The Messier 80 globular cluster in the constellation Scorpius is located about 28,000 light-years from the Sun and contains hundreds of thousands of stars. Messier 80 (also known as M80 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 or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by ^[1]

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The Galactic Center is the rotational center of the Milky Way Galaxy. This article is about artificial satellites For natural satellites also known as moons see Natural satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. Gravitation is a natural Phenomenon by which objects with Mass attract one another The name of this category of star cluster is derived from the Latin globulus—a small sphere. Star clusters are groups of Stars which are gravitationally bound Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. A globular cluster is sometimes known more simply as a globular.

Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk. 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 Globular clusters are fairly common; there are about 158^[2] currently known globular clusters in the Milky Way, with perhaps 10–20 more undiscovered. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[3] Large galaxies can have more: Andromeda, for instance, may have as many as 500. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda ^[4] Some giant elliptical galaxies, such as M87,^[5] may have as many as 10,000 globular clusters. An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated Messier 87 (also known as M87, Virgo A or NGC 4486) is a giant Elliptical galaxy. These globular clusters orbit the galaxy out to large radii, 40 kiloparsecs (approximately 131 thousand light-years) or more. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by ^[6]

Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters. The Local Group is the group of galaxies that includes our galaxy the Milky Way. ^[7] The Sagittarius Dwarf and Canis Major Dwarf galaxies appear to be in the process of donating their associated globular clusters (such as Palomar 12) to the Milky Way. The Sagittarius Dwarf Elliptical Galaxy ( SagDEG) is an elliptically looped shaped 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 ^[8] This demonstrates how many of this galaxy's globular clusters were acquired in the past.

Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear. The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning the formation of the first galaxies the way It does appear clear that globular clusters are significantly different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy rather than as a separate galaxy. Dwarf elliptical galaxies, or dE's are elliptical galaxies that are much smaller than others classified as dE. However, recent conjectures by astronomers suggest that globular clusters and dwarf spheroidals may not be clearly separate and distinct types of objects. Dwarf spheroidal galaxy ( dSph) is a term in Astronomy applied to low luminosity Galaxies that are Companions to the Milky ^[9]

Observation history

The first globular cluster discovered was M22 in 1665 by Abraham Ihle, a German amateur astronomer. Messier 22 (also known as M22 or NGC 6656) is an elliptical Globular cluster in the Constellation Sagittarius near the Galactic Johann Abraham Ihle ( June 14, 1627 – c1699 was a German Amateur astronomer who discovered the first known Globular cluster, Omega Centauri or NGC 5139 is a Globular cluster of Stars seen in the constellation of Centaurus, discovered by Edmond Halley in Edmond Halley FRS (ˈɛdmənd ˈhɔːlɪ ( November 8, 1656 &ndash January 14, 1742) was an English Astronomer Globular Cluster M5 (also known as Messier Object 5 or NGC 5904) is a Globular cluster in the Constellation Serpens. Gottfried Kirch ( Kirche, Kirkius) (December 18 1639—July 25 1710 was a German astronomer. The Great Globular Cluster in Hercules (also known as the Hercules Globular Cluster, Messier Object 13, Messier 13, M13, or NGC 6205 Messier 71 (also known as M71 or NGC 6838) is a Globular cluster in the Constellation Sagitta. Jean-Philippe Loys de Chéseaux (1718&ndash1751 was an astronomer from Lausanne in Switzerland. Messier 4 (also known as M4 or NGC 6121) is a Globular cluster in the Constellation Scorpius. Globular Cluster M15 (also known as Messier Object 15 or NGC 7078) is a Globular cluster in the Constellation Giovanni Domenico ( Jean-Dominique) Maraldi ( April 17, 1709 – November 14, 1788) was an Italian -born Messier 2 (also known as M2 or NGC 7089) is a Globular cluster in the Constellation Aquarius. Messier 22 (also known as M22 or NGC 6656) is an elliptical Globular cluster in the Constellation Sagittarius near the Galactic ^[10] However, due to the small aperture of early telescopes, individual stars within a globular cluster were not resolved until Charles Messier observed M4. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. Angular resolution describes the resolving power of any image forming device such as an optical or Radio telescope, a Microscope, a Camera Charles Messier ( June 26, 1730 &ndash April 12, 1817) was a French astronomer most notable for publishing an Messier 4 (also known as M4 or NGC 6121) is a Globular cluster in the Constellation Scorpius. 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. Abbé Nicolas Louis de Lacaille ( March 15, 1713 &ndash March 21, 1762) was a French Astronomer. 47 Tucanae ( NGC 104) or just 47 Tuc is a Globular cluster located in the Constellation Tucana. NGC 4833 is a Globular cluster discovered by Abbe Lacaille during his 1751-1752 journey to South Africa and catalogued in 1755 Messier 55 (also known as M55 or NGC 6809) is a Globular cluster in the Constellation Sagittarius. Messier 69 (also known as M69 or NGC 6637) is a Globular cluster in the Constellation Sagittarius. NGC 6397 is a Globular cluster in the Ara Constellation. It is located about 7200 Light-years from Earth, making it one of the The M before a number refers to the catalogue of Charles Messier, while NGC is from the New General Catalogue by John Dreyer. The New General Catalogue ( NGC) is the best-known catalogue of Deep sky objects in Amateur astronomy. John Louis Emil Dreyer ( February 13 1852 &ndash 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. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British 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.

