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Encyclopedia > Globular cluster
The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars.
The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars.[1]

A globular cluster is a spherical collection of stars that orbits a galaxy core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shape, and relatively high stellar density towards their core. Globular clusters contain considerably more stars than the less dense galactic, or open clusters. Download high resolution version (758x795, 157 KB)Image for M80 & Globular cluster Source: http://hubblesite. ... Download high resolution version (758x795, 157 KB)Image for M80 & Globular cluster Source: http://hubblesite. ... Globular Cluster M80 Globular Cluster M80 (also known as Messier Object 80 or NGC 6093) is a globular cluster in the constellation Scorpius. ... Scorpius (Latin for scorpion, symbol , Unicode ♏) is one of the constellations of the zodiac. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ... A sphere (< Greek σφαίρα) is a perfectly symmetrical geometrical object. ... The Pleiades, an open cluster of stars in the constellation of Taurus. ... The Galactic Center is the rotational center of the Milky Way galaxy. ... It has been suggested that Satellite orbit be merged into this article or section. ... An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. ...


A globular cluster is sometimes known more simply as globular. The word globular is derived from the Latin globus and the English suffix "-ular", and means to have the shape of a globe or globule. Latin was the language originally spoken in the region around Rome called Latium. ... The English language is a West Germanic language that originates in England. ...


Globular clusters are fairly numerous; there are about 150 currently known globular clusters in the Milky Way, with perhaps 10–20 more undiscovered.[2] Large galaxies can have more: Andromeda, for instance, may have as many as 500.[3] Some giant elliptical galaxies, such as M87,[4] may have as many as 10,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 40 kiloparsecs or more.[5] The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy which forms part of the Local Group. ... The Andromeda Galaxy (also known as Messier 31, M31, or NGC 224; older texts often call it the Andromeda Nebula) is a barred spiral galaxy approximately 2. ... An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ... The jet emitted by M87 in this image is thought to be caused by a supermassive black hole at the galaxys center. ... Stellar parallax motion The parsec (symbol pc) is a unit of length used in astronomy. ...


Every galaxy of sufficient mass in the local group has an associated group of globular clusters, and almost every large galaxy has been found to possess a system of globular clusters.[6] The Sagittarius Dwarf and Canis Major Dwarf galaxies appear to be in the process of donating their associated globular clusters to the Milky Way, such as Palomar 12.[7] This demonstrates how many of this galaxy's globular clusters were acquired in the past. A member of the Local Group of galaxies, irregular galaxy Sextans A is 10 million light years distant. ... The Sagittarius Dwarf Elliptical Galaxy (Sag DEG) is a satellite galaxy of the Milky Way Galaxy. ... The Canis Major Dwarf Galaxy is located in the same part of the sky as the constellation of Canis Major. ... Palomar 12 is a globular cluster in the constellation Capricornus that belongs to the halo of the Milky Way galaxy. ...

Contents


Observation history

Early Globular Cluster Discoveries
Cluster name Discovered by Year
M22 Abraham Ihle 1665
ω Cen Edmond Halley 1677
M5 Gottfried Kirch 1702
M13 Edmond Halley 1714
M71 Philippe Loys de Chéseaux 1745
M4 Philippe Loys de Chéseaux 1746
M15 Jean-Dominique Maraldi 1746
M2 Jean-Dominique Maraldi 1746

