Globular cluster
A globular cluster (sometimes known more simply as a globular) is a spherical collection of stars that orbits a galaxy core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shape, and relatively high stellar density towards their core. Globular clusters contain considerably more stars than the less dense galactic, or open clusters.
Globular clusters are fairly numerous; there are about 150 currently known globular clusters in the Milky Way (with perhaps 10–20 more undiscovered), and larger galaxies such as Andromeda tend to have more (Andromeda may have as many as 500). Some giant elliptical galaxies, such as M87, may have as many as 10,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 100 kiloparsecs or more.
Every galaxy of sufficient mass in the local group has an associated group of globular clusters, and almost every large galaxy has been found to possess a system of globular clusters.[2] The Sagittarius Dwarf and Canis Major Dwarf galaxies appear to be in the process of donating their associated globular clusters to the Milky Way (such as Palomar 12) demonstrating how many of this galaxy's globular clusters were acquired in the past.
Observation history
The first globular cluster discovered was M22 in 1665 by Abraham Ihle. However, due to the small aperture of early telescopes, individual stars within a globular cluster were not resolved until Charles Messier observed M4. Additional globular clusters were individually discovered as shown in the table below.
| Cluster name | Discovered by | Year |
|---|---|---|
| M22 | Abraham Ihle | 1665 |
| NGC 5139 | Edmond Halley | 1677 |
| M5 | Gottfried Kirch | 1702 |
| M13 | Edmond Halley | 1714 |
| M71 | Philippe Loys de Chéseaux | 1745 |
| M4 | Philippe Loys de Chéseaux | 1746 |
| M15 | Jean-Dominique Maraldi | 1746 |
| M2 | Jean-Dominique Maraldi | 1746 |
Subsequently, Abbe Lacaille would list NGC 104, NGC 4833, M55, M69, and NGC 6397 in his 1751–52 catelogue. (The M before a number refers to the catalogue of Charles Messier, while NGC is from the New General Catalogue by John Dreyer.)
William Herschel began a survey program in 1782 using larger telescopes and was able to resolve the stars in all 33 of the known globular clusters. In addition he found 37 additional clusters. In Herschel's 1789 catalog of deep sky objects, his second such, he became the first to use the name globular cluster as their description. (The word globular is derived from the latin globus and the English suffix "-ular", which means to have the shape of a globe or globule.)
The number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947. There are now a total of 151 globular clusters that have been discovered in the Milky Way galaxy, out of an estimated total of 180 ± 20. [3] (Some undiscovered globular clusters may be hidden behind the gas and dust of the Milky Way.)
Beginning in 1914, Harlow Shapley had begun a series of studies of globular clusters that were published in about 40 scientific papers. He examined the cepheid variables in the clusters and would use their period–luminosity relationship for distance estimates.
Of the globular clusters within our Milky Way, the majority are found in the vicinity of the galactic core, and the large majority lie on the side of the celestial sky centered on the core. In 1918 this strongly asymmetrical distribution was used by Harlow Shapley to make a determination of the overall dimensions of the galaxy. By assuming a roughly spherical distribution of globular clusters around the galaxy's center, he used the positions of the clusters to estimate the position of the sun relative to the galactic center.[4] While his distance estimate was significantly in error, it did demonstrate that the dimensions of the galaxy were much greater than had been previously thought. (Shapley's estimate was, however, within the same order of magnitude of the currently accepted value.)
Shapley was subsequently assisted in his studies of clusters by Henrietta Swope and Helen Battles Sawyer (later Hogg). In 1927–29, Harlow Shapley and Helen Sawyer began categorizing clusters according to the amount of concentration the system has toward the core. The most concentrated stars were identified as Class I, with successively diminishing concentrations ranging to Class XII. This became known as the Shapley–Sawyer Concentration Class. (It is sometimes given with numbers (Class 1–12) rather than roman numerals.)[5]
Composition
Globular clusters are generally composed of hundreds of thousands of old stars, similar to the bulge of a spiral galaxy but confined to a volume of only a few cubic parsecs. Some globular clusters (like Omega Centauri in our Milky Way, and G1 in M31) are extraordinarily massive clusters, weighing as many as several million solar masses. Some globular clusters (like M15) have extremely massive cores which are expected to harbor black holes.
