The globular cluster Omega Centauri, NGC 5139

To the unaided eye this glorious globular cluster has the appearance of a hazy star and was frequently confused with Halley's comet when it drifted through Centaurus in 1986. One of the richest in the Milky Way, Omega Cen contains several million stars, but unlike its southern rival, 47 Tucanae, it has a relatively open structure. Like most galactic globular clusters, the stellar population of Omega Cen identifies it as one of the oldest objects associated with the Milky Way, indeed its age is comparable to that of the Universe itself. The cluster contains a large number of RR Lyrae variable stars which enable its distance to be determined as 17,000 light years.

Entry from NGC 2000.0 (R.W. Sinnott, Ed.) © Sky Publishing Corporation, 1988:

NGC 5139  Gb 13 26.8  -47 29 s  Cen  36.3   3.7  !!!, glob. cl. , omega Cen