Hubbry Logo
Globular clusterGlobular clusterMain
Open search
Globular cluster
Community hub
Globular cluster
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Globular cluster
Globular cluster
Globular cluster
View on Wikipedia
from Wikipedia

Globular cluster
Messier 2
Characteristics
TypeStar cluster
Mass range1K M☉ - >1M M☉[1]
Size range10-300 ly across[1]
Density~2 stars/cubic ly [1]
Average luminosity~25 000 L☉[1]
External links
inline Media category
inline Q11276
Additional Information
DiscoveredAbraham Ihle, 1665

A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It 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 though globular clusters were long held to be the more luminous of the two, discoveries of outliers had made the distinction between the two less clear by the early 21st century.[3] Their name is derived from Latin globulus (small sphere). Globular clusters are occasionally known simply as "globulars".

Although one globular cluster, Omega Centauri, was observed in antiquity and long thought to be a star, recognition of the clusters' true nature came with the advent of telescopes in the 17th century. In early telescopic observations, globular clusters appeared as fuzzy blobs, leading French astronomer Charles Messier to include many of them in his catalog of astronomical objects that he thought could be mistaken for comets. Using larger telescopes, 18th-century astronomers recognized that globular clusters are groups of many individual stars. Early in the 20th century the distribution of globular clusters in the sky was some of the first evidence that the Sun is far from the center of the Milky Way.

Globular clusters are found in nearly all galaxies. In spiral galaxies like the Milky Way, they are mostly found in the outer spheroidal part of the galaxy – the galactic halo. They are the largest and most massive type of star cluster, tending to be older, denser, and composed of lower abundances of heavy elements than open clusters, which are generally found in the disks of spiral galaxies. The Milky Way has more than 150 known globulars, and there may be many more.

Both the origin of globular clusters and their role in galactic evolution are unclear. Some are among the oldest objects in their galaxies and even the universe, constraining estimates of the universe's age. Star clusters were formerly thought to consist of stars that all formed at the same time from one star-forming nebula, but nearly all globular clusters contain stars that formed at different times, or that have differing compositions. Some clusters may have had multiple episodes of star formation, and some may be remnants of smaller galaxies captured by larger galaxies.

History of observations

[edit]

The first known globular cluster, now called M 22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer.[4][5][6] The cluster Omega Centauri, easily visible in the southern sky with the naked eye, was known to ancient astronomers like Ptolemy as a star, but was reclassified as a nebula by Edmond Halley in 1677,[7] then finally as a globular cluster in the early 19th century by John Herschel.[8][9][10] The French astronomer Abbé Lacaille listed NGC 104, NGC 4833, M 55, M 69, and NGC 6397 in his 1751–1752 catalogue.[a] The low resolution of early telescopes prevented individual stars in a cluster from being visually separated until Charles Messier observed M 4 in 1764.[11][b][12]

Early globular cluster discoveries
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]

Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center
NGC 7006 is a highly concentrated, Class I globular cluster.

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 Galactic Center.[21] He correctly concluded that the Milky Way's center is in the Sagittarius constellation and not near the Earth. He overestimated the distance, finding typical globular cluster distances of 10–30 kiloparsecs (33,000–98,000 ly);[22] the modern distance to the Galactic Center is roughly 8.5 kiloparsecs (28,000 ly).[e][23][24][25] Shapley's measurements indicated the Sun is relatively far from the center of the galaxy, contrary to what had been inferred from the observed uniform distribution of ordinary stars. In reality most ordinary stars lie within the galaxy's disk and are thus obscured by gas and dust in the disk, whereas globular clusters lie outside the disk and can be seen at much greater distances.[20]

Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center
The Messier 80 globular cluster in the constellation Scorpius is located about 30,000 light-years from the Sun and contains hundreds of thousands of stars.[26]

The count of known globular clusters in the Milky Way has continued to increase, reaching 83 in 1915, 93 in 1930, 97 by 1947,[19] and 157 in 2010.[27][28] The number of known globular clusters in the Milky Way reached 158 by the end of 2010, according to the European Southern Observatory, before two new globular clusters were discovered as part of the ESO's VISTA (Visible and Infrared Survey Telescope for Astronomy) infrared survey, known as Variables in the Vía Láctea (VVV) survey, bringing the total to 160 known globular clusters.[29] The two discovered by VISTA in 2011 are named VVV CL001 and VVV CL002.[30][31]

Additional, undiscovered globular clusters are believed to be in the galactic bulge[32] or hidden by the gas and dust of the Milky Way.[33] For example, most of the Palomar Globular Clusters have only been discovered in the 1950s, with some located relatively close-by yet obscured by dust, while others reside in the very far reaches of the Milky Way halo. The Andromeda Galaxy, which is comparable in size to the Milky Way, may have as many as five hundred globulars.[34] Every galaxy of sufficient mass in the Local Group has an associated system of globular clusters, as does almost every large galaxy surveyed.[35] Some giant elliptical galaxies (particularly those at the centers of galaxy clusters), such as M 87, have as many as 13,000 globular clusters.[36]

Classification

[edit]
Main article: Shapley–Sawyer Concentration Class

Shapley was later assisted in his studies of clusters by Henrietta Swope and Helen Sawyer Hogg. In 1927–1929, Shapley and Sawyer categorized clusters by the degree of concentration of stars toward each core. Their system, known as the Shapley–Sawyer Concentration Class, identifies the most concentrated clusters as Class I and ranges to the most diffuse Class XII.[f][37] Astronomers from the Pontifical Catholic University of Chile proposed a new type of globular cluster on the basis of observational data in 2015: Dark globular clusters.[38]

Formation

[edit]
Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center
NGC 2808 contains three distinct generations of stars.[39]
NASA image

The formation of globular clusters is poorly understood.[40] Globular clusters have traditionally been described as a simple star population formed from a single giant molecular cloud, and thus with roughly uniform age and metallicity (proportion of heavy elements in their composition). Modern observations show that nearly all globular clusters contain multiple populations;[41] the globular clusters in the Large Magellanic Cloud (LMC) exhibit a bimodal population, for example. During their youth, these LMC clusters may have encountered giant molecular clouds that triggered a second round of star formation.[42] This star-forming period is relatively brief, compared with the age of many globular clusters.[43] It has been proposed that this multiplicity in stellar populations could have a dynamical origin. In the Antennae Galaxy, for example, the Hubble Space Telescope has observed clusters of clusters – regions in the galaxy that span hundreds of parsecs, in which many of the clusters will eventually collide and merge. Their overall range of ages and (possibly) metallicities could lead to clusters with a bimodal, or even multimodal, distribution of populations.[44]

A small fuzzy white ball in the center of a speckled black backdrop
Globular star cluster Messier 54[45]

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 starburst regions and in interacting galaxies.[46] Some globular clusters likely formed in dwarf galaxies and were removed by tidal forces to join the Milky Way.[47] In elliptical and lenticular galaxies there is a correlation between the mass of the supermassive black holes (SMBHs) at their centers and the extent of their globular cluster systems. The mass of the SMBH in such a galaxy is often close to the combined mass of the galaxy's globular clusters.[48]

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.[49]

Many of the Milky Way's globular clusters have a retrograde orbit (meaning that they revolve around the galaxy in the reverse of the direction the galaxy is rotating),[50] including the most massive, Omega Centauri. Its retrograde orbit suggests it may be a remnant of a dwarf galaxy captured by the Milky Way.[51][52]