The first globular cluster discovered was M22 in 1665 by Abraham Ihle, a German amateur astronomer.[8] However, due to the small aperture of early telescopes, individual stars within a globular cluster were not resolved until Charles Messier observed M4. The first eight globular clusters discovered are shown in the table. Subsequently, Abbé Lacaille would list NGC 104, NGC 4833, M55, M69, and NGC 6397 in his 1751–52 catalogue. The M before a number refers to the catalogue of Charles Messier, while NGC is from the New General Catalogue by John Dreyer. Globular Cluster M22 (also known as Messier Object 22, Messier 22, M22, or NGC 6656) is a globular cluster in the Sagittarius constellation. ... A small region at the heart of Omega Centauri, containing some 50,000 stars (NASA/STScI) Omega Centauri or NGC 5139 is a globular cluster of stars orbiting our galaxy, the Milky Way. ... Portrait of Edmond Halley painted around 1687 by Thomas Murray (Royal Society, London) Portrait of Edmond Halley Bust of Edmond Halley in the Museum of the Royal Greenwich Observatory Edmond Halley (sometimes Edmund, November 8, 1656 – January 14, 1742) was an English astronomer, geophysicist, mathematician, meteorologist, and physicist. ... Globular Cluster M5 (also known as Messier Object 5, Messier 5, M5, or NGC 5904) is a globular cluster in the Serpens constellation. ... Gottfried Kirch (Kirche, Kirkius) (December 18, 1639—July 25, 1710) was a German astronomer. ... Messier Object 13, the Great Globular Cluster in Hercules; one of the most prominent and best known globular clusters of the Northern celestial hemisphere. ... Globular Cluster M71 (also known as Messier Object 71, Messier 71, M71, or NGC 6838) is a globular cluster in the Sagitta constellation. ... Portrait of Jean-Phillippe de Chesaux Jean-Philippe Loys de Cheseaux (1718–1751) was an astronomer from Lausanne in Switzerland. ... Globular Cluster M4 (also known as Messier Object 4, Messier 4, M4, or NGC 6121) is a globular cluster in the Scorpius constellation. ... Globular Cluster M15 (also known as Messier Object 15, Messier 15, M15, or NGC 7078) is a globular cluster in the Pegasus constellation. ... Globular Cluster M2 (also known as Messier Object 2, Messier 2, M2, or NGC 7089) is a globular cluster in the Aquarius constellation. ... Globular Cluster M22 (also known as Messier Object 22, Messier 22, M22, or NGC 6656) is a globular cluster in the Sagittarius constellation. ... Definitions of Aperture in the 1707 Glossographia Anglicana Nova For other uses, see Aperture (disambiguation). ... 50 cm refracting telescope at Nice Observatory. ... Angular resolution describes the resolving power of a telescope. ... Charles Messier Charles Messier (June 26, 1730 – April 12, 1817) was a French astronomer who in 1774 published a catalogue of 45 deep sky objects such as nebulae and star clusters. ... Globular Cluster M4 (also known as Messier Object 4, Messier 4, M4, or NGC 6121) is a globular cluster in the Scorpius constellation. ... Abbé Nicolas Louis de Lacaille (March 15, 1713 – March 21, 1762) was a French astronomer. ... Globular Cluster 47 Tucanae (also known as 47 Tucanae, Globular Cluster NGC 104, or NGC 104) is a globular cluster in the Tucana constellation. ... NGC 4833 is a globular cluster discovered by Abbe Lacaille during his 1751-1752 journey to South Africa, and catalogued in 1755. ... Globular Cluster M55 (also known as Messier Object 55, Messier 55, M55, or NGC 6809) is a globular cluster in the Sagittarius constellation. ... Globular Cluster M69 (also known as Messier Object 69 or NGC 6637) is a globular cluster in the constellation Sagittarius. ... Globular Cluster NGC 6397 (also known as NGC 6397) is a globular cluster in the Ara constellation. ... The New General Catalogue (NGC) is the most well-known catalogue of deep sky objects in amateur astronomy. ... John Louis Emil Dreyer (February 13, 1852 – September 14, 1926) was a Danish-Irish astronomer. ...


William Herschel began a survey program in 1782 using larger telescopes and was able to resolve the stars in all 33 of the known globular clusters. In addition he found 37 additional clusters. In Herschel's 1789 catalog of deep sky objects, his second such, he became the first to use the name globular cluster as their description. Sir Wilhelm Friedrich Herschel, FRS KH (November 15, 1738 – August 25, 1822) was a German-born British astronomer and composer who became famous for discovering the planet Uranus. ...


The number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947. A total of 151 globular clusters have now been discovered in the Milky Way galaxy, out of an estimated total of 180 ± 20.[2] These additional, undiscovered globular clusters are believed to be hidden behind the gas and dust of the Milky Way. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy which forms part of the Local Group. ...


Beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers. He examined the cepheid variables in the clusters and would use their period–luminosity relationship for distance estimates. Harlow Shapley in his earlier years. ... Cepheid in the Spiral Galaxy M100 A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. ...

