- NGC 5139 Omega Centauri
NGC 5139 is the brightest, largest, and most massive globular cluster associated with our Milky Way Galaxy. With a visual magnitude of 3.9, this cluster was bright enough to be classified as a "star" by early astronomers. In 150 AD, Ptolemy catalogued it as a star in his publication the Almagest. The German Johann Bayer was the one who designated it as the star ω in the constellation Centaurus in his 1603 publication of Uranometria. The Englishman Edmund Halley, making telescopic observations from the southern island of St. Helena in 1677, was the first to note that this object was not a single star but a "luminous patch". While both De Cheseaux and Lacaille included it in thieir lists of "nebulous objects", it was the Scottish astronomer James Dunlop in 1826 who first recognized this object as a cluster of stars.
NGC 5139 is located at a distance of 15,800 LY from us. It has a physical size of around 150 LY and contains around 10 million stars. It is so large that astronomers speculate that it is actually the remnant core of a dwarf galaxy that was absorbed by our Milky Way Galaxy some time in the past. The chemistry of its stars and its motion within our galaxy support this possibility. There is also some evidence that NGC 5139 contains a black hole near its center.
While Omega Centauri is a spectacular object, it is difficult to observe for astronomers in the Northern Hemisphere. With a Declination of -47.5°, it always rides close to the southern horizon for observers in North America and Europe (if it's visible at all).
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