Introduction

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring.

Distribution

Open stars clusters are collections of stars with similar ages, chemical compositions, and distances from the Sun. Open clusters are strongly concentrated close to the galactic plane where they form from cold dense clouds of molecular gas and dust. There are over 1,000 known open clusters in our galaxy, but the actual number may be up to ten times higher. The number of stars in open clusters, typically from hundreds to a few thousands, makes them only loosely bound by gravity and thus vulnerable to dynamical disruption in the dense traffic of the galactic disk. Accordingly, most open clusters are younger than a few hundred million years, and older clusters are preferentially found at greater distances from the galactic center.

Open clusters are strongly concentrated toward the Milky Way. They form a flattened disklike system 2,000 light-years thick, with a diameter of about 30,000 light-years. The younger clusters serve to trace the spiral arms of the Galaxy, since they are found invariably to lie in them. Very distant clusters are hard to detect against the rich Milky Way background. A classification based on central concentration and richness is used and has been extended to nearly 1,000 open clusters. Probably about half the known open clusters contain fewer than 100 stars, but the richest have 1,000 or more. The largest have apparent diameters of several degrees, the diameter of the Taurus cluster being 400 arc minutes (nearly seven arc degrees) and that of the Perseus cluster being 240 arc minutes.

The linear diameters range from the largest, 75 light-years, down to 5 light-years. Increasingly, it has been found that a large halo of actual cluster members surrounds the more-noticeable core and extends the diameter severalfold. Cluster membership is established through common motion, common distances, and so on. Tidal forces and stellar encounters lead to the disintegration of open clusters over long periods of time as stars “evaporate” from the cluster.

Stars of all spectral classes from O to M (high to low temperatures) are found in open clusters, but the frequency of types varies from one cluster to another, as does concentration near the centre. In some (O or OB clusters), the brightest stars are blue, very hot spectral types O or B. In others, they are whitish yellow, cooler spectral type F. High-luminosity stars are more common than in the solar neighbourhood, and dwarfs are much more scarce. The brightest stars in some open clusters are 150,000 times as bright as the Sun. The luminosity of the brightest stars at the upper end of the main sequence varies in clusters from about −8 to −2 visual magnitude. (Visual magnitude is a magnitude measured through a yellow filter, the term arising because the eye is most sensitive to yellow light.)

Because of the high luminosity of their brightest stars, some open clusters have a total luminosity as bright as that of some globular clusters (absolute magnitude of −8), which contain thousands of times as many stars. In the centre of rich clusters, the stars may be only one light-year apart. The density can be 100 times that of the solar neighbourhood. In some, such as the Pleiades and the Orion clusters, nebulosity is a prominent feature, while others have none. In clusters younger than 25 million years, masses of neutral hydrogen extending over three times the optical diameter of the cluster have been detected with radio telescopes. Many of the OB clusters mentioned above contain globules—relatively small, apparently spherical regions of absorbing matter. The most-numerous variables connected with young open clusters are the T Tauri type ( and related stars that occur by the hundreds in some nebulous regions of the sky. Conspicuously absent from open clusters is the type most common in globular clusters, the RR Lyrae stars. Other variables include eclipsing binary stars (both Algol type and contact binaries), flare stars, and spectrum variables, such as Pleione. The last-named star, one of the Pleiades, is known to cast off shells of matter from time to time, perhaps as a result of its high rotational speed (up to 322 km/sec). About two dozen open clusters are known to contain Population I Cepheids, and since the distances of these clusters can be determined accurately, the absolute magnitudes of those Cepheids are well determined. This has been of paramount importance in calibrating the period-luminosity relation for Cepheids, and thus in determining the distance scale of the universe.

The colour- or spectrum-magnitude diagram derived from the individual stars holds vital information. Colour-magnitude diagrams are available for about 200 clusters on the UBV photometric system, in which colour is measured from the amount of light radiated by the stars in the ultraviolet, blue, and visual (yellow) wavelength regions. In young clusters, stars are found along the luminous bright blue branch, whereas in old clusters, beyond a turnoff only a magnitude or two brighter than the Sun, they are red giants and supergiants.

