harvard University astronomer, Dr. Harlow Shapley. This catalogue appeared in the specialised book simply known as “Star Clusters” (1930) - being the first specifically on all clusters. It catalogue contain some 250 cluster examples containing only their fundemental properties. Although much of the information contained is now obsolete, the data remained in use for many decades.

After Shapley, the main catalogue for clusters and associations was the Alter, et.al “Catalogue of Star Clusters and Associations” first published in 1958 with subsequent editions in 1966, 1970 and 1981. The main catalogue has all the known references or open, globulars and associations in a huge card file system, which comes in a single purple coloured box that weights just over 5kg! The updates of the 1981 edition have several additional supplemental books. In writing text on clusters it is particularly useful as it references the original data giving a pertinent view on the development of any selected cluster. (Having my own personal copy certainly helps!) Some of the historial parts of the observational descriptions of clusters (found elsewhere in these pages) have been determined using this reference. Needless to say much of this work is now obsolete, as much of the information in now available on the net through the either the CDS - Centre de Données astronomiques de Strasbourg and the ADS - Astrophysical Data Service.

One of the most recent catalogue in use is the ‘5th Lund-Strasbourg Star Catalogue’ or Lund Catalogue - being the ‘5th Open Cluster Data Catalogue‘ (OCD) (1987) that was first published by the Lund Observatory in 1981 by Göstå Lyndå. This contains a list of some 1 151 open clusters with each being identified by the IAU Cluster Designation, followed by the usual or traditional designation by the discovery and the cluster.

The IAU in 1970 based many of these designations on a decreed from IAU Commission 37. (21). Although considered useful, its usage seems to be on the decline - probably because it uses the older 1950 co-ordinate system.

Lyngå’s 5th OCD is useful because it also contains a listing of many of the important open cluster parameters. It includes such normal things as the age and size of the cluster, but includes far more astrophysical data, such as E(B-V) magnitude or colour excess, the [Fe/H] (Iron to Hydrogen Ratio) and the observed turn-off colour point.
Other sections of the catalogue includes the sub-division of known variable star types within each cluster.

A more recent edition has not been produced, but a newer one is by the W.S. Dias et.al. “New Catalog of Optically Visible Open Clusters and Candidates”, originally published in Astro.&Astroph., 389, 871 (2002). This contains more recent updates but is given in a less extensive summary. This contains some 1600 open star clusters being 25% larger than the older 5th OCD.

This catalogue given additional information including an estimation of the mass of the cluster and whether or not the cluster has available photometry for analysis.

One of the more useful of the catalogues available is the Internet based WEBDA (http://obswww.unige.ch/webda/). This useful Swiss based site is the work of the Laboratory of Astronomy at the Ecole Polytechnique (EPFL) in Lausanne, which began in 2002 and is presently developed and maintained by Jean-Claude Mermillio. WEBDA allows fast access to all current data on all open clusters. Also the site also gives some general information on “Cluster Parameters” (2002) by A.L. Tadross, P. Werner, A. Osman and M. Marie M. (At http://obswww.unige.ch/webda/tadross.html) that contains information on the top two-hundred (200) clusters.

Another useful catalogue for amateurs on Open Clusters appears in the Willman-Bell book “Star Clusters” by (2004)

The Role of Open Clusters in Astronomy

Star Clusters have held a prominent role in the development of astronomy. Studies of clusters have revealed discoveries in the type and nature of stars. It has made it possible for astronomers to understand the evolution of stars, from their creation to their demise. This information has also been used to divulge the structure of the Milky Way. (18) Based on current evolution theory, stars are born in nebulae not in ones and twos, but into clusters. Most of the individual stars end up within a star cluster as either singles or 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. Membership of individual stars to the star cluster cannot be understood unless we can measure the individual stars orbital paths within the cluster. Determining this motion will disclose whether a particular star is a member of the star cluster. Disclosure of these stellar motions can only be ascertained using the many photographic surveys of the heavens over the last one hundred years or so. (19) The photographic process 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 5kms^-1. It is relatively easy to tell if a star is not a member, as the velocity will appear quite different from this ‘average’. Additional data from this technique can be determined by observing Doppler shifts seen in the stellar spectrum via spectroscopy.

Most important of all for the astronomer is the production of the Colour-Magnitude Diagram. From this diagram, first realised in the 1920’s, the ages of the stars within the cluster can be calculated, including the age of the cluster itself. Furthermore, we can extend this data to include deduction of stellar masses, sizes, luminosities and densities.

Open Star Clusters:
Classification and Descriptions

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

Shapley / Melotte Classification

Shapley and Melotte set up an rough initial two-dimensional array of classification. One parameter related 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 set a physical division between the open clusters and globulars were later 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’ ie. 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 the clusters M21, 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%

Shapley’s second parameter* is based on the colour of the cluster. It can roughly be divided in groups;

a. The Blue or Pleiades cluster type
b. The Yellow or Hyades type

* This colour classification was quickly discontinued after its conception because it did not relate to ages or evolution sequences.

