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Messier 44, a cluster in the constellation of Cancer

A star cluster is a group of stars held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters, tight groups of ten thousand to millions of old stars which are gravitationally bound; and open clusters, less tight groups of stars, generally containing fewer than a few hundred members.

As they move through the galaxy, over time, open clusters become disrupted by the gravitational influence of giant molecular clouds, so that the clusters we observe are often young. Even though they are no longer gravitationally bound, they will continue to move in broadly the same direction through space and are then known as stellar associations, sometimes referred to as moving groups. Globular clusters, with more members and more mass, remain intact for far longer and the globular clusters observed are usually billions of years old.

Star clusters visible to the naked eye include the Pleiades and Hyades open clusters, and the globular cluster 47 Tucanae.

Open cluster

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The Pleiades, an open cluster dominated by hot blue stars surrounded by reflection nebulosity
Main article: Open cluster

Open clusters are very different from globular clusters. Unlike the spherically distributed globulars, they are confined to the galactic plane, and are almost always found within spiral arms. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as Messier 67 (the closest and most observed old open cluster) for example.[1] They form in H II regions such as the Orion Nebula.

Open clusters typically have a few hundred members and are located in an area up to 30 light-years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as "evaporation".

The most prominent open clusters are the Pleiades and Hyades in Taurus. The Double Cluster of h+Chi Persei can also be prominent under dark skies. Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die.

A subset of open clusters constitute a binary or aggregate cluster.[2] New research indicates Messier 25 may constitute a ternary star cluster together with NGC 6716 and Collinder 394.[3]

Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by Cepheids variable stars, which are then used as standard candles. Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe (Hubble constant). Indeed, the open cluster NGC 7790 hosts three classical Cepheids which are critical for such efforts.[4][5]

Embedded cluster

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The embedded Trapezium cluster seen in X-rays which penetrate the surrounding clouds
Main article: Embedded cluster

Embedded clusters are groups of very young stars that are partially or fully encased in interstellar dust or gas which is often impervious to optical observations. Embedded clusters form in molecular clouds, when the clouds begin to collapse and form stars. There is often ongoing star formation in these clusters, so embedded clusters may be home to various types of young stellar objects including protostars and pre-main-sequence stars. An example of an embedded cluster is the Trapezium Cluster in the Orion Nebula. In ρ Ophiuchi cloud (L1688) core region there is an embedded cluster.[6]

The embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through radiation pressure, stellar winds and outflows, or supernova explosions. In general less than 30% of cloud mass is converted to stars before the cloud is dispersed, but this fraction may be higher in particularly dense parts of the cloud. With the loss of mass in the cloud, the energy of the system is altered, often leading to the disruption of a star cluster. Most young embedded clusters disperse shortly after the end of star formation.[7]

The open clusters found in the Galaxy are former embedded clusters that were able to survive early cluster evolution. However, nearly all freely floating stars, including the Sun,[8] were originally born into embedded clusters that disintegrated.[7]

Globular cluster

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The globular cluster Messier 15 photographed by HST
Main article: Globular cluster

Globular clusters are roughly spherical groupings ranging from 10 thousand to several million stars packed into regions ranging from 10 to 30 light-years across. They commonly consist of very old Population II stars – just a few hundred million years younger than the universe itself – which are mostly yellow and red, with masses less than two solar masses.[9] Such stars predominate within clusters because hotter and more massive stars have exploded as supernovae, or evolved through planetary nebula phases to end as white dwarfs. Yet a few rare blue stars exist in globulars, thought to be formed by stellar mergers in their dense inner regions; these stars are known as blue stragglers.

In the Milky Way galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the Galactic Center, orbiting the center in highly elliptical orbits. In 1917, the astronomer Harlow Shapley made the first respectable estimate of the Sun's distance from the Galactic Center, based on the distribution of globular clusters.

Until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However, greatly improved distance measurements to globular clusters using the Hipparcos satellite and increasingly accurate measurements of the Hubble constant resolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less.

Our Galaxy has about 150 globular clusters,[9] some of which may have been captured cores of small galaxies stripped of stars previously in their outer margins by the tides of the Milky Way, as seems to be the case for the globular cluster M79. Some galaxies are much richer in globulars than the Milky Way: The giant elliptical galaxy M87 contains over a thousand.

A few of the brightest globular clusters are visible to the naked eye; the brightest, Omega Centauri, was observed in antiquity and catalogued as a star, before the telescopic age. The brightest globular cluster in the northern hemisphere is M13 in the constellation of Hercules.

Super star cluster

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Main article: Super star cluster

Super star clusters are very large regions of recent star formation, and are thought to be the precursors of globular clusters. Examples include Westerlund 1 in the Milky Way.[10]

Intermediate forms

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Messier 68, a loose globular cluster whose constituent stars span a volume of space more than a hundred light-years across

In 2005, astronomers discovered a new type of star cluster in the Andromeda Galaxy, which is, in several ways, very similar to globular clusters although less dense. No such clusters (which also known as extended globular clusters) are known in the Milky Way. The three discovered in Andromeda Galaxy are M31WFS C1[11] M31WFS C2, and M31WFS C3.