With a few notable exceptions, each globular cluster appears to have a definite age. That is, all the stars in a cluster are at the same stage in stellar evolution, suggesting that they formed at the same time. Globular clusters are typically the oldest objects in the Galaxy, and were among the first collections of stars to form.
Metallicity
Globular clusters normally consist of Population II stars, which have a low metallicity compared to Population I stars such as the Sun. (To astronomers, metals consist of all elements heavier than Helium, such as Lithium and Carbon.)
The Dutch astronomer Pieter Oosterhoff noticed that there appear to be two populations of globular clusters, which became known as Oosterhoff groups. The second group has a slightly longer period of RR Lyrae variable stars.[6] Both groups have weak lines of metallic elements. But the lines in the stars of Oosterhoff type I (OoI) cluster are not quite as weak as those in type II (OoII). Hence type I are referred to as "metal-rich" while type II are "metal-poor".
These two populations have been observed in many galaxies (especially massive elliptical galaxies). Both groups are of similar ages (nearly as old as the universe itself) but differ in their metal abundances. Many scenarios have been suggested to explain these subpopulations, including violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In our Milky Way, the metal-poor clusters are associated with the halo and the metal-rich clusters with the Bulge.
In the Milky Way it has been discovered that the large majority of the low metallicity clusters are aligned along a plane in the outer part of the galaxy's halo. This result argues in favor of the view that type II clusters in the galaxy were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been previously thought. The difference between the two cluster types would then be explained by a time delay between when the two galaxies formed their cluster systems.[7]
Exotic components
Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars occur relatively often. Due to these chance encounters, some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries, are much more common in globular clusters. A blue straggler is formed from the merger two stars, possibly as a result of an encounter with a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, and thus differs from the main sequence stars.[8]
Astronomers have searched for the existence of black holes within globular clusters since the 1970s. However the resolution requirements for this task are exacting, and it was only with the Hubble space telescope that the first confirmed discoveries have been made. In independent programs, a 4,000 solar mass intermediate-mass black hole has been discovered in the globular cluster M15 and a 20,000 solar mass black hole in the G1 cluster in the Andromeda Galaxy.[9]
These are of particular interest because they are the first black holes discovered that were intermediate in mass between the conventional stellar-mass black hole and the supermassive black holes discovered at the cores of galaxies. The mass of these intermediate mass black holes are proportional to the mass of the clusters, following a trend previously-discovered among supermassive black holes and their surrounding galaxies.
Color-magnitude diagram
The Hertzsprung-Russell diagram (HR-diagram) is a graph of a large sample of stars that plots their visual absolute magnitude against their color index. The color index, B−V, is the difference between the magnitude of the star in blue light, or B, and the magnitude in visual light (green-yellow), or V. Large positive values indicate a red star with a cool surface temperature, while negative values imply a blue star with a hotter surface.
When the stars near the Sun are plotted on an HR diagram, it displays a distribution of stars of various masses, ages, and compositions. Many of the stars lie relatively close to a sloping curve with increasing absolute magnitude as the stars are hotter, known as main sequence stars. However the diagram also typically includes stars that are in later stages of their evolution and have wandered away from this main sequence curve.
