Globular cluster
Globular cluster | |
---|---|
Characteristics | |
Type | Star cluster |
Mass range | 1K M☉ - >1M M☉[1] |
Size range | 10-300 ly across[1] |
Density | ~2 stars/cubic ly [1] |
Average luminosity | ~25 000 L☉[1] |
External links | |
Media category | |
Q11276 | |
Additional Information | |
Discovered | Abraham Ihle, 1665 |
A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars,[2] all orbiting in a stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies, and the distinction between the two is not always clear.[3] Their name is derived from Latin globulus (small sphere). Globular clusters are occasionally known simply as "globulars".
Although one globular cluster,
Globular clusters are found in nearly all
Both the origin of globular clusters and their role in
History of observations
The first known globular cluster, now called
Cluster name | Discovered by | Year |
---|---|---|
M 22[5] | Abraham Ihle
|
1665 |
ω Cen[c][13] | Edmond Halley | 1677 |
M 5[14](p 237)[15] | Gottfried Kirch | 1702 |
M 13[14](p 235) | Edmond Halley | 1714 |
M 71[16] | Philippe Loys de Chéseaux
|
1745 |
M 4[16] | Philippe Loys de Chéseaux | 1746 |
M 15[17] | Jean-Dominique Maraldi
|
1746 |
M 2[17] | Jean-Dominique Maraldi | 1746 |
When William Herschel began his comprehensive survey of the sky using large telescopes in 1782, there were 34 known globular clusters. Herschel discovered another 36 and was the first to resolve virtually all of them into stars. He coined the term globular cluster in his Catalogue of a Second Thousand New Nebulae and Clusters of Stars (1789).[18][d][19] In 1914, Harlow Shapley began a series of studies of globular clusters, published across about forty scientific papers. He examined the clusters' RR Lyrae variables (stars which he assumed were Cepheid variables) and used their luminosity and period of variability to estimate the distances to the clusters. RR Lyrae variables were later found to be fainter than Cepheid variables, causing Shapley to overestimate the distances.[20]
A large majority of the Milky Way's globular clusters are found in the halo around the galactic core. In 1918, Shapley used this strongly asymmetrical distribution to determine the overall dimensions of the galaxy. 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
The count of known globular clusters in the Milky Way has continued to increase, reaching 83 in 1915, 93 in 1930, 97 by 1947,
Classification
Shapley was later assisted in his studies of clusters by
Formation
The formation of globular clusters is poorly understood.
Observations of globular clusters show that their stars primarily come from regions of more efficient star formation, and from where the interstellar medium is at a higher density, as compared to normal star-forming regions. Globular cluster formation is prevalent in
No known globular clusters display active star formation, consistent with the hypothesis that globular clusters are typically the oldest objects in their galaxy and were among the first collections of stars to form. Very large regions of star formation known as super star clusters, such as Westerlund 1 in the Milky Way, may be the precursors of globular clusters.[46]
Many of the Milky Way's globular clusters have a
Composition
Globular clusters are generally composed of hundreds of thousands of
Globular clusters can contain a high density of stars; on average about 0.4 stars per cubic parsec, increasing to 100 or 1000 stars/pc3 in the core of the cluster.[52] In comparison, the stellar density around the Sun is roughly 0.1 stars/pc3.[53] The typical distance between stars in a globular cluster is about one light year,[54] but at its core the separation between stars averages about a third of a light year – thirteen times closer than the Sun is to its nearest neighbor, Proxima Centauri.[55]
Globular clusters are thought to be unfavorable locations for planetary systems. Planetary orbits are dynamically unstable within the cores of dense clusters because of the gravitational perturbations of passing stars. A planet orbiting at one
Some globular clusters, like Omega Centauri in the Milky Way and Mayall II in the Andromeda Galaxy, are extraordinarily massive, measuring several million solar masses (M☉) and having multiple stellar populations. Both are evidence that supermassive globular clusters formed from the cores of dwarf galaxies that have been consumed by larger galaxies.[58] About a quarter of the globular cluster population in the Milky Way may have been accreted this way,[59] as with more than 60% of the globular clusters in the outer halo of Andromeda.[60]
Heavy element content
Globular clusters normally consist of
The Dutch astronomer Pieter Oosterhoff observed two special populations of globular clusters, which became known as Oosterhoff groups. The second group has a slightly longer period of RR Lyrae variable stars.[62] While both groups have a low proportion of metallic elements as measured by spectroscopy, the metal spectral lines in the stars of Oosterhoff type I (Oo I) cluster are not quite as weak as those in type II (Oo II),[62] and so type I stars are referred to as metal-rich (e.g. Terzan 7[63]), while type II stars are metal-poor (e.g. ESO 280-SC06[64]). These two distinct populations have been observed in many galaxies, especially massive elliptical galaxies. Both groups are nearly as old as the universe itself and are of similar ages. Suggested scenarios to explain these subpopulations include violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In the Milky Way, the metal-poor clusters are associated with the halo and the metal-rich clusters with the bulge.[65]