Distances can be determined by many methods—geometric, photometric,and spectroscopic—with corrections for interstellar absorption. For the very nearest clusters, direct (trigonometric) parallaxes may be obtained, and these are inversely proportional to the distance.Distances can be derived from proper motions, apparent magnitudes of the brightest stars, and spectroscopically from individual bright stars. Colour-magnitude diagrams, fitted to a standard plot of the main sequence, provide a common and reliable tool for determining distance. The nearest >open cluster is the nucleus of the Ursa Major group at a distance of 65 light-years; the farthest clusters are thousands of light-years away.

Motions, including radial velocities and proper motion, have been measured for thousands of cluster stars. The radial velocities of open cluster stars are much smaller than those of globular clusters, averaging tens of kilometres per second, but their proper motions are larger. Open clusters share in the galactic rotation. Used with galactic-rotation formulas, the radial velocities provide another means of distance determination.

A few clusters are known as moving clusters because the convergence of the proper motions of their individual stars toward a “convergent point” is pronounced. The apparent convergence is caused by perspective: the cluster members are really moving as a swarm in almost parallel directions and with about the same speeds. The Hyadesis the most-prominent example of a moving cluster. The Hyades stars are converging with a velocity of 45 km/sec toward the point in the sky with position coordinates right ascension 94 arc degrees, declination +7.6 arc degrees.) The Ursa Major group, another moving cluster, occupies a volume of space containing the Sun, but the Sun is not a member. The cluster consists of a compact nucleus of 14 stars and an extended stream.

Stellar groups are composed of stars presumed to have been formed together in a batch, but the members are now too widely separated to be recognized as a cluster.

Of all the open clusters, the Pleiades is the best known and perhaps the most thoroughly studied. This cluster, with a diameter of 35 light-years at a distance of 380 light-years, is composed of about 500 stars and is 100 million years old. Near the Pleiades in the sky but not so conspicuous, the Hyades is the second nearest cluster at 150 light-years. Its stars are similar to those in the solar neighbourhood, and it is an older cluster (about 615 million years in age). Measurements of the Hyades long formed a basis for astronomical determinations of distance and age because its thoroughly studied main sequence was used as a standard. The higher-than-usual metal abundance in its stars, however, complicated matters, and it is no longer favoured in this way. Coma Berenices, located 290 light-years away, is an example of a “poor” cluster, containing only about 40 stars. There are some extremely young open clusters. Of these, the one associated with theOrion Nebula, which is some 4 million years old, is the closest, at a distance of 1,400 light-years. A still younger cluster is NGC 6611, some of the stars in which formed only a few hundred thousand years ago. At the other end of the scale, some open clusters have ages approaching those of the globular clusters. M67 in the constellation Cancer is 4.5 billion years old, and NGC 188 in Cepheus is 6.5 billion years of age. The oldest known open cluster, Collinder 261 in the southern constellation of Musca, is 8.9 billion years old.

OB and T associations

The chief distinguishing feature of the members of a is that the large majority of constituent stars have similar physical characteristics. An OB associationconsists of many hot blue-giant stars, spectral classes O and B, and a relatively small number of other objects. A T association consists of cooler dwarf stars, many of which exhibit irregular variations in brightness. The stars clearly must be relatively close to each other in space, though in some cases they might be widely dispersed in the sky and are less closely placed than in the open clusters.

The existence of an OB association is usually established through a study of the space distribution of early O- and B-type stars. It appears as a concentration of points in a three-dimensional plot of galactic longitude and latitude and distance. More than 70 have been cataloged and are designated by constellation abbreviation and number (e.g., Per OB 1 in the constellation Perseus). In terms of dimensions, they are larger than open clusters, ranging from 100 to 700 light-years in diameter, and usually contain one or more open clusters as nuclei. They frequently contain a special type of multiple star, the Trapezium> (named for its prototype in Orion), as well as supergiants, binaries, gaseous nebulas, and globules. Associations are relatively homogeneous in age. The best distance determinations are from spectroscopic parallaxes of individual stars—i.e., estimates of their absolute magnitudes made from studies of their spectra. Most of those known are closer than 10,000 light-years, with the nearest association, straddling the boundary between Centaurus and Crux, at 385 light-years.

Associations appear to be almost spherical, though rapid elongation would be expected from the shearing effect of differential galactic rotation. Expansion, which is on the order of 10 km/sec, may well mask the tendency to elongate, and this is confirmed in some. Tidal forces break up an association in less than 10 million years through differences in the attraction by an outside body on members in different parts of the association.

