Theory Globular Clusters

a famed globular cluster. The Great Cluster in Hercules. picture courtesy site 'Amateur Astronomy'

Globular clusters are associations of several hundred thousand or millions, of stars, in spherical-shaped clumps. The large mass in the rich stellar centre of the globular cluster pulls the stars inward to form a ball of stars. The word globulus, from which these clusters take their name, is Latin for small sphere. Globular clusters are about a hundred light-years in diameter. Globular clusters are mostly linked to galaxies; they are located in their halos, forming a sphere around galaxies. Elliptical galaxies typically have a few thousand globular clusters, while spirals contain a few hundred. Globular clusters mays find their origin into proto-globular clusters, as such pockets of gas eventually collapse and form fully-fledged globular clusters. Famed Centaurus A is suspected to harbour as many as 2000 globular clusters as many of those are brighter and more massive than the ones orbiting the Milky Way. There are about 150-200 globular clusters around our Milky Way, the most of which are grouped around the central hub of it. In most globular clusters, stars are densely packed and closer to each other than in the usual interstellar medium: a tenth of light-year apart instead of 5-6 light-years. Stars are orbiting the center of the cluster on very elliptical orbits. Stars in globular clusters are usually older and less massive than the Sun. Stars in the globular cluster are 10-13 billions years old, dating back to the birth of galaxies, being of help to study the origins of them. In our Milky Way, globular clusters distances from the Sun range from 7000-8000 light-years to more than 100,000. Globular clusters are orbiting the Galaxy on long term orbits and these orbits cross galactic plane as they mostly now are lying closer to the galactic center than we do. A groupe of 15 globular clusters, known as the Palomar clusters, from survey plates by the first Palomar Observatory Sky Survey in the 1950s, are faint for cause of remoteness or hidden behind blankets of dust, or a small number of remaining stars. A globular cluster seen in the outer halo of the Large Magellanic Cloud, a satellite-dwarf galaxy to the Milky Way is populated by middle-aged stars, several billions of years old. Some globular clusters from our own Milky Way Galaxy as seen from Earth are seen slightly out of shape as the gravitational forces work upon them as they are orbiting close to the galactic center. Some globular clusters around the Andromeda Galaxy are holding a black hole, as that is not the case for our Milky Way Galaxy

Most of galaxies are featuring globular clusters, like the M87 galaxy, for example, which holds thousands. One of those, after passing through two supermassive black holes at the center of M87, was accelerated to more than two million miles per hour and ejected into the intergalactic void. There are even globular clusters with the Magelanic Clouds. Massive galaxies tend to have both metal-poor (appearing blue) and metal-rich (appearing red) globular clusters. Red clusters are believed to form as a galaxy forms, while the blue clusters are later brought in as smaller satellites are swallowed by the central galaxy. A mix of both hints to that a galaxy keeps cannibalizing galaxies. Globular clusters may also be found, orphaned in galaxy clusters from their home galaxies through galaxy tidal interactions (in a cluster of about 1,000 galaxies, 22,426 globular star clusters are found, for example)

->A study by the Hubble Space Telescope in October 2006, confirmed that in globular clusters, like thought, stars are sorting out according to their mass. Heavier stars slow down and sink towards the cluster's core, as lighter one get speedier and move to the periphery. The stars density, in the cluster's core, on the other hand, is much heavy as it's sparsed in the periphery of the globular cluster

->Globular cluster might be younger than thought, as most of them would be in their adolescent period instead of middle, a study by NASA's Chandra X-rays Telescope found in April 2008

