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OPEN STAR CLUSTERS : 3 of 10



History of Open Star Clusters

Visual Discoveries

Of all the Open Clusters, the two most famous are the northern sky asterisms known as The Hyades and The Pleiades, have existed since ancient times. They are both these clusters are mentioned in Homers The Iliad and also appear in the Bible. Perhaps the most famous biblical quote appears in Job 9, 9;

[He] who made the Bear and Orion, the Pleiades and the chambers of the south. (10)

According to Richard Allens Star-Names and their Lore and Meanings, the first written record of individual stars in these clusters were from Chinese annals in 2,357 BC, being star, Alcyone, then only 3° from the then vernal equinox. (11) The next identified record was 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.

Claudius Ptolemy (c.90 AD-168 AD) in c.138 AD found the next bright open cluster being M7 (NGC 6475) in the tail of Scorpius, who also noted in the Almagest …a lucid spot. Another group is the Coma Berenices Star Cluster (12) (Mel 111) surrounding the star 15 Coma Berenices. Another three clusters nebulosities are 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, which 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 opalescence, haze or lucid spots scattered occasionally along the Milky Way or among the constellations that follow this pathway.

New Discoveries in the 17th Century

Since the advent of the telescope, as first gazed through by Galileo Galilei (1564-1642), found that some, if not all, of the Milky Way and the unresolved nebulae consisted of many many faint stars invisible to the naked-eye. Although he may have never seen explicitly saw any open clusters, the implications was clear that most, if not all, of these nebulae 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 the central star. This notion was finally concluded in 1864, when Higgins discovered many of the nebulae were true clouds of hot and luminous incandescent gas.

First to examine the nature of the star clusters was the recently discovered records of the Sicilian born Giovanni Batista Horierna (1597-1660). He examined and discovered several new objects sometime before 1654, including the southern open clusters Canis Majors 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 Cluster of Perseus (NGC 869 & NGC 884), Scorpius M6 and M7, and NGC 6231. Hodierna examined these nebulae in some detail, categorising them into three principal types (13).


Image of NGC 6231
20cm. 803× 25mm Kellner (24′ arcmin)

Edmond Halley (1656-1742) in 1677 again independently 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 23rd May 1764.(14) Philippe Loys de Cheseaux (1781-1751) was the first to identify M7s 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 and clusters that were hitherto unknown. This included the second most prominent globular star cluster, now known as 47 Tucanae / NGC 104 and the magnificent 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 many of the best clusters in the southern skies, probably being; The Jewel Box / NGC 4755, the Omicron or Velorum Cluster / IC 2395. The Southern Pleiades / IC 2602, The Football Cluster / NGC 3532 (See Image below) and NGC 2516 in Carina, and Centaurus, NGC 3766.


Image of NGC 3532
20cm. 33× 60mm Plössel (55′ arcmin)

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 of 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 or 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. Initially, 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 kinds of objects, but it was really throughout the 17th Century that the accidental and quite sporadically discovery of several more of these clusters occurred.

After Charles Messier made his discoveries, most of the other northern clusters were found by Sir William Herschel during in the 1780s-90s during his scanning of the heavens which he referred as star-gauging observations in the hope of determining the overall Milky Way structure.

First person to recognise the physical relationship of the observed stars in the open clusters were physically related was in 1767 by Reverend John Michell (1724-1793). He soon calculated that the statistical probability of even just one group of stars — like the open star cluster Pleiades — being just chance alignment was very small I.e. 496,000:1. This conclusion appeared in the Philosophical Transactions of the Royal Society, 57, 234-264 (1767).

From the southern hemisphere point of view, 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 1820s, James Dunlop (Δ) found several of these in his famous 629 deep-sky object catalogue, many of which found after the Brisbane Paramatta Star Catalogue (PSC) observations in 1827. Less than ten years later, even more southern cluster were found by Sir John Herschel between 1834 and 1838 from his location in South Africa.

Discovery of southern and northern clusters remained static for almost fifty years until the introduction of Dreyers New General Catalogue or NGC in 1888. Many modern catalogues, like Dias (2002), recognise the total of 501 New General Catalogue (NGC) clusters (31%). In the early 1900s, made many additions of wide clusters. Most of these were discovered with the advent of a astrophotography — the first group of twenty-eight (28) became incorporated into the Index Catalogue (IC). Some of these clusters include; IC 2602 (Southern Pleiades), IC 2395 (Omicron Velorum Cluster), IC 4665 (Ophiuchus) and IC 2944 (West of the Cross in Centaurus).