M75 is a highly-concentrated, Class I globular cluster.
M75 is a highly-concentrated, Class I globular cluster.

Of the globular clusters within our Milky Way, the majority are found in the vicinity of the galactic core, and the large majority lie on the side of the celestial sky centered on the core. In 1918 this strongly asymmetrical distribution was used by Harlow Shapley to make a determination of the overall dimensions of the galaxy. By 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.[9] While his distance estimate was significantly in error, it did demonstrate that the dimensions of the galaxy were much greater than had been previously thought. Shapley's estimate was, however, within the same order of magnitude of the currently accepted value. Image File history File links Messier75. ... Image File history File links Messier75. ... Globular Cluster M75 (also known as Messier Object 75 or NGC 6864) is a globular cluster in the constellation Sagittarius. ... An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. ...


Shapley was subsequently assisted in his studies of clusters by Henrietta Swope and Helen Battles Sawyer (later Hogg). In 1927–29, Harlow Shapley and Helen Sawyer began categorizing clusters according to the amount of concentration the system has toward the core. The most concentrated clusters were identified as Class I, with successively diminishing concentrations ranging to Class XII. This became known as the Shapley–Sawyer Concentration Class. (It is sometimes given with numbers (Class 1–12) rather than roman numerals.)[10] Henrietta Hill Swope (1902 - 1980) was a U.S. astronomer. ... Helen Battles Sawyer Hogg, CC, FRSC (August 1, 1905 – January 28, 1993) was a prolific astronomer noted for her research into globular clusters. ... The system of Roman numerals is a numeral system originating in ancient Rome, and was adapted from Etruscan numerals. ...

Composition

Globular clusters are generally composed of hundreds of thousands of old stars, similar to the bulge of a spiral galaxy but confined to a volume of only a few cubic parsecs. Some globular clusters, like Omega Centauri in our Milky Way and G1 in M31, are extraordinarily massive, weighing as many as several million solar masses. Some globular clusters (like M15) have extremely massive cores which are expected to harbor black holes.[11] In astronomy, a bulge is a huge, tightly packed group of stars. ... A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. ... Stellar parallax motion The parsec (symbol pc) is a unit of length used in astronomy. ... A small region at the heart of Omega Centauri, containing some 50,000 stars (NASA/STScI) Omega Centauri or NGC 5139 is a globular cluster of stars orbiting our galaxy, the Milky Way. ... The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy which forms part of the Local Group. ... Mayall II, G1, SKHB 1, or HBK 0-1 is a globular cluster in M31, the Andromeda Galaxy. ... The Andromeda Galaxy (also known as Messier 31, M31, or NGC 224; older texts often call it the Andromeda Nebula) is a barred spiral galaxy approximately 2. ... In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ... Globular Cluster M15 (also known as Messier Object 15, Messier 15, M15, or NGC 7078) is a globular cluster in the Pegasus constellation. ... This article is about the astronomical body. ...


While globular clusters can contain a high density of stars, they are not thought to be favorable locations for the survival of planetary systems. Planetary orbits are dynamically unstable within the cores of dense clusters due to the perturbations of passing stars. A planet orbiting at 1 astronomical unit around a star that is within the core of a dense cluster such as 47 Tucanae would only survive on the order of 108 years.[12] However, there has been at least one planetary system found orbiting a pulsar (PSR B1620−26) that belongs to the globular cluster M 4.[13] The astronomical unit (AU or au or a. ... Globular Cluster 47 Tucanae (also known as 47 Tucanae, Globular Cluster NGC 104, or NGC 104) is a globular cluster in the Tucana constellation. ... Composite Optical/X-ray image of the Crab Nebula pulsar, showing surrounding nebular gases stirred by the pulsars magnetic field and radiation. ... PSR B1620-26 is also called PSR J1623-2631 (or or ) // System Configuration PSR B1620-26 is a pulsar in the globular cluster Messier 4 (or M4: see Messier object), about 5,600 light years from Earth in the constellation Scorpius. ... Globular Cluster M4 (also known as Messier Object 4, Messier 4, M4, or NGC 6121) is a globular cluster in the Scorpius constellation. ...