These new-found star clusters contain hundreds of thousands of stars, a similar number to globular clusters. The clusters also share other characteristics with globular clusters, e.g. the stellar populations and metallicity. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are thus much greater. The clusters have properties intermediate between globular clusters and dwarf spheroidal galaxies.[12]

How these clusters are formed is not yet known, but their formation might well be related to that of globular clusters. Why M31 has such clusters, while the Milky Way has not, is not yet known. It is also unknown if any other galaxy contains this kind of clusters, but it would be very unlikely that M31 is the sole galaxy with extended clusters.[12]

Another type of cluster are faint fuzzies which so far have only been found in lenticular galaxies like NGC 1023 and NGC 3384. They are characterized by their large size compared to globular clusters and a ringlike distribution around the centres of their host galaxies. As the latter they seem to be old objects.[13]

Astronomical significance

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Artist's impression of an exoplanet orbiting a star in the cluster Messier 67[14]

Star clusters are important in many areas of astronomy. The reason behind this is that almost all the stars in old clusters were born at roughly the same time.[15] Various properties of all the stars in a cluster are a function only of mass, and so stellar evolution theories rely on observations of open and globular clusters. This is primarily true for old globular clusters. In the case of young (age < 1Gyr) and intermediate-age (1 < age < 5 Gyr), factors such as age, mass, chemical compositions may also play vital roles.[16] Based on their ages, star clusters can reveal a lot of information about their host galaxies. For example, star clusters residing in the Magellanic Clouds can provide essential information about the formation of the Magellanic Clouds dwarf galaxies. This, in turn, can help us understand many astrophysical processes happening in our own Milky Way Galaxy. These clusters, especially the young ones can explain the star formation process that might have happened in our Milky Way Galaxy.

Clusters are also a crucial step in determining the distance scale of the universe. A few of the nearest clusters are close enough for their distances to be measured using parallax. A Hertzsprung–Russell diagram can be plotted for these clusters which has absolute values known on the luminosity axis. Then, when similar diagram is plotted for a cluster whose distance is not known, the position of the main sequence can be compared to that of the first cluster and the distance estimated. This process is known as main-sequence fitting. Reddening and stellar populations must be accounted for when using this method.

Nearly all stars in the Galactic field, including the Sun, were initially born in regions with embedded clusters that disintegrated. This means that properties of stars and planetary systems may have been affected by early clustered environments.[17] This appears to be the case for our own Solar System, in which chemical abundances point to the effects of a supernova from a nearby star early in our Solar System's history.

Star cloud

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Scutum Star Cloud with open cluster Messier 11 at lower left

Technically not star clusters, star clouds are large groups of many stars within a galaxy, spread over very many light-years of space. Often they contain star clusters within them. The stars appear closely packed, but are not usually part of any structure.[18] Within the Milky Way, star clouds show through gaps between dust clouds of the Great Rift, allowing deeper views along our particular line of sight.[19] Star clouds have also been identified in other nearby galaxies.[20] Examples of star clouds include the Large Sagittarius Star Cloud, Small Sagittarius Star Cloud, Scutum Star Cloud, Cygnus Star Cloud, Norma Star Cloud, and NGC 206 in the Andromeda Galaxy.

Nomenclature

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In 1979, the International Astronomical Union's 17th general assembly recommended that newly discovered star clusters, open or globular, within the Galaxy have designations following the convention "Chhmm±ddd", always beginning with the prefix C, where h, m, and d represent the approximate coordinates of the cluster centre in hours and minutes of right ascension, and degrees of declination, respectively, with leading zeros. The designation, once assigned, is not to change, even if subsequent measurements improve on the location of the cluster centre.[21] The first of such designations were assigned by Gosta Lynga in 1982.[22][23]

See also

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Wikimedia Commons has media related to:
Star cluster (category)

References

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Star cluster

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A star cluster is a gravitationally bound collection of stars that originated from the same , forming simultaneously through the collapse of gas and dust under gravity, and remaining cohesive for periods ranging from millions to billions of years depending on their type and environment. These clusters serve as fundamental building blocks in galactic structure, providing insights into , processes, and the chemical enrichment of galaxies, with the alone hosting approximately 150 globular clusters and thousands of open clusters. Star clusters are broadly classified into three main types based on their density, age, and binding: open clusters, globular clusters, and super star clusters. Open clusters are relatively loose groupings of tens to a few thousand young stars, typically less than 1 billion years old, spanning just a few light-years and often located in the spiral arms of galaxies like the ; a prominent example is the , which contains over 1,000 confirmed member stars. In contrast, globular clusters are dense, spherical aggregates of tens of thousands to millions of ancient stars, aged 8 to 13 billion years, with diameters of 50 to 450 light-years, primarily residing in the halo of galaxies; , located about 17,000 light-years from , exemplifies this type as one of the largest known. Super star clusters are extremely massive young clusters, potentially evolving into globular clusters, found in starburst galaxies. The formation of star clusters begins in dense regions of molecular clouds where gravitational instability triggers the collapse of gas pockets, leading to the birth of multiple over approximately 1 million years, often in a hierarchical manner with subclusters merging over time. Evolutionarily, open clusters tend to dissolve within hundreds of millions of years due to internal dynamical relaxation and external tidal forces from the galactic disk, while globular clusters endure longer, potentially hosting multiple generations of and even intermediate-mass holes in their cores. These systems are crucial for astronomers, as their study—facilitated by telescopes like Hubble and —reveals the history of rates, the distribution of heavy elements in the , and the dynamical processes shaping galaxies over .

Fundamentals

Definition and overview

A star cluster is a gravitationally bound group of stars that share a common origin, having formed simultaneously from the same . These assemblies typically contain anywhere from tens to millions of stars, held together by their mutual gravity over timescales ranging from millions to billions of years. Unlike smaller binary or multiple star systems, which involve only a handful of stars, or vast galaxies comprising billions of diverse stellar populations, star clusters occupy an intermediate scale, providing isolated laboratories for studying and interactions. The recognition of star clusters dates back to the early telescope era, when in 1610 resolved portions of the into individual stars, demonstrating that its hazy appearance arose from unresolved stellar concentrations, including clusters. This observation marked the first clear evidence of clustered stellar distributions within our . Building on this, in the 1780s initiated systematic surveys, publishing his Catalogue of One Thousand New Nebulae and Clusters of Stars in 1786, which documented numerous such groupings and laid foundational catalogs for deeper astronomical study. Star clusters have since proven essential for probing , as their co-eval stars—born under similar conditions—reveal evolutionary sequences across a single system, from main-sequence youth to post-main-sequence remnants. In the , approximately 10,000 open clusters and 150 to 200 globular clusters are estimated to exist, with total stellar masses spanning 100 to 1,000,000 solar masses depending on cluster type and age. These structures not only trace galactic structure but also inform models of and dynamical processes.

