Messier 54 — Globular Cluster in Sagittarius

M54 (NGC 6715) is a globular cluster located in the constellation Sagittarius, shining at magnitude 7.6 with an apparent diameter of about 12 arcminutes. What makes M54 extraordinary is that it does not belong to the Milky Way at all — it is the nuclear core of the Sagittarius Dwarf Elliptical Galaxy (SagDEG), a small galaxy currently being absorbed by our own. At a distance of approximately 87,400 light-years, M54 was the first globular cluster discovered to belong to another galaxy. The cluster is intrinsically very luminous, with an absolute magnitude of about -10.0, making it one of the most luminous globular clusters known — comparable in brightness to Omega Centauri. It has a mass of roughly 1.5 million solar masses and a true diameter of about 150 light-years. M54 is extremely dense at its center and may harbor an intermediate-mass black hole of about 9,400 solar masses.

Located about 1.5 degrees south-southwest of the star Zeta Sagittarii (Ascella), at the base of the Sagittarius Teapot asterism. In binoculars, M54 appears as a small, moderately bright fuzzy star. Through a 4-inch telescope it shows a bright, compact core with a small halo — noticeably more concentrated than many other globulars. Resolving individual stars is extremely difficult due to the cluster's great distance; even a 12-inch telescope under excellent conditions only hints at granularity in the outer halo. Its southerly declination (-30 degrees) means observers at mid-northern latitudes should look for it when it transits the meridian during summer evenings. Best observed from July through September.

Discovered by Charles Messier on July 24, 1778, who described it as 'a very faint nebula.' For over two centuries it was assumed to be a normal Milky Way globular cluster. In 1994, astronomers discovered the Sagittarius Dwarf Elliptical Galaxy, and subsequent studies in the mid-1990s revealed that M54 lies at the very center of this dwarf galaxy, making it the first globular cluster conclusively shown to belong to an extragalactic system being cannibalized by the Milky Way.