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Globular Star Clusters

[M Globular]Click icon to view globular clusters of Messier's catalog

>> Messier's Globular Clusters;Links

The icon shows47 Tucanae (NGC 104).

Globular clusters are gravitationally bound concentrations of approximatelyten thousand to one million stars, spread over a volume of several tens to about 200 light years in diameter.

The first globular cluster discovered, but then taken for a nebula, wasM22 in Sagittarius, which was probably discovered byAbraham Ihle in 1665. This discovery was followed by that of southernOmega Centauri (NGC 5139) byEdmond Halley on his 1677 journey to St. Helena. This "nebula" had been known but classified as star since ancient times. Next followed the discovery ofM5 in Serpens Caput byGottfried Kirch in 1702, and that ofM13 in Hercules, again by Halley, in 1714.De Chéseaux'slist of (21) nebulae of 1746 contains, in addition, two new globular clusters,M71 andM4, whileJean-Dominique Maraldi discoveredM15 andM2 in September of this year (1746).Guillaume Legentil possibly or probablydiscoveredNGC 6712 in 1749.Nicholas Louis de Lacaille'scatalog of (42) southern"nebula" of 1751-52 contains 7 globular clusters (among them 4 new ones), whileMessier's catalog of 110 objects containsa total of 29 globulars, 20 of them new discoveries.Charles Messier was the first to resolve one globular cluster,M4, but still referred to the other 28 of these objects in his catalog as "round nebulae." Thus, in summer 1782, beforeWilliam Herschel started his comprehensive deep sky survey with large telescopes, there were 34 globular clusters known. Herschel himself discovered 36 new globulars (plus 2 rediscoveries: M107 and NGC 6712), bringing thenumber of known globulars to 70. He was the first to resolve virtually all of them into stars, and coined the term "globular cluster" in the discussion adjacent to his second catalog of 1000 deepsky objects (1789).

These early known globulars are all belonging to ourMilky Way Galaxy, but some of them have been integrated, or have immigrated into the Galactic Globular Cluster system only recently, includingM54 (noted 1994) andM79 (noted 2003).

Melotte's list of 1915 already lists 83 globulars,Shapley (1930) has 93, one of them in the Large Magellanic Cloud (NGC 1651).Helen Sawyer (Hogg, 1947) lists 99 Milky Way globular clusters, 97 of which are still considered as such; her 1959 list (Sawyer Hogg 1959) has 118 entries (115 real). Further lists ofArp (1965),Alcaino (1973) andKukarkin (1974) have between 119 and 131 clusters.Becvar lists 100, theSky Catalogue 2000.0 lists 138 globulars, a number given for 1987 also byKenneth Glyn Jones (1991).The more recent compilations ofWebbink (1985)andDjorgovski and Meylan (1993) have 154 and 143 (both 141 real), respectively.W.E. Harris' database of 1994 contains 142 (the same 141 real objects), that of 1996 and 1997 have 146, the 1999 version has 147, the version of February 2003 contains 150, andthe most current revision (December 2010) version contains 157, with 8 morecandidates not sufficiently confirmed yet.The curremt catalog of Holger Baumgardt contains data for 168 Galactic globularclusters (Baumgardt 2023).Of the 168 known globular clusters of ourMilky Way galaxy, 104 are in the NGC catalog, and3 more in the IC catalog of J.L.E. Dreyer.

Since their discovery and resolution, globular clusters were alwaysthought or suspected to be physical objects, agglomerations or swarms of stars held together by their mutual gravity, all close together and at about the same distance. The proof for this correct guess, or working model, came only with spectroscopy, showing that the stars of these clusters have radial velocities as expected for such swarms, and perfectly match in color-magnitude or Hertzsprung-Russell diagram, i.e. they represent a population of stars of about same age (also see below), all at about the same distance.