Composition

[edit]
A loose scattering of small dull white dots on a black background with a few brighter coloured stars
Djorgovski 1's stars contain hydrogen and helium, but not much else. In astronomical terms they are metal-poor.[53]

Globular clusters are generally composed of hundreds of thousands of low-metal, old stars. The stars found in a globular cluster are similar to those in the bulge of a spiral galaxy but confined to a spheroid in which half the light is emitted within a radius of only a few to a few tens of parsecs.[40] They are free of gas and dust,[54] and it is presumed that all the gas and dust was long ago either turned into stars or blown out of the cluster by the massive first-generation stars.[40]

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.[55] In comparison, the stellar density around the Sun is roughly 0.1 stars/pc3.[56] The typical distance between stars in a globular cluster is about one light year,[57] 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.[58]

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 astronomical unit around a star that is within the core of a dense cluster, such as 47 Tucanae, would survive only on the order of a hundred million years.[59] There is a planetary system orbiting a pulsar (PSR B1620−26) that belongs to the globular cluster M4, but these planets likely formed after the event that created the pulsar.[60]

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.[61] About a quarter of the globular cluster population in the Milky Way may have been accreted this way,[62] as with more than 60% of the globular clusters in the outer halo of Andromeda.[63]

Heavy element content

[edit]

Globular clusters normally consist of Population II stars which, compared with Population I stars such as the Sun, have a higher proportion of hydrogen and helium and a lower proportion of heavier elements. Astronomers refer to these heavier elements as metals (distinct from the material concept) and to the proportions of these elements as the metallicity. Produced by stellar nucleosynthesis, the metals are recycled into the interstellar medium and enter a new generation of stars. The proportion of metals can thus be an indication of the age of a star in simple models, with older stars typically having a lower metallicity.[64]

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.[65] 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),[65] and so type I stars are referred to as metal-rich (e.g. Terzan 7[66]), while type II stars are metal-poor (e.g. ESO 280-SC06[67]). 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.[68]

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.[69]

Exotic components

[edit]
Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center
Messier 53 contains an unusually large number of a type of star called blue stragglers.[70][71]

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 blue stragglers, millisecond pulsars, and low-mass X-ray binaries – which are much more common in globular clusters.[72] How blue stragglers form remains unclear, but most models attribute them to interactions between stars, such as stellar mergers, the transfer of material from one star to another, or even an encounter between two binary systems.[73][74] The resulting star has a higher temperature than other stars in the cluster with comparable luminosity and thus differs from the main-sequence stars formed early in the cluster's existence.[75] Some clusters have two distinct sequences of blue stragglers, one bluer than the other.[74]

Hundreds of white-ish dots scattered on a black background, concentrated towards the center
Globular cluster M15 may have an intermediate-mass black hole at its core,[76] but this claim is contested.[77]
Simulation of stellar motions in Messier 4
Simulation of stellar motions in Messier 4, where astronomers suspect that an intermediate-mass black hole could be present.[78][79] If confirmed, the black hole would be in the center of the cluster, and would have a sphere of influence (black hole) limited by the red circle.

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.[80] Both X-ray and radio emissions from Mayall II appear consistent with an intermediate-mass black hole;[81] however, these claimed detections are controversial.[82]

The heaviest objects in globular clusters are expected to migrate to the cluster center due to mass segregation. One research group pointed out that the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15[77] and Mayall II.[83] Observations from 2018 find no evidence for an intermediate-mass black hole in any globular cluster, including M15, but cannot definitively rule out one with a mass of 500–1000 M☉.[84] Finally, in 2023, an analysis of HST and the Gaia spacecraft data from the closest globular cluster, Messier 4, revealed an excess mass of roughly 800 M☉ in the center of this cluster, which appears to not be extended. This could thus be considered as kinematic evidence for an intermediate-mass black hole[78][79] (even if an unusually compact cluster of compact objects like white dwarfs, neutron stars or stellar-mass black holes cannot be completely discounted).

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.[82][85]

Hertzsprung–Russell diagrams

[edit]
A scattering of dots on a black background, most yellow and aligned in a roughly vertical band down the center, with some white dots extending in two arms to the left and a few red dots scattered on the right of the image
H–R diagram for the globular cluster M3. There is a characteristic "knee" in the curve at magnitude 19 where stars begin entering the giant stage of their evolutionary path, the main-sequence turnoff.

Hertzsprung–Russell diagrams (H–R diagrams) of globular clusters allow astronomers to determine many of the properties of their populations of stars. A 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 a 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.[86]

Constructing a 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).[87] 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 a H–R diagram of another set of stars, a method known as spectroscopic parallax or main-sequence fitting.[88]

Properties

[edit]

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.[87][89] 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.[41] Observations with the Wide Field Camera 3, installed in 2009 on the Hubble Space Telescope, made it possible to distinguish these slightly different curves.[90]

The most massive main-sequence stars have the highest luminosity and will be the first to evolve into the giant star stage. As the cluster ages, stars of successively lower masses will do the same. Therefore, the age of a single-population cluster can be measured by looking for those stars just beginning to enter the giant star stage, which form a "knee" in the H–R diagram called the main-sequence turnoff, bending to the upper right from the main-sequence line. The absolute magnitude at this bend is directly a function of the cluster's age; an age scale can be plotted on an axis parallel to the magnitude.[87]

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 NGC 2808 discerned three close, but distinct, main sequences.[91] Further, the placements of the cluster stars in a H–R diagram (including the brightnesses of distance indicators) can be influenced by observational biases. One such effect, called blending, arises when the cores of globular clusters are so dense that observations see multiple stars as a single target. The brightness measured for that seemingly single star is thus incorrect – too bright, given that multiple stars contributed.[92] In turn, the computed distance is incorrect, so the blending effect can introduce a systematic uncertainty into the cosmic distance ladder and may bias the estimated age of the universe and the Hubble constant.[93]

Consequences

[edit]

The blue stragglers appear on the H–R diagram as a series diverging from the main sequence in the direction of brighter, bluer stars.[74] White dwarfs (the final remnants of some Sun-like stars), which are much fainter and somewhat hotter than the main-sequence stars, lie on the bottom-left of a H–R diagram. Globular clusters can be dated by looking at the temperatures of the coolest white dwarfs, often giving results as old as 12.7 billion years.[94] In comparison, open clusters are rarely older than about half a billion years.[95] The ages of globular clusters place a lower bound on the age of the entire universe, presenting a significant constraint in cosmology. Astronomers were historically faced with age estimates of clusters older than their cosmological models would allow,[96] but better measurements of cosmological parameters, through deep sky surveys and satellites, appear to have resolved this issue.[97][98]

Studying globular clusters sheds light on how the composition of the formational gas and dust affects stellar evolution; the stars' evolutionary tracks vary depending on the abundance of heavy elements. Data obtained from these studies are then used to study the evolution of the Milky Way as a whole.[99]

Morphology

[edit]
Ellipticity of globular clusters
Galaxy Ellipticity[100]
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.[101][102]

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 relaxation time, related to the characteristic length of time a star needs to cross the cluster and the number of stellar masses.[103] The relaxation time varies by cluster, but a typical value is on the order of one billion years.[104][105]

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 oblate spheroids in shape, while those in the Large Magellanic Cloud are more elliptical.[106]

Radii

[edit]
"Tidal radius" redirects here; not to be confused with Roche limit.
Hundreds of white-ish dots scattered on a black background, concentrated towards the center, with some brighter red and blue dots scattered across the frame
NGC 411 is classified as an open cluster.[107]