As all the stars of a globular cluster are at approximately the same distance from us their absolute magnitudes differ from their visual magnitude by about the same amount. The main sequence stars in the globular cluster will fall along a line that is believed to be comparable to similar stars in the solar neighborhood. (The accuracy of this assumption is confirmed by comparable results obtained by comparing the magnitudes of nearby short-period variables, such as RR Lyrae stars and cepheid variables, with those in the cluster.)[10]
By matching up these curves on the HR diagram, the absolute magnitude of main sequence stars in the cluster can also be determined. This in turn provides a distance estimate to the cluster, based on the visual magnitude of the stars. The difference between the relative and absolute magnitude (the bolometric correction) yields this distance estimate.[11]
When the stars of a particular globular cluster are plotted on an HR diagram, nearly all of the stars fall upon a relatively well-defined curve. This differs from the HR diagram of stars near the Sun, which lumps together stars of differing ages and origins. The shape of the curve for a globular cluster is characteristic of a grouping of stars that were formed at the same time and from the same materials, differing only in their initial mass. As the position of each star in the HR diagram varies with age, the shape of the curve for a globular cluster can be used to measure the overall age of the collected stars.[12]
The most massive main sequence stars in a globular cluster will also have the highest absolute magnitude, and these will be the first to evolve into the giant star stage. As the cluster ages, stars of successively lower masses will also enter the giant star stage. Thus the age of a cluster can be measured by looking for the stars that are just beginning to enter the giant star stage. This forms a "knee" in the HR diagram, bending to the upper right from the main sequence line. The absolute magnitude at this bend is directly a function of the globular cluster, and the age range can be plotted on an axis parallel to the magnitude.
By this means it has been shown, for example, that the cluster NGC 1818 is only about 40 million years in age, while M4 may be as old as 12.7 thousand million years.[13] The later cluster, and other similar clusters, place a bounds on the age limit of the entire universe. This lower limit has been a significant constraint in Cosmology.
Evolutionary studies of globular clusters can also be used to determine changes due to the starting composition of the gas and dust that formed the cluster. That is, the change in the evolutionary tracks due to the abundance of heavy elements. (Heavy elements in astronomy are considered to be all elements more massive than Helium.) The data obtained from studies of globular clusters are then used to study the evolution of the Milky Way as a whole.[14]
Morphology
In contrast to open clusters, most globular clusters remain gravitationally-bound for time periods comparable to the life spans of the majority of their stars. (A possible exception is when strong tidal interactions with other large masses result in the dispersement of the stars.)
| Galaxy | Ellipticity[15] |
|---|---|
| Milky Way | 0.07±0.04 |
| LMC | 0.16±0.05 |
| SMC | 0.19±0.06 |
| M31 | 0.09±0.04 |
Although globular clusters generally appear spherical in form, ellipticities can occur due to tidal interactions. Clusters within the Milky Way and the Andromeda Galaxy are typically oblate spheroids in shape, while those in the Large Magellanic Clouds are more elliptical.[16]
Radii
Astronomers characterize the morphology of a globular cluster by means of standard radii. These are the core radius (rc), the half-light radius (rh) and the tidal radius (rc). The overall luminosity of the cluster steadily decreases with distance from the core, and the core radius is the distance at which the apparent surface luminosity has dropped by half. A comparable quantity is the half-light radius, or the distance from the core at which half the total luminosity from the cluster is received. This is typically larger than the core radius.
Note that the half-light radius includes stars in the outer part of the cluster that happen to lie along the line of sight, so theorists will also use the half-mass radius (rm)—the radius from the core that contains half the total mass of the cluster. When the half-mass radius of a cluster is small relative to the overall size, it has a dense core. An example of this is the Globular Cluster M3, which has an overall visible dimension of about 18″, but a half-mass radius of only 1.12″.[17]
Finally the tidal radius is the distance from the center of the globular cluster at which the external gravitation of the galaxy has more influence over the stars in the cluster than does the cluster itself. This is the distance at which the individual stars belonging to a cluster can be separated away by the galaxy. The tidal radius of M3 is about 38″.
