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Open star clusters contain tens to thousands of gravitationally bound stars. Several hundred are known from our own galaxy, and are popular objects for amateur observers. Some of their basic characteristics are described, with emphasis upon their evolution, using primarily southern hemisphere examples.
Keywords: open cluster, galactic cluster, star formation, cluster evolution, luminosity function
Open star clusters contain tens to thousands of gravitationally bound stars. Several hundred are known from our own galaxy, and many are popular objects for amateur observers. The most famous, and easiest to see with the unaided eye, is the Pleiades cluster in Taurus (see below). Other good examples are the incomparable NGC 3532 in Carina, NGC 4755 (the ‘Jewel Box’) in Crux, M44 (Praesepe, also known as the Beehive) in Cancer, M6 in Scorpius and, for northern hemisphere sky-watchers, the double cluster c and h Persei.
In older publications they are usually called ‘galactic clusters,’ because we see them inside the body of our galaxy, but now it is more common to refer to them as ‘open clusters’ because their member stars are much less concentrated than those of globular clusters. They are different to globulars in many other respects too: their loose structures contain anywhere from a few dozen to a few thousand stars, the brightest of which are mostly blue in colour; they are irregular in shape and there is a great range in size (1-20 pc) and in number of members; distribution is concentrated about the galactic plane. Many of the younger clusters contain gas and dust.
Most open clusters are young: generally less than a few hundred million years old. They are rich in the youngest and most heavy element-rich stars. Over millions of years, the tidal gravitational forces in the host galaxy tend to shred these clusters of thousands of stars, scattering the stars into the general population of stars wandering interstellar space.
The Sun was probably formed in such a cluster ~5 billion years ago, but the rest of the stars in its family have long since been dispersed.
Unlike globular clusters, which have a Gaussian luminosity function, the number of open clusters increases monotonically towards fainter luminosities (fig. 1).
Open clusters (relative to globular clusters) are young clusters of metal rich Population I stars. They are believed to originate from large cosmic gas and dust clouds (see below), and to continue to orbit the galaxy through the disk. Since all the stars in a cluster formed from the same diffuse nebula, they are all of similar initial chemical composition.
The process of formation takes only a relatively short time compared to the overall lifetime of the cluster, so evolution of the birth cloud (e.g. by supernova enrichment) is not thought to contribute a significant effect.
Size Range and Concentration
The number of member stars varies from as few as ten to a few thousands. Multiple systems are common.
The true diameters of open clusters are typically between 1 and 20 pc and about 80% of them are between 2 and 6 pc. In general, the diameter is smaller for clusters where there are many stars, highly concentrated. However, since the exact limits of clusters are difficult to determine, these generalisations are quite approximate.
The density or concentration of member stars varies widely: a factor of at least 300 separates the very sparse Hyades from a very dense open cluster such as M11. Generally there is less of an increase in concentration towards the centre than in globular clusters.
These characteristics have led to various classification attempts. Trumpler distinguished four groups on the basis of concentration, ranging from (I) highly concentrated, so that the cluster stands out clearly against the background, to (IV) where the cluster looks like an accidental accumulation of stars in the general field. An additional Arabic number indicates whether the absolute magnitudes of all the stars of the cluster (1) are about the same, (2) are spread uniformly over a wider range, or (3) are distributed among a few very bright and a large number of weaker stars. Finally, the letters p, m and r indicate whether the cluster is poor (fewer than 50 members), moderate (50-100 members), or rich (> 100 members). According to Trumpler's system, the Hyades is classified II 3m and the Pleiades II 3r.
Open clusters are the youngest formations in the galaxy (cf. globular clusters). They form along the galactic plane where there is an abundance of dust and gas, hence the vast majority are found near the galactic equator. However, as they orbit the galactic centre, their orbits become perturbed by their passage through interstellar clouds or close approaches to other massive bodies. Although the effect of any one encounter is usually minor, the effect is cumulative.
Open clusters are believed to originate from large cosmic gas (predominantly H II) and dust clouds, which typically have temperatures around 10K and densities in the order of 108 atoms/m3.
