Globular cluster

[[Image:M80.jpg|thumb|right|300px|The {{Globular Cluster M80}} in the constellation {{Scorpius}} is located about 28,000 {{light year}}s from the Sun and contains hundreds of thousands of stars. The {{Sagittarius Dwarf Elliptical Galaxy|Sagittarius Dwarf}} and {{Canis Major Dwarf Galaxy|Canis Major Dwarf}} galaxies appear to be in the process of donating their associated globular clusters to the Milky Way (such as {{Palomar 12}}) demonstrating how many of this galaxy's globular clusters were acquired in the past.

Observation history

Early Globular Cluster Discoveries

Cluster name	Discovered by!Year
{{Globular Cluster M22|M22}}	Abraham Ihle	1665
{{Omega Centauri|ω Cen}}	{{Edmond Halley}}	1677
{{Messier 5|M5}}	{{Gottfried Kirch}}	1702
{{Messier 13|M13}}	Edmond Halley	1714
{{Globular Cluster M71|M71}}	{{Philippe Loys de Chéseaux}}	1745
{{Globular Cluster M4|M4}}	Philippe Loys de Chéseaux	1746
{{Messier 15|M15}}	Jean-Dominique Maraldi	1746
{{Globular Cluster M2|M2}}	Jean-Dominique Maraldi	1746

The first globular cluster discovered was {{Globular Cluster M22|M22}} in 1665 by Abraham Ihle. However, due to the small {{aperture}} of early {{telescope}}s, individual stars within a globular cluster were not {{Angular resolution|resolved}} until {{Charles Messier}} observed {{Globular Cluster M4|M4}}. The first eight globular clusters discovered are shown in the table. Subsequently, Abbe Lacaille would list {{47 Tucanae|NGC 104}}, {{NGC 4833}}, {{Globular Cluster M55|M55}}, {{Globular Cluster M69|M69}}, and {{NGC 6397}} in his 1751â€"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}}.)

{{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. (The word globular is derived from the {{Latin language|Latin}} globus and the {{English language|English}} suffix "-ular", which means to have the shape of a globe or globule.)

The number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947. There are now a total of 151 globular clusters that have been discovered in the {{Milky Way}} galaxy, out of an estimated total of 180 ± 20. (Some undiscovered globularclusters may be hidden behind the gas and dust of the Milky Way.)

Beginning in 1914, {{Harlow Shapley}} had begun a series of studies of globular clusters that were published in about 40 scientific papers. He examined the {{cepheid variable}}s in the clusters and would use their periodâ€"luminosity relationship for distance estimates.

{{Image:Messier75.jpg|left|thumb|150px|{{Globular Cluster M75|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. 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.)

Shapley was subsequently assisted in his studies of clusters by Henrietta Swope and Helen Battles Sawyer (later Hogg). In 1927â€"29, Harlow Shapley and Helen Sawyer began categorizing clusters according to the amount of concentration the system has toward the core. The most concentrated stars were identified as Class I, with successively diminishing concentrations ranging to Class XII. This became known as the Shapleyâ€"Sawyer Concentration Class. (It is sometimes given with numbers (Class 1–12) rather than roman 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 clusters, weighing as many as several million solar masses. Some globular clusters (like M15) have extremely massive cores which are expected to harbor black holes.

With a few notable exceptions, each globular cluster appears to have a definite age. That is, all the stars in a cluster are at the same stage in stellar evolution, suggesting that they formed at the same time. Globular clusters are typically the oldest objects in the Galaxy, and were among the first collections of stars to form.

Metallicity

Globular clusters normally consist of [[Metal-poor|Population IIstars]], which have a low metallicity compared toPopulation I stars such as the Sun. (Toastronomers, metals consist of all elements heavier than Helium,such as Lithium and Carbon.)

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. 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). 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). Both groups are of similar ages (nearly as old as the universe itself) but differ in their metalabundances. 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.

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.

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 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.

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

Astronomers have searched for the existence of black holes within globular clusters since the 1970s. However the resolution requirements for this task are exacting, and it was 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.

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 bolometric correction) yields this distance estimate.

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.

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. 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.

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.