Luminosity
In measuring the luminosity curve of a globular cluster as a function of radius, most clusters in the Milky Way steadily increase in luminosity up to a certain distance from the core, then the luminosity levels off. Typically this distance is about 1–2 parsecs from the core. However about 20% of the globular clusters have undergone a process termed "core collapse". In this type of cluster, the luminosity continues to steadily increase all the way to the core region.[18] An example of a core-collapsed globular is M15.
Core-collapse is thought to occur when the more massive stars in a globular encounter their less massive companions. As a result of the encounters the larger stars tend to lose kinetic energy and start to settle toward the core. Over a lengthy period of time this leads to a concentration of massive stars near the core.
The overall luminosities of the globular clusters within the Milky Way and M31 can be modelled by means of a gaussian curve. This gaussian can be represented by means of a average magnitude Mv and a variance σ. This distribution of globular cluster luminosities is called the Globular Cluster Luminosity Function (GCLF). (For the Milky Way, Mv = −7.20±0.13, σ=1.1±0.1 magnitudes. [19]) The GCLF has also been used as a "standard candle" for measuring the distance to other galaxies, under the assumption that the globular clusters in remote galaxies follow the same principles as they do in the Milky Way.
N-body simulations
Computing the interactions between the stars within a globular cluster requires solving what is termed the N-body problem. That is, each of the stars within the cluster continually interacts with the other N−1 stars, where N is the total number of stars in the cluster. The CPU computational "cost" for a simulation increases in proportion to N3', so the potential computing requirements to accurately simulate such a cluster can be enormous.[20] An efficient method of mathematically simulating the N-body dynamics of a globular cluster is done by sub-dividing into small volumes and velocity ranges, and using probabilities to describe the locations of the stars. The motions are then described by means of a formula called the Fokker-Planck equation. This can be solved by a simplified form of the equation, or by running Monte Carlo simulations and using random values. However the simulation becomes more difficult when the effects of binaries and the interaction with external gravitation forces (such as from the Milky Way galaxy) must also be included.[21]
The results of N-body simulations have shown that the stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that results in an increase in velocity, some of the stars gain sufficient energy to be able to depart the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation.[22]
Binary stars form a significant portion of the total population of stellar systems, with up to half of all stars occuring in binary systems. Numerical simulations of globular clusters have demonstrated that binaries can hinder and even reverse the process of core collapse in globular clusters. When a star in a cluster has a gravitational encounter with a binary system, a possible result is that the binary becomes more tightly bound and kinetic energy is added to the solitary star. When the massive stars in the cluster are sped up by this process, it reducing the contraction at the core and limits core collapse.[8]
Tidal encounters
When a globular cluster has a close encounter with a large mass, such as the core region of a galaxy, it undergoes a tidal interaction. The difference in the pull of gravity between the part of the cluster nearest the mass and the pull on the furthest part of the cluster results in a tidal effect. A "tidal shock" occurs whenever the orbit of a cluster takes it through the plane of a galaxy.
As a result of a tidal shock, streams of stars can be pulled away from the cluster halo, leaving only the core part of the cluster. These tidal interaction effects create tails of stars that can extend up to several degrees of arc away from the cluster.[23] These tails typically both precede and follow the cluster along its orbit. The tails can accumulate significant portions of the original mass of the cluster, and can form clump-like features.[24]
The globular cluster Palomar 5, for example, is near the perihelion of its orbit after passing through the Milky Way. Streams of stars extend outward toward the front and rear of the orbital path of this cluster, stretching out to distances of 13,000 light years.[25] Tidal interactions have stripped away much of the mass from Palomar 5, and further interactions as it passes through the galactic core will transform it into a long stream of stars orbiting the Milky Way halo.
Tidal interactions add kinetic energy into a globular cluster, dramatically increasing the evaporation rate and shrinking the size of the cluster. Not only does tidal shock strip off the outer stars from a globular cluster, but the increased evaporation accelerates the process of core collapse.
