Brightest globular cluster , located in the constellation Centaurus.
It has a magnitude of 3.7 and is visible to the unaided eye as a faint luminous patch. One of the nearer globular clusters (about 17,000 light-years away), it is estimated to contain hundreds of thousands of stars, including several hundred variable stars . John Herschel (see Herschel family ) was the first to recognize it as a star cluster and not a nebula .