A Swarm of Ancient Stars - GPN-2000-000930

A globular cluster is a

spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has cir ...

al conglomeration of

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

Globular A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member ...

clusters are bound together by

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars. Their name is derived from

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

(small sphere). Globular clusters are occasionally known simply as "globulars". Although one globular cluster, Omega Centauri, was observed in antiquity and long thought to be a star, recognition of the clusters' true nature came with the advent of telescopes in the 17th century. In early telescopic observations globular clusters appeared as fuzzy blobs, leading French astronomer Charles Messier to include many of them in his catalog of astronomical objects that he thought could be mistaken for

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...

s. Using larger telescopes, 18th-century astronomers recognized that globular clusters are groups of many individual stars. Early in the 20th century the distribution of globular clusters in the sky was some of the first evidence that the Sun is far from the center of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

. Globular clusters are found in nearly all

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

. In

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''galactic halo. They are the largest and most massive type of

star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clust ...

, tending to be older, denser, and composed of lower abundances of heavy elements than

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...

s, which are generally found in the

disk Disk or disc may refer to: * Disk (mathematics), a geometric shape * Disk storage Music * Disc (band), an American experimental music band * ''Disk'' (album), a 1995 EP by Moby Other uses * Disk (functional analysis), a subset of a vector sp ...

s of spiral galaxies. The Milky Way has more than 150 known globulars, and there may be many more. 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 The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...

, constraining estimates of the universe's age. Star clusters were formerly thought to consist of stars that all

form Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens. Form also refers to: *Form (document), a document (printed or electronic) with spaces in which to write or enter data ...

ed at the same time from one star-forming nebula, but nearly all globular clusters contain stars that formed at different times, or that have differing compositions. Some clusters may have had multiple episodes of star formation, and some may be remnants of smaller galaxies captured by larger galaxies.

History of observations

The first known globular cluster, now called M 22, was discovered in 1665 by

Abraham Ihle Johann Abraham Ihle (14 June 1627 – c.1699) was a German amateur astronomer who discovered the first known globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a ...

, a German amateur astronomer. The cluster Omega Centauri, easily visible in the southern sky with the naked eye, was known to ancient astronomers like

Ptolemy Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importanc ...

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 Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wor ...

. The French astronomer Abbé Lacaille listed NGC 104, , M 55, M 69, and in his 1751–1752 catalogue. The label M before a number refers to Charles essier's catalogue, while NGC is from ''the ew eneral atalogue'' by

John Dreyer John Dreyer may refer to: *John Louis Emil Dreyer, Danish-Irish astronomer *John Dreyer (footballer) John Brian Dreyer (born 11 June 1963) is an English former professional footballer and manager who played in defence and midfield. He is curren ...

. The low resolution of early

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

s prevented individual stars in 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 Harlow Shapley (November 2, 1885 – October 20, 1972) was an American scientist, head of the Harvard College Observatory (1921–1952), and political activist during the latter New Deal and Fair Deal. Shapley used Cepheid variable stars to estim ...

began a series of studies of globular clusters, published across about forty scientific papers. He examined the clusters' RR Lyrae variables (stars which he assumed were Cepheid variables) and used their luminosity and period of variability to estimate the distances to the clusters. It was later found that RR Lyrae variables are fainter than Cepheid variables, causing Shapley to overestimate the distances. NGC 7006 (HST)

A large majority of the Milky Way's globular clusters are found in the celestial sky around the galactic core. In 1918 Shapley used this strongly asymmetrical distribution to determine the overall dimensions of the galaxy. Assuming a roughly spherical distribution of globular clusters around the galaxy's center, he used the positions of the clusters to estimate the position of the Sun relative to the galactic center. He correctly concluded that the Milky Way's center is in the

Sagittarius constellation Sagittarius is one of the constellations of the zodiac and is located in the Southern celestial hemisphere. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Its ...

and not near the Earth. He overestimated the distance, finding typical globular cluster distances of ; the modern distance to the Galactic Center is roughly . Shapley's measurements indicated the Sun is relatively far from the center of the galaxy, contrary to what had been inferred from the observed uniform distribution of ordinary stars. In reality most ordinary stars lie within the galaxy's disk and are thus obscured by gas and dust in the disk, whereas globular clusters lie outside the disk and can be seen at much greater distances. 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 the

galactic bulge In astronomy, a galactic bulge (or simply bulge) is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges w ...

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 the

Local Group The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...

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.

