As globular clusters go, Omega Centauri is the king.
At an estimated 5 million solar masses with a diameter of around 150 light years, it’s by far and away the largest of the 150 or so globular clusters known in the Milky Way. So large in fact, that it’s the only one to be visible to the naked eye — originally catalogued by Ptolemy around 2000 years ago as a star. A dense clump of gravitationally bound stars, only an average of 0.1 light year apart in it’s centre.
Image: GRAS-013 – 10m exposure – 23/7/08
The thing is, Omega Cen is quite unusual as globular clusters go. With an intermediate mass black hole at it’s heart, it contains stars of different ages. Abnormal, because most globular clusters are all around the same age. Like open clusters, globulars normally form all at once. In fact it’s so unusual that many believe that Omega Cen used to be the core of a small galaxy that was greedily devoured by the Milky Way.
It’s certainly true that The Milky Way has a steady diet of small galaxies (it’s eating a couple even as you read this). Plus, Omega Cen is old. 12 billion years old. That’s at least 2 billion years older than the Milky Way’s disk, if that puts things into some semblance of perspective. Perhaps Omega Cen was eaten before its black hole could grow into a behemoth like Sagittarius A*…