Astronomers characterize the morphology (shape) of a globular cluster by means of standard radii: the core radius (rc), the half-light radius (rh), and the tidal or Jacobi radius (rt). The radius can be expressed as a physical distance or as a subtended angle in the sky. Considering a radius around the core, the surface luminosity of the cluster steadily decreases with distance, and the core radius is the distance at which the apparent surface luminosity has dropped by half.[108] A comparable quantity is the half-light radius, or the distance from the core containing half the total luminosity of the cluster; the half-light radius is typically larger than the core radius.[109][110]

Most globular clusters have a half-light radius of less than ten parsecs (pc), although some globular clusters have very large radii, like NGC 2419 (rh = 18 pc) and Palomar 14 (rh = 25 pc).[111] The half-light radius includes stars in the outer part of the cluster that happen to lie along the line of sight, so theorists also use the half-mass radius (rm) – the radius from the core that contains half the total mass of the cluster. A small half-mass radius, relative to the overall size, indicates a dense core. Messier 3 (M3), for example, has an overall visible dimension of about 18 arc minutes, but a half-mass radius of only 1.12 arc minutes.[112]

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.[113] This is the distance at which the individual stars belonging to a cluster can unbound by the galaxy. The tidal radius of M3, for example, is about forty arc minutes,[114] or about 113 pc.[115]

Mass segregation, luminosity and core collapse

[edit]

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.[116][117]

Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center
47 Tucanae is the second most luminous globular cluster in the Milky Way, after Omega Centauri.

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 kinetic energy from the core region and leading the region's remaining stars to occupy a more compact volume. When this gravothermal instability occurs, the central region of the cluster becomes densely crowded with stars, and the surface brightness of the cluster forms a power-law cusp.[118] A massive black hole at the core could also result in a luminosity cusp.[119] Over a long time, this leads to a concentration of massive stars near the core, a phenomenon called mass segregation.[120]

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.[121][122] 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.[123]

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.[124] 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.[125]

Hundreds of white-ish dots scattered on a black background, concentrated towards the center
Globular cluster NGC 1854 is located in the Large Magellanic Cloud.[126]

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.[127]

The overall luminosities of the globular clusters within the Milky Way and the Andromeda Galaxy each have a roughly Gaussian distribution, with an average magnitude Mv and a variance σ2. This distribution of globular cluster luminosities is called the Globular Cluster Luminosity Function (GCLF). For the Milky Way, Mv = −7.29 ± 0.13, σ = 1.1 ± 0.1. The GCLF has been used as a "standard candle" for measuring the distance to other galaxies, under the assumption that globular clusters in remote galaxies behave similarly to those in the Milky Way.[128]

N-body simulations

[edit]
Main article: N-body simulation

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.[129][130] 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.[131] In 2010 a low-density globular cluster's lifetime evolution was able to be directly computed, star-by-star.[132]

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.[133] The typical time scale for the evaporation of a globular cluster is 1010 years.[103] The ultimate fate of a globular cluster must be either to accrete stars at its core, causing its steady contraction,[134] or gradual shedding of stars from its outer layers.[135]

Binary stars form a significant portion of stellar systems, with up to half of all field stars and open cluster stars occurring in binary systems.[136][137] The present-day binary fraction in globular clusters is difficult to measure, and any information about their initial binary fraction is lost by subsequent dynamical evolution.[138] 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 reduces the contraction at the core and limits core collapse.[75][139]

Intermediate forms

[edit]
Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center
Messier 10 lies about 15,000 light-years from Earth, in the constellation of Ophiuchus.[140]

Cluster classification is not always definitive; objects have been found that can be classified in more than one category. For example, BH 176 in the southern part of the Milky Way has properties of both an open and a globular cluster.[141]

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 to 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.[142] The formation of these extended clusters is likely related to accretion.[143] 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.[142]

Tidal encounters

[edit]

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.[123][144]

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.[145] 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.[146] 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.[147]

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.[148] Palomar 12, for example, most likely originated in the Sagittarius Dwarf Spheroidal but is now associated with the Milky Way.[149][150] Tidal interactions like these add kinetic energy into a globular cluster, dramatically increasing the evaporation rate and shrinking the size of the cluster.[103] The increased evaporation accelerates the process of core collapse.[103][151]

Planets

[edit]

Astronomers are searching for exoplanets of stars in globular star clusters.[152] A search in 2000 for giant planets in the globular cluster 47 Tucanae came up negative, suggesting that the abundance of heavier elements – low in globular clusters – necessary to build these planets may need to be at least 40% of the Sun's abundance. Because terrestrial planets are built from heavier elements such as silicon, iron and magnesium, member stars have a far lower likelihood of hosting Earth-mass planets than stars in the solar neighborhood. Globular clusters are thus unlikely to host habitable terrestrial planets.[153]

A giant planet was found in the globular cluster Messier 4, orbiting a pulsar in the binary star system PSR B1620-26. The planet's eccentric and highly inclined orbit suggests it may have been formed around another star in the cluster, then "exchanged" into its current arrangement.[154] The likelihood of close encounters between stars in a globular cluster can disrupt planetary systems; some planets break free to become rogue planets, orbiting the galaxy. Planets orbiting close to their star can become disrupted, potentially leading to orbital decay and an increase in orbital eccentricity and tidal effects.[155] In 2024, a gas giant or brown dwarf was found to closely orbit the pulsar "M62H", where the name indicates that the planetary system belongs to the globular cluster Messier 62.[156]

See also

[edit]

Footnotes

[edit]

References

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Globular cluster

View on Grokipedia
from Grokipedia
A globular cluster is a tightly bound, spherically symmetric collection of stars that share a common origin, typically containing tens of thousands to millions of stars packed into a dense volume spanning 50 to 450 light-years across. These ancient stellar systems are among the oldest structures in the , with stars dating back 8 to 13 billion years, making them key relics from the early epochs of galaxy formation. Globular clusters orbit in the outer halo regions of galaxies, far from the dense disk where younger form, and the alone hosts approximately 150 such clusters, many of which follow retrograde orbits suggestive of possible capture from external sources. Unlike looser open clusters found in galactic arms, globular clusters maintain their spherical shape due to the intense gravitational binding of their massive stellar populations, often appearing as reddish "beehives" of light when observed. These clusters formed from massive molecular clouds in the 's infancy, potentially hosting multiple generations of stars that provide insights into , chemical enrichment, and the dynamical history of their host galaxies. Astronomers study them extensively using telescopes like Hubble to probe phenomena such as blue stragglers—unusually hot stars formed through stellar mergers—and to test models of in dense environments. Notable examples include , the largest known in the with around 10 million stars, and M80, a densely packed swarm visible in the constellation .