Classification

Shapley was later assisted in his studies of clusters by

Henrietta Swope Henrietta Hill Swope (October 26, 1902 – November 24, 1980) was an American astronomer who studied variable stars. In particular, she measured the period-luminosity relation for Cepheid stars, which are bright variable stars whose periods of va ...

and Helen Sawyer Hogg. In 1927–1929 Shapley and Sawyer categorized clusters by the degree of concentration of stars toward each core. Their system, known as the Shapley–Sawyer Concentration Class, identifies the most concentrated clusters as Class I and ranges to the most diffuse Class XII. The Concentration Class is sometimes given with Arabic numerals (Classes 1–12) rather than

Roman numeral Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, eac ...

s. In 2015 astronomers from the

Pontifical Catholic University of Chile The Pontifical Catholic University of Chile (''PUC or UC Chile'') ( es, Pontificia Universidad Católica de Chile) is one of the six Catholic Universities existing in the Chilean university system and one of the two pontifical universities i ...

proposed a new type of globular cluster on the basis of observational data:

Dark globular cluster Dark globular cluster is a proposed type of globular star clusters that has an unusually high mass for the number of stars within it. Proposed in 2015 on the basis of observational data, dark globular clusters are believed to be populated by obje ...

Formation

The formation of globular clusters 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 In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

(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 The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...

(LMC) exhibit a bimodal population, for example. During their youth, these LMC clusters may have encountered giant molecular clouds that triggered a second round of star formation. 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 The Antennae Galaxies (also known as NGC 4038/NGC 4039 or Caldwell 60/Caldwell 61) are a pair of interacting galaxies in the constellation Corvus. They are currently going through a starburst phase, in which the collision of clouds of gas and du ...

, for example, the Hubble Space Telescope has observed clusters of clustersregions 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. The globular star cluster Messier 54

Observations of globular clusters show that their stars primarily come from regions of more efficient star formation, and from where the interstellar medium is at a higher density, as compared to normal star-forming regions. Globular cluster formation is prevalent in

starburst MicroPro International Corporation was an American software company founded in 1978 in San Rafael, California. They are best known as the publisher of WordStar, a popular early word processor for personal computers. History Founding and early su ...

regions and in

interacting galaxies Interacting galaxies (''colliding galaxies'') are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a ...

. 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 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 cluster A super star cluster (SSC) is a very massive young open cluster that is thought to be the precursor of a globular cluster. These clusters called "super" because they are relatively more luminous and contain more mass than other young star clusters. ...

s, such as Westerlund 1 in 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.

Composition

Globular clusters are generally composed of hundreds of thousands of low-metal, old stars. The stars found in a globular cluster are similar to those in the bulge of a

spiral galaxy Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''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 a high density of stars; on average about 0.4stars per cubic parsec, increasing to 100 or 1000stars/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 yearthirteen times closer than the Sun is to its nearest neighbor,

Proxima Centauri Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...

. 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 The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...

around a star that is within the core of a dense cluster, such as 47 Tucanae, would survive only on the order of a hundred million years. There is a planetary system orbiting a

pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...

( PSRB1620−26) 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 mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es () and having multiple stellar populations. Both are evidence that supermassive globular clusters formed from the cores of dwarf galaxies that have been consumed by larger galaxies. 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.

Heavy element content

Globular clusters normally consist of Population II stars which, compared with Population I stars such as the Sun, have a higher proportion of hydrogen and helium and a lower proportion of heavier elements. Astronomers refer to these heavier elements as ''metals'' (distinct from the material concept) and to the proportions of these elements as the metallicity. Produced by

stellar nucleosynthesis Stellar nucleosynthesis is the creation (nucleosynthesis) of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...

, the metals are recycled into the

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

and enter a new generation of stars. The proportion of metals can thus be an indication of the age of a star in simple models, with older stars typically having a lower metallicity. The Dutch astronomer

Pieter Oosterhoff Pieter Theodorus Oosterhoff (30 March 1904, Leeuwarden - 14 March 1978, Leiden) was a Dutch astronomer. He was the co-administrator, along with Jan Oort, of the Leiden Observatory in the Netherlands. His published papers are primarily in regard ...

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 Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

, the metal spectral lines in the stars of Oosterhoff typeI (OoI) cluster are not quite as weak as those in typeII (OoII), and so typeI stars are referred to as ''metal-rich'' (e.g. Terzan 7), while typeII 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 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. In the Milky Way a large majority of the metal-poor clusters are aligned on a plane in the outer part of the galaxy's halo. This observation supports the view that typeII 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.

Exotic components

Close interactions and near-collisions of stars occur relatively often in globular clusters because of their high star density. These chance encounters give rise to some exotic classes of starssuch as blue stragglers, millisecond pulsars, and

low-mass X-ray binaries X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', which ...