In such dense environments, binary star systems are as frequent -or maybe higher- as in less compact ones however. Most globular clusters binaries are Sun-like stars orbiting a white dwarf or a neutron star. Such systems are X-ray binaries where matter falling from the normal star onto the smaller, yields X-rays. With so many stars such close, interactions occurs: stars get close enough to form binary stars or to modify existing binary systems. Interactions may take the form of "three-star exchange collisions" (an isolated neutron star may modify an existing system of usual stars; less massive of both stars is ejected and the larger pairs with the neutron star) or of tidal captures (a neutron star grazes a normal star and captures it). In the usual interstellar medium, only one in a billion stars is a member of an X-ray binary system containing a neutron star. In globular clusters, fraction is more like one in a million. The fact that globular clusters display a same overall age, beacuse they -and their stars- formed in a short period of time, does not hide however that astronomers have observed differences. Because bright, high-mass stars burn up their fuel quite quickly, and globular clusters are very old, there should only be low-mass stars still shining within them. That is not the case however as clusters are still young at heart. In some circumstances, indeed, stars can be given a new burst of life, receiving extra fuel that bulks them up and substantially brightens them. This can happen if one star pulls matter off a close neighbour, or if they collide. Such reinvigorated stars are called 'blue stragglers.' Blue stragglers are old stars that really succeed looking younger. Blue stragglers are bluer, more luminous and more massive than they should be. Main theory about that is that such stars were not born as big as we see them today as they received additional material at some point in their life. That might result either from a star merger or a transfer of material between two members of a binary. As heavier stars sink towards the centre of a cluster as the cluster ages, blue stragglers’ high masses mean they are strongly affected by this process. From the whole globular clusters in the Milky Way Galaxy, a few clusters appeared young, with blue straggler stars distributed throughout, while a larger group appears old, with the blue stragglers clumped in the centre. A third group is in the process of aging, with the stars closest to the core migrating inwards first, then stars ever further out progressively sinking towards the centre. In the case of fast-aging clusters, the central shift can be complete within a few hundred million years, while for the slowest it would take several times the current age of the Universe. As a globular cluster’s heaviest stars sink towards the centre, the cluster eventually experiences a phenomenon called core collapse, where the centre of the cluster bunches together extremely densely. The processes leading towards core collapse are quite well understood, and revolve around the number, density and speed of movement of the stars. One important thing for globular clusters is that energy released in the formation of close binary systems might keep central parts of the cluster from collapsing to form a massive black hole (gravitational interactions expel stellar black holes). Some objects in the globular clusters, called 'ultraluminous X-ray source' (ULX) are among the objects which emits the more X-rays in the Universe, albeit less than the galactic black holes, might be, for some of them, black holes with masses between about a hundred and a thousand times that of the Sun, and might torn some stars apart in a cluster. The legions of stars in a globular cluster orbit about a shared center of gravity. Some stars maintain relatively circular orbits, while others loop out into the cluster's fringes. As the stars interact with each other over time, lighter stars tend to pick up speed and migrate out toward the cluster's edges, while the heavier stars slow and congregate in orbits toward the center. That produces the denser, brighter centers characteristic of so-called core-collapsed clusters, with even more stars than average. About a fifth of the more than 150 globular clusters in the Milky Way have undergone a core collapse. Globular star clusters sort out stars according to their mass where lower mass stars rob momentum from more massive stars. The result is that heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to the edge, a process known as 'mass segregation.' In a globular cluster like 47 Tucanae, for example, such a migration will take 40 million years, until concerned stars reach a location appropriate for their mass. The traveling stars encounter few interactions as they migrate outward, because the density of stars decreases as one gets distant from a globular cluster's center. Gravitational forces in a globular cluster sort stars over time, pushing less dense ones to the outside, with a very dense inner core. Because of their large masses and great ages, globular clusters are thought to have produced a large number of stellar-mass black holes, as it was assumed that almost all black holes would disappear from globular clusters after a short time, a stellar mass black hole, at 4 solar masses was found by early 2018 in the NGC 3201 globular cluster

In many clusters, all the stars seem to have formed at the same time, although in others we see distinct populations of stars that are different ages. Age of stars within a globular cluster depends upon the mass of the later. The smaller the mass, a single generation of stars, the larger, several ones, due to gravity as more massive globulars manage to grab more gas and dust, which can then be transformed into new stars. Globular clusters usually contain two or more populations of stars with different amounts of light chemical elements such as carbon, nitrogen and sodium for example. Another point about globular clusters is that their stars are mostly aged and that few heavy elements are found. Stars in a globular cluster are the oldest in the Universe and did not manage to come in successive varieties. Some large, about 3 times the usual size, globular clusters, on the other hand, might have a gravity enough to retain the heavy elements spewn off by the very first stars formed there, and be thus able to trigger successive waves of stars formation during a tremendous 200 million years! Some astronomers think that such globular clusters however might not be some, but dwarf galaxies instead, stripped from most of their components by gravitational interaction with our Milky Way Galaxy. An example of that would be the famed globular cluster Omega Centauri, which some consider in fact like a dwarf galaxy. On a other hand, the majority of clusters which hold few elements heavier than hydrogen and helium, typically a sign of stars that were born early in the Universe’s history, are found along a plane in the Milky Way’s halo suggesting that such clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system. The predominant elements, generally within globular clusters are hydrogen and helium, with only traces of others. Metal-poor clusters are classified Oosterhoff type II (OoII). The composition of globular cluster helps to figure out the age of those. In a globular cluster, when the most massive stars created in the first burst of formation explode as supernovae they blow away the gas needed to form further stars, but the gas reservoir can later be replenished by less massive stars which last longer and shed their gas less violently. After this gas flows to the dense central regions of the star cluster, a second phase of star formation can take place. This cycle can continue a few times, at which time the remaining gas reservoir is thought to be too small to form any new stars