History and Discovery

Early Observations

The earliest known reference to a globular cluster appears in the 2nd-century AD Almagest by Claudius Ptolemy, where (NGC 5139) is cataloged as a single star in the constellation , though its fuzzy, nebulous appearance to the distinguished it from point-like stars. This object, now recognized as the largest globular cluster in the , was similarly noted in ancient Arabic star catalogs for its hazy form, but without telescopic resolution, it remained classified among fixed stars or clouds. Telescopic observations began in the mid-17th century, with Italian astronomer Hodierna describing several diffuse celestial objects in his 1654 treatise De systemate orbis cometici deque admirandis coeli characteribus, including nebulous patches that encompassed early sightings of deep-sky features later identified as star clusters. By 1714–1715, English astronomer published the first dedicated list of six "luminous patches or nebulous stars" in the Philosophical Transactions of the Royal Society, explicitly including and the Hercules globular cluster M13 (NGC 6205) as objects too distant or compact to resolve into individual stars with contemporary telescopes, marking them as distinct from planets or comets. These early accounts treated globular clusters as a subset of "nebulae," non-resolvable misty regions in the sky. In the late 18th century, French astronomer systematically cataloged 29 globular clusters as entries in his famous 1781 list of 110 nebulae and clusters, such as M2, M3, M4, M5, M10, M12, M13, M15, M22, M28, M53, M54, M55, M62, M69, M70, M79, M80, M92, M107, and others, all described as round, hazy patches without stellar resolution to guard against confusion with comets. advanced this understanding in the 1780s and 1790s through extensive sweeps with his large reflectors, resolving many of these objects into dense swarms of faint stars; for instance, on July 15, 1781, he observed M13 as "a fine cluster of small stars gradually much compressed in the middle; irregular borders and brighter towards the center," confirming its stellar nature and distinguishing resolvable clusters from irresolvable "true" nebulae or distant galaxies. Herschel estimated distances to several clusters by assuming they were comparable in intrinsic size to the nearest known ones, placing them thousands of light-years away and suggesting a halo distribution around the . The 20th century brought transformative insights, with Harlow Shapley's analysis of 69 globular clusters using periods to calibrate distances, revealing their concentration toward the in Sagittarius, implying the Milky Way's center lies about 50,000 light-years from the Sun and spans hundreds of thousands of light-years in diameter—revolutionizing prior assumptions of a small, Sun-centered . In the , measurements of clusters, pioneered by astronomers like H.C. Plummer and , demonstrated systematic motions consistent with orbital revolution around this distant , with average velocities of around 90–100 km/s indicating a rotating halo system rather than random distribution. These observations established globular clusters as tracers of the galaxy's overall and dynamics.

Classification Systems

One of the earliest classification systems for globular clusters was introduced by in 1918, which categorized clusters into concentration classes based on their central star density and the resolvability of individual stars in the core. This scheme, later refined into the widely used Shapley-Sawyer concentration classes I through XII (with class I indicating the highest central concentration and class XII the lowest), provided a qualitative measure of cluster compactness derived from photographic plates. Modern structural classifications build on analytical models, particularly the King models, which describe cluster density profiles using parameters such as concentration (c = log(r_t / r_c), where r_t is the tidal radius and r_c the core radius), core radius, and half-light radius (the radius enclosing half the cluster's light). Baumgardt and Mackey (2017) applied these parameters to derive structural properties for globular clusters by fitting N-body simulations to observed velocity dispersions and profiles, enabling assessments of dynamical evolution and mass estimates. Integrated classifications assess clusters via their combined light, assigning spectral types typically ranging from F to based on absorption features like the G band and calcium H and lines, which reflect dominant stellar populations. Observations through , a low-extinction region toward the , have been key in calibrating these types and linking them to indicators, revealing variations from metal-poor (earlier types) to metal-rich clusters. Globular clusters are divided into subsystems based on position, , and chemistry, distinguishing metal-poor halo populations ([Fe/H] < -1.0) from metal-rich disk or bulge populations ([Fe/H] > -0.8). Zinn's 1985 classification further subdivided the halo into inner (R_GC < 8 kpc) and outer (R_GC > 8 kpc) components, with outer halo clusters showing greater age spreads and lower metallicities, suggesting prolonged formation in an extended galactic structure. The Harris catalog, first published in 1996 and updated in 2010, compiles parameters for over 150 globular clusters, including absolute visual magnitude (typically M_V ≈ -6 to -10, indicating luminosities of 10^4 to 10^5 solar), ellipticity (ε < 0.2 for most, reflecting near-spherical shapes), and other metrics like horizontal branch morphology. Extragalactic classifications extend these schemes to systems like M31 (Andromeda), where surveys identify over 500 globular clusters, often using Hubble Space Telescope imaging to resolve faint, distant objects (M_V > -4) in the outer halo and distinguish them from background galaxies via size and color criteria.

Physical Characteristics

Morphology and Dimensions

Globular clusters exhibit a high degree of spherical symmetry, characterized by a centrally concentrated distribution of stars that decreases outward, forming a dense core surrounded by a more diffuse halo. This morphology arises from their gravitational binding, with the stellar density peaking at the center and tapering off toward the periphery. Key structural parameters include the core radius rcr_c, which encloses the region of highest density and typically ranges from 1 to 10 parsecs (pc); the half-mass radius rhr_h, containing half the cluster's stellar mass and often around 1–5 pc; and the tidal radius rtr_t, marking the boundary where the cluster's gravity balances the tidal forces from the host galaxy, generally spanning 50–200 pc. For instance, the globular cluster M13 has a tidal radius of approximately 70 pc, while the exceptionally large Omega Centauri features a tidal radius of about 70 pc, highlighting variations in size across the population. The density profiles of globular clusters are commonly modeled using the King model, an empirical formulation based on a lowered that accounts for tidal . The model's concentration parameter c=log⁡(rt/rc)c = \log(r_t / r_c) typically ranges from 0.5 to 2.5, with higher values indicating more centrally concentrated clusters; this profile fits the observed and density distributions well for most systems. Observed dimensions of globular clusters reflect their compact nature, with typical half-light radii (projected equivalent of half-mass) of 1–5 pc and total diameters ranging from 50 to 300 pc, though outliers like extend to larger scales. These sizes are derived from resolved imaging and photometry, revealing a total concentrated within the inner regions while the outer halo fades gradually until the tidal limit. While most globular clusters are nearly spherical, with ellipticities e<0.2e < 0.2 (where e=1−b/ae = 1 - b/a and b/ab/a is the axis ratio), some display mild oblateness or flattening due to internal rotation or external influences. For example, NGC 1650 in the Large Magellanic Cloud exhibits oblate characteristics, with a measured ellipticity greater than average, contributing to a small subset of non-spherical profiles. Variations in morphology include core-collapsed clusters, which exhibit steeper central density profiles approaching power-law cusps with slopes of ρ∝r−2\rho \propto r^{-2} or steeper, contrasting with the flatter cores of uncollapsed systems.

Composition and Metallicity

Globular clusters are predominantly composed of old, low-mass stars with ages ranging from 10 to 13 billion years, reflecting their formation in the early universe. These clusters typically contain 10410^4 to 10610^6 solar masses of stars, with the initial mass function skewed toward stars below 1 solar mass, leading to a current population dominated by low-mass main-sequence stars and evolved giants. The metallicity of globular clusters, expressed as [Fe/H], spans a range from approximately -2.5 to -0.5 dex, with a characteristic bimodal distribution featuring a metal-poor peak around [Fe/H] ≈\approx -1.5 and a metal-rich peak near [Fe/H] ≈\approx -0.7. This distribution is derived primarily through high-resolution spectroscopy targeting absorption lines such as those of Ca II, which provide precise measurements of iron and other elemental abundances in individual cluster stars. For example, the globular cluster M13 exhibits [Fe/H] = -1.65, illustrating the metal-poor end of this spectrum. Heavy element abundances in globular clusters show distinct patterns, including α\alpha-element enhancements of [α/Fe]≈+0.3[\alpha/Fe] \approx +0.3 dex relative to iron, arising from core-collapse supernovae in the early chemical enrichment phases, alongside generally low levels of s-process elements due to the metal-poor environments. While clusters host single-age stellar populations, chemical inhomogeneities exist among subgroups, though the overall composition remains uniform in age. Approximately 90% of the stellar content consists of main-sequence stars and red giants, with few massive stars present owing to the advanced age and evolution of higher-mass members. Observational characterization of composition and metallicity relies on integrated light photometry for broad surveys and high-resolution spectroscopy for detailed abundances, with recent data from the and revealing subtle metallicity gradients, such as metal-rich stars being more centrally concentrated in some clusters.