Physical properties

Star clusters vary significantly in their physical properties, which reflect their formation environments and dynamical histories. Open clusters generally span diameters of 1 to 10 parsecs, presenting a relatively sparse and irregular structure with stars distributed in a loose configuration. In contrast, globular clusters are larger, with diameters typically ranging from 20 to over 100 parsecs, and exhibit a characteristic core-halo structure: a centrally concentrated core of higher stellar surrounded by an extended, lower- halo. The total masses of star clusters range from approximately 10210^2 to 10510^5 solar masses (M⊙M_\odot) for open clusters, while globular clusters are more massive, spanning 10410^4 to 10610^6 M⊙M_\odot. in star clusters, particularly young ones, is predominantly contributed by massive stars, which outshine lower-mass members due to their high radiative output during early evolutionary stages. Dynamically, star clusters are self-gravitating systems governed by the , which states that for a stable, bound configuration, twice the total KK plus the total WW equals zero: 2K+W=02K + W = 0. This relation allows estimation of cluster mass from observed velocities. Velocity dispersions in star clusters typically range from 1 to 10 km/s, with lower values in open clusters and higher in the denser cores of globulars. The two-body relaxation time, during which stars exchange energy and velocities to approach , scales as 10810^8 to 10910^9 years, longer in more massive globular clusters than in open ones. Metallicity and age in star clusters are inferred from color-magnitude diagrams (CMDs), which plot stellar colors (related to temperature) against magnitudes (related to luminosity). The main-sequence turnoff point, where stars leave the hydrogen-burning main sequence, serves as a primary age indicator, with younger clusters (ages 10710^7 to 10910^9 years) showing turnoffs at brighter, hotter stars and older globular clusters (101010^{10} years) at fainter, cooler ones. Metallicity, or the abundance of elements heavier than helium, is determined by comparing the shape and position of features like the red giant branch in the CMD to theoretical models, with metal-poor clusters displaying steeper branches.

Formation and Evolution

Mechanisms of formation

Star clusters form primarily within giant molecular clouds (GMCs), vast reservoirs of cold, dense interstellar gas where initiates the process. The initial collapse is often triggered by external perturbations such as shock waves from supernovae explosions, density waves in galactic spiral arms, or collisions between smaller clouds, which compress the gas and increase local densities sufficiently to overcome supporting pressures. Once perturbed, regions within the GMC become unstable to gravitational fragmentation via the , where the cloud's mass exceeds the critical Jeans mass, allowing self-gravity to dominate thermal and turbulent support, leading to the formation of dense cores that collapse into protostellar systems. These mechanisms operate on scales of tens to hundreds of parsecs in GMCs, fostering the hierarchical assembly of multiple stars and subclusters. The efficiency of star formation in these collapsing regions is typically low, with only 20-30% of the available gas mass converting into stars before the process halts. This limited efficiency arises from feedback mechanisms generated by the nascent stars themselves, including that heats and disperses surrounding gas, mechanical winds from massive stars that inject and , and eventual supernovae explosions that drive powerful outflows. These feedbacks regulate the by injecting and , preventing further accretion and expelling much of the residual gas, thereby setting the stellar content of the emerging cluster. In cluster-forming environments, this interplay ensures that while dense cores form efficiently, the overall GMC does not fully , leaving behind a structured . Recent advancements in numerical simulations have provided deeper insights into cluster formation, particularly highlighting the role of high star formation efficiency in producing compact structures. For instance, the SIEGE project, a series of zoom-in cosmological simulations with sub-parsec resolution and individual star feedback, demonstrates that dense stellar clusters emerge in environments with elevated efficiencies, forming compact clumps on scales of 1-3 parsecs with stellar surface densities up to nearly 104M⊙10^4 M_\odot pc−2^{-2}. In low-metallicity clouds with abundances Z=10−6Z = 10^{-6} to 10−2Z⊙10^{-2} Z_\odot, such simulations reveal that reduced cooling allows for the direct collapse of supermassive stars (masses exceeding 103M⊙10^3 M_\odot), which in turn drive the rapid assembly of dense clusters by concentrating stellar mass and enhancing local gravitational binding. In the context of the early , Λ\LambdaCDM cosmological simulations from 2025 illustrate how cosmic dawn clusters—formed during the first billion years—achieve high stellar densities in metal-poor environments due to the pristine gas conditions that favor massive and minimal opacity. These models, using high-resolution hydrodynamics in AREPO, show that primordial halos host clusters with densities exceeding 105M⊙10^5 M_\odot pc−2^{-2}, shaped by inefficient metal enrichment and intense fields that promote hierarchical merging of protostellar systems. Such findings underscore the universality of collapse and feedback processes across cosmic epochs, from metal-poor origins to present-day GMCs.