The distribution of the globular clusters in ourMilky Way galaxy is concentrated around the galactic center in the Sagittarius -- Scorpius -- Ophiuchus region: Of the 138 Milky Way globulars listed in the Sky Catalog 2000, these constellations contain 29, 18, and 24 globulars, respectively, so a total of 71 clusters, or 51.4 percent (though one must admit that of the 29 clusters listed in Sagittarius, probably four are members of theSagittarius Dwarf Elliptical Galaxy discovered 1994, and not really of the Milky Way, among themM54; however, these are in process to be integratedinto the Milky Way's halo). Current counting (July 2011) brings these numbers to 35, 23 and 33 globulars in Sagittarius, Scorpius and Ophiuchus, respectively, thus a total of 91 clusters of a total of 151, or 57.9 percent. Of the 88 constellations, only 46 or little more than half contain any globularcluster at all; Ara and Serpens follow with 5 each, Hercules and Aquila with 4 clusters each, and no further constellation contains more than 3.

Of the 157 known clusters, 141 (89.8 percent) are concentrated in the hemisphere centered on Sagittarius, while only 16 globulars (10.2 percent) are on the opposite side of us (among themM79). Of these 16, four (including M79) are suspected to be members of the remnant globular cluster system of theCanis Major Dwarf galaxy discovered in 2003, which again is going to be integrated into the Milky Way's Galactic Halo.

This pronounced anisotropy in the distribution of globular clusters was of historic importance whenHarlow Shapley, in 1917, derived from it that the center of our galaxy is lying at a considerable distance in the direction of Sagittarius and not close to our solar system as had been thought previously (however, he significantly overestimated the size of the Milky Way as a whole, as well as the size of the globular cluster system and our distance from the Galactic Center).

Woltjer (1975) investigated the distribution of theglobular clusters of out Milky Way and classified them as halo globulars or disk-like globulars.

Radial velocity measurements have revealed that most globulars are movingin highly excentric elliptical orbits that take them far outside the MilkyWay; they form a halo of roughly spherical shape which is highly concentratedto the Galactic Center, but reaches out to a distance of several 100,000 light years, much more than the dimension of the Galaxy's disk. As they don't participate in the Galaxy's disk rotation, they can have highrelative velocities of several 100 km/sec with respect to our solar system;this is what shows up in the radial velocity measurements.Ninkovic (1983) has estimated excentricities of globular cluster orbits.

To determine the physical orbits of globular clusters, it is required to knowtheir proper motions in addition to the radial velocities.Cudworth and Hanson (1993) undertook some first rough determinations of proper motions with respect to background galaxies.From these and similar measurements,Van den Bergh (1995) estimated perigalactic distances, andDaupholeet.al. (1996) calculated firstapproximate orbits. Much more acurate data for proper motions became onlyavailable from astrometrical data obtained with ESA's Hipparcos satellitein 1997, from which space motions (Geffertet.al. 1997) and approximate orbits (Broscheet.al. 1997) could bedetermined.

Classification of globular clusters from physical, structural parametershas been introduced in the 1920s by Harlow Shapley and Helen Sawyer Hogg(Shapley, Sawyer (1927);Shapley 1930, II.5, pp. 11-14).They introduced Concentration Classes (sometimes referred to as Shapley-Sawyer Concentration Classes), named by numbers, typically denotedRoman, from I-XII, or 1-12, with I denoting the most concentrated or denseststructure, XII the least concentrated. These concentration classes are givenin these pages for each cluster.Quick overview for the 29 Messier Globular Clusters:

The classification scheme with Concentration Classes can well be illustratedwith Messier's globular clusters:

Spectroscopic study of globular clusters shows that they are much lower in heavy element abundance than stars such as the Sun that form in the disks of galaxies. Thus, globular clusters are believed to be very old and consisted from an earlier generation of stars (Population II), which have formed from the more primordial matter present in the young galaxy just after (or even before) its formation.The disk stars, by contrast, have evolved through many cycles of starbirth and supernovae, which enrich the heavy element concentration in star-forming clouds and may also trigger their collapse.

