Omega Centauri, NGC 5139

Arguably the finest globular cluster in the sky, omega Centauri is a glittering collection of an estimated 5 million stars occupying a region of space about 160 light years across and 16,000 light years away. It has a mass of around 4x10⁶ solar masses and is classified as VIII (rather loosely concentrated towards the centre) on the Shapley-Sawyer globular cluster concentration scale (1927).

Even a cursory examination of the image below reveals that the cluster contains (old) yellow stars and (young) blue stars, an observation which is contrary to the general understanding that stars in globular clusters formed at the same time. That the stars in omega Centauri have a range of ages and metallicities implies that star formation has continued throughout the 12 billion-year age of the cluster and it has evolved in a different way to other such objects. An explanation that satisfies these properties is that omega Centauri is the remnant core of a satellite galaxy that has had its outer regions assimilated by the Milky Way. Measurements of star motions near the core have been interpreted to suggest that an  approximately 1x10⁴ solar mass black hole resides there.