Evolutionary stages

Star clusters undergo a series of evolutionary stages following their formation, driven by internal dynamical processes and interactions with their galactic environment. The initial infant or molecular phase sees young clusters embedded within their natal molecular clouds, where they remain shrouded in dense gas and for approximately 1-10 million years (Myr), resulting in high levels of that obscure observations. During this period, ongoing and gas accretion contribute to the cluster's growth, with stellar feedback from massive stars beginning to influence the surrounding medium. The transition to the next stage occurs with gas expulsion, triggered by feedback mechanisms such as , winds, and supernovae from massive , which rapidly remove the residual natal gas on timescales of 10-100 Myr. If the efficiency is below 50%, this sudden mass loss—often 50-90% of the initial cluster mass—leads to a violent expansion, increasing the dispersion and potentially unbinding a significant of , with expansion factors reaching 3-4 times the original size. This phase marks the emergence of the cluster as a gas-free , though the dynamical reconfiguration can last several crossing times, approximately 20,000 years for a 10 cluster. As clusters age, dynamical relaxation becomes dominant, where gravitational interactions among redistribute energy and , leading to mass segregation and, in dense systems, core collapse. The two-body relaxation timescale, which governs these processes, is given by τrel∝Nln⁡N(rh3G[M](/page/M))1/2\tau_\mathrm{rel} \propto \frac{N}{\ln N} \left( \frac{r_h^3}{G [M](/page/M)} \right)^{1/2}, where NN is the number of stars, rhr_h the half-mass , MM the total mass, and GG the ; for typical parameters like M=104M⊙M = 10^4 M_\odot and half-mass density ρh=104M⊙pc−3\rho_h = 10^4 M_\odot \mathrm{pc}^{-3}, τrh≈18\tau_\mathrm{rh} \approx 18 Myr. Core collapse in such systems occurs on a timescale of about 0.2 τrh\tau_\mathrm{rh}, or roughly 3 Myr, after which the cluster may expand or stabilize through binary interactions. Disruption mechanisms ultimately determine the longevity of clusters, with tidal shocks from the galactic disk and disk passages causing significant mass loss on timescales of 10910^9 years for open clusters, while evaporation through stellar escapes via two-body relaxation contributes steadily, particularly in tidally limited systems. Recent 2025 high-resolution simulations of star clusters in a like WLM demonstrate the full lifecycle, from embedded birth to complete dispersal over hundreds of Myr, highlighting how tidal interactions strip stars into tails and fully dissolve clusters within ~500 Myr in galactic contexts. These processes are modulated by the density, with shocks dominating early dissolution rates. Open clusters have shallow potential wells and are vulnerable to tidal forces, with few persisting beyond 1 Gyr, whereas globular clusters, benefiting from deeper potentials and higher es, exhibit greater stability and can endure for over 10 Gyr. Analytical models predict dissolution times scaling as tdis≈1.7(Mi/104M⊙)0.67t_\mathrm{dis} \approx 1.7 (M_i / 10^4 M_\odot)^{0.67} Gyr for initial masses 102<Mi<105M⊙10^2 < M_i < 10^5 M_\odot, underscoring the mass dependence of survival in the Galactic disk.

Classification and Types

Open clusters

Open clusters are gravitationally bound aggregates of , typically comprising 10210^2 to 10310^3 members, that form in the disks of spiral galaxies such as the . These systems are characterized by relatively low stellar densities, ranging from 0.1 to 10 per cubic , which contributes to their loose structure and irregular shapes. With ages spanning 10710^7 to 10910^9 years, open clusters represent young to intermediate-age populations that trace recent episodes. They are predominantly found in or near spiral arms, where dense facilitates their birth and initial cohesion. Notable examples illustrate their diversity and accessibility. The (M45), located about 136 parsecs away, is a classic young with an age of roughly 100 million years and over 500 confirmed members, many of which are hot, blue B-type stars visible to the naked eye. The Hyades, the nearest at approximately 46 parsecs, offers a closer view of an older system around 625 million years old, containing about 200 stars and serving as a benchmark for studies due to its proximity. Open clusters originate from the fragmentation and collapse of giant molecular clouds (GMCs) in the galactic disk, where turbulent processes trigger hierarchical star formation. Unlike more isolated structures, their positions in the crowded disk expose them to strong tidal fields from the galaxy's and nearby mass concentrations, leading to elevated disruption rates over gigayear timescales. This tidal influence gradually strips outer stars, causing many clusters to dissolve within a few hundred million years, dispersing their members into the field population. In terms of diversity, open clusters vary from sparse, poor associations with fewer than 100 stars to rich, compact groups like the Jewel Box (NGC 4755), a visually striking cluster in containing hundreds of stars, including massive red supergiants and blue giants, at a distance of about 1,900 parsecs. Recent simulations of dense open clusters highlight their potential for dynamic interactions; in environments with high stellar densities, repeated collisions between massive stars can produce very massive stars that collapse into intermediate-mass black holes (100–10,000 solar masses). Such processes underscore the role of open clusters in seeding exotic objects within galactic disks. Young open clusters often emerge from embedded phases shrouded in residual GMC material, transitioning to exposed states as gas disperses.

Globular clusters

Globular clusters are tightly bound, spheroidal collections of that in the halos of galaxies, typically containing between 10510^5 and 10610^6 . These systems exhibit ages ranging from 10 to 13 billion years, making them among the oldest stellar aggregates in the and providing key insights into early galactic formation. Their central densities are exceptionally high, reaching 10310^3 to 10510^5 per cubic , with a near-spherical that concentrates toward the core while thinning out toward the periphery. This structure arises from gravitational binding and dynamical relaxation over billions of years. In terms of orbital dynamics, globular clusters follow highly eccentric paths around the within the halo, experiencing significant tidal interactions that shape their evolution. These orbits often have eccentricities exceeding 0.5, leading to periodic close approaches (perigalactica) where tidal forces are strongest, potentially stripping outer stars. The tidal radius, which delineates the boundary beyond which stars are no longer bound to the cluster, is defined by the Jacobi surface—a zero-velocity contour in the rotating frame of the cluster-galaxy system. This surface accounts for the combined gravitational influences of the cluster's mass and the galaxy's tidal field, influencing the cluster's long-term survival and mass loss. A defining feature of globular clusters is the presence of multiple stellar populations, indicating sequential episodes of within the same system. These populations show variations in chemical abundances, particularly elevated and sodium levels in later generations alongside depletions in oxygen, as evidenced by anticorrelations in spectroscopic data from clusters like NGC 6752. This suggests that material enriched by processes such as star pollution or early massive star winds was recycled into subsequent star-forming clouds. A 2025 study published in used cosmological simulations to demonstrate the emergence of globular clusters and globular-cluster-like dwarfs in the early universe, linking their formation to the hierarchical assembly of halos around z ≈ 6–10. Recent simulations from 2025 have further revealed a new class of ancient star systems in the that mimic traditional globular clusters in appearance but harbor hidden internal structures, such as embedded remnants or disrupted cores. These findings, derived from high-resolution N-body models, suggest that up to dozens of such systems may lurk undetected, offering clues to the galaxy's accretion and the survival of primordial clusters against tidal disruption.

