85-633: The Aquarius Dwarf is a dwarf irregular galaxy , first catalogued in 1959 by the DDO survey . It is located within the boundaries of the constellation of Aquarius . It is a member of the Local Group of galaxies, albeit an extremely isolated one; it is one of only a few known Local Group members for which a past close approach to the Milky Way or Andromeda Galaxy can be ruled out, based on its current location and velocity. Local Group membership
170-516: 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. In 2010 a low-density globular cluster's lifetime evolution was able to be directly computed, star-by-star. Completed N-body simulations have shown that stars can follow unusual paths through the cluster, often forming loops and falling more directly toward
255-520: A blue compact dwarf galaxy ( BCD galaxy ) is a small galaxy which contains large clusters of young, hot, massive stars . These stars, the brightest of which are blue, cause the galaxy itself to appear blue in colour. Most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies . Because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become very violent when forming. BCD galaxies cool in
340-464: 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. This star-forming period is relatively brief, compared with the age of many globular clusters. It has been proposed that this multiplicity in stellar populations could have a dynamical origin. In the Antennae Galaxy , for example,
425-492: A cluster from being visually separated until Charles Messier observed M 4 in 1764. 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). In 1914, Harlow Shapley began
510-518: A cluster of thousands of stars can be enormous. 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
595-441: 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. 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
680-554: A few hundred to one hundred thousand stars , making them the faintest galaxies in the Universe . UFDs resemble globular clusters (GCs) in appearance but have very different properties. Unlike GCs, UFDs contain a significant amount of dark matter and are more extended. UFDs were first discovered with the advent of digital sky surveys in 2005, in particular with the Sloan Digital Sky Survey (SDSS). UFDs are
765-411: 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 . They are free of gas and dust, 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. Globular clusters can contain
850-440: A globular cluster must be either to accrete stars at its core, causing its steady contraction, or gradual shedding of stars from its outer layers. 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. The present-day binary fraction in globular clusters is difficult to measure, and any information about their initial binary fraction
935-420: A half-mass radius of only 1.12 arc minutes. 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. This is the distance at which the individual stars belonging to a cluster can be separated away by the galaxy. The tidal radius of M3, for example,
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#17327987336741020-490: A high density of stars; on average about 0.4 stars per cubic parsec, increasing to 100 or 1000 stars/pc in the core of the cluster. In comparison, the stellar density around the Sun is roughly 0.1 stars/pc . The typical distance between stars in a globular cluster is about one light year, but at its core the separation between stars averages about a third of a light year – thirteen times closer than
1105-781: A lower metallicity. 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. 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), and so type I stars are referred to as metal-rich (e.g. Terzan 7 ), while type II stars are metal-poor (e.g. ESO 280-SC06 ). These two distinct populations have been observed in many galaxies, especially massive elliptical galaxies. Both groups are nearly as old as
1190-448: 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
1275-500: 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. A massive black hole at the core could also result in a luminosity cusp. Over a long time, this leads to a concentration of massive stars near the core, a phenomenon called mass segregation . The dynamical heating effect of binary star systems works to prevent an initial core collapse of
1360-518: A much later time than the ancient UFDs. These galaxies have not been observed in our Universe so far. Ultra-compact dwarf galaxies (UCD) are a class of very compact galaxies with very high stellar densities, discovered in the 2000s. They are thought to be on the order of 200 light years across, containing about 100 million stars. It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by tidal interactions , travelling through
1445-893: 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. 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. How blue stragglers form remains unclear, but most models attribute them to interactions between stars, such as stellar mergers ,
1530-457: 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 . Constructing an H–R diagram requires knowing the distance to the observed stars to convert apparent into absolute magnitude. Because all
1615-399: 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
1700-462: 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. Their name is derived from Latin globulus (small sphere). Globular clusters are occasionally known simply as "globulars". Although one globular cluster, Omega Centauri ,
1785-608: A star needs to cross the cluster and the number of stellar masses. The relaxation time varies by cluster, but a typical value is on the order of one billion years. 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. Astronomers characterize
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#17327987336741870-527: Is M60-UCD1 , about 54 million light years away, which contains approximately 200 million solar masses within a 160 light year radius; the stars in its central region are packed 25 times more densely than stars in Earth's region in the Milky Way. M59-UCD3 is approximately the same size as M60-UCD1 with a half-light radius , r h , of approximately 20 parsecs but is 40% more luminous with an absolute visual magnitude of approximately −14.6. This makes M59-UCD3
1955-437: Is a graph of a large sample of stars plotting their absolute magnitude (their luminosity , or brightness measured from a standard distance), as a function of their color index . The color index, roughly speaking, measures the color of the star; positive color indices indicate a reddish star with a cool surface temperature, while negative values indicate a bluer star with a hotter surface. Stars on an H–R diagram mostly lie along
2040-832: Is a small galaxy composed of about 1000 up to several billion stars , as compared to the Milky Way 's 200–400 billion stars. The Large Magellanic Cloud , which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition. One theory states that most galaxies, including dwarf galaxies, form in association with dark matter , or from gas that contains metals. However, NASA 's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases with low metallicity . These galaxies were located in