A large majority of the metal-poor clusters in the Milky Way are aligned on a plane in the outer part of the galaxy's halo. This observation supports the view that type II clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as was 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.[66]
Exotic components
Close interactions and near-collisions of stars occur relatively often in globular clusters because of their high star density. These chance encounters give rise to some exotic classes of stars – such as
Astronomers have searched for black holes within globular clusters since the 1970s. The required resolution for this task is exacting; it is only with the Hubble Space Telescope (HST) that the first claimed discoveries were made, in 2002 and 2003. Based on HST observations, other researchers suggested the existence of a 4,000 M☉(solar masses) intermediate-mass black hole in the globular cluster M15 and a 20,000 M☉ black hole in the Mayall II cluster of the Andromeda Galaxy.[76] Both X-ray and radio emissions from Mayall II appear consistent with an intermediate-mass black hole;[77] however, these claimed detections are controversial.[78]
The heaviest objects in globular clusters are expected to migrate to the cluster center due to
The confirmation of intermediate-mass black holes in globular clusters would have important ramifications for theories of galaxy development as being possible sources for the supermassive black holes at their centers. The mass of these supposed intermediate-mass black holes is proportional to the mass of their surrounding clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies.[78][81]
Hertzsprung–Russell diagrams
Hertzsprung–Russell diagrams (H–R diagrams) of globular clusters allow astronomers to determine many of the properties of their populations of stars. An H–R diagram is a graph of a large sample of stars plotting their absolute magnitude (their luminosity, or brightness measured from a standard distance), as a function of their color index. The color index, roughly speaking, measures the color of the star; positive color indices indicate a reddish star with a cool surface temperature, while negative values indicate a bluer star with a hotter surface. Stars on an H–R diagram mostly lie along a roughly diagonal line sloping from hot, luminous stars in the upper left to cool, faint stars in the lower right. This line is known as the main sequence and represents the primary stage of stellar evolution. The diagram also includes stars in later evolutionary stages such as the cool but luminous red giants.[82]
Constructing an H–R diagram requires knowing the distance to the observed stars to convert apparent into absolute magnitude. Because all the stars in a globular cluster have about the same distance from Earth, a color–magnitude diagram using their observed magnitudes looks like a shifted H–R diagram (because of the roughly constant difference between their apparent and absolute magnitudes).[83] This shift is called the distance modulus and can be used to calculate the distance to the cluster. The modulus is determined by comparing features (like the main sequence) of the cluster's color–magnitude diagram to corresponding features in an H–R diagram of another set of stars, a method known as spectroscopic parallax or main-sequence fitting.[84]
Properties
Since globular clusters form at once from a single giant molecular cloud, a cluster's stars have roughly the same age and composition. A star's evolution is primarily determined by its initial mass, so the positions of stars in a cluster's H–R or color–magnitude diagram mostly reflect their initial masses. A cluster's H–R diagram, therefore, appears quite different from H–R diagrams containing stars of a wide variety of ages. Almost all stars fall on a well-defined curve in globular cluster H–R diagrams, and that curve's shape indicates the age of the cluster.[83][85] A more detailed H–R diagram often reveals multiple stellar populations as indicated by the presence of closely separated curves, each corresponding to a distinct population of stars with a slightly different age or composition.[38] Observations with the Wide Field Camera 3, installed in 2009 on the Hubble Space Telescope, made it possible to distinguish these slightly different curves.[86]
The most massive main-sequence stars have the highest luminosity and will be the first to evolve into the
The morphology and luminosity of globular cluster stars in H–R diagrams are influenced by numerous parameters, many of which are still actively researched. Recent observations have overturned the historical paradigm that all globular clusters consist of stars born at exactly the same time, or sharing exactly the same chemical abundance. Some clusters feature multiple populations, slightly differing in composition and age; for example, high-precision imagery of cluster
Consequences
The blue stragglers appear on the H–R diagram as a series diverging from the main sequence in the direction of brighter, bluer stars.