This system is now found only in the older observational catalogues and books and has fallen out of favour because of its obvious astrophysical limitations. This is due to the 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

The system devised by Shapley and Melotte was superseded by the Trumpler System which had it origins in 1930. (20) It is superior than all the earlier systems because it gives an abbreviated indication of the nature of the cluster and describes details on the stellar membership. This four part structure in the system is as follows;

1. 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 region’s. 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

The 5th Lund Open Cluster Data file suggests an extension to the richness scale. This adds two additional catergories, bring very poor (vp) and very rich (vr), and although not said, these are clusters that are ‘vp’ less than 20 stars or ‘vr’ if they contain more the 250 stars. I have not seen this classification beyond the 6th OCD Catalogue, though observationally, given 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.
Ie. NGC 3293 and NGC 3247 (Both in Carina).

Examples of Classification

Magnitudes of Open Clusters

Observationally, we may measure the apparent magnitude of star clusters in terms of the magnitude of the brightest star but in some listings the 5th brightest star. Often some apparent magnitudes are listed as an integrated magnitude, where all the starlight is combined into one magnitude. The fainter star clusters are typically quoted as photographic magnitudes. Each of these we have discovered by the photographic process as concentrations of the star field and by star counts. More recent cluster studies have produced additional studies into the brightness of open clusters. In 1983, Skiff started to measure the magnitude of the cluster by photometry. This calculates the entire visual magnitude. It is made by observing the brighter components, and then calculating the total visual magnitude. From this value, the apparent magnitude of the component stars can be made. However these quoted magnitudes are misnomers because it does not reflect the 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 Parameters of Open Clusters

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 obtained by using photometry using a B filter or a photographic plate which has been exposed through a 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 a large number of fainter stars. (22) Care should be taken when writing a stellar magnitude for a cluster because sometimes the source may not specify the magnitude type. For visual observers, the use of the 5th magnitude star or the integrated magnitude is probably the more realistic when estimating visibility in a telescope.

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

Identification of Star Clusters

The task to identify faint open clusters is sometimes not an easy as first assumed. The number of stars and the angular size from a catalogue can give a rough assessment of the appearance in the eyepiece. This, however, assumes that you know the size of the apparent field of view, the magnification and the limiting magnitude of the telescope. Yet the cluster’s geometric parameters and the magnitude range or quantity of observable stars become the major unpredictable variation. A few clusters smaller in size than the Pleiades, the Hyades or NGC 2516 maybe naked-eye objects in any clear and dark nighttime sky. (And assuming reasonably good eyesight!) Stellar numbers increase substantially with optical aid (like binoculars), however, resolution will not improve too much, unless medium-sized telescopes are used. To do justice to any tight open cluster, or even some globular star clusters, either 20cm or 25cm telescope will improve resolution tremendously, especially if the location has good seeing. It is certain 25cm telescope could study all clusters contained within the NGC and IC catalogues. (The basis of both these catalogues is 30cm telescope, so the description will meet the standards of advanced amateurs.) Amateurs should also be aware that other catalogues of star clusters can be identified in medium-sized telescopes. With the advent of the larger Dobsonian telescopes, the clusters far out weigh those written in the non-NGC and non-IC catalogues.

Identification of the majority of clusters are mainly limited to the Milky Way. The fainter stars that lie in the cluster are normally difficult to identify because they blend with the foreground stars. Most of the outlying stars will normally be misidentified. If the cluster lies beyond the Milky Way then the identification of outlying stars is far more easier to distinguish.

Some clusters can also be seen associated with nebulosity. In crisp skies, the reflection nebulae can sometimes be glimpsed in medium to large telescopes using low magnification and averted vision. Most are much easier to photograph, especially using blue sensitive film, or a CCD (possibly with a blue filter.) The newer clusters may also be associated with the emission nebula. In these cases, it would be best to use a red filter. Photographers using film or CCD’s have to show some caution because the brightness of the nebula may eliminate the stars if the exposure is too long. M8 and M16 can be telescopically seen with both stars and nebulosity. NGC 2174-2175, NGC 2467 and NGC 6960 have nebulosity that can be easily photographed.

A few clusters show distinct visual colour. Most of the newer clusters display a distinct blue colouration. Some of these clusters, like M7 in Scorpius and NGC 2156 in Carina has the single bright red star among all the other blue stars, that can appear quite attractive. A few shows some coloured range - like that of the Jewel Box is the most stunning of this type. Older clusters, normally not favoured by amateurs, can show distinct yellowish or orangish colouration due to their distance and the amount of interstellar absorption. Most are uninteresting because they do not have much variation in apparent magnitude.(23) For most observers, these clusters are not high on the observing list. Once observed most are not reexamined.

Examples of all of these clusters types can be seen on the charts in Table 1.