2125-399: Is about forty arc minutes, or about 113 pc. 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
2210-469: Is directly a function of the cluster's age; an age scale can be plotted on an axis parallel to the magnitude. 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
2295-414: Is lost by subsequent dynamical evolution. 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
2380-485: Is poorly understood. 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; the globular clusters in the Large Magellanic Cloud (LMC) exhibit
2465-540: Is the distance at which the apparent surface luminosity has dropped by half. A comparable quantity is the half-light radius, or the distance from the core containing half the total luminosity of the cluster; the half-light radius is typically larger than the core radius. 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 (r h = 18 pc) and Palomar 14 (r h = 25 pc). The half-light radius includes stars in
2550-510: The Dark Ages within the first billion years after the Big Bang . More than 20 known dwarf galaxies orbit the Milky Way, and recent observations have also led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri , is in fact the core of a dwarf galaxy with a black hole at its centre, which was at some time absorbed by the Milky Way. In astronomy ,
2635-538: The Galactic Center . 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); the modern distance to the Galactic Center is roughly 8.5 kiloparsecs (28,000 ly). Shapley's measurements indicated the Sun is relatively far from
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2720-643: The Hubble Space Telescope is required in order to study its stellar populations in detail. RR Lyrae stars have been discovered in Aquarius Dwarf, indicating the existence of stars more than 10 billion years old, but the majority of its stars are much younger (median age 6.8 billion years). Among Local Group galaxies, only Leo A has a younger mean age, leading to the suggestion that delayed star formation could be correlated with galaxy isolation. Dwarf galaxy A dwarf galaxy
2805-478: The Leo Ring , a cloud of hydrogen and helium around two massive galaxies in the constellation Leo . Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies , resulting in stellar streams and eventually galaxy merger . There are many dwarf galaxies in the Local Group ; these small galaxies frequently orbit larger galaxies, such as
2890-423: The Local Group has an associated system of globular clusters, as does almost every large galaxy surveyed. Some giant elliptical galaxies (particularly those at the centers of galaxy clusters ), such as M 87 , have as many as 13,000 globular clusters. 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
2975-486: The Mayall ;II cluster of the Andromeda Galaxy. Both X-ray and radio emissions from Mayall II appear consistent with an intermediate-mass black hole; however, these claimed detections are controversial. 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
3060-652: The Milky Way , the Andromeda Galaxy and the Triangulum Galaxy . A 2007 paper has suggested that many dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact . Streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them. A 2018 study suggests that some local dwarf galaxies formed extremely early, during
3145-403: 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
3230-518: The galactic bulge or hidden by the gas and dust of the Milky Way. 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. Every galaxy of sufficient mass in
3315-403: 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
3400-442: 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. Hertzsprung–Russell diagrams (H–R diagrams) of globular clusters allow astronomers to determine many of the properties of their populations of stars. An H–R diagram
3485-404: 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
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3570-597: 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
3655-465: 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
3740-549: 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 multiple, distribution of populations. Observations of globular clusters show that their stars primarily come from regions of more efficient star formation, and from where
3825-487: The Milky Way, may be the precursors of globular clusters. 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), including the most massive, Omega Centauri. Its retrograde orbit suggests it may be a remnant of a dwarf galaxy captured by the Milky Way. Globular clusters are generally composed of hundreds of thousands of low-metal , old stars. The stars found in
3910-429: The Sun is to its nearest neighbor, Proxima Centauri . 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
3995-514: The center of the cluster, even without a black hole, in both M15 and Mayall II. 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 ☉ . 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
4080-532: 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. The count of known globular clusters in the Milky Way has continued to increase, reaching 83 in 1915, 93 in 1930, 97 by 1947, and 157 in 2010. Additional, undiscovered globular clusters are believed to be in
4165-482: 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 (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
4250-421: The cluster are sped up by this process, it reduces the contraction at the core and limits core collapse. 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. In 2005 astronomers discovered a new, "extended" type of star cluster in
4335-493: 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. The overall luminosities of
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#17327987336744420-440: 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. 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. Core collapse may be divided into three phases. During
4505-480: 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. 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
4590-593: The coolest white dwarfs, often giving results as old as 12.7 billion years. In comparison, open clusters are rarely older than about half a billion years. 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, but better measurements of cosmological parameters, through deep sky surveys and satellites, appear to have resolved this issue. Studying globular clusters sheds light on how
4675-424: 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. The typical time scale for the evaporation of a globular cluster is 10 years. The ultimate fate of
4760-441: The cores of dwarf galaxies that have been consumed by larger galaxies. About a quarter of the globular cluster population in the Milky Way may have been accreted this way, as with more than 60% of the globular clusters in the outer halo of Andromeda. 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