Globular clusters typically contain a number of variable stars, in particularRR Lyrae stars which were once called "Cluster Variables" because of theirabundance in globulars. Their frequency is varying from cluster to cluster.See Christine Clement'sCatalog of Variable Stars in Globulars (Clement 2001).A small number of four globular clusters also containsplanetary nebulae; these appear to be so rare because of the short liftime ofplanetary nebulae. They also contain a large number of white dwarfs and, at least in some cases,many neutron stars, some of which show up as pulsars. To the end of 2024, pulsars have been detected in 45 globular clusters,see Paulo Freire'sPulsars in globular clusters catalog (Freire 2024).

[M5 CMD]The H-R diagrams for globular clusters (here shown forM5) typically have short main sequences and prominent horizontal branches, this again represents very old stars that have evolved past giant or supergiant phases.

Comparison of the measured HRD of each globular cluster with theoreticalmodel HRDs derived from the theory of stellar evolution provides the possibility to derive, or estimate, the age of that particular cluster.It is perhaps a bit surprising that almost all globular clusters seem to be of about the same age; there seems to be a physical reason that they all formedin a short period of time in the history of the universe, and this period was apparently long ago when the galaxies were young.Semi-recent estimates yield an age of 12 to 20 billion years; the best value for observation is perhaps 12 to 16 billion (see e.g. the discussion atM92).As their age is crucial as a lower limit for the age of our universe, it wassubject to vivid and continuous discussion since decades.In early 1997, the discussion of the age of the globular clusters got revivedbecause of the general modifications of the distance scale of the universe, implied by results of ESA's astrometrical satellite Hipparcos: These results suggest that galaxies and many galactic objects, including the globular clusters, may be at a 10 per cent larger distance; therefore, the intrinsical brightness of all their stars must be about 20 % higher. Considering the various relations which are important forunderstanding stellar structure and evolution, they should also be roughly15 % younger, in a preliminary off-hand estimate.

As globular clusters follow their orbits around the Milky Way's Galacticcenter through the billion years, they are subject to a variety ofdisturbations:

Although significantly slower compared to the less densely packed and less populatedopen clusters, these disturbations are tending to disrupt the clusters.The currently existing globulars are just the survivors of a perhaps significantly larger population, the rest of which has been disrupted and spread their stars throughout the Galactic halo. The process of destruction still works, and it was estimated that about half of the Milky Way globulars will cease to exist within the next 10 billion years.

Our galaxy has a system of perhaps about 180 to 200 globular clusters (including the 29 Messier globulars, of which two may have immigrated onlyrecently:M54 andM79).Most other galaxies have globular cluster systems as well, in some cases (e.g., forM87) containing several thousands of globulars!A small number of the known extragalacric globulars is in the reach of largeramateur telescopes; see Jim Shield'sExtragalactic Globulars webpage for examples.

While all the globulars in our Milky Way, and our big companion, theAndromeda Galaxy M31, are old, otherLocal Group galaxies as theLarge and theSmall Magellanic Cloud as well as theTriangulum Galaxy M33 also contain considerablyyounger globular star clusters, which can be concluded with certainty fromspectroscopic investigations.These galaxies contain also extremely large diffuse nebulae with masses ofthe order of globular clusters, clear candidates for future young globularscurrently in formation, notably theTarantula Nebula NGC 2070 in the LMC andNGC 604 in M33.A large number of over 100 young globular clusters have been detected recently inM82, an irregular galaxy beyond the Local Group.

Messier's globular clusters:M2,M3,M4,M5,M9,M10,M12,M13,M14,M15,M19,M22,M28,M30,M53,M54,M55,M56,M62,M68,M69,M70,M71,M72,M75,M79,M80,M92,M107.
Otherearly known globular clusters:NGC 104 (47 Tucanae),NGC 4833,NGC 5139 (Omega Centauri),NGC 6397,NGC 6712.

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References
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Binary Star Systems

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