With a few notable exceptions, each globular cluster appears to have a definite age. That is, most of the stars in a cluster are at approximately the same stage in stellar evolution, suggesting that they formed at about the same time. Globular clusters are typically the oldest objects in the Galaxy, and were among the first collections of stars to form. In astronomy, stellar evolution is the sequence of changes that a star undergoes during its lifetime; the hundreds of thousands, millions or billions of years during which it emits light and heat. ...


Metallicity

Globular clusters normally consist of Population II stars, which have a low metallicity compared to Population I stars such as the Sun. (To astronomers, metals includes all elements heavier than Helium, such as Lithium and Carbon.) Metal-poor is a term that is used to describe the chemical make up of an astronomical object. ... The Sun is the name given to the star of our solar system. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/gray Atomic mass 6. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ...


The Dutch astronomer Pieter Oosterhoff noticed that there appear to be two populations of globular clusters, which became known as Oosterhoff groups. The second group has a slightly longer period of RR Lyrae variable stars.[14] Both groups have weak lines of metallic elements. But the lines in the stars of Oosterhoff type I (OoI) cluster are not quite as weak as those in type II (OoII).[14] Hence type I are referred to as "metal-rich" while type II are "metal-poor". Pieter Theodorus Oosterhoff (1904-1978) was a Dutch astronomer. ... Hertzsprung-Russell diagram adapted from Powell. ... Most stars are of almost constant luminosity. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ...


These two populations have been observed in many galaxies (especially massive elliptical galaxies). Both groups are of similar ages (nearly as old as the universe itself) but differ in their metal abundances. Many scenarios have been suggested to explain these subpopulations, including violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In our Milky Way, the metal-poor clusters are associated with the halo and the metal-rich clusters with the Bulge.[15] An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ... The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy which forms part of the Local Group. ...


In the Milky Way it has been discovered that the large majority of the low metallicity clusters are aligned along a plane in the outer part of the galaxy's halo. This result argues in favor of the view that type II clusters in the galaxy were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been 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.[16]


Exotic components

Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars occur relatively often. Due to these chance encounters, some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries, are much more common in globular clusters. A blue straggler is formed from the merger of two stars, possibly as a result of an encounter with a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, and thus differs from the main sequence stars.[17] Blue stragglers are stars in open or globular clusters that are hotter and bluer than other cluster stars having the same luminosity. ... A millisecond pulsar (MSP), often referred to as recycled pulsar, is a pulsar with a rotational period in the range of about 1-10 milliseconds. ... Low-mass X-ray binaries (LMXBs) are binary stars where one of the components is either a black hole or neutron star. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ...

Globular cluster M15 has a 4,000-solar mass black hole at its core. NASA image.
Enlarge
Globular cluster M15 has a 4,000-solar mass black hole at its core. NASA image.

Astronomers have searched for black holes within globular clusters since the 1970s. However the resolution requirements for this task are exacting, and it is only with the Hubble space telescope that the first confirmed discoveries have been made. In independent programs, a 4,000 solar mass intermediate-mass black hole has been discovered in the globular cluster M15 and a 20,000 solar mass black hole in the G1 cluster in the Andromeda Galaxy.[18] Image File history File links Download high-resolution version (541x800, 98 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Globular cluster ... Image File history File links Download high-resolution version (541x800, 98 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Globular cluster ... Globular Cluster M15 (also known as Messier Object 15, Messier 15, M15, or NGC 7078) is a globular cluster in the Pegasus constellation. ... In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ... A black hole is an object predicted by general relativity with a gravitational field so strong that nothing can escape it — not even light. ... NASA logo Listen to this article · (info) This audio file was created from an article revision dated 2005-09-01, and does not reflect subsequent edits to the article. ... A black hole is an object predicted by general relativity with a gravitational field so strong that nothing can escape it — not even light. ... The Hubble Space Telescope (HST) is a telescope in orbit around the Earth. ... In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ... An Intermediate-mass black hole (IMBH) is a black hole whose mass is significantly more than stellar black holes (a few tens of the mass of Sun) yet far less than supermassive black holes (a few millions of the mass of Sun). ... M15 refers to: Messier 15 — a Messier object and a globular cluster in the Pegasus constellation. ... G1 can be: The cellular G1 phase The globular cluster G1 (astronomy) PowerPC G1, aka PowerPC 601 RealPlayer G1 aka RealPlayer 7 Canon PowerShot G1 camera G1 star, a subclass of G-class stars Group 1 of the periodic table Good One, an Internet Slang word This is a disambiguation... The Andromeda Galaxy (also known as Messier 31, M31, or NGC 224; older texts often call it the Andromeda Nebula) is a barred spiral galaxy approximately 2. ...


