Omega Centauri - NGC 5139

Omega Centauri (NGC 5139, or Caldwell 80) is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. At a distance of about 17,000 light-years (5,200 parsecs) from Earth, Omega Centauri is one of the few globular clusters visible to the naked eye - it appears almost as large as the full Moon when seen from a dark location away from light pollution. It is the brightest, largest and, at 4 million solar masses, the most massive-known globular cluster within the Milky Way.

Although Omega Centauri contains several million Population II stars (low metallicity stars containing relatively little of the elements heavier than helium), overall, the cluster contains stars with a range of metallicities. On this basis, it is likely that the stars present within the cluster did not all form at the same time and so have a range of ages. This, in turn, has led to speculation that Omega Centauri represents the core of a dwarf galaxy that was disrupted and absorbed by the Milky Way.

The stars in the core of Omega Centauri are so closely packed that they are estimated to average only 0.1 light-year away from each other. Observations of the internal dynamics of and relative motions of the stars made by the Hubble Space Telescope and Gemini Observatory on Cerro Pachón in Chile have been used to propose the presence of an intermediate-mass black hole at the center of the cluster. These studies showed that stars are bunching up near the center of Omega Centauri, as evidenced by the gradual increase in starlight near the centre. Using instruments at the Gemini Observatory to measure the speed of stars swirling in the cluster's core, it was found that stars closer to the core are moving faster than those farther away. This measurement was interpreted to mean that unseen matter at the core is interacting gravitationally with nearby stars. By comparing these results with standard models, it has been concluded that the most likely cause is the gravitational pull of a dense, massive object such as a black hole. [Wikipedia]