The actual process of formation takes only a relatively short time compared to the overall lifetime of the cluster, so that all member stars are of similar age. Also, as all the stars in a cluster formed from the same diffuse nebula, they are all of similar initial chemical composition.
Since the diameter of a distant cluster is small relative to its distance from the Earth, to a first order approximation, all of the stars may be regarded as being at the same distance.
Owing to these properties, open clusters are of great interest for astrophysicists.
Since the member stars of any one cluster are of similar age, composition and distance, any differences we observe (e.g. in spectrum and apparent magnitude) are presumed to be due solely to their different masses. Heavier stars evolve faster.
Comparing the "standard" HRD, derived from nearby stars with sufficiently well known distances, or the theory of stellar evolution, with the measured HRD of star clusters, provides a reliable method to determine the distance of star clusters. Comparing their HRD with stellar theory provides a reasonable way to estimate the age of star clusters.
As clusters age, the cumulative effect of encounters with clouds or other massive bodies, is turbulence within the cluster itself. Some of the low mass K and M stars are accelerated to escape velocity and leave the cluster entirely. Older clusters gradually become impoverished in the low-mass, low luminosity red and yellow dwarf stars, and eventually disperse altogether. Virtually all stars form in clusters but most, including our Sun, have become scattered as the clusters disperse.
The evolution of open clusters is best illustrated by studying clusters of different ages, such as the examples in the following section.
NGC 2264, the cluster associated with the famous cone nebula in Monoceros, is a very young cluster of stars lying in front of and slightly embedded in a dark cloud, about 740 to 800 pc (2400 – 2600 light years) away. The hottest stars of the cluster have already arrived at the main sequence but stars cooler than about 10,000K mostly fall above the main sequence. "A natural interpretation is that all the stars in the cluster formed at approximately the same time. But the more massive ones evolved faster. These have already contracted to the main sequence, while the less massive ones (about 3 solar masses and less) are still pre-main sequence stars" (Zeilik 1991, p. 329).
This cluster probably started forming about 2 million years ago.
|NGC 6231 is another exceptionally young cluster – perhaps as young as three million years – located in Scorpius and containing z Scorpii. It is rich in extremely hot, high-luminosity, young giant and supergiant stars. One cluster member, z1 Scorpii, has an absolute magnitude of -8.7 in visual wavelengths (a luminosity of 250,000 suns) and a bolometric absolute magnitude of -10.8. It is one of the most brilliant stars known from our galaxy. This cluster is also notable for containing two of the rare Wolf-Rayet stars.|
|NGC 1432/5, better known as the Pleiades, is a somewhat older open cluster, including many blue giants, subtending about 1° of sky at 125 pc (410 light years) distance (Þ 7 light years diameter). However, the visible "cluster" is only the core of a volume up to 9 pc (30 light years) across and containing as many as 500 member stars. The cluster began forming within the past 50 to 60 million years and the youngest members may be as young as 2 million years. It includes BU Tauri, also known as Pleione, a shell star which throws off rings of gas at irregular intervals, causing it to fluctuate unpredictably between magnitudes 5.0 and 5.5. A dust cloud which surrounds the stars, forming a faint reflection nebula around the brightest stars, is thought to have moved into the cluster recently, rather than being the remnant of the star forming cloud.|
|The Hyades is another bright cluster, surrounding the foreground star a Tauri, usually known as Aldebaran. The visual grouping comprises about 200 stars, subtending about 5 degrees of sky at 45 pc (150 light years, Þ 13 light years diameter). Again, however, this is only the core of the true extent of the cluster which may be over 18 pc (60 light years) across and includes many hundred stars. This cluster is estimated to have begun forming 500 million years ago, so is about ten times older than the Pleiades.|
in Coma Berenices is another cluster, containing around 40 stars
centred around 12 Comae Berenices. Also about 500 million years old,
the Coma cluster was star-poor from its origin and therefore
particularly susceptible to star loss. Already it has no members
fainter than absolute magnitude +6. This cluster is 80 pc (260
light years; Malin & Frew 1995, p. 213) away and subtends 5°
which translates to 22 light years in diameter. However,
several of the bright stars in the area are unrelated line-of-sight
stars: they include g ,
7, and 18 Comae. The Comae cluster appears almost at the
galactic pole; as far from the galactic plane as it can
get. However, this is an artefact of its relative proximity;
at 80 pc it still lies well within the galactic disc. It has a radial
velocity of zero, so it is not leaving the disc.
