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Globular Clusters

IN SHORT - Globular clusters are a number of stars gathered by gravity into spherical-shaped, a hundred light-year wide clumps. Most galaxy clusters come with galaxies, in their halo. Stars in the globular clusters are much old, something like 10-13 billion years of age, with the heavier stars tending to the center of the cluster, increasing the density there.

X-ray binary systems, one of the stars is a neutron star, are more frequent in the globular cluster than in the other environments in the Universe, with frequent gravitational interactions leading to a creation, or a modification of binary pairs. It might too that those interactions prevent the collapse of the central part of the clusters, and the formation of a black hole

Globular clusters are associations of several hundred thousand or millions, of stars, in spherical-shaped clumps. 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. There are about 150-200 globular clusters around our Milky Way. 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

A study by Hubble 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. 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)

Another point about globular clusters is that their stars are mostly aged and that few heavy elements are found. 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