A good example of an OB association is Per OB 1, at a distance of some 7,500 light-years, which spreads out from the double cluster and χ Persei. A large group of 20 supergiant stars of spectral type M belongs to Per OB 1. Associations with red supergiants may be in a relatively advanced evolutionary stage, almost ready to disintegrate.

The T associations (short for T Tauri associations) are formed by groups of T Tauri stars associated with the clouds of interstellar matter (nebulas) in which they occur. About three dozen are recognized. A T Tauri star is characterized by irregular variations of light, low luminosity, and hydrogen line (H-alpha) emission. It is a newly formed star of intermediate mass that is still in the process of contraction from diffuse matter. The small motions of T Tauri stars relative to a given nebula indicate that they are not field stars passing through the nebula. They are found in greatest numbers in regions with bright O- and B-type stars.

T associations occur only in or near regions of galactic nebulosity, either bright or dark, and only in obscured regions showing the presence of dust. Besides T Tauri stars, they include related variables, nonvariable stars, and Herbig-Haro objects—small nebulosities 10,000 astronomical units in diameter, each containing several starlike condensations in configurations similar to the Trapezium, Theta Orionis, in the sword of Orion. These objects are considered to be star groups at the very beginning of life.

The constellation of Cygnus has five T associations, and Orion and Taurus have four each. The richest is Ori T2, with more than 400 members; it has a diameter of 50 by 90 light-years and lies at a distance of 1,300 light-years around the variable star T Ori.

 

Dynamics

Seen from intergalactic space, the Milky Way Galaxy would appear as a giant luminous pinwheel, with more than 150 globular clusters dotted around it. The richest parts of the spiral arms of the pinwheel would be marked by dozens of open clusters. If this panorama could be seen as a time-lapse movie, the great globular clusters would wheel around the galactic centre in elliptical orbits with periods of hundreds of millions of years. The open clusters and stellar associations would be seen to form out of knots of diffuse matter in the spiral arms, gradually disperse, run through their life cycle, and fade away, while the Sun pursued its course around the galactic centre for billions of years.

Young open clusters and associations, occupying the same region of space as clouds of ionized hydrogen (gaseous nebulas), help to define the spiral arms. A concentration of clusters in the bright inner portion of the Milky Way between galactic longitudes 283° and 28° indicates an inner arm in Sagittarius. Similarly, the two spiral arms of Orion and Perseus are defined between 103° and 213°, with a bifurcation of the Orion arm. Associations show the existence of spiral structure in the Sun’s vicinity. Older clusters, whose main sequence does not reach to the blue stars, show no correlation with spiral arms because in the intervening years their motions have carried them far from their place of birth.

All the O- and B-type stars in the Galaxy might have originated in OB associations. The great majority, if not all, of the O-type stars were formed and still exist in clusters and associations. Though only 10 percent of the total number of B-type stars are now in OB associations or clusters, it is likely that all formed in them. At the other (fainter) end of the range of stellar luminosities, the number of dwarf variable stars in the nearby T associations is estimated at 12,000. These associations are apparently the main source of low-luminosity stars in the neighbourhood of the Sun.

While large numbers of associations have formed and dispersed and provided a population of stars for the spiral arms, the globular clusters have survived relatively unchanged except for the evolutionary differences that time brings. They are too massive to be disrupted by the tidal forces of the Galaxy, though their limiting dimensions are set by these forces when they most closely approach the galactic centre. Impressive as they are individually, their total mass of 10 million suns is small compared with the mass of the Galaxy as a whole—only about 1/10,000. Their substance is that of the Galaxy in a very early stage. The Galaxy probably collapsed from a gaseous cloud composed almost entirely of hydrogen and helium. About 14 billion years ago, before the last stages of the collapse, matter forming the globular clusters may have separated from the rest. The fact that metal-rich clusters are near the galactic nucleus while metal-poor clusters are in the halo or outer fringes may indicate a nonuniform distribution of elements throughout the primordial mass. However, there is evidence that galaxies are given to cannibalism, in which smaller galaxies merge with larger ones that do not necessarily have the same properties. This has complicated the picture of chemical evolution. The case of the globular cluster Omega Centauri suggests this merging also may happen on smaller scales. Its stars are unusual, perhaps unique, in having a variety of chemical compositions, as though they came from more than one earlier cluster.