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. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[3] These additional, undiscovered globular clusters are believed to be hidden behind the gas and dust of the Milky Way.

Beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers. Harlow Shapley ( November 2 1885 &ndash October 20 1972) was an American Astronomer. He examined the cepheid variables in the clusters and would use their period–luminosity relationship for distance estimates. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation

M75 is a highly-concentrated, Class I globular cluster. Messier 75 (also known as M75 or NGC 6864) is a Globular cluster in the Constellation Sagittarius.

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. ^[11] 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. His error was due to the fact that dust in the Milky Way diminished the amount of light from a globular cluster that reached the earth, thus making it appear farther away. Shapley's estimate was, however, within the same order of magnitude of the currently accepted value. 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's measurements also indicated that the Sun was relatively far from the center of the galaxy, contrary to what had previously been inferred from the apparently nearly even distribution of ordinary stars. In reality, ordinary stars lie within the galaxy's disk and are thus often obscured by gas and dust, whereas globular clusters lie outside the disk and can be seen at much further distances.

Shapley was subsequently assisted in his studies of clusters by Henrietta Swope and Helen Battles Sawyer (later Hogg). Henrietta Hill Swope (1902 &ndash 1980 was an American astronomer. Helen Battles Sawyer Hogg CC, FRSC ( August 1, 1905 &ndash January 28, 1993) was a prolific astronomer In 1927–29, Harlow Shapley and Helen Sawyer began categorizing clusters according to the degree 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. The Shapley–Sawyer Concentration Class is a classification system on a scale of one to twelve using Roman numerals for Globular clusters according to their concentration (It is sometimes given with numbers [Class 1–12] rather than Roman numerals. Roman numerals are a Numeral system originating in ancient Rome, adapted from Etruscan numerals. )^[12]

Composition

Globular clusters are generally composed of hundreds of thousands of low-metal, old stars. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen The type of stars found in a globular cluster are similar to those in the bulge of a spiral galaxy but confined to a volume of only a few cubic parsecs. In Astronomy, a bulge is a tightly packed group of Stars The term almost exclusively refers to the central group of stars found in most spiral galaxies. A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 They are free of gas and dust and it is presumed that all of the gas and dust was long ago turned into stars.

While globular clusters can contain a high density of stars (on average about 0. 4 stars per cubic parsec, increasing to 100 or 1000 stars per cubic parsec in the core of the cluster),^[13] they are not thought to be favorable locations for the survival of planetary systems. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 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. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the 47 Tucanae ( NGC 104) or just 47 Tuc is a Globular cluster located in the Constellation Tucana. ^[14] However, there has been at least one planetary system found orbiting a pulsar (PSR B1620−26) that belongs to the globular cluster M4. Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves PSR B1620-26 is a Binary star system approximately 12400 Light-years away in the Constellation of Scorpius. Messier 4 (also known as M4 or NGC 6121) is a Globular cluster in the Constellation Scorpius. ^[15]

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. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime All known globular clusters appear to have no active star formation, which is consistent with the view that globular clusters are typically the oldest objects in the Galaxy, and were among the first collections of stars to form. Very large regions of star formation known as super star clusters, such as Westerlund 1 in the Milky Way, may be the precursors of globular clusters. A super star cluster ( SSC) is a very large region of Star formation thought to be the precursor of a Globular cluster Westerlund 1 (sometimes Wd1) is the most massive compact young Star cluster in the Local group and is about 5 kpc away The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[16]