Super star clusters

Super star clusters represent the most massive and dense end of young star cluster populations, containing between 10510^5 and 10710^7 stars with total masses greater than 10510^5 M⊙_\odot. These clusters are remarkably compact, with effective radii typically less than 10 pc, and are characterized by very young ages, generally under 100 Myr, often in the range of 1–10 Myr. Their high stellar densities, exceeding 10410^4 stars per pc³ in central regions, foster intense interactions among massive stars, leading to elevated rates of stellar collisions and binary formations. These clusters predominantly form in turbulent, high-pressure environments within merging or starburst galaxies, such as the (NGC 4038/4039), where interactions trigger rapid gas inflows and . Host galaxies exhibit elevated rates, ranging from 100 to 1000 M⊙_\odot/yr, far surpassing quiescent spirals, which drives the efficient assembly of such massive systems from giant molecular clouds with masses up to 10810^8 M⊙_\odot. In these settings, feedback from supernovae and stellar winds shapes the , potentially regulating further cluster formation. If they withstand the strong tidal fields and dynamical disruptions in their host galaxies, super star clusters may evolve into long-lived analogs over gigayears, losing low-mass stars through while retaining a core of massive remnants. Recent studies from 2025 highlight the role of extremely massive stars—up to thousands of solar masses—in the formation of the universe's earliest super star clusters, where their powerful winds and explosions enriched the gas, facilitating the chemical signatures observed in ancient globulars. Prominent examples include , the most massive known cluster in the with a of approximately 10510^5 M⊙_\odot and an age of 3.5–5 Myr, hosting hundreds of thousands of stars within a radius under 6 light-years. Another is MGG 11 in the M82, a compact cluster with a around 10610^6 M⊙_\odot, an age of 6–12 Myr, and a radius of about 2 pc, notable for its potential to harbor intermediate-mass black holes from stellar mergers.

Special and Intermediate Forms

Embedded clusters

Embedded clusters represent the earliest observable stage of star cluster formation, where young stellar groups remain deeply embedded within their natal molecular clouds. These clusters typically have ages less than 10 million years (Myr) and are characterized by high levels of obscuration, with visual extinctions (A_V) exceeding 10 magnitudes due to surrounding dust and gas. The stellar populations are dominated by protostars and pre-main-sequence stars such as objects, which are still accreting material from their birth environments. This phase occurs during active , with embedded clusters forming preferentially in giant molecular clouds that provide the dense gas reservoirs necessary for clustered star birth. In terms of structure, embedded clusters exhibit a centralized distribution of , often with radii of a few parsecs, surrounded by dense gas envelopes that foster ongoing . Prominent features include bipolar outflows and Herbig-Haro jets driven by the youngest massive , which pierce through the obscuring material and indicate dynamic interactions between the forming and their gaseous surroundings. Mass segregation begins to emerge early in this stage, with more massive concentrating toward the cluster center, potentially inherited from the initial density structure of the clumps or resulting from competitive accretion processes. This early dynamical organization sets the stage for the cluster's future evolution while the system remains gas-rich. The transition from the embedded phase to exposed open clusters is triggered by gas expulsion, primarily through feedback mechanisms like and from massive , which disperse the remaining material within a few Myr. This rapid dispersal unbinds a significant fraction of the unless the cluster's velocity dispersion is sufficiently low to retain binding after mass loss. A well-studied example is the Orion Nebula Cluster (ONC), which contains approximately 2,000 and has an age of 1-2 Myr, currently emerging from its embedded state as evidenced by its partial exposure and ongoing gas clearing. Recent observations with the (JWST) have provided unprecedented insights into embedded clusters in nearby galaxies, revealing their timescales and structures during the embedded-to-exposed transition. For instance, JWST NIRCam imaging of emerging young star clusters in the galaxy M83 has shown that the embedded phase lasts about 1-3 Myr, with clusters partially unveiling as dust is cleared by stellar feedback. In nearby systems, such as those observed in the PHANGS-JWST survey, embedded clusters appear as compact, dust-enshrouded sources in mid-infrared bands, highlighting their role in . These 2024-2025 datasets underscore how embedded clusters serve as precursors to the globular and populations seen today.

Associations and moving groups

Stellar associations represent loosely bound or unbound groups of that share a common origin but lack persistent gravitational binding, distinguishing them from denser clusters. OB associations primarily consist of young, massive O and B-type , typically aged 10-50 million years, formed in regions of active within giant molecular clouds. In contrast, T associations comprise lower-mass , including T Tauri-type pre-main-sequence objects, with ages ranging from 10 to 100 million years, often exhibiting variability in brightness due to their youth. Moving groups, a related , are co-moving streams of identified through kinematic similarities, such as shared proper motions and radial velocities, forming overdensities in velocity without spatial concentration. These structures typically span 10-100 parsecs in extent and contain 10-1000 stars, with low stellar densities that render them unstable to galactic tidal forces. Expansion arises from residual velocities imparted during formation, often triggered by explosions of massive stars within the group, which impart and drive dispersal into the . Such dynamics allow associations to retain some spatial coherence for tens of millions of years before fully integrating with the galactic field population. Prominent examples include the -Centaurus association, the largest nearby OB association at approximately 130 parsecs distance, comprising three main subgroups—Upper Scorpius, Upper Centaurus-Lupus, and Lower Centaurus-Crux—with ages ranging from 5 to 17 million years and hosting hundreds of B-type stars. The moving group, aged around 20-25 million years, exemplifies a young co-moving stream notable for its high fraction of debris disks, including the iconic disk around Beta Pictoris itself, which harbors evidence of planetary formation. As intermediate forms between gravitationally bound clusters and dispersed field stars, associations provide key insights into the transition from clustered to distributed stellar populations following gas expulsion and dynamical relaxation. Recent 2025 simulations of star cluster formation and dispersal, incorporating radiation-magnetohydrodynamics, demonstrate how initial cluster cores evolve into unbound associations through feedback processes, bridging the gap between dense birth environments and the galactic field over 10-100 million years. These models highlight the role of supernova-driven expansion in populating moving groups, consistent with observations of nearby systems.