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, 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 In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...

stars formed early in the cluster's existence. Some clusters have two distinct sequences of blue stragglers, one bluer than the other. STSci-2002-18

Astronomers have searched for

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

s within globular clusters since the 1970s. The required resolution for this task is exacting; it is only with the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

(HST) that the first claimed discoveries were made, in 2002 and 2003. Based on HST observations, other researchers suggested the existence of a (solar masses) intermediate-mass black hole in the globular cluster

M15 M15 or M-15 may refer to: In science * Messier 15 (M15), a globular cluster in the constellation Pegasus In firearms and military equipment * M15 mine, a United States anti-tank mine * M15 rifle, a variant of the M14, a United States military rif ...

and a black hole in the Mayall II cluster of the Andromeda Galaxy. Both

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

and

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...

emissions from MayallII 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 In astronomy, dynamical mass segregation is the process by which heavier members of a gravitationally bound system, such as a star cluster, tend to move toward the center, while lighter members tend to move farther away from the center. Equipar ...

. One research group pointed out that the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15 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 . The confirmation of intermediate-mass black holes in globular clusters would have important ramifications for theories of galaxy development as being possible sources for the supermassive black holes at their centers. The mass of these supposed intermediate-mass black holes is proportional to the mass of their surrounding clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies.

Hertzsprung–Russell diagrams

Hertzsprung–Russell diagram The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosity, luminosities versus their stellar classifications or eff ...

s (H–R diagrams) of globular clusters allow astronomers to determine many of the properties of their populations of stars. An H–R diagram is a graph of a large sample of stars plotting their

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...

(their

luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

, or brightness measured from a standard distance), as a function of their color index. The color index, roughly speaking, measures the color of the star; positive color indices indicate a reddish star with a cool surface temperature, while negative values indicate a bluer star with a hotter surface. Stars on an H–R diagram mostly lie along a roughly diagonal line sloping from hot, luminous stars in the upper left to cool, faint stars in the lower right. This line is known as the main sequence and represents the primary stage of

stellar evolution Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...

. The diagram also includes stars in later evolutionary stages such as the cool but luminous

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

s. Constructing an H–R diagram requires knowing the distance to the observed stars to convert apparent into absolute magnitude. Because all the stars in a globular cluster have about the same distance from Earth, a color–magnitude diagram using their observed magnitudes looks like a shifted H–R diagram (because of the roughly constant difference between their apparent and absolute magnitudes). This shift is called the distance modulus and can be used to calculate the distance to the cluster. The modulus is determined by comparing features (like the main sequence) of the cluster's color–magnitude diagram to corresponding features in an H–R diagram of another set of stars, a method known as spectroscopic parallax or main-sequence fitting.

Properties

Since globular clusters form at once from a single giant molecular cloud, a cluster's stars have roughly the same age and composition. A star's evolution is primarily determined by its initial mass, so the positions of stars in a cluster's H–R or color–magnitude diagram mostly reflect their initial masses. A cluster's H–R diagram, therefore, appears quite different from H–R diagrams containing stars of a wide variety of ages. Almost all stars fall on a well-defined curve in globular cluster H–R diagrams, and that curve's shape indicates the age of the cluster. A more detailed H–R diagram often reveals multiple stellar populations as indicated by the presence of closely separated curves, each corresponding to a distinct population of stars with a slightly different age or composition. Observations with the

Wide Field Camera 3 The Wide Field Camera 3 (WFC3) is the Hubble Space Telescope's last and most technologically advanced instrument to take images in the visible spectrum. It was installed as a replacement for the Wide Field and Planetary Camera 2 during the first s ...

, 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 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 The turnoff point for a star refers to the point on the Hertzsprung-Russell diagram where it leaves the main sequence after its main fuel is exhaustedthe main sequence turnoff. By plotting the turnoff points of individual stars in a star cluster ...

, bending to the upper right from the main-sequence line. The absolute magnitude at this bend is directly a function of the cluster's age; an age scale can be plotted on an axis parallel to the magnitude. The morphology and luminosity of globular cluster stars in H–R diagrams are influenced by numerous parameters, many of which are still actively researched. Recent observations have overturned the historical paradigm that all globular clusters consist of stars born at exactly the same time, or sharing exactly the same chemical abundance. Some clusters feature multiple populations, slightly differing in composition and age; for example, high-precision imagery of cluster NGC 2808 discerned three close, but distinct, main sequences. 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 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 incorrecttoo bright, given that multiple stars contributed. The computed distance is in turn incorrect, so the blending effect can introduce a systematic uncertainty into the

cosmic distance ladder The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...

and may bias the estimated age of the universe and the

Hubble constant Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...