Stellar Populations and Evolution

Hertzsprung-Russell Diagrams

The Hertzsprung-Russell (HR) diagram for globular clusters plots stellar luminosity against effective temperature, often derived from color-magnitude diagrams (CMDs) where color approximates temperature and apparent magnitude scales with luminosity after distance correction. These diagrams reveal distinct evolutionary sequences characteristic of old, coeval stellar populations: the main sequence (MS) of hydrogen-fusing dwarfs; the main-sequence turn-off (TO) point, indicating the exhaustion of core hydrogen in the cluster's most massive unevolved stars; the red giant branch (RGB), tracing helium-shell burning ascent; the horizontal branch (HB), where core helium fusion occurs in lower-mass stars; and the asymptotic giant branch (AGB), marking advanced thermal pulsing phases. In typical globular clusters with metallicity [Fe/H] ≈ -1.5 and ages of about 12 Gyr, the TO corresponds to stars of roughly 0.8 M_⊙, reflecting the cluster's formation in the early universe when only lower-mass stars could have survived to the present. The HB morphology varies from blue (hotter, He-burning stars) to red (cooler), primarily driven by helium abundance variations, as higher helium content increases the envelope mass loss efficiency and shifts stars to higher temperatures at fixed luminosity. Theoretical isochrones, which model evolutionary tracks for a given age and composition, show the MS luminosity scaling as L ∝ M^{3.5} for low-mass stars, providing a framework to fit observed CMDs and infer cluster parameters. High-resolution observations, such as Hubble Space Telescope (HST) CMDs of the cluster M4, place the TO at V ≈ 17 mag, highlighting the faint end of the MS and subgiant branch transition. Metal-poor clusters ([Fe/H] < -1.5) display bluer HBs due to reduced metal-line blanketing, which lowers opacity and allows hotter surface temperatures. Recent James Webb Space Telescope (JWST) imaging in 2025 has resolved fainter MS segments in Omega Centauri down to unprecedented depths, clarifying the split between its helium-normal and helium-enhanced populations. Ages of globular clusters are estimated by comparing the TO luminosity or the vertical magnitude span from the TO to the RGB base, ΔV(TO-RGB) ≈ 3 mag, to isochrone models calibrated for 12 Gyr populations, confirming their role as ancient benchmarks.

Multiple Populations and Exotic Components

Globular clusters exhibit chemical inhomogeneities among their member stars, manifesting as multiple stellar populations characterized by anticorrelations in light element abundances, such as the Na-O anticorrelation, where sodium-rich stars are oxygen-poor. This phenomenon is observed in nearly all massive Galactic globular clusters, with evidence for distinct first-generation (pristine composition) and second-generation (enriched) stars. The second generation often shows helium enhancement, with spreads up to ΔY ≈ 0.03, as inferred from spectroscopic and photometric studies that reveal variations in the helium abundance driving evolutionary discrepancies. These multiple populations are thought to arise from pollution of the intracluster medium by ejecta from first-generation stars, particularly asymptotic giant branch (AGB) stars or massive star binaries, which process material through high-temperature proton-capture reactions and release it via winds or explosions. In AGB pollution models, the hot bottom burning in intermediate-mass AGB stars (4–8 M⊙) produces the observed Na-O and Al-Mg anticorrelations by converting oxygen to sodium and magnesium to aluminum, with the enriched gas then accreting onto protostars or forming new stars in the cluster core. Alternative scenarios involve fast-rotating massive stars or supermassive stars as polluters, but AGB ejecta best explain the helium enrichment without excessive heavy-element pollution. Observations in clusters like NGC 6752 confirm these anticorrelations across red giant branch stars, supporting self-enrichment within the cluster environment. The presence of multiple populations leads to distinctive features in color-magnitude diagrams, including multiple main sequences due to helium-enhanced stars evolving faster and appearing bluer, and extended horizontal branches where helium-rich populations populate the blue end, prolonging core helium burning. These effects broaden the horizontal branch morphology, with super-helium-rich subpopulations (ΔY > 0.03) contributing to extreme blue extensions observed in clusters like NGC 2808. Recent photometry has further resolved these sequences in extragalactic clusters, reinforcing the helium-driven interpretations. Exotic stellar components also arise from interactions enabled by the dense cluster environment. Blue stragglers, which lie above the main-sequence turnoff and are brighter than expected for the cluster age, form primarily through mass transfer in primordial binaries or direct stellar collisions, with the former dominating in less dense regions and collisions in cores exceeding 10³ M⊙ pc⁻³. Their luminosity follows the main-sequence relation, approximated as LBS/LTO≈(MBS/MTO)4L_{\rm BS} / L_{\rm TO} \approx (M_{\rm BS} / M_{\rm TO})^4, where masses are typically 1.2–1.8 M⊙ compared to the turnoff mass of ~0.8 M⊙, making them up to 10 times more luminous. Millisecond pulsars, recycled neutron stars spun up by accretion, are found in 10–20% of globular clusters, with 47 Tucanae hosting over 40 such objects, many in binary systems. X-ray binaries, including low-mass variants with neutron star or black hole accretors, are overabundant in clusters due to dynamical formation via exchanges in dense cores, accounting for ~10% of Galactic X-ray sources despite clusters comprising <0.1% of stellar mass. Variable stars like RR Lyrae, pulsating horizontal branch members, serve as standard candles for distance measurements in globular clusters, with their period-luminosity-metallicity relation yielding precisions of ~1–2% when calibrated against cluster distances. These components highlight the role of multiple populations and interactions in shaping cluster evolution.