See also
References
- ^ "Hubble Images a Swarm of Ancient Stars". HubbleSite News Desk. Space Telescope Science Institute. 1999-07-01. Retrieved 2006-05-26.
{{cite news}}: Check date values in:|date=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Harris, William E. (1991). [http://adsabs.harvard.edu/abs/1991ARA&A..29..543H
accessdate=2006-06-02 "Globular cluster systems in galaxies beyond the Local Group"]. Annual Review of Astronomy and Astrophysics. 29: 543–579.
{{cite journal}}: Check|url=value (help); Missing pipe in:|url=(help); line feed character in|url=at position 50 (help) - ^ Ashman, Keith M.; Zepf, Stephen E. (1992). "The formation of globular clusters in merging and interacting galaxies". Astrophysical Journal, Part 1. 384: 50–61. Retrieved 2006-05-27.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Shapley, Harlow (1918). "Globular Clusters and the Structure of the Galactic System". Publications of the Astronomical Society of the Pacific. 30 (173): 42+. Retrieved 2006-05-30.
{{cite journal}}: Unknown parameter|author_link=ignored (help) - ^ Hogg, Helen Battles Sawyer (1965). "Harlow Shapley and Globular Clusters". Publications of the Astronomical Society of the Pacific. 77: 336–46.
- ^ T. S. van Albada, Norman Baker, (1973). "On the Two Oosterhoff Groups of Globular Clusters". Astrophysical Journal. 185: 477–498.
{{cite journal}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link) - ^ Y.W. Lee, S.J. Yoon (2002). "On the Construction of the Heavens". An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way. 297: 578. Retrieved 2006-06-01.
- ^ a b V. C. Rubin, W. K. J. Ford (1999). "A Thousand Blazing Suns: The Inner Life of Globular Clusters". Mercury. 28: 26. Retrieved 2006-06-02.
- ^ D. Savage; N. Neal; R. Villard; R. Johnson; H. Lebo (2002-09-17). "Hubble Discovers Black Holes in Unexpected Places". HubbleSite. Space Telescope Science Institute. Retrieved 2006-05-25.
{{cite news}}: Check date values in:|date=(help)CS1 maint: multiple names: authors list (link) - ^ Shapley, H. (1917). "Studies based on the colors and magnitudes in stellar clusters. I,II,III". Astrophysical Journal. 45: 118–141. Retrieved 2006-05-26.
- ^ Martin, Schwarzschild (1958). Structure and Evolution of Stars. Princeton University Press.
- ^ Sandage, A.R. (1957). "Observational Approach to Evolution. III. Semiempirical Evolution Tracks for M67 and M3". Astrophysical Journal. 126: 326. Retrieved 2006-05-26.
- ^ B.M.S. Hansen, J. Brewer, G.G. Fahlman, B.K. Gibson, R. Ibata, M. Limongi, R.M. Rich, H.B. Richer, M.M. Shara, P.B. Stetson (2002). "The White Dwarf Cooling Sequence of the Globular Cluster Messier 4". Astrophysical Journal Letters. 574: L155. Retrieved 2006-05-26.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ "Ashes from the Elder Brethren — UVES Observes Stellar Abundance Anomalies in Globular Clusters" (Press release). 2001-03-01. Retrieved 2006-05-26.
{{cite press release}}: Check date values in:|date=(help) - ^ A. Staneva, N. Spassova, V. Golev (1996). "The Ellipticities of Globular Clusters in the Andromeda Galaxy". Astronomy and Astrophysics Supplement. 116: 447–461. Retrieved 2006-05-31.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ C. S. Frenk & S. D. M. White (1980). "The ellipticities of Galactic and LMC globular clusters". Monthly Notices of the Royal Astronomical Society. 286 (3): L39–L42. Retrieved 2006-05-31.
- ^ Benacquista, Matthew J. (1994). "The Stellar Population of the Globular Cluster M 3. I. Photographic Photometry of 10 000 Stars". Astronomy and Astrophysics. 290: 69–103. Retrieved 2006-05-29.