These are of particular interest because they are the first black holes discovered that were intermediate in mass between the conventional stellar-mass black hole and the supermassive black holes discovered at the cores of galaxies. The mass of these intermediate mass black holes is proportional to the mass of the clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies. The Pleiades, an open cluster of stars in the constellation of Taurus. ... Top: artists conception of a supermassive black hole drawing material from a nearby star. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ...


On the other hand, these claims of intermediate mass black holes have met with some skepticism. The heaviest objects in globular clusters are expected to sink to the cluster center due to mass segregation. These will be white dwarfs and neutron stars in an old stellar population like a globular cluster. As pointed out in two papers by Holger Baumgardt and collaborators, the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15[19] and G1[20].


Color-magnitude diagram

The Hertzsprung-Russell diagram (HR-diagram) is a graph of a large sample of stars that plots their visual absolute magnitude against their color index. The color index, B−V, is the difference between the magnitude of the star in blue light, or B, and the magnitude in visual light (green-yellow), or V. Large positive values indicate a red star with a cool surface temperature, while negative values imply a blue star with a hotter surface. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude (CM) diagram) shows the relationship between absolute magnitude, luminosity, classification, and surface temperature of stars. ... In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us (in the absence of interstellar extinction!). It allows the overall brightnesses of objects to be compared without regard to distance. ... In astronomy, color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. ... In thermodynamics, temperature is the physical property of a system that underlies the common notions of hot and cold —something that is hotter has the greater temperature. ...


When the stars near the Sun are plotted on an HR diagram, it displays a distribution of stars of various masses, ages, and compositions. Many of the stars lie relatively close to a sloping curve with increasing absolute magnitude as the stars are hotter, known as main sequence stars. However the diagram also typically includes stars that are in later stages of their evolution and have wandered away from this main sequence curve. The Sun is the name given to the star of our solar system. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ...


As all the stars of a globular cluster are at approximately the same distance from us, their absolute magnitudes differ from their visual magnitude by about the same amount. The main sequence stars in the globular cluster will fall along a line that is believed to be comparable to similar stars in the solar neighborhood. (The accuracy of this assumption is confirmed by comparable results obtained by comparing the magnitudes of nearby short-period variables, such as RR Lyrae stars and cepheid variables, with those in the cluster.)[21] The apparent magnitude (m) of a star, planet or other heavenly body is a measure of its apparent brightness; that is, the amount of light received from the object. ... RR Lyrae is a variable star in the Lyra constellation. ... Cepheid in the Spiral Galaxy M100 A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. ...


By matching up these curves on the HR diagram, the absolute magnitude of main sequence stars in the cluster can also be determined. This in turn provides a distance estimate to the cluster, based on the visual magnitude of the stars. The difference between the relative and absolute magnitude, the distance modulus, yields this estimate of the distance.[22] The distance modulus is a way of expressing distances which is often used in astronomy to express the distance to galaxies and clusters of galaxies. ...


When the stars of a particular globular cluster are plotted on an HR diagram, nearly all of the stars fall upon a relatively well-defined curve. This differs from the HR diagram of stars near the Sun, which lumps together stars of differing ages and origins. The shape of the curve for a globular cluster is characteristic of a grouping of stars that were formed at approximately the same time and from the same materials, differing only in their initial mass. As the position of each star in the HR diagram varies with age, the shape of the curve for a globular cluster can be used to measure the overall age of the collected stars.[23]

Color-magnitude diagram for the globular cluster Messier 3. Note the characteristic "knee" in the curve at magnitude 19 where stars begin entering the giant stage of their evolutionary path.
Color-magnitude diagram for the globular cluster Messier 3. Note the characteristic "knee" in the curve at magnitude 19 where stars begin entering the giant stage of their evolutionary path.

