Globular cluster

In Astronomy, a globular cluster is a stellar Amas very dense, containing typically a hundred thousands of stars, distributed spherically, in a volume of some Parsec S of ray. Their density is thus definitely higher than that of the open Amas S.

One counts 150 globular clusters in our galaxy. But there undoubtedly exist about it of different, undetectable because masked by the galactic center.

Some of them, like Omega Centauri of our Galaxy, are very massive: several million solar masses.

The globular clusters belong to the galactic halation, they orbit around the galactic center at a distance from 1-100 kpc. It is by their study that Harlow Shapley, in 1918, could determine the position of the Sun within the Galaxy. Indeed, it was thought before the sun was in the middle of the galaxy because the distribution of observable stars appeared uniform. When one took into account the distance from the globular clusters, it appeared that their distribution was strongly asymmetrical and that the observable part of the galactic disc constituted of it only one fraction, the remainder being darkened by gas and the dust of the galactic disc.

The majority of the globular clusters are very old and were probably formed at the same time as their galaxy host. Nevertheless, certain globular clusters of blue color were recently observed and their color is, normally, representative of hot and young stars. One does not know globular clusters yet so can be formed relatively late in the life of a galaxy, but it is probable that their formation is related to catastrophic events, like those accompanying the collision by two galaxies. As the globular clusters contain the oldest stars of a galaxy, they contribute in an important way to the study of the evolution of the stars and the galaxies.

Certain stars of the particular type, like the slow blue ones (Blue straggler S), the Pulsar S-milliseconds or binary X of low mass (Low-farmhouse X-ray binaries), are much more common in the globular clusters.

Because the density of stars in the globular clusters is very high, the collisions or quasi-collisions between stars are sometimes possible there, contrary to the other areas of a galaxy.

History of the observations

The first globular cluster M22 was discovered in 1665 by Abraham Ihle, an astronomer German amateur. Because of the weak opening of the telescopes of this time, the individual stars of the clusters could not be solved. The first to obtain this level of detail was Charles Messier when it observed the cluster M4. The first eight clusters observed appear in the table opposite. Later, Abbé Lacaille listed the clusters NGC 104, NGC 4833, M55, M69 and NGC 6397 in its catalog going back to 1751-52. The M before the number of the cluster refers to the catalog of Charles Messier, while NGC comes from the catalog New General Catalogs established by John Dreyer.

William Herschel began a programme of observation in 1782, using a larger telescope able to separate stars from the 33 known globular clusters at this time. In the passing, He discovered 37 new clusters. The first to use the term Globular Amas was Herschel in its catalog of the remote objects of 1789.

The number of globular clusters discovered increased regularly, reaching 83 into 1915,93 in 1930 and 97 about 1947. On the whole, 151 globular clusters are listed in our galaxy, on an estimated total of 180 ± 20. It is thought that many clusters are hidden behind the cloud of gas and dust of the galactic core.

At the beginning of 1914, Harlow Shapley began a series from studies of the globular clusters, published in forty scientific articles. It observed variable stars céphéides in the clusters, which enabled him to determine their distance (in correspondence with their luminosity).

The majority of the globular clusters of the Milky Way are observed near the galactic core and a majority appears in the part of the celestial sky centered on the core. In 1918 this very asymmetrical distribution was used by Harlow Shapley to determine dimensions of our galaxy as a whole. By adopting the approach that globular clusters followed a more or less spherical distribution around the center of the galaxy, it used their position to calculate the position of the sun in the Milky Way. The distances found being obviously very incorrect, he deduced from it that dimensions of the local galaxy were quite higher than it was thought. This error was due to the fact that galactic dust decreased the quantity of light arriving at the ground and source of the clusters, revealing them more remote. At final, the estimates of Shapley were in the same order of magnitude as those in progress currently.

Measurements of Shapley also stated that the Sun was relatively far from the center of the Galaxy, contrary to the data of its time resting on the regular distribution of ordinary stars in the sky. In fact, the ordinary stars are often darkened stalemate the gas and dust of the galactic disc whereas the globular clusters are apart from this disc and can be seen of much further.

Henrietta Swope and Helen Battles Sawyer took part later in work of Shapley. Between 1927 and 1929, Harlow Shapley and Helen Sawyer started to classify the clusters according to their star density. The densest clusters are thus said of class I, to the least dense clusters of class XII. The system of H. Shapley and H.B. Sawyer exit of this classification is used today.

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