​                                                             NGC5139 - Omega Centauri

 

      Omega Centauri (ω Cen) or NGC 5139, is a globular cluster in the constellation of Centaurus that was discovered by Edmond Halley in 1677. Located at a distance of 15,800 light-years , it is the largest globular cluster in the Milky Way galaxy. Omega Centauri is so distinctive from the other galactic globular clusters that it is thought to have an alternate origin as the core remnant of a disrupted dwarf galaxy.

 

      In 150 A.D., Greco-Roman writer and astronomer Ptolemy catalogued this object in his Almagest as a star on the horse's back, "Quae est in principio scapulae". German lawyer and cartographer Johann Bayer used Ptolemy's data to designate this object "Omega Centauri" with his 1603 publication of Uranometria. Using a telescope from the South Atlantic island of Saint Helena, English astronomer Edmond Halley rediscovered this object in 1677, listing it as a non-stellar object. In 1715, it was published by Halley among his list of six "luminous spots or patches" in the Philosophical Transactions of the Royal Society.

 

      Swiss astronomer Jean-Philippe de Cheseaux included Omega Centauri in his 1746 list of 21 nebulae, as did French astronomer Lacaille in 1755, who gave it the catalogue number L I.5. It was first recognized as a globular cluster by Scottish astronomer James Dunlop in 1826, who described it as a "beautiful globe of stars very gradually and moderately compressed to the centre".

 

      Omega Centauri is one of the few globular clusters visible to the naked eye—and appears almost as large as the full Moon when seen from a dark, rural area. It is the brightest, largest and at 4 million solar masses the most massive known globular cluster associated with our galaxy. Of all the globular clusters in the Local Group of galaxies, only Mayall II in the Andromeda Galaxy is brighter and more massive. Orbiting through the Milky Way, Omega Centauri contains several million Population II stars and is about 12 billion years old.

 

      The stars in the core of Omega Centauri are so crowded that they are estimated to average only 0.1 light years away from each other. The internal dynamics have been analyzed using measurements of the radial velocities of 469 stars. The members of this cluster are orbiting the center of mass with a peak velocity dispersion of 7.9 km s−1. The mass distribution inferred from the kinematics is slightly more extended than, though not strongly inconsistent with, the luminosity distribution.

 

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