The most massive main sequence stars in a globular cluster will also have the highest absolute magnitude, and these will be the first to evolve into the giant star stage. As the cluster ages, stars of successively lower masses will also enter the giant star stage. Thus the age of a cluster can be measured by looking for the stars that are just beginning to enter the giant star stage. This forms a "knee" in the HR diagram, bending to the upper right from the main sequence line. The absolute magnitude at this bend is directly a function of the globular cluster, and the age range can be plotted on an axis parallel to the magnitude. Image File history File links Download high-resolution version (796x673, 83 KB) Summary Color-magnitude diagram of M3. ... Image File history File links Download high-resolution version (796x673, 83 KB) Summary Color-magnitude diagram of M3. ... Photo of Messier 3 taken with a 24 telescope in Boulder, Colorado. ... Giant star is a star that has stopped fusing hydrogen in its core. ... Giant star is a star that has stopped fusing hydrogen in its core. ...


By this means it has been shown, for example, that the cluster NGC 1818 is only about 40 million years in age, while M4 may be as old as 12.7 billion years.[24] The later cluster, and other similar clusters, place a bounds on the age limit of the entire universe. This lower limit has been a significant constraint in cosmology. NGC 1818 is a rare young globular cluster, estimated to be only about 40 million years old. ... In astronomy, geology, and paleontology, mya is an acronym for million years ago and is used as a unit of time to denote length of time before the present. ... Globular Cluster M4 (also known as Messier Object 4, Messier 4, M4, or NGC 6121) is a globular cluster in the Scorpius constellation. ... One thousand million (1,000,000,000) is the natural number following 999,999,999 and preceding 1,000,000,001. ... Cosmology, as a branch of astrophysics, is the study of the large-scale structure of the universe and is concerned with fundamental questions about its formation and evolution. ...


Evolutionary studies of globular clusters can also be used to determine changes due to the starting composition of the gas and dust that formed the cluster. That is, the change in the evolutionary tracks due to the abundance of heavy elements. (Heavy elements in astronomy are considered to be all elements more massive than Helium.) The data obtained from studies of globular clusters are then used to study the evolution of the Milky Way as a whole.[25] General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ...


Morphology

In contrast to open clusters, most globular clusters remain gravitationally-bound for time periods comparable to the life spans of the majority of their stars. (A possible exception is when strong tidal interactions with other large masses result in the dispersement of the stars.)


At present the formation of globular clusters remains a poorly understood phenomenon. However, observations of globular clusters shows that these stellar formations arise primarily in regions of efficient star formation, and where the interstellar medium is at a higher density than in normal star-forming regions. Globular cluster formation is prevalant in starburst regions and in interacting galaxies.[26] In astronomy, starburst is a generic term to describe a region of space with a much higher than normal star formation. ... Interacting galaxies (Colliding galaxies) is the result of gravity of two close galaxies. ...


After they are formed, the stars in the globular cluster begin to gravitationally interact with each other. As a result the velocity vectors of the stars are steadily modified, and the stars lose any history of their original velocity. The characteristic interval for this to occur is the relaxation time. This is related to the characteristic length of time a star needs to cross the cluster as well as the number of stellar masses in the system.[27] The value of the relaxation time varies by cluster, but the mean value is on the order of 109 years. Relaxation time is a general physics concept for the characteristic time in which a system relaxes under certain changes in external conditions. ...

Ellipticity of Globulars
Galaxy Ellipticity[28]
Milky Way 0.07±0.04
LMC 0.16±0.05
SMC 0.19±0.06
M31 0.09±0.04

Although globular clusters generally appear spherical in form, ellipticities can occur due to tidal interactions. Clusters within the Milky Way and the Andromeda Galaxy are typically oblate spheroids in shape, while those in the Large Magellanic Clouds are more elliptical.[29] The Large Magellanic Cloud (LMC for short) is a dwarf galaxy that orbits our own galaxy, the Milky Way. ... The Small Magellanic Cloud (SMC for short) is a dwarf galaxy in orbit around the Milky Way Galaxy. ... The Andromeda Galaxy (also known as Messier 31, M31, or NGC 224; older texts often call it the Andromeda Nebula) is a barred spiral galaxy approximately 2. ... Oblate also refers to a member of the Roman Catholic religious order of the Missionary Oblates of Mary Immaculate, or in some cases to a lay or religious person who has officially associated himself (or herself) with a monastic community such as the Benedictines for reasons of personal enrichment without... The Large Magellanic Cloud (LMC for short) is a dwarf galaxy that orbits our own galaxy, the Milky Way. ...