Some globular clusters, like Omega Centauri in our Milky Way and G1 in M31, are extraordinarily massive (several million solar masses) and contain multiple stellar populations. Omega Centauri or NGC 5139 is a Globular cluster of Stars seen in the constellation of Centaurus, discovered by Edmond Halley in The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply Mayall II ( M31 G1) also known as NGC-224-G1, SKHB 1, GSC 27882139, HBK 0-1, M31GC J003247+393440 The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. Both can be regarded as evidence that supermassive globular clusters are in fact the cores of dwarf galaxies that are consumed by the larger galaxies. Several globular clusters (like M15) have extremely massive cores which may harbor black holes,^[17] although simulations suggest that a less massive black hole or central concentration of neutron stars or massive white dwarfs explain observations equally well. Globular Cluster M15 (also known as Messier Object 15 or NGC 7078) is a Globular cluster in the Constellation A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e

Metallic content

Globular clusters normally consist of Population II stars, which have a low metallic content compared to Population I stars such as the Sun. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen The Sun (Sol is the Star at the center of the Solar System. (To astronomers, metals includes all elements heavier than helium, such as lithium and carbon. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 )

The Dutch astronomer Pieter Oosterhoff noticed that there appear to be two populations of globular clusters, which became known as Oosterhoff groups. The Netherlands ( Dutch:, ˈnedərlɑnt is the European part of the Kingdom of the Netherlands, which consists of the Netherlands the Netherlands Pieter Theodorus Oosterhoff (1904-1978 was a Dutch Astronomer. The second group has a slightly longer period of RR Lyrae variable stars. RR Lyrae variables are Variable stars often used as Standard candles RR Lyrae are pulsating Horizontal branch stars with a mass of around half the For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson ^[18] Both groups have weak lines of metallic elements. 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 But the lines in the stars of Oosterhoff type I (OoI) cluster are not quite as weak as those in type II (OoII). ^[18] Hence type I are referred to as "metal-rich" while type II are "metal-poor".

These two populations have been observed in many galaxies (especially massive elliptical galaxies). An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated 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. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[19]

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. ^[20]

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. Blue stragglers ( BSS) 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 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 formed at the beginning of the cluster. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness ^[21]

Globular cluster M15 has a 4,000-solar mass black hole at its core. Globular Cluster M15 (also known as Messier Object 15 or NGC 7078) is a Globular cluster in the Constellation The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e NASA image. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program

Astronomers have searched for black holes within globular clusters since the 1970s. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e The resolution requirements for this task, however, are exacting, and it is only with the Hubble space telescope that the first confirmed discoveries have been made. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into In independent programs, a 4,000 solar mass intermediate-mass black hole has been suggested to exist based on HST observations in the globular cluster M15 and a 20,000 solar mass black hole in the Mayall II cluster in the Andromeda Galaxy. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and 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 the Sun) yet Globular Cluster M15 (also known as Messier Object 15 or NGC 7078) is a Globular cluster in the Constellation Mayall II ( M31 G1) also known as NGC-224-G1, SKHB 1, GSC 27882139, HBK 0-1, M31GC J003247+393440 The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda ^[22] Both x-ray and radio emissions from Mayall II appear to be consistent with an intermediate-mass black hole. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. ^[23]

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. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 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.

Claims of intermediate mass black holes have been met with some skepticism. The densest 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 or cluster of 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^[24] and Mayall II. ^[25]

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 Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard In Astronomy, the color index is a simple numerical expression that determines the color of an object which in the case of a Star gives its Temperature 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. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally 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. The Sun (Sol is the Star at the center of the Solar System. 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. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness However the diagram also typically includes stars that are in later stages of their evolution and have wandered away from this main sequence curve.

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 apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 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. RR Lyrae is a Variable star in the Lyra constellation It is the prototype of the RR Lyrae variables star class A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation )^[26]

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. The distance modulus is a way of expressing distances that is often used in Astronomy. ^[27]

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. ^[28]

Color-magnitude diagram for the globular cluster M3. Messier 3 (also known as M3 or NGC 5272) is a Globular cluster in the Constellation Canes Venatici. 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. A giant star is a Star with substantially larger Radius and Luminosity than a Main sequence star of the same surface temperature. As the cluster ages, stars of successively lower masses will also enter the giant star stage. A giant star is a Star with substantially larger Radius and Luminosity than a Main sequence star of the same surface temperature. 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 age of globular cluster, so an age scale can be plotted on an axis parallel to the magnitude.

In addition, globular clusters can be dated by looking at the temperatures of the coolest white dwarfs. Typical results for globular clusters are that they may be as old as 12. 7 billion years. ^[29] This is in contrast to open clusters which are only tens of millions of years old.