Studying globular clusters sheds light on how the composition of the formational gas and dust affects stellar evolution; the stars'
Morphology
Galaxy | Ellipticity[96] |
---|---|
Milky Way | 0.07±0.04 |
LMC | 0.16±0.05 |
SMC | 0.19±0.06 |
M31 | 0.09±0.04 |
In contrast to open clusters, most globular clusters remain gravitationally bound together for time periods comparable to the lifespans of most of their stars. Strong tidal interactions with other large masses result in the dispersal of some stars, leaving behind "tidal tails" of stars removed from the cluster.[97][98]
After formation, the stars in the globular cluster begin to interact gravitationally with each other. The velocities of the stars steadily change, and the stars lose any history of their original velocity. The characteristic interval for this to occur is the
Although globular clusters are generally spherical in form, ellipticity can form via tidal interactions. Clusters within the Milky Way and the Andromeda Galaxy are typically
Radii
Astronomers characterize the morphology (shape) of a globular cluster by means of standard radii: the core radius (rc), the
Most globular clusters have a half-light radius of less than ten parsecs (pc), although some globular clusters have very large radii, like
The tidal radius, or Hill sphere, 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.[109] 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, for example, is about forty arc minutes,[110] or about 113 pc.[111]
Mass segregation, luminosity and core collapse
In most Milky Way clusters, the surface brightness of a globular cluster as a function of decreasing distance to the core first increases, then levels off at a distance typically 1–2 parsecs from the core. About 20% of the globular clusters have undergone a process termed "core collapse". The luminosity in such a cluster increases steadily all the way to the core region.[112][113]
Models of globular clusters predict that core collapse occurs when the more massive stars in a globular cluster encounter their less massive counterparts. Over time, dynamic processes cause individual stars to migrate from the center of the cluster to the outside, resulting in a net loss of
The dynamical heating effect of binary star systems works to prevent an initial core collapse of the cluster. When a star passes near a binary system, the orbit of the latter pair tends to contract, releasing energy. Only after this primordial supply of energy is exhausted can a deeper core collapse proceed.[117][118] In contrast, the effect of tidal shocks as a globular cluster repeatedly passes through the plane of a spiral galaxy tends to significantly accelerate core collapse.[119]
Core collapse may be divided into three phases. During a cluster's adolescence, core collapse begins with stars nearest the core. Interactions between binary star systems prevents further collapse as the cluster approaches middle age. The central binaries are either disrupted or ejected, resulting in a tighter concentration at the core.[120] The interaction of stars in the collapsed core region causes tight binary systems to form. As other stars interact with these tight binaries they increase the energy at the core, causing the cluster to re-expand. As the average time for a core collapse is typically less than the age of the galaxy, many of a galaxy's globular clusters may have passed through a core collapse stage, then re-expanded.[121]
The HST has provided convincing observational evidence of this stellar mass-sorting process in globular clusters. Heavier stars slow down and crowd at the cluster's core, while lighter stars pick up speed and tend to spend more time at the cluster's periphery. The cluster 47 Tucanae, made up of about one million stars, is one of the densest globular clusters in the Southern Hemisphere. This cluster was subjected to an intensive photographic survey that obtained precise velocities for nearly fifteen thousand stars in this cluster.[123]
The overall luminosities of the globular clusters within the Milky Way and the Andromeda Galaxy each have a roughly
N-body simulations