Radii

Astronomers characterize the morphology of a globular cluster by means of standard radii. These are the core radius (rc), the half-light radius (rh) and the tidal radius (rc). The overall luminosity of the cluster steadily decreases with distance from the core, 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 at which half the total luminosity from the cluster is received. This is typically larger than the core radius.


Note that the half-light radius includes stars in the outer part of the cluster that happen to lie along the line of sight, so theorists will also use the half-mass radius (rm)—the radius from the core that contains half the total mass of the cluster. When the half-mass radius of a cluster is small relative to the overall size, it has a dense core. An example of this is the Globular Cluster M3, which has an overall visible dimension of about 18 arc seconds, but a half-mass radius of only 1.12 arc seconds.[30] Photo of Messier 3 taken with a 24 telescope in Boulder, Colorado. ... A second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 ≈ 7. ...


Finally the tidal radius 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 is about 38″.


Luminosity

In measuring the luminosity curve of a globular cluster as a function of radius, most clusters in the Milky Way steadily increase in luminosity up to a certain distance from the core, then the luminosity levels off. Typically this distance is about 1–2 parsecs from the core. However about 20% of the globular clusters have undergone a process termed "core collapse". In this type of cluster, the luminosity continues to steadily increase all the way to the core region.[31] An example of a core-collapsed globular is M15. Globular Cluster M15 (also known as Messier Object 15, Messier 15, M15, or NGC 7078) is a globular cluster in the Pegasus constellation. ...

47 Tucanae is the second most luminous globular cluster in the Milky Way, after Omega Centauri.
47 Tucanae is the second most luminous globular cluster in the Milky Way, after Omega Centauri.

Core-collapse is thought to occur when the more massive stars in a globular encounter their less massive companions. As a result of the encounters the larger stars tend to lose kinetic energy and start to settle toward the core. Over a lengthy period of time this leads to a concentration of massive stars near the core. Image File history File links Download high-resolution version (2024x1543, 505 KB) Summary Globular cluster 47 Tucanae (also known as NGC 104). ... Image File history File links Download high-resolution version (2024x1543, 505 KB) Summary Globular cluster 47 Tucanae (also known as NGC 104). ... Globular Cluster 47 Tucanae (also known as 47 Tucanae, Globular Cluster NGC 104, or NGC 104) is a globular cluster in the Tucana constellation. ... A small region at the heart of Omega Centauri, containing some 50,000 stars (NASA/STScI) Omega Centauri or NGC 5139 is a globular cluster of stars orbiting our galaxy, the Milky Way. ... Kinetic energy (SI unit: the joule) is energy that a body possesses as a result of its motion. ...


The overall luminosities of the globular clusters within the Milky Way and M31 can be modelled by means of a gaussian curve. This gaussian can be represented by means of an average magnitude Mv and a variance σ. This distribution of globular cluster luminosities is called the Globular Cluster Luminosity Function (GCLF). (For the Milky Way, Mv = −7.20±0.13, σ=1.1±0.1 magnitudes.)[32] The GCLF has also been used as a "standard candle" for measuring the distance to other galaxies, under the assumption that the globular clusters in remote galaxies follow the same principles as they do in the Milky Way. This image is a Galaxy Evolution Explorer observation of the large galaxy in Andromeda, Messier 31. ... Probability density function of Gaussian distribution (bell curve). ... A standard candle is an astronomical object that has a known luminosity. ...