The ages of globular clusters place a bound on the age limit of the entire universe. This lower limit has been a significant constraint in cosmology. Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its During the early 1990s, astronomers were faced with age estimates of globular clusters that appeared older than cosmological models would allow. However, better measurements of cosmological parameters through deep sky surveys and satellites such as COBE have resolved this issue as have computer models of stellar evolution that have different models of mixing.

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. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical ) The data obtained from studies of globular clusters are then used to study the evolution of the Milky Way as a whole. ^[30]

In globular clusters a few stars known as blue stragglers are observed, apparently continuing the main sequence in the direction of brighter, bluer stars. Blue stragglers ( BSS) are Stars in open or Globular clusters that are hotter and bluer than other cluster stars having the same Luminosity The origins of these stars is still unclear, but most models suggest that these stars are the result of mass transfer in multiple star systems.

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 dispersal of the stars. )

At present the formation of globular clusters remains a poorly understood phenomenon. It remains uncertain whether the stars in a globular cluster form in a single generation, or are spawned across multiple generations over a period of several hundred million years. This star-forming period is relatively brief, however, compared to the age of many globular clusters. ^[31] Observations of globular clusters show 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 prevalent in starburst regions and in interacting galaxies. Interacting galaxies ( Colliding galaxies) are the result of one Galaxy 's Gravity disturbing another galaxy ^[32]

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. Relaxation time is a general concept in Physics for the Characteristic time in which a system changes to an Equilibrium condition from a non-equilibrium 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. ^[33] The value of the relaxation time varies by cluster, but the mean value is on the order of 10⁹ years.

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 Cloud are more elliptical. An oblate Spheroid is a rotationally symmetric Ellipsoid having a polar axis shorter than the diameter of the equatorial circle whose plane The Large Magellanic Cloud (LMC is a nearby Satellite galaxy of our own galaxy the Milky Way. ^[35]

Radii

Astronomers characterize the morphology of a globular cluster by means of standard radii. These are the core radius (r_c), the half-light radius (r_h) and the tidal radius (r_t). 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 within 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 (r_m)—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 Messier 3 (M3), which has an overall visible dimension of about 18 arc minutes, but a half-mass radius of only 1. Messier 3 (also known as M3 or NGC 5272) is a Globular cluster in the Constellation Canes Venatici. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. 12 arc minutes. ^[36]

Almost all globular clusters have a half-light radius of less than 10 pc. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 Although there are well-established globular clusters with very large radii (i. e. NGC 2419 (R_h = 18 pc) and Palomar 14 (R_h = 25 pc). NGC 2419 is a Globular cluster in the constellation Lynx. It was discovered by William Herschel on December 31, 1788. ^[9]

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 arc minutes.

Mass segregation and luminosity

In measuring the luminosity curve of a given globular cluster as a function of distance from the core, most clusters in the Milky Way steadily increase in luminosity as this distance decreases, 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. ^[37] An example of a core-collapsed globular is M15. Globular Cluster M15 (also known as Messier Object 15 or NGC 7078) is a Globular cluster in the Constellation

47 Tucanae - the second most luminous globular cluster in the Milky Way, after Omega Centauri. 47 Tucanae ( NGC 104) or just 47 Tuc is a Globular cluster located in the Constellation Tucana. Omega Centauri or NGC 5139 is a Globular cluster of Stars seen in the constellation of Centaurus, discovered by Edmond Halley in

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. The kinetic energy of an object is the extra Energy which it possesses due to its motion Over a lengthy period of time this leads to a concentration of massive stars near the core, a phenomenon called mass segregation. In Astronomy, dynamical mass segregation is the process by which heavier members of a gravitationally bound system such as a Star cluster or cluster of

The Hubble Space Telescope has been used to provide 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 globular star cluster 47 Tucanae, which is made up of about 1 million stars, is one of the densest globular clusters in the Southern Hemisphere. 47 Tucanae ( NGC 104) or just 47 Tuc is a Globular cluster located in the Constellation Tucana. This cluster was subjected to an intensive photographic survey, which allowed astronomers to track the motion of its stars. Precise velocities were obtained for nearly 15,000 stars in this cluster. ^[38]

The different stages of core-collapse may be divided into three phases. During a globular cluster's adolescence, the process of core-collapse begins with stars near the core. However, the interactions between binary star systems prevents further collapse as the cluster approaches middle age. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. Finally, the central binaries are either disrupted or ejected, resulting in a tighter concentration at the core.