Formation and Models

Theoretical Formation Scenarios

Theoretical formation scenarios for globular clusters primarily revolve around two broad paradigms: in-situ formation within the host galaxy and hierarchical assembly through mergers and accretion events in the early universe. In the in-situ model, globular clusters are thought to have originated from the collapse of dense gas clouds in the galactic bulges or halos approximately 13 billion years ago, shortly after the , during a period of intense star formation. This process likely involved the rapid coalescence of smaller stellar aggregates, such as open clusters, into more massive, gravitationally bound systems capable of surviving dynamical evolution. Metallicity distributions in these clusters serve as tracers of their in-situ origins, with metal-poor populations indicating formation in primordial environments. In contrast, hierarchical models propose that globular clusters assembled from subclusters within dwarf galaxies that were later accreted into larger host galaxies, such as the . For instance, outer halo clusters in the are associated with the Gaia-Sausage-Enceladus merger, a significant accretion event around 8-11 billion years ago that contributed multiple globular clusters to the galactic halo. These models align with the Lambda-CDM framework, where clusters form through successive mergers of stellar groups in dark matter mini-halos at high redshifts. A critical aspect of both scenarios is the role of feedback mechanisms, particularly from supernovae and stellar radiation, which regulate star formation and determine the final cluster masses. In these models, initial cloud masses on the order of 107M⊙10^7 M_\odot undergo partial disruption due to explosive feedback, leaving behind bound remnants with current typical masses around 105M⊙10^5 M_\odot. This feedback rapidly expels residual gas, quenching further star formation and preserving the cluster's integrity against complete dissolution. Observational evidence supporting these early formation epochs includes the remarkable uniformity in globular cluster ages, with most dated to 12-13 billion years, consistent with a -era origin. Recent studies, such as a 2025 analysis of high-redshift mergers, demonstrate how globular cluster-like dwarfs emerge from such events, providing direct links to hierarchical assembly. However, challenges persist in explaining the multiple stellar populations observed in many clusters, as models must account for sequential star formation episodes without excessive dynamical disruption of the nascent systems. A November 2025 study proposes that extremely massive stars (1,000–10,000 solar masses) formed via an inertial-inflow model in turbulent early universe gas clouds could resolve this by enriching clusters with helium, nitrogen, and oxygen through powerful stellar winds within 1–2 million years, creating abundance anomalies characteristic of multiple populations without relying on supernovae. These stars may have collapsed into detectable by , linking globular cluster formation to early galaxy evolution. Numerical simulations play a pivotal role in understanding the formation and early evolution of globular clusters by directly modeling their dynamical and hydrodynamical processes over cosmic timescales. N-body simulations, which track the gravitational interactions among individual stars, have been instrumental in elucidating relaxation dynamics, including the prediction and evolution of core collapse. In his seminal 1971 work, Hénon developed a theoretical framework for relaxation in stellar systems, predicting that dense cores in star clusters would undergo gravitational collapse due to two-body encounters, a process later confirmed through numerical modeling. Modern N-body codes, such as NBODY6 and its extensions like NBODY6++, enable high-fidelity simulations of systems with up to 10510^5 stars over gigayear timescales, capturing phenomena like mass segregation and binary interactions that shape cluster structure. Hydrodynamical simulations further link globular cluster formation to the early universe by incorporating gas dynamics, star formation, and feedback in cosmological contexts. Recent ultra-high-resolution hydrodynamical zoom-in simulations from the University of Hertfordshire trace the 13.8 billion-year history of dwarf galaxies, revealing that globular clusters form in dense gas clumps at redshifts z∼10−15z \sim 10-15, during the era of the first galaxy assembly. These models demonstrate how clusters emerge from turbulent, metal-poor environments in proto-galaxies, with about half forming via centralized star formation bursts and the rest through hierarchical mergers of smaller clumps. Ties to the early universe underscore globular clusters' origins in the first generations of galaxies and dwarf systems, where they formed amid the cosmic web's initial collapse and survived the epoch of reionization. These ancient structures, with ages approaching the universe's age, likely originated in low-mass dwarfs that later merged into larger galaxies, preserving their compact, metal-poor nature through tidal stripping. Recent predictions from cosmological simulations suggest the existence of a new class of globular cluster-like dwarf galaxies, compact systems with uniform stellar populations that mimic traditional clusters but formed as isolated entities in the high-redshift universe. Observational links to these simulations are emerging from high-resolution surveys that catalog globular cluster candidates and probe their progenitors at high redshifts. The PHANGS-HST survey has produced catalogs of likely globular clusters in 17 nearby spiral galaxies, providing statistical samples that align with simulation predictions for cluster properties and formation efficiencies at z>6z > 6. Similarly, early data from the (VRO) in 2025 has revealed details of outer-halo stars in , including unresolved binaries, offering insights into the cluster's extended structure and potential early accretion history that match dynamical models. A key timescale in these N-body models is the two-body relaxation time, which governs the rate of energy redistribution among stars and influences core collapse: trelax≈N8ln⁡N(rh3GM)1/2,t_\mathrm{relax} \approx \frac{N}{8 \ln N} \left( \frac{r_h^3}{G M} \right)^{1/2}, where N∼105N \sim 10^5 is the number of stars, rhr_h is the half-mass radius, MM is the total mass, and GG is the ; for typical globular clusters, this yields trelax∼108−109t_\mathrm{relax} \sim 10^8 - 10^9 years, allowing significant evolution over Hubble time.

Dynamics and Interactions

Internal Dynamics

The internal dynamics of globular clusters are primarily governed by two-body relaxation, a process in which gravitational encounters between stars cause diffusion in velocity space, gradually randomizing their orbits and driving the system toward . This relaxation leads to the equipartition of , where the velocity dispersion σ\sigma of stars scales inversely with the of their , σ∝M−1/2\sigma \propto M^{-1/2}, meaning more massive stars move more slowly than lighter ones; however, observations indicate partial equipartition with a weaker mass dependence. The characteristic timescale for this process, known as the relaxation time, varies from about 10810^8 to 10910^9 years across typical globular clusters, depending on their and stellar content. A key consequence of two-body relaxation is mass segregation, whereby heavier lose kinetic energy more efficiently in encounters and "sink" toward the cluster core, while lighter are ejected to the outer regions. For instance, , which are among the most massive in the main population, are observed to concentrate in the centers of many clusters. This phenomenon has been directly confirmed through data from the DR3 release (building on DR2), which provided improved kinematic measurements for in over 150 globular clusters, revealing systematic velocity gradients consistent with mass-dependent segregation. In dense environments, prolonged relaxation can trigger core collapse, a runaway process termed the gravothermal catastrophe, where energy flows inward from the core to the halo, causing the central density to surge by orders of magnitude. Approximately 20% of globular clusters exhibit signs of having undergone this phase, with (M15) serving as a prototypical example due to its extremely high central concentration. The collapse is typically arrested by dynamical heating from hard binary stars formed through interactions, resulting in a post-collapse "bounce" that stabilizes the core; the overall timescale for core collapse is approximately 340 times the central relaxation time (or 12 to 19 times the half-mass relaxation time). Core collapse profoundly influences the of globular clusters, as the increasing central enhances stellar interactions and brightens region through mechanisms like binary hardening and tidal captures. Direct N-body simulations demonstrate that following the bounce, undergoes gravothermal expansion driven by binary energy output, leading to a gradual redistribution of luminosity and a more extended profile over subsequent relaxation times. Recent advancements in computational modeling, such as the high-resolution simulations conducted at the Institute for Astrophysics in , have traced the complete dynamical life-cycles of globular clusters from formation through core collapse and eventual dispersal, incorporating realistic and binary populations to predict internal structural changes over billions of years.

External Tidal Effects

Globular clusters experience significant external tidal effects from the gravitational influence of their host galaxy, primarily through the tidal field that limits the cluster's extent and drives mass loss over time. The tidal radius rtr_t, which defines the boundary beyond which stars are no longer bound to the cluster, is approximated by the formula rt=(GMcl2Vg2)1/3Rg2/3,r_t = \left( \frac{G M_\mathrm{cl}}{2 V_g^2} \right)^{1/3} R_g^{2/3}, where GG is the gravitational constant, MclM_\mathrm{cl} is the cluster mass, VgV_g is the galactic orbital velocity, and RgR_g is the galactocentric distance. This expression, derived under the assumption of a flat rotation curve, balances the cluster's self-gravity against the differential galactic tidal forces. Close encounters with galactic components such as the bulge, disk, or other clusters perturb the clusters' stars, leading to ejection and mass loss. Disk shocking, in particular, induces impulsive perturbations during pericentric passages, resulting in an estimated mass loss of 10-20% over a gigayear for clusters in the inner . These interactions preferentially strip low-mass stars from the outer envelopes, altering the cluster's mass function and structure, with inner clusters experiencing more intense stripping due to stronger tidal fields compared to their outer counterparts. For instance, the outer cluster NGC 288 exhibits extended tidal tails indicative of ongoing but moderate stripping, while inner clusters show more pronounced mass deficits and disrupted candidates like Palomar 5 highlight advanced . Tidal disruption often manifests as stellar streams, elongated tails of escaped stars trailing and leading the cluster along its orbit. The Palomar 5 stream exemplifies this, extending over a projected length of approximately 10 kpc and revealing the cluster's ongoing dissolution. Recent models forecast around 80 incomplete globular cluster streams in the , suggesting the current observed population is severely incomplete, particularly beyond 15 kpc from the . Internal relaxation processes facilitate this stripping by populating the outer regions with loosely bound stars vulnerable to external perturbations. Gaia proper motions have revolutionized the study of these dynamics, particularly for obscured clusters. For UKS 1, a bulge-obscured cluster, DR2 data yield mean proper motions of (μαcos⁡δ,μδ)=(−2.59,−3.42)±(0.52,0.44)(\mu_\alpha \cos \delta, \mu_\delta) = (-2.59, -3.42) \pm (0.52, 0.44) mas yr−1^{-1} , indicating a highly eccentric with pericenter distances under 1.4 kpc and maximum height above the plane below 0.5 kpc, consistent with strong tidal influence in the inner Galaxy. Updated analyses incorporating later releases continue to refine these orbits, highlighting enhanced stripping in such environments.

