The great Roman (Greco-Egyptian) mathematician and polymath Claudius Ptolemy is credited with listing 48 - more than half - of the constellations that adorn the modern western star lore. An exceptional visual observer, he carefully catalogued the stars visible to him in Egypt in the second century of the Common Era. In 150 CE, he produced the Almagest ('Great Work') which included several treatises on the nature of the cosmos, and it remained an influential astronomical work for over a thousand years.
But Ptolemy and others of his era did not have access to telescopes or any other optical aid for observing the stars, so it's no surprise that when looking at the constellation Centaurus, he failed to notice that one of its members - Omega - was more impressive than it seemed. He catalogued it as a star, and its status would not be challenged for about 1,500 years. In 1677, English astronomer Edmund Halley was charting the southern stars from the island of Saint Helena in the South Atlantic. There he determined for the first time that Omega Centauri was distinctly 'non-stellar', appearing as a fuzzy nebulous patch in his telescope.
It would take over 50 years for its true nature to be understood. In the 1830s, English astronomer John Herschel (son of William Herschel) got a clear look at Omega Centauri, with sufficient resolution to resolve an apparently spherical cloud of stars - a cluster of enormous size not confined to the Milky Way, but orbiting around it. He classified it as a 'globular cluster'.
The name is very appropriate. These objects are, more or less, globe-shaped star clusters that typically contain tens or hundreds of thousands of stars. Larger than the 'open' clusters we find inside the Galaxy's spiral arms, they are so populated as to be substantially more dense on average. Most globular clusters contain a well-defined core region that is brighter, surrounded by a fainter halo - noticeably sparse by comparison. In this regard, Omega Centauri is typical, but in terms of scale it is extraordinary.
Precise observations of Omega Centauri have revealed that it lies about 17,000 light-years away and has a diameter of approximately 180 light-years. Other clusters exist at a similar size, but no other comes close to the mass or sheer population of Omega Centauri. At four million solar masses, it is estimated to contain about ten million individual stars! Nothing else even comes close.
The Hubble Space Telescope has produced an astonishing, colourful portrait of Omega Centauri's core, which shows stars of several different populations, crowded together at an estimated average distance of just 0.1 light-years from one another. If were to move to a planet orbiting one of these stars, we would not only see hundreds of thousands in our night sky, but likely several thousand in our daylight sky as well.
Data from Hubble and the Gemini Observatory suggest that a black hole is lurking at the heart of Omega Centauri, causing the stars around it to orbit measurably faster than those farther out. This finding, combined with the discovery of stars of varying ages, has led astronomers to speculate that the cluster could actually be a disrupted dwarf galaxy.
Globular clusters usually comprise a single population of stars of the same age, with a few stragglers that wander in and become gravitationally bound. Omega Centauri's oldest stars are about 11.5 billion years old, but the cluster is also home to many younger stars too. This suggests that active star formation once took place there, until it was starved of gas by its host galaxy - the Milky Way.