Star clouds

Star clouds are apparent concentrations of stars visible along the plane of the , resulting from the alignment of unrelated stars at varying distances along our rather than forming physically bound groups. Unlike true star clusters, where members are held together by mutual , star clouds consist of stars that are not dynamically associated, creating an of through projection effects. These features appear as irregular, hazy patches of against the galactic background, often spanning several degrees across the sky. The characteristics of star clouds include mixed stellar populations with diverse ages, distances, and compositions, leading to no shared proper motions or coherent . Apparent surface densities seem high due to the superposition of field from multiple galactic depths, but in three dimensions, these dilute significantly, revealing sparse distributions. Observational challenges arise from interstellar dust obscuration along the , which can further enhance the visual clustering effect while complicating measurements. Prominent examples include the Sagittarius star clouds, such as the (Messier 24), a bright patch approximately 1.5 degrees wide located about 10,000 light years away, containing embedded open clusters but dominated by unrelated foreground and background stars. Another is the Cygnus Star Cloud, the brightest section of the northern , spanning the constellation Cygnus and appearing as a large, unassociated aggregation visible to the under . Astrometry from the mission has distinguished these by demonstrating disparate parallaxes and proper motions among the stars, confirming their non-physical nature. Historically, early telescopic observations often mistook star clouds for true clusters or nebulae due to limited resolution and lack of kinematic data; for instance, cataloged the in 1764 as a nebulous object before recognizing its stellar composition. Advances in and studies in the , culminating in Gaia's precise measurements, resolved this confusion by revealing the unrelated origins of the stars involved.

Observation and Detection

Methods of study

Optical and infrared photometry serves as a foundational method for studying star clusters, enabling the construction of color-magnitude diagrams (CMDs) that reveal stellar evolutionary stages, cluster ages, and metallicities through comparisons with theoretical isochrones. By plotting stars in color-magnitude space, astronomers identify main-sequence turnoffs to estimate ages ranging from young open clusters (tens of millions of years) to ancient globular clusters (billions of years), while the position and slope of the provide metallicity indicators, often calibrated against spectroscopic benchmarks. observations, particularly in the near- and mid-IR bands, penetrate dust-obscured regions to access embedded clusters, lifting age-metallicity degeneracies when combined with optical data. Proper motion measurements from the Gaia mission further refine cluster membership by distinguishing co-moving stars from field contaminants, utilizing data from releases spanning 2016 to 2022, including the Focused Product Release in 2023. Gaia's astrometric precision, achieving microarcsecond-level proper motions in Data Release 3 (2022) and the subsequent Focused Product Release (2023), allows identification of cluster extents including tidal tails, with membership probabilities derived from clustering algorithms on 5D phase-space data (position and velocity). The mission concluded science operations in early 2025. This has expanded catalogs of known clusters, revealing over 2,000 open clusters with well-defined boundaries within 2 kpc of the Sun. Spectroscopic techniques provide detailed kinematic and chemical insights, measuring radial velocities to confirm membership and probe internal dynamics, while abundance patterns trace cluster origins and enrichment histories. High-resolution spectra yield radial velocities with precisions below 1 km/s, enabling the separation of cluster members from foreground/background stars and the mapping of velocity dispersions that indicate and virial states. Chemical abundances of elements like iron, sodium, and oxygen are derived from line strengths, revealing homogeneous compositions within clusters that distinguish them from the galactic field and inform on contributions to stellar populations. Integral field units (IFUs), such as on the VLT, extend this to 2D , resolving velocity fields across cluster cores to study , segregation, and dynamical interactions in dense environments like young massive clusters. Multi-wavelength approaches integrate observations across the to uncover phenomena obscured at single wavelengths, such as emissions from binaries and radio signatures of ionized gas. telescopes like detect compact sources in clusters, identifying accreting binaries whose luminosities (10^{30}-10^{36} erg/s) signal dynamical interactions and help estimate total cluster masses via binary fractions. Radio , using facilities like the VLA, maps H II regions around massive stars in embedded clusters, tracing ionization fronts and feedback that shape cluster dispersal, with fluxes indicating stellar content and ages under 10 Myr. Recent (JWST) mid-infrared observations from 2022 to 2025 have revealed dust-enshrouded super star clusters in distant galaxies, using MIRI to penetrate extinction and resolve structures down to scales, exposing young populations with masses exceeding 10^5 solar masses. Numerical simulations complement observations by modeling cluster and formation under gravitational and feedback physics. N-body simulations track the long-term dynamics of thousands to millions of , incorporating two-body relaxation, , and tidal fields to predict dissolution timescales and escaper populations over gigayears. Tools like NBODY6++GPU enable high-fidelity runs that reveal how initial conditions, such as , influence core collapse and binary formation rates in globular clusters. High-resolution cosmological simulations, achieving sub-parsec resolution by 2025, integrate hydrodynamics and to simulate cluster formation within galaxies from z ~ 10 to the present, capturing feedback from individual that regulates efficiency and cluster survival. These models, such as those in the SIEGE suite, demonstrate that dense clusters form in turbulent giant molecular clouds, with survival rates below 10% due to early gas expulsion.