The Coma cluster contains only about 100 solar masses which, spread out over its 22 light year diameter, yields a density of only 1/60th solar masses per cubic light year. The cluster is believed to be in equilibrium right on the point of falling apart, which explains the loss of all low-mass members of the cluster. At 500 million years of age, the Coma cluster is probably on its third circuit about the galactic centre and will dissociate entirely long before it reaches the age of the next cluster we consider: NGC 752.
in Andromeda is an ancient cluster, located about 400 pc (1300 light
years) away from earth. The cluster is characterised by two
interesting peculiarities that are consequences of its estimated 1.7
billion years age. First, it lies about 180 pc (600 light
years) out of the plane of the Milky Way, where most open clusters are
found. Although clusters are often moved out of the galactic plane by
their passage through interstellar clouds or close approaches to other
massive bodies, it takes a very long time for the orbital plane to
become as inclined as that of NGC 752. Second, its 70 or 80 members all
have absolute magnitudes between +1 and +4. It has therefore
lost all of its ordinary stars fainter than absolute magnitude +4: the
faint, low-mass dwarf stars that are usually a clusters most numerous
The stars of NGC 752, and it's contemporaries NGC 2158 and NGC 7789, are metal-poor by a factor of 2. Arp 1962 suggests that such clusters formed in regions where star formation is slower and the inter-stellar gas is metal-poor; "toward the outside of the galaxy" (pp. 72-73; also see Burnham 1989a, pp. 153-154).
| Other ancient
star clusters are NGC 2682 (M67) in Cancer, NGC 188
in Cepheus, and NGC 6791 in Lyra.
"The ages of the old disk star clusters M67, NGC 188, and NGC 6791 are derived on the basis of theoretical solar calibrated isochrones. The ages of M67 and NGC 188 were found to be about 4.0 +1.0 or -0.5 Gyr (close to the age of the sun) and 6.5 +1.5 or -0.5 Gyr, respectively. The age of NGC 6791 was found to be about 1.0 Gyr larger than that of NGC 188, assuming that the two clusters have the same metallicity. If, however, NGC 6791 is more metal rich, its age is less certain; it could be as low as 6.5 Gyr" (Demarque et al. 1992, Abstract).
A more recent source (Kaler 2001, p. 193) notes that NGC 188 and NGC 6791, both of which "are accessible with a decent amateur telescope, ... are estimated to be about eight billion years old, three billion more than the Sun. (When the Sun was born, they were still losing their class F stars, which have since evolved into white dwarfs.) NGC 6791 may in fact approach 10 billion years, depending on whose study one adopts. The oldest known is an obscure open cluster in Auriga called 'Berkeley 17,' which seems to fall between 10 and 13 billion years old."
Arp, Halton 1962: Intermediate-Age Star Clusters. Astrophysical Journal 136: 66-74.
Burnham, Robert 1989a: Burnham’s Celestial Handbook. Volume 1 – Andromeda to Cetus. Dover.
Demarque, Pierre; Guenther, D. B.; Green, E. M. 1992: Solar Calibration and the Ages of the Old Disk Clusters M67, NGC 188, and NGC 6791. Astronomical Journal v. 103, pp. 151-162.
Kaler, James B. 2001: Extreme stars at the edge of creation. Cambridge, 236 pp.
Malin, David; Frew, David J. 1995: Hartung's Astronomical Objects for Southern Telescopes. A Handbook for Amateur Observers. Melbourne University Press. 428 pp.
Van den Bergh, Sidney; Lafontaine, Andy 1984: Luminosity Function of the Integrated Magnitudes of Open Clusters. Astronomical Journal 89: 1822-1824.
Zeilik, Michael 1991: Astronomy: The Evolving Universe (6th ed). Wiley.
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