Astrophysical Significance

Planetary Systems

Planetary systems around stars in globular clusters are expected to be rarer than in the field, with models indicating that 10–50% of primordial systems may be disrupted over the cluster's lifetime due to the clusters' high stellar densities, elevated velocities, and intense radiation environments that disrupt forming or existing planets. No confirmed Jupiter-mass exoplanets have been detected orbiting main-sequence stars in globular clusters as of 2025, though upper limits from transit surveys suggest rates below 6% for hot Jupiters with periods of 1–36 days. As of late 2025, ongoing surveys including JWST infrared observations continue to yield no confirmed exoplanets around main-sequence stars in globular clusters, supporting the low occurrence expectations. Searches for exoplanets in globular clusters have employed multiple detection methods, yielding candidates but no definitive confirmations around typical stars. In 1999, observations of microlensing events in the cluster M22 identified six short-duration anomalies suggestive of low-mass objects, potentially free-floating planets or with masses around 60 times that of per main-sequence star, though their nature remains ambiguous. monitoring of over 1,000 stars in , conducted with high precision down to 10 m/s, detected no signals in 2000, setting stringent upper limits on close-in giant planets. More recently, the K2 mission's light curves of M4 in 2019 revealed variable stars but no transiting exoplanets, providing occurrence upper limits of 16% for sub-Neptune-sized planets with 1–10 day periods. Theoretical models highlight significant hurdles to planetary formation and retention in globular clusters. Dynamical interactions during core segregation lead to frequent stellar encounters that eject planets; simulations indicate that 10–50% of primordial systems are disrupted over a cluster's lifetime, with up to 90% of wide-orbit lost within 1 Gyr due to these processes. Additionally, the low metallicities typical of globular cluster stars (often [Fe/H] < -1) hinder the formation of rocky planetary cores necessary for gas giant accretion, as reduced solid material limits efficient core buildup in protoplanetary disks. Emerging observational capabilities offer new prospects for detecting planetary remnants in globular clusters. As of 2025, the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) targets outer halo regions for free-floating planets via microlensing, potentially revealing ejected objects from ancient cluster dynamics with sensitivities to Mars-sized worlds. The James Webb Space Telescope (JWST) holds promise for infrared transit searches in metal-poor environments, where its sensitivity could probe faint signals around dim cluster stars despite crowding challenges. A notable example is the planetary companion to the millisecond pulsar PSR B1620-26 in M4, announced in the 1990s with an estimated mass of 2.5 Jupiter masses and an orbital period of 95 years; initially hailed as the oldest known planet (~12.7 Gyr), it is now understood to be a captured planet likely formed around the progenitor star before being acquired by the binary system, rather than formed via standard processes around the current stars. Such systems underscore the harsh conditions in globular clusters, where high radiation and dynamical instability render habitability improbable for any surviving planets.

Role in Galaxy Evolution

Globular clusters (GCs) serve as precise cosmic clocks due to their uniform ages, typically around 12-13 billion years, which provide constraints on the epoch of reionization at redshifts z ≈ 6-10. Their formation shortly after the positions them as potential contributors to the ionizing radiation that reionized the intergalactic medium, with metal-poor GCs potentially accounting for a significant fraction of the required photons. A 2025 study in Astronomy & Astrophysics utilized GC ages to outline the Milky Way's assembly timeline, linking their homogeneity to early hierarchical merging events and reinforcing cosmological models of galaxy formation. As tracers of galactic mergers, GC kinematics reveal accreted stellar populations, with approximately 11% (19 out of ~170) of Milky Way GCs associated with the Gaia-Sausage-Enceladus (GSE) progenitor dwarf galaxy. Detailed 3D kinematic analyses of 30 Galactic GCs demonstrate distinct velocity profiles and specific angular momentum distributions that differentiate in-situ clusters from those captured during mergers like GSE, offering insights into the dynamical history of the Milky Way halo. GCs, often characterized as second-generation systems with multiple stellar populations showing anticorrelations in light elements, inform chemical enrichment processes in the early universe. Their compositions provide evidence for variations in the initial mass function (IMF) and supernova feedback within progenitor dwarf galaxies, particularly linking them to ultra-faint dwarfs where bottom-heavy IMFs enhance low-mass star formation and metal-poor environments. This connection suggests a shared origin, with GCs potentially forming from the disrupted remnants of these faint systems during early mergers. In extragalactic contexts, GC systems in spiral galaxies exhibit specific frequencies SNS_N (number of GCs per unit galaxy luminosity in the V-band) ranging from ~1 to 5, as quantified in the 2025 PHANGS-HST survey of 17 nearby spirals. Galactic halos act as factories for GC formation, with mergers stripping and depositing clusters into the outer envelopes, influencing the overall GC population scaling with host galaxy mass. Looking ahead, the Vera C. Rubin Observatory, operational since 2025, will enable comprehensive mapping of GC streams from disrupted satellites, revealing merger remnants across the Milky Way halo. Concurrently, JWST observations are identifying high-z (z > 6) progenitors of metal-poor GCs, such as nitrogen-enhanced galaxies at z ≈ 8-10, bridging early cluster formation to present-day systems.