Computing the gravitational interactions between stars within a globular cluster requires solving the N-body problem. The naive computational cost for a dynamic simulation increases in proportion to N 2 (where N is the number of objects), so the computing requirements to accurately simulate a cluster of thousands of stars can be enormous.[125][126] A more efficient method of simulating the N-body dynamics of a globular cluster is done by subdivision into small volumes and velocity ranges, and using probabilities to describe the locations of the stars. Their motions are described by means of the Fokker–Planck equation, often using a model describing the mass density as a function of radius, such as a Plummer model. 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.[127] In 2010 a low-density globular cluster's lifetime evolution was able to be directly computed, star-by-star.[128]
Completed N-body simulations have shown that stars can follow unusual paths through the cluster, often forming loops and falling more directly toward the core than would a single star orbiting a central mass. Additionally, some stars gain sufficient energy to escape the cluster due to gravitational interactions that result in a sufficient increase in velocity. Over long periods of time this process leads to the dissipation of the cluster, a process termed evaporation.[129] The typical time scale for the evaporation of a globular cluster is 1010 years.[99] The ultimate fate of a globular cluster must be either to accrete stars at its core, causing its steady contraction,[130] or gradual shedding of stars from its outer layers.[131]
Intermediate forms
Cluster classification is not always definitive; objects have been found that can be classified in more than one categories. For example, BH 176 in the southern part of the Milky Way has properties of both an open and a globular cluster.[137]
In 2005 astronomers discovered a new, "extended" type of star cluster in the Andromeda Galaxy's halo, similar to the globular cluster. The three new-found clusters have a similar star count as globular clusters and share other characteristics, such as stellar populations and metallicity, but are distinguished by their larger size – several hundred light years across – and some hundred times lower density. Their stars are separated by larger distances; parametrically, these clusters lie somewhere between a globular cluster and a dwarf spheroidal galaxy.[138] The formation of these extended clusters is likely related to accretion.[139] It is unclear why the Milky Way lacks such clusters; Andromeda is unlikely to be the sole galaxy with them, but their presence in other galaxies remains unknown.[138]
Tidal encounters
When a globular cluster comes close to a large mass, such as the core region of a galaxy, it undergoes a tidal interaction. The difference in gravitational strength between the nearer and further parts of the cluster results in an asymmetric, tidal force. A "tidal shock" occurs whenever the orbit of a cluster takes it through the plane of a galaxy.[119][140]
Tidal shocks can pull stars away from the cluster halo, leaving only the core part of the cluster; these trails of stars can extend several degrees away from the cluster.[141] These tails typically both precede and follow the cluster along its orbit and can accumulate significant portions of the original mass of the cluster, forming clump-like features.[142] The globular cluster Palomar 5, for example, is near the apogalactic point 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 to distances of 13,000 light years. Tidal interactions have stripped away much of Palomar 5's mass; further interactions with the galactic core are expected to transform it into a long stream of stars orbiting the Milky Way in its halo.[143]
The Milky Way is in the process of tidally stripping the Sagittarius Dwarf Spheroidal Galaxy of stars and globular clusters through the Sagittarius Stream. As many as 20% of the globular clusters in the Milky Way's outer halo may have originated in that galaxy.[144] Palomar 12, for example, most likely originated in the Sagittarius Dwarf Spheroidal but is now associated with the Milky Way.[145][146] Tidal interactions like these add kinetic energy into a globular cluster, dramatically increasing the evaporation rate and shrinking the size of the cluster.[99] The increased evaporation accelerates the process of core collapse.[99][147]
Planets
Astronomers are searching for exoplanets of stars in globular star clusters.