In a study of star clusters, a time panorama unfolds—from the oldest objects existing in the Galaxy, the globular clusters, through clusters in existence only half as long, to extremely young open clusters and associations that have come into being since humans first trod Earth.

Extra-Galactic

Clusters have been discovered and studied in many external galaxies,particularly members of the Local Group (a group of about 40 stellar systems to which the Galaxy belongs). At their great distances classification is difficult, but it has been accomplished from studies of the colours of the light from an entire cluster (integrated colours) or, for relatively few, from colour-magnitude diagrams.

Clusters have been found by the hundreds in some of the nearest galaxies. At the distance of the Magellanic Clouds, a cluster like the Pleiades would appear as a faint 15th-magnitude object, subtending 15 seconds of arc instead of several degrees. Nevertheless, it is estimated that the Small Magellanic Cloud, at a distance of 200,000 light-years, contains about 2,000 open clusters. In the Large Magellanic Cloud, at a distance of 163,000 light-years, over 1,200 of an estimated 4,200 have been cataloged. Most of them are young blue-giant open clusters such as NGC 330 and NGC 1866. The open clusters contain some Cepheid variables and in chemical composition are similar to, but not exactly the same as, those of the Galaxy. The globular =clusters fall into two distinct groups. Those of the first group, the red, have a large metal deficiency similar to the globular clusters in= the Galaxy, and some are known to contain RR Lyrae variables. The globular clusters of the second group are large and circular in outline,= with colours much bluer than normal galactic globular clusters and with ages of about one million to one billion years. They are similar to the open clusters of the Magellanic Clouds but are very populous. The= observed differences between clusters in the Galaxy and the Magellanic Clouds result from small differences in helium or heavy-element= abundances. There are at least 122 associations with a mean diameter of 250 light-years, somewhat richer and larger than in the Galaxy. Sixteen of the associations contain coexistent clusters. Also, 15 star clouds =(aggregations of many thousands of stars dispersed over hundreds or even =thousands of light-years) are recognized.

In the great Andromeda spiral M31 some 2.2 million light-years away, about 400 globular clusters are known. Colour studies of some of these clusters reveal that they have a higher metal content than globular clusters of the Galaxy. Nearly 200 OB associations are known, with distances up to 80,000 light-years from the nucleus. The diameters of their dense cores are comparable to those of galactic associations. NGC 206 (OB 78) is the richest star cloud in M31, having a total mass of 200,000 suns and bearing a strong resemblance to the double cluster in Perseus. Some globular clusters have been found around the dwarf elliptical companions to M31, NGC 185, and NGC 205.

M33 in the constellation Triangulum—a spiral galaxy with thick, loose arms (an Sc system in the Hubble classification scheme)—has about 300 known clusters, not many of which have globular characteristics. Of the six dwarf spheroidal galaxies in the Local Group, only the one in the constellation Fornax has clusters. Its five globular clusters are similar to the bluest globular clusters of the Galaxy. No clusters have= been discovered in the irregular galaxies NGC 6822 and IC 1613.

Beyond the Local Group, at a distance of 45 million light-years, the giant elliptical galaxy in the Virgo cluster of galaxies is surrounded by an estimated 13,000 globular star clusters. Inspection of other elliptical galaxies in Virgo shows that they too have globular clusters whose apparent magnitudes are similar to those in M87, though their stellar population is substantially smaller. It appears that the mean absolute magnitudes of globular clusters are constant and independent of the absolute luminosity of the parent galaxy.

The total number of clusters now known in external galaxies far exceeds the number known in the Milky Way system.

History

They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, as well as losing cluster members through internal close encounters. Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud.

Importance to Research

Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest away. Open clusters are very important objects in the study of stellar evolution. Because the stars are all of very similar age and chemical composition, the effects of other more subtle variables on the properties of stars are much more easily studied than they are for isolated stars.

These qualities make open clusters important cosmic laboratories for studies of fundamental astrophysics such as: the formation of stars, stellar evolution, dynamical interactions between stars, and the chemical and dynamical evolution and structure of the disk of our Galaxy. Nearby open clusters play a key role in calibrating our measure of cosmic distances.

Examples


Jewelbox Cluster


Hyades Cluster


Persius Double Cluster


Beehive Cluster


Messier 35


Pleiades Cluster