N-body simulations

 N-body problem. That is, each of the stars within the cluster continually interacts with the other N−1 stars, where N is the total number of stars in the cluster. The CPU computational "cost" for a simulation increases in proportion to N3, so the potential computing requirements to accurately simulate such a cluster can be enormous.[33] An efficient method of mathematically simulating the N-body dynamics of a globular cluster is done by sub-dividing into small volumes and velocity ranges, and using probabilities to describe the locations of the stars. The motions are then described by means of a formula called the Fokker-Planck equation. This can be solved by a simplified form of the equation, or by running Monte Carlo simulations and using random values. However 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.[34] 

The results of N-body simulations have shown that the stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that results in an increase in velocity, some of the stars gain sufficient energy to be able to depart the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation.[35] The typical time scale for the evaporation of a globular cluster is 1010 years.[27] The n-body problem is the problem of finding, given the initial positions, masses, and velocities of n bodies, their subsequent motions as determined by classical mechanics, i. ... CPU can stand for: in computing: Central processing unit in journalism: Commonwealth Press Union in law enforcement: Crime prevention unit in software: Critical patch update, a type of software patch distributed by Oracle Corporation in Macleans College is often known as Ash Lim. ... The Fokker-Planck equation (named after Adriaan Fokker and Max Planck; also known as the Kolmogorov Forward equation) describes the time evolution of the probability density function of position and velocity of a particle. ... Monte Carlo methods are algorithms for solving various kinds of computational problems by using random numbers (or more often pseudo-random numbers), as opposed to deterministic algorithms. ...


Binary stars form a significant portion of the total population of stellar systems, with up to half of all stars occurring in binary systems. 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.[17] A binary star system consists of two stars both orbiting around their barycenter. ...


Tidal encounters

When a globular cluster has a close encounter with a large mass, such as the core region of a galaxy, it undergoes a tidal interaction. The difference in the pull of gravity between the part of the cluster nearest the mass and the pull on the furthest part of the cluster results in a tidal force. A "tidal shock" occurs whenever the orbit of a cluster takes it through the plane of a galaxy. Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ...


As a result of a tidal shock, streams of stars can be pulled away from the cluster halo, leaving only the core part of the cluster. These tidal interaction effects create tails of stars that can extend up to several degrees of arc away from the cluster.[36] These tails typically both precede and follow the cluster along its orbit. The tails can accumulate significant portions of the original mass of the cluster, and can form clump-like features.[37]


The globular cluster Palomar 5, for example, is near the perihelion 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 out to distances of 13,000 light years.[38] Tidal interactions have stripped away much of the mass from Palomar 5, and further interactions as it passes through the galactic core will transform it into a long stream of stars orbiting the Milky Way halo. This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ...


Tidal interactions add kinetic energy into a globular cluster, dramatically increasing the evaporation rate and shrinking the size of the cluster.[27] Not only does tidal shock strip off the outer stars from a globular cluster, but the increased evaporation accelerates the process of core collapse.


See also

This is a list of globular clusters. ... Plummer model or Plummers model is a density profile and potential pair that is used to describe the distribution of position and velocities of particles in dynamical systems. ...

References

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General resources

  • NASA Astrophysics Data System has a collection of past articles, from all major astrophysics journals and many conference proceedings.
  • SCYON is a newsletter dedicated to star clusters.
  • MODEST is a loose collaboration of scientists working on star clusters.

Books

  • Binney, James; Tremaine, Scott (1987). Galactic Dynamics, Princeton University Press, Princeton, New Jersey.
  • Heggie, Douglas; Hut, Piet (2003). The Gravitational Million-Body Problem: A Multidisciplinary Approach to Star Cluster Dynamics, Cambridge University Press.
  • Spitzer, Lyman (1987). Dynamical Evolution of Globular Clusters, Princeton University Press, Princeton, New Jersey.

Review Articles

  • Elson, Rebecca; Hut, Piet; Inagaki, Shogo (1987). Dynamical evolution of globular clusters. Annual review of astronomy and astrophysics 25 565. NASA ADS
  • Meylan, G.; Heggie, D. C. (1997). Internal dynamics of globular clusters. The Astronomy and Astrophysics Review 8 1. NASA ADS

External links

  • Globular Clusters, SEDS Messier pages
  • Milky Way Globular Clusters
  • Catalogue of Milky Way Globular Cluster Parameters by William E. Harris, McMaster University, Ontario, Canada.
  • A galactic globular cluster database by Marco Castellani, Rome Astronomical Observatory, Italy.
  • Key stars have different birthdays - Stars in globular clusters are born in several bursts, rather than all at once.

 
 

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