A giant planet was found in the globular cluster
See also
- List of globular clusters
- Extragalactic Distance Scale
- Kraken galaxy
- Leonard-Merritt mass estimator
- Polytrope
Footnotes
- ^ The label M before a number refers to Charles Messier's catalogue, while NGC is from the New General Catalogue by John Dreyer.
- ^
From page 437: Le 8 Mai 1764, j'ai découvert une nébuleuse ... de 25d 55′ 40″ méridionale.
"On 8 May 1764, I discovered a nebula near Antares, and on its parallel; it is a [source of] light which has little extension, which is dim, and which is seen with difficulty; by using a good telescope to see it, one perceives very small stars in it. Its right ascension was determined to be 242° 16′ 56″, and its declination, 25° 55′ 40″ south."[11](p 437) - ^ Omega Centauri was known in antiquity, but Halley discovered its nature as a nebula.
- ^
From page 218, discussing the shapes of star clusters, Herschel wrote:
"And thus, from the above-mentioned appearances, we come to know that there are globular clusters of stars nearly equal in size, which are scattered evenly at equal distances from the middle, but with an encreasing [sic] accumulation towards the center."[18](p 218) - ^
Harlow Shapley's error was aggravated by interstellar dustin the Milky Way, which absorbs and diminishes the amount of light from distant objects (such as globular clusters), thus making them appear to be farther away.
- Roman numerals.
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- ^ Pooley, Dave. "Globular Cluster Dynamics: the importance of close binaries in a real N-body system". self-published. Archived from the original on June 19, 2010. Retrieved April 7, 2021.
- S2CID 118504277.
- .
- S2CID 119275313.
- S2CID 202542401.
- ^ "Globular Cluster M10". ESA/Hubble Picture of the Week. Retrieved June 18, 2012.
- Bibcode:1995A&A...300..726O.
- ^ S2CID 6215035.
- .
- doi:10.1086/181018.
- Bibcode:2003AAS...20311226L.
- Bibcode:2004DDA....35.0303D.
- ^ Staude, Jakob (June 3, 2002). "Sky Survey Unveils Star Cluster Shredded By The Milky Way". Image of the Week (Press release). Sloan Digital Sky Survey. Archived from the original on June 29, 2006. Retrieved April 9, 2021.
- .
- S2CID 118898193.
- .
- doi:10.1086/303441.
- ^ Ricard, Elise (January 15, 2016). "Planet locations, a supernova, and a black hole". Space Friday. California Academy of Sciences. Retrieved May 15, 2016.
- S2CID 18179704.
- Bibcode:2008ASPC..398..119S.
- S2CID 119083161.
- ISSN 0035-8711.
Further reading
Books
- ISBN 978-0-691-08444-2.
- ISBN 978-0-521-77486-4.
- ISBN 978-0-691-08460-2.
Review articles
- Elson, Rebecca; Hut, Piet; Inagaki, Shogo (1987). "Dynamical evolution of globular clusters". Annual Review of Astronomy and Astrophysics. 25: 565. .
- Gratton, R.; Bragaglia, A.; Carretta, E.; et al. (2019). "What is a globular cluster? An observational perspective". The Astronomy and Astrophysics Review. 27 (1): 8. S2CID 207847491.
- Meylan, G.; Heggie, D. C. (1997). "Internal dynamics of globular clusters". The Astronomy and Astrophysics Review. 8 (1–2): 1–143. S2CID 119059312.
External links
- Globular Clusters, Students for the Exploration and Development of Space 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
- Catalogue of structural and kinematic parameters and galactic orbits of globular clusters by Holger Baumgardt, University of Queensland, Australia
- SCYON, a newsletter dedicated to star clusters.
- MODEST, a loose collaboration of scientists working on star clusters.