M5 est un amas globulaire situé dans la constellation du Serpent. Il fut découvert par Gottfried Kirch en , puis indépendamment par Charles Messier en. L’enfance tourmentée des amas globulaires. Séminaire le 26 Jan à 11h Intervenant: Corinne Charbonnel. (Observatoire de Genève). L’enfance. Download Citation on ResearchGate | Sur l’evolution dynamique des amas globulaires / | “Serie A, no. ” Thesis (doctoral)–Universite de Paris, }.
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A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravitywhich gives them their spherical shapes and relatively high stellar densities toward their centers.
The name of this category of star cluster is derived from the Latin globulus —a small sphere. A globular cluster is sometimes known more simply as a globular. Globular clusters are found in the halo of a galaxy and contain considerably more stars and are much older than the less dense open clusterswhich are found in the disk of a galaxy. Globular clusters are fairly common; there are about  to  currently known globular clusters in the Milky Waywith perhaps 10 to 20 more still undiscovered.
Andromeda Galaxyfor instance, may have as many as 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.
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. 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.
The first known globular cluster, now called M22was discovered in by Abraham Ihlea German amateur astronomer. When William Herschel began his comprehensive survey of the sky using large telescopes in there were 34 known globular clusters. Herschel discovered another 36 himself and was the first to resolve virtually all of them into stars. The number of globular clusters discovered glohulaires to increase, reaching 83 in93 in and 97 by Beginning inHarlow Goobulaires began a series of studies of globular clusters, published in about 40 scientific papers.
He examined the RR Lyrae variables in the clusters which he assumed were Cepheid variables and used their period—luminosity relationship for distance estimates.
Later, it was found that RR Lyrae variables are fainter than Cepheid variables, which caused Shapley to overestimate the distances of the clusters. Of the globular clusters within the Milky Way, the majority are found in a halo around the galactic core, and the large majority are located in the celestial sky centered on the core.
Inthis strongly asymmetrical distribution was used by 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. His error was due to interstellar dust in the Milky Way, which amws and diminishes the amount of light from distant objects, such as globular clusters, that reaches the Earth, thus making them appear to be more distant than they are.
Shapley’s measurements also indicated that the Sun is relatively far from the center of the galaxy, also contrary to what had previously been inferred from the apparently amws even distribution of ordinary stars. In reality, most ordinary stars lie within the galaxy’s disk and those globulares that lie in the direction of the galactic centre and beyond are thus obscured by gas and dust, whereas globular clusters lie outside the disk and can be seen at much further distances.
In —29, Shapley and Sawyer categorized clusters according to the degree of concentration each system has toward its 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.
M5 (amas globulaire) — Wikipédia
The formation of globular clusters remains a poorly understood phenomenon and 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. In many globular clusters, most of the stars are at approximately the same stage in stellar evolutionsuggesting that they formed at about the same time.
During their youth, these LMC clusters may have encountered giant molecular clouds that triggered a second round of star formation. qmas
In the Antennae galaxyfor example, the Hubble Space Telescope has observed clusters of clusters, regions in the galaxy that span hundreds of parsecs, where many of the clusters will eventually collide and merge. Many of them present a significant range in ages, hence possibly metallicitiesand their merger could plausibly lead to clusters with a bimodal or even multiple distribution of populations. 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. The mass of the SMBH in such a galaxy is often close to the combined mass of the galaxy’s globular clusters.
No known globular clusters display active star formation, 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 clusterssuch as Westerlund 1 in the Milky Waymay be the precursors of globular clusters. Globular clusters are generally composed of hundreds of thousands of low-metalold stars.
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 million cubic parsecs. They are free of gas and dust and it is presumed that all of the gas and dust was long ago turned into stars. Globular clusters can contain a high density of stars; on average about 0. Globular clusters are not thought to be favorable locations for the survival of planetary systems.
Planetary orbits are dynamically unstable within the cores of dense clusters because of 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 10 8 years.
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 although simulations suggest that a less massive black hole or central concentration of neutron stars or massive white dwarfs explain observations equally well.
Globular clusters normally consist of Population II starswhich have a low proportion of elements other than hydrogen and helium when compared to Population I stars such as the Sun. Astronomers refer to these heavier elements as metals and to the proportions of these elements as the metallicity.
These elements are produced by stellar nucleosynthesis and then are recycled into the interstellar mediumwhere they enter the next generation of stars. Hence the proportion of metals can be an indication of the age of a star, with older stars typically having a lower metallicity. 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.
These two populations have been observed in many galaxies, especially massive elliptical galaxies. Both groups are nearly as old as the universe itself and are of similar ages, 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 the Milky Waythe metal-poor clusters are associated with the halo and the metal-rich clusters with the bulge. 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. 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 stragglersmillisecond pulsars and low-mass X-ray binariesare 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. Astronomers have searched for black holes within globular clusters since the s. 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. 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.
Claims of intermediate mass black holes have been met with some skepticism. The heaviest objects in globular clusters are expected to migrate to the cluster center due to mass segregation. 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  and Mayall II.
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.
Large positive values indicate a red star with a cool surface temperaturewhile negative values imply a blue star with a hotter surface. 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. 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 variableswith those in the cluster.
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 modulusyields this estimate of the distance.
When the stars of a particular globular cluster are plotted on an HR diagram, in many cases nearly all of the stars fall upon a hlobulaires 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 star population. However, the above-mentioned historic process of determining the age and distance to globular clusters is not as robust as first thought, since the morphology globulalres luminosity of globular cluster stars in color-magnitude diagrams are influenced by numerous parameters, many of which are still being actively researched.
Certain clusters even display populations that are absent from other globular clusters e. The historical paradigm that all globular clusters consist of stars born at exactly the same time, or sharing exactly the same chemical abundance, has likewise been overturned e. One such effect is called blending, and it arises because the cores of globular clusters are so dense that in low-resolution observations multiple unresolved stars may appear as a single target. Thus the brightness measured for that seemingly single star e.
The most massive main-sequence stars will also have the highest absolute magnitude, and these will be the first to evolve into the anas star stage. As the cluster ages, stars of successively lower masses will also enter the giant star stage.
Thus the age of a single population 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 gobulaires white dwarfs.
Typical results for globular clusters are that they may be as old as 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. Historically, astronomers were faced with age estimates of globular clusters that appeared older than cosmological models would anas. However, better measurements of cosmological parameters through deep sky surveys and satellites such as the Hubble Space Telescope appear to have resolved this issue.