4845-417: 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. 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
4930-597: 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. 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 . The formation of globular clusters
5015-419: The dispersal of some stars, leaving behind "tidal tails" of stars removed from the cluster. 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
5100-488: The distance to other galaxies, under the assumption that globular clusters in remote galaxies behave similarly to those in the Milky Way. 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 (where N is the number of objects), so the computing requirements to accurately simulate
5185-410: The distances. 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
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#17327987336745270-399: 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. The HST has provided convincing observational evidence of this stellar mass-sorting process in globular clusters. Heavier stars slow down and crowd at
5355-524: 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. 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
5440-500: The globular clusters within the Milky Way and the Andromeda Galaxy each have a roughly Gaussian distribution , with an average magnitude M v and a variance σ . This distribution of globular cluster luminosities is called the Globular Cluster Luminosity Function (GCLF). For the Milky Way, M v = −7.29 ± 0.13 , σ = 1.1 ± 0.1 . The GCLF has been used as a " standard candle " for measuring
5525-789: The hearts of rich clusters. UCDs have been found in the Virgo Cluster , Fornax Cluster , Abell 1689 , and the Coma Cluster , amongst others. In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team. The first ever relatively robust studies of the global properties of Virgo UCDs suggest that UCDs have distinct dynamical and structural properties from normal globular clusters. An extreme example of UCD
5610-439: 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 . Some globular clusters likely formed in dwarf galaxies and were removed by tidal forces to join the Milky Way. In elliptical and lenticular galaxies there is a correlation between the mass of the supermassive black holes (SMBHs) at their centers and
5695-402: The main sequence) of the cluster's color–magnitude diagram to corresponding features in an H–R diagram of another set of stars, a method known as spectroscopic parallax or main-sequence fitting. 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
5780-402: The morphology (shape) of a globular cluster by means of standard radii: the core radius ( r c ), the half-light radius ( r h ), and the tidal or Jacobi radius ( r t ). 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
5865-409: The most dark matter -dominated systems known. Astronomers believe that UFDs encode valuable information about the early Universe , as all UFDs discovered so far are ancient systems that have likely formed very early on, only a few million years after the Big Bang and before the epoch of reionization . Recent theoretical work has hypothesised the existence of a population of young UFDs that form at
5950-533: The order of a hundred million years. 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. 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
6035-401: The outer part of the cluster that happen to lie along the line of sight, so theorists also use the half-mass radius ( r m ) – 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
6120-453: 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. A more detailed H–R diagram often reveals multiple stellar populations as indicated by
6205-474: The presence of closely separated curves, each corresponding to a distinct population of stars with a slightly different age or composition. Observations with the Wide Field Camera 3 , installed in 2009 on the Hubble Space Telescope, made it possible to distinguish these slightly different curves. The most massive main-sequence stars have the highest luminosity and will be the first to evolve into
6290-410: The process of forming new stars . The galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies. Nearby examples include NGC 1705 , NGC 2915 , NGC 3353 and UGCA 281 . Ultra-faint dwarf galaxies (UFDs) are a class of galaxies that contain from
6375-431: 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. Further, the placements of the cluster stars in an H–R diagram (including the brightnesses of distance indicators) can be influenced by observational biases. One such effect, called blending, arises when
6460-476: 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. The first known globular cluster, now called M 22 , was discovered in 1665 by Abraham Ihle , a German amateur astronomer. The cluster Omega Centauri , easily visible in
6545-543: The second densest known galaxy. Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have supermassive black holes weighing 13% and 18% of the galaxies' masses. Globular cluster 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, all orbiting in
6630-449: 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, then finally as a globular cluster in the early 19th century by John Herschel . The French astronomer Abbé Lacaille listed NGC 104 , NGC 4833 , M 55 , M 69 , and NGC 6397 in his 1751–1752 catalogue. The low resolution of early telescopes prevented individual stars in
6715-413: 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). 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
6800-460: The transfer of material from one star to another, or even an encounter between two binary systems. 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. Some clusters have two distinct sequences of blue stragglers, one bluer than the other. Astronomers have searched for black holes within globular clusters since
6885-469: The universe and the Hubble constant . The blue stragglers appear on the H–R diagram as a series diverging from the main sequence in the direction of brighter, bluer stars. 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 an H–R diagram. Globular clusters can be dated by looking at the temperatures of
6970-422: 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. A large majority of the metal-poor clusters in the Milky Way are aligned on
7055-412: The way to the core region. 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
7140-465: Was firmly established only in 1999, with the derivation of a distance based on the tip of the red-giant branch method. Its distance from the Milky Way of 3.2 ±0.2 Mly (980 ±40 kpc) means that Aquarius Dwarf is quite isolated in space. It is one of the least luminous Local Group galaxies to contain significant amounts of neutral hydrogen and support to ongoing star formation, although it does so only at an extremely low level. Because of its large distance,
7225-517: 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
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