NGC 6540 is a globular cluster of stars in the souther constellation Sagittarius, positioned about 4.66° away from the Galactic Center. It was discovered by German-British astronomer Wilhelm Herschel on May 24, 1784, with an 18.7-inch mirror telescope, who described the cluster as "pretty faint, not large, crookedly extended, easily resolvable". It has an apparent visual magnitude of 9.3 with an angular diameter of about 9.5 arcminutes. The cluster is located at a distance of 12 kly (3.7 kpc) from the Sun, and 14 kly (4.4 kpc) from the Galactic Center. It was originally thought to be an open cluster before being designated a globular. The cluster includes a peculiar X-ray source of uncertain type.
References
External links NGC 6540 Robert Burnham Jr., Burnham's Celestial Handbook: An observer's guide to the universe beyond the solar system, vol 3, p. 1556