A 2008 study by Dr. John Fregeau of 13 globular clusters in the Milky Way shows that three of them have unusually large number of X-ray sources, or X-ray binaries, suggesting the clusters are middle-aged. Previously, these globular clusters had been classified as being in old age because they had very tight concentrations of stars in their centers, another litmus test of age used by astronomers. The implication is that most globular clusters, including the other ten studied by Fregeau, are not in middle age, as previously thought, but are actually in adolescence.

"It's remarkable that these objects, which are thought to be some of the oldest in the Universe, may really be very immature," said Fregeau whose paper appears in The Astrophysical Journal. The Astrophysical Journal (abbreviated to ApJ or Astrophys J) is a Scientific journal covering Astronomy and Astrophysics. "This would represent a major change in thinking about the current evolutionary status of globular clusters. "^[39]

The overall luminosities of the globular clusters within the Milky Way and M31 can be modeled by means of a gaussian curve. In Mathematics, a Gaussian function (named after Carl Friedrich Gauss) is a function of the form f(x = a e^{- { (x-b^2 \over 2 This gaussian can be represented by means of 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 = −7. 20±0. 13, σ=1. 1±0. 1 magnitudes. )^[40] 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. A standard candle is an astronomical object that has a known Luminosity.

N-body simulations

Computing the interactions between the stars within a globular cluster requires solving what is termed the N-body problem. 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 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 naive CPU computational "cost" for a dynamic simulation increases in proportion to N³,^[41] so the potential computing requirements to accurately simulate such a cluster can be enormous. ^[42] 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. The Fokker–Planck equation describes the Time evolution of the Probability density function of the position of a particle and can be generalized to other observables This can be solved by a simplified form of the equation, or by running Monte Carlo simulations and using random values. Monte Carlo methods are a class of Computational Algorithms that rely on repeated Random sampling to compute their results 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. ^[43]

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 result in an increase in velocity, some of the stars gain sufficient energy to escape the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation. ^[44] The typical time scale for the evaporation of a globular cluster is 10¹⁰ years. ^[33]

Binary stars form a significant portion of the total population of stellar systems, with up to half of all stars occurring in binary systems. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. 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. ^[21]

Intermediate forms

The distinction between cluster types is not always clear-cut, and objects have been found that blur the lines between the categories. For example, BH 176 in the southern part of the Milky Way has properties of both an open and a globular cluster. ^[45]

In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy, which is, in several ways, very similar to globular clusters. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. The clusters also share other characteristics with globular clusters, e. g. the stellar populations and metallicity. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters. Parametrically, these clusters lie somewhere between a (low dark-matter) globular cluster and a (dark matter-dominated) dwarf spheroidal galaxy. Dwarf spheroidal galaxy ( dSph) is a term in Astronomy applied to low luminosity Galaxies that are Companions to the Milky ^[46]

How these clusters are formed is not yet known, but their formation might well be related to that of globular clusters. Why M31 has such clusters, while the Milky Way does not, is not yet known. It is also unknown if any other galaxy contains these types of clusters, but it would be very unlikely that M31 is the sole galaxy with extended clusters. ^[46]

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. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced A "tidal shock" occurs whenever the orbit of a cluster takes it through the plane of a galaxy.

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. ^[47] 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. ^[48]

The globular cluster Palomar 5, for example, is near the perigalactic point of its orbit after passing through the Milky Way. Palomar 5 is a Globular cluster discovered by Walter Baade in 1950 In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from 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. ^[49] Tidal interactions have stripped away much of the mass from Palomar 5, and further interactions as it passes through the galactic core are expected to transform it into a long stream of stars orbiting the Milky Way halo.

Tidal interactions add kinetic energy into a globular cluster, dramatically increasing the evaporation rate and shrinking the size of the cluster. ^[33] Not only does tidal shock strip off the outer stars from a globular cluster, but the increased evaporation accelerates the process of core collapse. The same physical mechanism may be at work in Dwarf spheroidal galaxies such as the Sagittarius Dwarf, which appears to be undergoing tidal disruption due to its proximity to the Milky Way. Dwarf spheroidal galaxy ( dSph) is a term in Astronomy applied to low luminosity Galaxies that are Companions to the Milky

See also

• List of globular clusters
• Plummer model
• Extragalactic Distance Scale

References

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 article describes how stars in globular clusters are born in several bursts, rather than all at once
• Globular Clusters Blog News, papers and preprints on Galactic Globular Clusters

• Clickable Messier Object table including globular clusters