Notable examples

The , also known as Messier 45 (M45), is a prominent open star cluster in the constellation Taurus, containing over 1,000 stars loosely bound by and located approximately 440 light-years from . Visible to the as a hazy patch, it has held significant cultural importance across civilizations, including in Polynesian astronomy where it was observed as a cluster of seven stars linked to navigational and mythological traditions. Its young age of about 100 million years makes it a key example for studying early in loose aggregates. Another well-known is the , or Messier 44 (M44), situated in the constellation Cancer at a distance of approximately 577 light-years and comprising around 1,000 stars. Recognized in ancient astronomical records for its visibility as a diffuse glow, it drifts through interstellar gas unrelated to its formation, highlighting the dynamic environments of such systems. Among globular clusters, (NGC 5139) stands out as the largest in the , harboring an estimated 10 million stars and spanning a diameter of about 150 light-years. Its complexity is evident in multiple stellar populations with varying metallicities and helium abundances, suggesting it may be the stripped core of a rather than a typical . In the southern sky, (NGC 104) is the second-brightest after , located approximately 14,500 light-years away and containing tens of thousands of stars packed into a dense core. With an age of approximately 10.5 billion years, it exemplifies the dense, ancient stellar environments that probe the early . For embedded and super star clusters, NGC 3603 in the Milky Way's Carina constellation serves as an analog to extragalactic super clusters, featuring a massive young core of hot, luminous stars embedded in a nebula 20,000 light-years distant. Its surrounding bubble of ionized gas and dense Bok globules illustrate intense star formation in obscured regions. Similarly, R136 in the Large Magellanic Cloud's 30 Doradus nebula represents an extreme case of density, with a compact core (0.1 parsec diameter) hosting over 65 O3-type stars and exhibiting mass segregation indicative of dynamical maturity despite its young age of 2-3 million years. Recent simulations in 2025 have revealed potential hidden globular cluster-like systems in the , emerging from turbulent molecular clouds and mimicking the density profiles of observed globulars through inertial inflows. Additionally, cosmological simulations of cosmic dawn have modeled ancient star clusters forming in high-density III environments, predicting compact aggregates with stellar densities up to 10^5 stars per cubic that survive feedback and contribute to early assembly. These examples underscore the diversity of star clusters detectable via advanced imaging and modeling techniques.

Significance in Astronomy

Role in galactic structure

Open clusters serve as key tracers of the galactic disk's structure, particularly in delineating spiral arms and reconstructing the . Young open clusters, with ages less than 80 million years, align closely with the positions of spiral arms in the , allowing astronomers to map arm and compute pattern speeds through backward integration of their orbits using data from missions like . These clusters reveal bursts of tied to arm passages, providing a timeline of disk activity over the past few hundred million years. Furthermore, radial age gradients in open clusters indicate inside-out disk , with younger clusters dominating the inner regions and progressively older ones outward, reflecting the propagation of fronts across the . In contrast, globular clusters act as building blocks of the , preserving evidence of ancient accretion events that assembled the . Many globulars exhibit kinematic signatures linking them to disrupted dwarf galaxies, such as the Gaia-Enceladus merger, which contributed a significant portion of the halo's stellar content around 10 billion years ago. Recent 2025 studies using high-resolution simulations demonstrate that globular clusters often form within dwarf galaxies during mergers, surviving tidal stripping to populate the 's halo and serve as relics of hierarchical assembly; this is supported by April 2025 Hubble observations of ongoing star cluster mergers in the nuclear regions of dwarf galaxies, providing direct evidence for such processes. These clusters' and multiple populations trace the merger history, with retrograde orbits indicating early accretion from systems. Star clusters also illuminate chemical evolution across galactic components, maintaining metallicity gradients that record enrichment processes. Open clusters in the disk exhibit a radial decrease in iron abundance ([Fe/H]) outward, with slopes around -0.06 dex kpc⁻¹, preserved due to their shared birth environments with field stars and minimal radial migration for older systems. This gradient flattens with time, signaling dilution by inward-migrating metal-rich gas or outward metal-poor infall. In mergers, super star clusters drive bulge formation by concentrating in dense, gas-rich environments, contributing to the central peak through rapid feedback and dynamical sinking. Tidal interactions between star clusters and the galactic potential induce dynamical heating, gradually thickening and stirring the disk over gigayears. Encounters with giant molecular clouds and spiral arms scatter cluster members, increasing velocity dispersions and eroding cluster cohesion, while the cumulative effect disperses stellar orbits to puff up the disk scale height. In time-variable tidal fields, such as during pericentric passages in mergers, clusters experience enhanced mass loss via "disk shocking," amplifying heating rates for both clusters and the surrounding disk population. This process links cluster disruption to long-term disk evolution, with simulations showing sustained energy injection over billions of years.

Insights into stellar populations

Star clusters provide ideal environments for studying stellar populations due to their shared age, composition, and distance, enabling the construction of precise Hertzsprung-Russell (HR) diagrams that reveal evolutionary stages. By plotting cluster members on the HR diagram, astronomers can overlay theoretical isochrones—curves representing stars of varying masses at a fixed age—to fit the observed turnoff point, which indicates the cluster's age and tests models of . For instance, the turnoff point marks where the most massive stars have exhausted their hydrogen fuel, directly linking initial mass to stellar lifetimes and providing constraints on mass-lifetime relations in models. This approach has been validated using grids of evolutionary tracks for masses from 0.8 to 120 solar masses, including pre-main-sequence phases with accretion, which align well with observations in young clusters like NGC 1893 and IC 1848. Globular clusters exhibit multiple stellar generations, characterized by variations in light elements like , carbon, , oxygen, sodium, and aluminum, resulting from pollution by earlier stellar populations. These second-generation stars form from material enriched by (AGB) stars of the first generation, which eject processed gas through winds, leading to abundance anomalies observed in high-temperature hydrogen-burning products. Recent simulations in 2025 have revealed a new class of ancient systems, termed globular cluster-like dwarfs, which form through hierarchical mergers in the early and may host undetected multiple populations, potentially explaining hidden ancient structures in the . The dense stellar environments of clusters promote dynamical interactions that foster binary star formation and mergers, particularly in young systems where high velocities lead to collisional runaways. These interactions can produce intermediate-mass (IMBHs) with masses between 100 and 10,000 solar masses through repeated mergers of stellar-mass , as demonstrated by 2024 simulations of dense young clusters; additionally, a 2025 study highlights the role of nuclear star clusters in boosting IMBH growth in low-mass galaxies via efficient dynamical processes. Such processes highlight clusters as key sites for understanding population dynamics and sources. Star clusters like Praesepe (the ) serve as laboratories for studies, hosting systems that reveal formation and survival in crowded environments. Recent surveys from 2023 to 2025, including analyses of young clusters, indicate occurrence rates of transiting planets around 54% for short-period orbits, suggesting that dynamical instabilities may disrupt outer systems but preserve inner hot sub-Neptunes at rates up to 107% in young populations. These findings underscore how cluster ages enable tracking of evolution, including interactions and atmospheric mass loss.