Consequences

The blue stragglers appear on the H–R diagram as a series diverging from the main sequence in the direction of brighter, bluer stars.

White dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

s (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 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 Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount (lexicographer), Thomas Blount's ''Glossographia'', and in 1731 taken up in ...

. 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 the composition of the formational gas and dust affects stellar evolution; the stars'

evolutionary track In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

s 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.

Morphology

In contrast to open clusters most globular clusters remain gravitationally bound together for time periods comparable to the lifespans of most of their stars. Strong tidal interactions with other large masses result in the dispersal of some stars, leaving behind "tidal tails" of stars removed from the cluster. After formation the stars in the globular cluster begin to interact gravitationally with each other. The velocities of the stars steadily change, and the stars lose any history of their original velocity. The characteristic interval for this to occur is the relaxation time, related to the characteristic length of time a star needs to cross the cluster and the number of stellar masses. 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 spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has circ ...

s in shape, while those in the Large Magellanic Cloud are more elliptical.

Radii

Astronomers characterize the morphology (shape) of a globular cluster by means of standard radii: the core radius (''r''_''c''), the

half-light radius Galaxy effective radius or half-light radius (R_e) is the radius at which half of the total light of a galaxy is emitted. This assumes the galaxy has either intrinsic circular symmetry, spherical symmetry or is at least circularly symmetric as vie ...

(''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 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 NGC 2419 (also known as Caldwell 25) is a globular cluster in the constellation Lynx. It was discovered by William Herschel on December 31, 1788. NGC 2419 is at a distance of about 300,000 light years from the Solar System and at the same distanc ...

(r_h = 18 pc) and

Palomar 14 Palomar 14 is a globular cluster located in the constellation Hercules. It is a member of the Palomar Globular Clusters group. Palomar 14 was discovered in 1958 by Sidney van den Bergh and Halton Arp during inspection of the photographic plates f ...

(r_h = 25 pc). The half-light radius includes stars in the outer part of the cluster that happen to lie along the line of sight, so theorists also use the half-mass radius (''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 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, is about forty arc minutes, or about 113 pc.

Mass segregation, luminosity and core collapse

In most Milky Way clusters the surface brightness of a globular cluster as a function of decreasing distance to the core first increases, then levels off at a distance typically 1–2 parsecs from the core. About 20% of the globular clusters have undergone a process termed "core collapse". In such a cluster the luminosity increases steadily all the way to the core region. Models of globular clusters predict 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 In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...

from the core region and leading the region's remaining stars to occupy a more compact volume. When this gravothermal instability occurs, the central region of the cluster becomes densely crowded with stars, and the surface brightness of the cluster forms a

power-law In statistics, a power law is a functional relationship between two quantities, where a relative change in one quantity results in a proportional relative change in the other quantity, independent of the initial size of those quantities: one qua ...

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

. The dynamical heating effect of binary star systems works to prevent an initial core collapse of the cluster. When a star passes near a binary system, the orbit of the latter pair tends to contract, releasing energy. Only after this primordial supply of energy is exhausted can a deeper core collapse proceed. In contrast, the effect of

tidal shock A tidal shock occurs when a star cluster or other distributed astronomical object passes by a large mass such as an interstellar cloud, resulting in gravitational perturbation on a time scale that is much less than the mean time for a star to comp ...

s 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 a cluster's adolescence, core collapse begins with stars nearest the core. Interactions between

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

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 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 stars of the Large Magellanic Cloud

The HST has provided convincing observational evidence of this stellar mass-sorting process in globular clusters. Heavier stars slow down and crowd at the cluster's core, while lighter stars pick up speed and tend to spend more time at the cluster's periphery. The cluster 47 Tucanae, made up of about one million stars, is one of the densest globular clusters in the Southern Hemisphere. This cluster was subjected to an intensive photographic survey that obtained precise velocities for nearly fifteen thousand stars in this cluster. The overall luminosities of the globular clusters within the Milky Way and the Andromeda Galaxy each have a roughly Gaussian distribution, with an average magnitude M_v and a variance σ². This distribution of globular cluster luminosities is called the Globular Cluster Luminosity Function (GCLF). For the Milky Way, M_v = , σ = . The GCLF has been used as a " standard candle" for measuring the distance to other galaxies, under the assumption that globular clusters in remote galaxies behave similarly to those in the Milky Way.

N-body simulations

Computing the gravitational interactions between stars within a globular cluster requires solving the N-body problem. The naive computational cost for a dynamic simulation increases in proportion to ''N''² (where N is the number of objects), so the computing requirements to accurately simulate a cluster of thousands of stars can be enormous. 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 In statistical mechanics, the Fokker–Planck equation is a partial differential equation that describes the time evolution of the probability density function of the velocity of a particle under the influence of drag forces and random forces, as ...

