Omega Centauri may be the leftover heart of a swallowed galaxy
For 1,500 years Omega Centauri was listed as a single star on the Centaur's back. It is actually a ball of roughly 10 million stars, the biggest cluster of its kind in our galaxy, and it may be all that remains of a small galaxy the Milky Way tore apart and absorbed.
Ptolemy catalogued it around 150 AD as a star on the Centaur's shoulder, and in 1603 Johann Bayer gave it the star-style label it still carries, omega. It took a telescope to reveal the truth. Edmond Halley, observing from the South Atlantic island of Saint Helena in 1677, noted it was not a star at all; 18th-century astronomers filed it with the nebulae; and in 1826 James Dunlop finally recognised a globular cluster, describing a lovely globe of stars growing gradually denser toward the middle.
It is a giant. About 17,000 light-years away and some 150 light-years across, it holds around 4 million times the Sun's mass, making it the brightest and heaviest globular cluster tied to the Milky Way. From a dark rural site it is visible to the naked eye and looks nearly as big as a full Moon. At its core the stars are packed so tightly that neighbours sit only about a tenth of a light-year apart on average.
Most globular clusters are made of stars born together, of similar age and chemistry. Omega Centauri contains a spread of ages and metal content, which is one reason astronomers think it began as the dense centre of a dwarf galaxy that our galaxy stripped away. Its motion fits that story, and Kapteyn's Star, now just 13 light-years from us, is thought to have come from it.
Its core may hide a medium-sized black hole, heavier than those left by single stars but far lighter than galactic giants. A 2008 study using Hubble and the Gemini Observatory in Chile found signs of one; later work moved the estimated centre and found no such signal. In 2024 researchers reported seven fast-moving central stars implying a black hole of at least 8,200 solar masses, a claim that has itself been questioned.
Source: Omega Centauri
