Messier 54 (also known as M54 or NGC 6715) is a globular cluster in the constellation Sagittarius. It was discovered by Charles Messier in 1778 and subsequently included in his catalog of comet-like objects.
Previously thought to be at a distance from Earth of about 50,000 light-years, it was discovered in 1994 that M54 was most likely not part of the Milky Way, but actually part of the Sagittarius Dwarf Elliptical Galaxy, (SagDEG), making it the first extragalactic globular cluster ever discovered, even if it wasn't recognized as such for nearly two and a quarter centuries.
Modern estimates now place M54 at a distance of some 87,000 light-years, translating into a true radius of 150 light-years across. It is one of the denser of the globulars, being of class III (I being densest and XII being the least dense). It shines with the luminosity of roughly 850,000 times that of the Sun and has an absolute magnitude of -10.0.