{{cite journal}}: More than one of|author=and|last=specified (help); line feed character in|author=at position 51 (help) - ^ S. Djorgovski, I. R. King (1986). "A preliminary survey of collapsed cores in globular clusters". Astrophysical Journal. 305: L61–L65. Retrieved 2006-05-29.
- ^ Secker, Jeff (1992). "A Statistical Investigation into the Shape of the Globular cluster Luminosity Distribution". Astronomical Journal. 104 (4): 1472–1481. Retrieved 2006-05-28.
- ^ Heggie, D. C. (1998). "Dynamical Simulations: Methods and Comparisons". In Johannes Andersen (ed.). Highlights of Astronomy Vol. 11A, as presented at the Joint Discussion 14 of the XXIIIrd General Assembly of the IAU, 1997. Kluwer Academic Publishers. pp. 591+. Retrieved 2006-05-28.
{{cite conference}}: Unknown parameter|booktitle=ignored (|book-title=suggested) (help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Benacquista, Matthew J. (2006). "Relativistic Binaries in Globular Clusters". Living Reviews in Relativity (lrr-2006-2). Retrieved 2006-05-28.
- ^ J. Goodman and P. Hut, ed. (1985). Dynamics of Star Clusters (International Astronomical Union Symposia). Springer. ISBN 9027719632.
- ^ A. Lauchner, R. Wilhelm, T.C. Beers, C. Allende Prieto (2003-12). "A Search for Kinematic Evidence of Tidal Tails in Globular Clusters". American Astronomical Society Meeting 203, #112.26. American Astronomical Society. Retrieved 2006-06-02.
{{cite conference}}: Check date values in:|date=(help); Unknown parameter|booktitle=ignored (|book-title=suggested) (help)CS1 maint: multiple names: authors list (link) - ^ P. Di Matteo, P. Miocchi, R. Capuzzo Dolcetta (2004-05). "Formation and Evolution of Clumpy Tidal Tails in Globular Clusters". American Astronomical Society, DDA meeting #35, #03.03. American Astronomical Society. Retrieved 2006-06-02.
{{cite conference}}: Check date values in:|date=(help); Unknown parameter|booktitle=ignored (|book-title=suggested) (help)CS1 maint: multiple names: authors list (link) - ^ Staude, Jakob (2002-06-03). "Sky Survey Unveils Star Cluster Shredded By The Milky Way". Image of the Week. Sloan Digital Sky Survey. Retrieved 2006-06-02.
General resources
- NASA Astrophysics Data System has a collection of past articles, from all major astrophysics journals and many conference proceedings.
- SCYON is a newsletter dedicated to star clusters.
- MODEST is a loose collaboration of scientists working on star clusters.
Books
- Binney, James; Tremaine, Scott (1987). Galactic Dynamics, Princeton University Press, Princeton, New Jersey.
- Heggie, Douglas; Hut, Piet (2003). The Gravitational Million-Body Problem: A Multidisciplinary Approach to Star Cluster Dynamics, Cambridge University Press.
- Spitzer, Lyman (1987). Dynamical Evolution of Globular Clusters, Princeton University Press, Princeton, New Jersey.
Review Articles
- Elson, Rebecca; Hut, Piet; Inagaki, Shogo (1987). Dynamical evolution of globular clusters. Annual review of astronomy and astrophysics 25 565. NASA ADS
- Meylan, G.; Heggie, D. C. (1997). Internal dynamics of globular clusters. The Astronomy and Astrophysics Review 8 1. NASA ADS
External links
- Globular Clusters, SEDS Messier pages
- Milky Way Globular Clusters
- Catalogue of Milky Way Globular Cluster Parameters by William E. Harris, McMaster University, Ontario, Canada.
- A galactic globular cluster database by Marco Castellani, Rome Astronomical Observatory, Italy.