(catalog number NGC 5139), the brightest globular star cluster. It is located in the southern constellation Centaurus. It has a magnitude of 3.7 and is visible to the unaided eye as a faint luminous patch. Omega Centauri is about 17,000 light-years from Earth and is thus one of the nearer globular clusters. It is estimated to contain hundreds of thousands of stars; several hundred variables have been observed in it. The English astronomer John Herschel in the 1830s was the first to recognize it as a star cluster and not a nebula.