Nomenclature and Catalogues

Naming conventions

Star clusters have been identified and named through a variety of historical, cultural, and systematic methods, reflecting their visibility and significance across human societies. Many prominent clusters bear mythological names rooted in ancient traditions; for instance, the is known as the Seven Sisters, drawing from where it represents the seven daughters of the Titan Atlas and the Oceanid Pleione, who were transformed into stars by . This name has parallels in numerous cultures worldwide, underscoring the cluster's enduring cultural resonance. Descriptive names, often inspired by visual appearance, also feature prominently in historical nomenclature. The Jewel Box cluster (NGC 4755) earned its moniker from astronomer John Herschel's 1830s observations, in which he likened its assemblage of bright, multicolored stars to a casket of jewels. Such informal or poetic designations complement more structured systems and persist in popular astronomy. Similarly, the Praesepe cluster (M44) is colloquially called the due to the dense, buzzing swarm-like pattern of its approximately 1,000 member visible to the . Systematic naming relies on astronomical catalogs rather than a unified convention, as no single universal standard governs all star clusters. The Messier catalog assigns "M" numbers to 110 bright deep-sky objects, including several clusters like M44 (Praesepe) and M45 (), compiled by in the to aid hunting. The (NGC), published in 1888 by John Louis Emil Dreyer, provides numerical designations such as NGC 4755 for the Jewel Box, encompassing thousands of non-stellar objects. The , introduced by in 1995, lists 109 additional objects (C numbers) visible to amateur astronomers, filling gaps in the Messier list, particularly for southern skies. This multiplicity of catalogs results in clusters often having multiple identifiers, such as the (M45), with associated nebulae like the Maia Nebula designated NGC 1432. Cultural significance extends to indigenous traditions, where clusters inform and cosmology, prompting modern to incorporate these perspectives and mitigate colonial legacies. For example, Aboriginal Australian oral histories associate the —irregular dwarf galaxies containing star clusters—with ancestral figures, such as campsites of an elder man and woman or creator brothers in Garadjari lore. The (IAU) has increasingly approved indigenous names for celestial features, including Australian Aboriginal terms for and clusters, to honor diverse heritages and avoid Eurocentric biases in official designations. This approach aligns with broader efforts to decolonize astronomical naming by prioritizing non-offensive, culturally sensitive terms.

Major catalogues

One of the earliest systematic catalogs including star clusters is Charles Messier's Catalogue des Nébuleuses et des Amas d'Étoiles, published in 1781, which lists 110 deep-sky objects observed to avoid confusion with comets, among them 56 star clusters comprising 27 open clusters and 29 globular clusters. expanded this effort with his Catalogue of One Thousand New Nebulae and Clusters of Stars in 1786, followed by a second catalog in 1789 containing another thousand objects, for a total of over 2,000 nebulae and star clusters primarily in the , many identified through his systematic sweeps with a 6.2-meter . These works laid the foundation for later inventories by emphasizing systematic observation of non-stellar objects. The late 19th and early 20th centuries saw more comprehensive compilations, notably John Louis Emil Dreyer's of Nebulae and Clusters of Stars (NGC), published in 1888, which integrated Herschel's findings and additional observations to list 7,840 deep-sky objects, including hundreds of star clusters, followed by two Index Catalogues (IC) in 1895 and 1908 adding 5,386 more entries for a combined total of approximately 13,226 objects visible from the . These catalogs provided standardized positions and descriptions, serving as references for both open and globular clusters despite including non-cluster objects like galaxies and nebulae. In the modern era, the New Catalogue of Optically Visible Open Clusters and Candidates (DAML), compiled by Wilton S. Dias and collaborators, has become a key resource for open clusters, with its 2021 update incorporating data from the infrared survey and astrometry to confirm parameters for 1,743 clusters and candidates out of 2,237 entries, focusing on Galactic disk populations. For globular clusters in the , William E. Harris's Catalogue of Parameters for Milky Way Globular Clusters, first published in 1996 and last updated in 2010, details structural, dynamical, and photometric properties for 157 confirmed systems, drawing on multiwavelength observations to refine distances, metallicities, and sizes. Extragalactic surveys have similarly advanced, with the Hubble Space Telescope's Advanced Camera for Surveys (ACS) observations in the 2000s contributing to catalogs of globular clusters in the (M31), such as the Revised Bologna Catalog by Galleti et al. (2004), which identifies 563 candidates through resolved-star photometry and across wide fields. More recently, (JWST) data from 2024 has enabled catalogs of young star clusters at high redshifts, providing insights into early cluster formation, while the PHANGS-HST collaboration, using data released in 2024, provides approximately 100,000 compact star clusters and associations in nearby galaxies like M83. These efforts highlight JWST's role in detecting faint, distant populations previously inaccessible. Gaia Data Release 3 (DR3), released in 2022, significantly enhanced cluster inventories through precise for billions of sources, enabling the identification of over 7,000 candidates in an all-sky search by Hunt & Reffert (2023), including about 1,200 previously unknown objects via and clustering, thus increasing the known Galactic count by thousands and improving completeness for faint or distant systems.

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