Descriptions of Some Bright Southern Open Clusters

Table 1. Selected List of Bright Open Star Clusters for Small Telescopes

Open Cluster Tables **NEW**

The Table above give the best cluster for small telescope. These have written text that I hope to post soon in a further update of this page. The new list in the link below has more bright cluster that can be seen in small to moderate telescopes.
These observing lists are divided as three (3) separate Tables, being as follows;

1) Fifty Best Southern Open Star Clusters
2) Fifty Best Northern Open Star Clusters
3) Best One Hundred Open Star Clusters

(SEE PAGE : “ The Best Open Clusters”)

References and Endnotes

1. Vosprosy Kosmgonii, Moscow, 1, 198, (1952)
2. Ap.J.,118, 318 (1953) and Ast.Naschr., 284, 73,(1958)
3. XI Congress at Berkeley / Transactions XIB, p.340 (1962)
4. Transactions of the IAU Commission 37, XIIB, p.337, 1966 and the three Tables on the subsequent p.354.
5. Further information on this nomenclature system, and the ‘standard’ Association and Cluster designations can be found in the ‘Catalogue of Star Clusters and Associations’ by Alter, J. Ruprecht and V. Vanýsek. Published by Akadémiai Kiadó, Budapest, 1970
6.
7. Sky Atlas 2000.0 has listed several of the OB Associations but does contain some northern hemisphere bias. For example, the Cha I Association is not list for example, and is possibly the most important of the southern associations. Also the proper motion studies of some of the other less prominent southern associations are not as complete which may account for some of this bias, but it is more likely the effect of the source used by Sky Atlas
8. Reiterating; this was first introduced by Ruprecht in 1966. (Trans., IAU XIIB p.348-356) The old designation used Roman Numerals ie IV Cep or Cep IV. This new system was recognised by the IAU in 1970 in the 'Catalogue of Stellar Clusters and Associations'. (Ref IAU 1970 AR)
9. This cluster is thought to be closely associated with the star clusters; Collinder 140, 135, 173 and NGC 2457 in Canis Major.
10.
11. Sir William Drummond in the 19th Century, according to Allen (pg.362) states that this refers to the shape of the tail of Scorpius on the southern horizon looks similar to a chamber. Here the Pleiades is opposite the constellation of Scorpius. Others think that the reference may be referring to the Hyades.
12. This position is likely important because after this date the changing places of the Pleiades over the ensuing centuries would have highlighted the effect of the Precession of the Equinoxes. In early antiquity, the vernal equinox would have been known as the "First Point of Taurus". Astrologically, the position of the Pleiades would have had highly significant importance, and this is suspected in a number of different cultures.
13. Not to be confused with the Coma Berenices galaxy cluster!
(13a) This is discussed in more detail in Archinal, B., Hynes, S. “Star Clusters” Section 1.3 ‘Early Telescopic Discoveries of Open Clusters’, pg.3 Published by Willmann-Bell.
14. M6 is the furthest southern object in the entire Messier list
(14a) Had these clusters been observed by the northern observer prior to this time the story of open clusters may have been very much different - especially if this had been made by a Galileo or William Herschel. The advancement to astronomy by these clusters alone may have shortened the finding of the stellar natures of clusters so much sooner (Ie. By c.50 to 100 years). Investigation of these objects in the southern skies did not really begin until the installation of the Great Melbourne telescope in the 1850’s.
15. I have included M24 as OSC (NGC 6603), but this Messier’ discovery is debatable. According to Sky Catalogue 2000.0, Messier only describes a simple brightening of the Milky Way. Some, like Kenneth Glyn Jones in ‘The Search for Nebula‘’ (1975) claim it is a “cluster with nebulosity.”
16. M104 to M110 were added by observations decided by several observers to produce an extended list. It was added during the 20th century. Méechain did find these objects using equipment similar to what Messier was using. The galaxy M104, for example, was added by Flammarion in 1921, Hogg in 1947, Gingerich in 1960 and Kenneth Glyn Jones in 1966.
17. The distribution of clusters occupying the first half of the catalogue, intermixed between M16 and M45 (Sixteen in all) were found between June and October 1764 in the first published forty-five objects. The later M46 to M48 were discovered on February 1771 (Three in all), M50 and M52 in early 1774, and the remainder between February 1777 and April 1781.
18. In 1944, it also revealed Baade's discovery of the difference in the galaxy of Population I and Population II stars. These two stellar types also became the fundamental basis for the difference between the open clusters and the globular clusters.
19. It was from these surveys that the very loose star clusters and associations were discovered.
20. Published in the Lick Observatory Bulletin XIV. (14),154 (1930)
21. Details can be found in “Celestial Designations” by Fenandoz, Larlet and Spite F. (1983) and “The First Dictionary of the Nomenclature of Celestial Objects” ; Astronomy and Astrophysics Supplement Series. 52 (4) 1.1 - 7.14 (1983)
22.The prime example for this type of cluster is NGC 2362 in Canis Major. It contains a single bright star (Tau Canis Majoris), the red star VY CMa with a multitude fainter stars, by some 4 to 8 magnitudes.
23. Clusters northern edge of the Milky Way and in the western side of the constellation of Centaurus are good example of these types of clusters.
24. This is also known as 'Velorum' or the ‘Velorum Cluster’.

[1] Globulars will be placed in a different set of NEW web pages... Coming soon!

Last Update : 29th April 2006

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