References

  1. https://science.nasa.gov/universe/star-clusters-inside-the-universes-stellar-collections/
  2. https://science.nasa.gov/mission/hubble/science/universe-uncovered/hubble-star-clusters/
  3. https://www.deepskycorner.ch/obj/ngc5139.en.php
  4. https://adsabs.harvard.edu/full/2009JBAA..119..165M
  5. http://www.messier.seds.org/xtra/similar/hodierna.html
  6. http://www.messier.seds.org/xtra/Bios/halley.html
  7. https://www.cambridge.org/core/books/construction-of-the-heavens/herschels-exploration-of-the-cosmos/67229CE49CFEE94AB039925AF1997358
  8. https://adsabs.harvard.edu/full/1979JHA....10..165H
  9. https://adsabs.harvard.edu/pdf/1918ApJ....48..154S
  10. https://adsabs.harvard.edu/full/1935LowOB...3..143E
  11. https://ui.adsabs.harvard.edu/abs/1918ApJ....48..154S/abstract
  12. http://www.messier.seds.org/xtra/supp/gc-class.html
  13. https://academic.oup.com/mnras/article/464/2/2174/2404627
  14. https://adsabs.harvard.edu/full/1956PASP...68..509M
  15. https://academic.oup.com/mnras/article/355/2/504/970158
  16. https://ui.adsabs.harvard.edu/abs/1985ApJ...293..424Z/abstract
  17. https://ui.adsabs.harvard.edu/abs/1996AJ....112.1487H/abstract
  18. https://iopscience.iop.org/article/10.1086/323457
  19. https://people.ast.cam.ac.uk/~vasily/Lectures/SDSG/sdsg_7_clusters.pdf
  20. https://iopscience.iop.org/article/10.1086/342017/pdf
  21. https://iopscience.iop.org/article/10.3847/2041-8213/abdbad
  22. https://academic.oup.com/mnras/article/401/3/1832/1094472
  23. https://iopscience.iop.org/article/10.1088/0004-637X/756/2/167/pdf
  24. https://iopscience.iop.org/article/10.1088/0004-6256/135/5/1731/pdf
  25. https://ui.adsabs.harvard.edu/abs/1983PASP...95..839V
  26. https://academic.oup.com/mnras/article/429/4/2881/1008546
  27. https://iopscience.iop.org/article/10.1088/0004-637X/691/2/946
  28. https://pmc.ncbi.nlm.nih.gov/articles/PMC34183/
  29. https://academic.oup.com/mnras/article/521/3/3991/7067267
  30. https://ntrs.nasa.gov/citations/19860052951
  31. https://ui.adsabs.harvard.edu/abs/2005AJ....129..303C/abstract
  32. https://ned.ipac.caltech.edu/level5/March03/McWilliam/paper.pdf
  33. https://www.aanda.org/articles/aa/abs/2024/07/aa49533-24/aa49533-24.html
  34. https://www.aanda.org/articles/aa/full_html/2024/09/aa50589-24/aa50589-24.html
  35. https://www.aanda.org/articles/aa/full_html/2018/08/aa32843-18/aa32843-18.html
  36. https://arxiv.org/abs/1312.4169
  37. https://indico.ph.tum.de/event/3237/contributions/1834/attachments/1617/1713/Weiss_thursday.pdf
  38. https://iopscience.iop.org/article/10.1088/0004-637X/775/2/134
  39. https://adsabs.harvard.edu/full/1997AJ....114..189A
  40. https://www.aanda.org/articles/aa/full_html/2025/09/aa55452-25/aa55452-25.html
  41. https://www.pnas.org/doi/10.1073/pnas.95.1.13
  42. https://www.aanda.org/articles/aa/full_html/2011/09/aa16955-11/aa16955-11.html
  43. https://ui.adsabs.harvard.edu/abs/2018ARA&A..56...83B/abstract
  44. https://www.mdpi.com/2218-1997/8/7/359
  45. https://iopscience.iop.org/article/10.3847/1538-4357/aaec67
  46. https://ui.adsabs.harvard.edu/abs/2016ApJ...831L..17V/abstract
  47. https://www.aanda.org/articles/aa/full_html/2019/02/aa29898-16/aa29898-16.html
  48. https://academic.oup.com/mnras/article/407/1/277/984869
  49. https://iopscience.iop.org/article/10.3847/1538-4357/abc882
  50. https://www.researchgate.net/publication/395649148_JWST_Photometry_of_Globular_Cluster_Populations_in_MACS04175-1154
  51. https://iopscience.iop.org/article/10.1088/0004-637X/777/2/106
  52. https://academic.oup.com/mnras/article/349/1/129/3101539
  53. https://www3.mpifr-bonn.mpg.de/staff/pfreire/47Tuc/
  54. https://www.pnas.org/doi/10.1073/pnas.0913903107
  55. https://www.nature.com/articles/s41550-023-02011-y
  56. https://pmc.ncbi.nlm.nih.gov/articles/PMC5832832/
  57. https://academic.oup.com/mnras/article/333/2/383/1019292
  58. https://www.aanda.org/articles/aa/full_html/2022/11/aa44530-22/aa44530-22.html
  59. https://ui.adsabs.harvard.edu/abs/2018ApJ...863L..28M/abstract
  60. https://academic.oup.com/mnras/article/319/4/1047/977639
  61. https://www.nature.com/articles/s41586-025-09494-x
  62. https://phys.org/news/2020-07-globular-clusters-universe-billion-years.html
  63. https://arxiv.org/abs/2206.10564
  64. https://phys.org/news/2025-11-extremely-massive-stars-forged-oldest.html
  65. https://carnegiescience.edu/news/new-class-star-systems-predicted
  66. https://link.springer.com/article/10.1007/s001590050008
  67. https://arxiv.org/pdf/astro-ph/9610076
  68. https://.org/abs/1603.00878
  69. https://ui.adsabs.harvard.edu/abs/1997A&ARv...8....1M/abstract
  70. https://academic.oup.com/mnras/article/482/4/5138/5173087
  71. https://www.aanda.org/articles/aa/full_html/2024/09/aa50573-24/aa50573-24.html
  72. https://arxiv.org/pdf/astro-ph/9605019
  73. http://archive.seds.org/hst/95-06.html
  74. https://arxiv.org/abs/astro-ph/9606182
  75. https://iopscience.iop.org/article/10.1086/178007
  76. https://.org/abs/astro-ph/9608160
  77. https://www.mpa-garching.mpg.de/1112717/hl202503
  78. https://adsabs.harvard.edu/pdf/1962AJ.....67..471K
  79. https://www.aanda.org/articles/aa/pdf/2001/38/aa1336.pdf
  80. https://iopscience.iop.org/article/10.3847/1538-4357/ada2ea
  81. https://www.aanda.org/articles/aa/full_html/2020/11/aa39328-20/aa39328-20.html
  82. https://arxiv.org/abs/2405.15851
  83. https://observatoiredeparis.psl.eu/streams-in-the-milky-way-halo-the-case-of-palomar.html
  84. https://iopscience.iop.org/article/10.3847/1538-4357/ab44d1
  85. https://iopscience.iop.org/article/10.3847/1538-3881/ab66b4
  86. https://academic.oup.com/mnras/article/381/1/334/998674
  87. https://www.nextbigfuture.com/2024/03/vera-rubin-telescope-could-detect-mars-mercury-and-pluto-sized-planets-from-2025-2035.html
  88. https://www.astronomy.com/science/ask-astro-have-there-been-any-exoplanets-discovered-in-globular-star-clusters/
  89. https://science.nasa.gov/exoplanet-catalog/psr-b1620-26-b/
  90. https://iopscience.iop.org/article/10.3847/0004-637X/827/1/54
  91. https://academic.oup.com/mnras/article/336/2/L33/1162507
  92. https://pmc.ncbi.nlm.nih.gov/articles/PMC6301084/
  93. https://www.aanda.org/articles/aa/full_html/2025/04/aa52812-24/aa52812-24.html
  94. https://iopscience.iop.org/article/10.3847/1538-4357/ac8159
  95. https://www.aanda.org/articles/aa/full_html/2025/02/aa52477-24/aa52477-24.html
  96. https://iopscience.iop.org/article/10.1088/0004-637X/771/1/29
  97. https://iopscience.iop.org/article/10.3847/0004-637X/831/2/204
  98. https://iopscience.iop.org/article/10.3847/1538-3881/ad1889
  99. https://academic.oup.com/mnras/article-abstract/542/2/591/8213529
  100. https://rubinobservatory.org/news/rubin-first-look/rhythm-stars
  101. https://iopscience.iop.org/article/10.3847/2041-8213/ad833d
Add your contribution
Related Hubs
User Avatar
No comments yet.