, often using a model describing the mass density as a function of radius, such as a

Plummer model The Plummer model or Plummer sphere is a density law that was first used by H. C. Plummer to fit observations of globular clusters. It is now often used as toy model in N-body simulations of stellar systems. Description of the model The Plumme ...

. 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 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 10¹⁰ years. The ultimate fate of 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 A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

form a significant portion of stellar systems, with up to half of all

field star This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outsid ...

s and

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 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 the cluster are sped up by this process, it reduces the contraction at the core and limits core collapse.

Intermediate forms

Cluster classification is not always definitive; objects have been found that can be classified in more than one categories. For example, BH 176 in the southern part of the Milky Way has properties of both an open and a globular cluster. In 2005 astronomers discovered a new, "extended" type of star cluster in the Andromeda Galaxy's halo, similar to the globular cluster. The three new-found clusters have a similar star count as globular clusters and share other characteristics, such as stellar populations and metallicity, but are distinguished by their larger sizeseveral hundred light years acrossand some hundred times lower density. Their stars are separated by larger distances; parametrically, these clusters lie somewhere between a globular cluster and a dwarf spheroidal galaxy. The formation of these extended clusters is likely related to accretion. It is unclear why the Milky Way lacks such clusters; Andromeda is unlikely to be the sole galaxy with them, but their presence in other galaxies remains unknown.

Tidal encounters

When a globular cluster comes close to a large mass, such as the core region of a galaxy, it undergoes a tidal interaction. The difference in gravitational strength between the nearer and further parts of the cluster results in an asymmetric, tidal force. A "tidal shock" occurs whenever the orbit of a cluster takes it through the plane of a galaxy. Tidal shocks can pull stars away from the cluster halo, leaving only the core part of the cluster; these trails of stars can extend several degrees away from the cluster. These tails typically both precede and follow the cluster along its orbit and can accumulate significant portions of the original mass of the cluster, forming clump-like features. The globular cluster

Palomar 5 Palomar 5 is a globular cluster and a member of the Palomar Globular Clusters group. It was discovered by Walter Baade in 1950, and independently found again by Albert George Wilson in 1955. After the initial name of Serpens, it was subsequentl ...

, for example, is near the apogalactic point of its orbit after passing through the Milky Way. Streams of stars extend outward toward the front and rear of the orbital path of this cluster, stretching to distances of 13,000 light years. Tidal interactions have stripped away much of Palomar5's mass; further interactions with the galactic core are expected to transform it into a long stream of stars orbiting the Milky Way in its halo. The Milky Way is in the process of tidally stripping the

Sagittarius Dwarf Spheroidal Galaxy The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters in ...

of stars and globular clusters through the

Sagittarius Stream In astronomy, the Sagittarius Stream is a long, complex structure made of stars that wrap around the Milky Way galaxy in an orbit that nearly crosses the galactic poles. It consists of Tidal stripping, tidally stripped stars from the Sagittarius Dw ...

. As many as 20% of the globular clusters in the Milky Way's outer halo may have originated in that galaxy.

Palomar 12 Palomar 12 is a globular cluster in the constellation Capricornus, and is a member of the Palomar Globular Clusters group. First discovered on the National Geographic Society – Palomar Observatory Sky Survey plates by Robert George Harri ...

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

Planets

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

eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...

and highly inclined orbit suggests it may have been formed around another star in the cluster, then "exchanged" into its current arrangement. The likelihood of close encounters between stars in a globular cluster can disrupt planetary systems; some planets break free to become rogue planets, orbiting the galaxy. Planets orbiting close to their star can become disrupted, potentially leading to

orbital decay Orbital decay is a gradual decrease of the distance between two orbiting bodies at their closest approach (the periapsis) over many orbital periods. These orbiting bodies can be a planet and its satellite, a star and any object orbiting it, or ...

and an increase in orbital eccentricity and tidal effects.

Footnotes

References

External links

Globular Clusters
Students for the Exploration and Development of Space Messier pages
Milky Way Globular Clusters

by William E. Harris, McMaster University, Ontario, Canada
A galactic globular cluster database
by Marco Castellani, Rome Astronomical Observatory, Italy
Catalogue of structural and kinematic parameters and galactic orbits of globular clusters
by Holger Baumgardt, University of Queensland, Australia
SCYON
a newsletter dedicated to star clusters.
MODEST
a loose collaboration of scientists working on star clusters. {{DEFAULTSORT:Globular Cluster Star clusters