Image of IC 2602 (Carina) :
5cm. Finder (4½°)

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 open clusters are simply catalogued by the observers 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;

OBSERVER RANGE YEAR ABB. NOTES
Basel 1 to 20 1979 Ba  
Berkeley 1 to 104 1960 Be  
BH 1 to 261 1975 VdB-Ha  
Bochum 1 to 15 1977 Bo  
Collinder 1 to 471 1931 Cr  
Czernik 1 to 45 1966 Cz  
Dolidze 1 to 47 1966 Do  
ESO fff-nNNN 133 OSC 1982 ESO All Sky
Haffner 1 to 26 1957 Haf Southern
Harvard 1 to 21 1979 Ha Older 'H'
Hogg 1 to 23 1965 Ho Southern
King 1 to 26 1949,66 Ki Northern
Loden 1 to 2326 1972-81 Lo  
Lyngå 1 to 15 1964 Lynga Southern Cen-Nor
Melotte 1 to 227 1915 Mel  
Pismis 1 to 27 1959 Pis Southern
Ruprecht 1 to 176 1960 Ru  
Stock 1 to 24 1956 Stock  
Trumpler 1 to 37 1930 Tr Mostly Southern
Turner 1 to 11 2002 Turner  
van der Bergh 1 to 152 1975 vdBergh  

NOTES:

* vdB-Ha van den Bergh and Hagen.
** fff-nNNN ESO field number plate :
Where; 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 1970s, there were an estimated 25,000 galactic open clusters in the Milky Way. These numbers have slowly risen to about 40,000 (1994), and some recent sources now even claim around 80,000 (2004). As a general subgroup, there are recognised about 1,600 (2002) well-established open star clusters that have been adequately investigated within about 4.0 kiloparsecs from the Sun. All of these are likely available to observe in amateur apertures.


Modern Open Cluster Catalogues

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 astrophysical book specifically on all clusters. This catalogue contains some 250 cluster examples with general fundamental properties. Although much of the information is now quite obsolete, the data in many sources still 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. Its main catalogue has all known references on open clusters, globulars and the associations, placed in a huge card file system. This once came in one purple coloured box that weights just over 5kg! Updates in the 1981 edition have several additional supplemental books. In writing text on clusters, this is particularly useful as the references the original data give a good view on the development of any selected cluster. (Having my own personal copy certainly helps!) Some of the historical 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 or 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.

In 1970, the IAU based many of these designations on the decree by IAU Commission 37. (18). Although considered useful, its usage seems to be on the decline — probably because it uses the older 1950 co-ordinate system.

Lyngås 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 this 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 A&A., 389, 871 (2002). This contains more recent updates but is given with less extensive summaries. Some 1600 open star clusters are listed, being 25% larger than the older 5th OCD. (See Webpage Open Clusters and Galactic Structure. It also has an important and useful List of Removed Clusters.)

Additional information given in this catalogue includes the mass estimation of the cluster in solar masses, and whether or not the cluster has any available photometry for useful analysis.

One of the more parts of these catalogues are the availability on the Internet based 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. It is presently developed and maintained by Jean-Claude Mermillion. WEBDA allows fast access to all current data on all open clusters. Also the site also gives some general information regarding Cluster Parameters” by A.L. Tadross, P. Werner, A. Osman and M. Marie (2002). This contains information on the top two-hundred (200) clusters.

Another very useful amateur open cluster catalogue appears within the Willman-Bell 498 page book Star Clusters by Brent A. Archinal and Steven J. Hynes (2003)



References and Endnotes

10. 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.

11. 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 Pleiades position would have had highly significant importance, and this is suspected in a number of different cultures.

12. Not to be confused with the Coma Berenices galaxy cluster!

13. 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 northern observers 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. Advancement to astronomy by these clusters alone may have shortened the finding of the stellar natures of clusters so much sooner. I.e. By about 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 1850s.

15. I have included M24 (NGC 6603) as OSC, but this actual Messier discovery of this is debatable. According to Sky Catalogue 2000.0, Messier only describes the simple brightening of the Milky Way. Some, like Kenneth Glyn Jones in The Search for Nebulae; (1975) claims it is 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 telescopic equipment similar to Messier. Galaxy M104, for example, was added by Flammarion in 1921, Hogg in 1947, Gingerich in 1960 and Kenneth Glyn Jones in 1966.

17. 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 (45) objects. 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. Details can be found in Celestial Designations by Fenandoz, Larlet and Spite F. (1983) and The First Dictionary of the Nomenclature of Celestial Objects; A&A.Sup.Ser., 52, 4, 1.1 – 7.14 (1983)



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