Messier 54 (also known as NGC 6715) is a globular cluster in the constellation Sagittarius. It was discovered by Charles Messier in July 1778, and subsequently included in his catalog of comet-like objects.
M 54 is easily found, close to the star ζ Sg, and has a visual magnitude of 7.6. However, M 54 is not resolvable into individual stars, even in larger telescopes, showing only a granulation around the edges of its 12'-diameter halo. The brightest cluster stars have an apparent magnitude around 15.5. The 2.1'-diameter core is bright but smooth.
Previously thought to be about 50,000 light-years away, it was discovered in 1994 that M 54 is most likely not part of the Milky Way. Instead, it belongs to the Sagittarius Dwarf Elliptical Galaxy, making it the first extragalactic globular cluster ever discovered. It is receding from us at 142 km/sec, about the same as the SagDEG (130 km/sec).
Modern estimates place M 54 at a distance of 87,000 light-years, giving it a true diameter of 300 light-years. It is about three times as distant as its two apparently close neighbors, M 69 and M 70, and has an absolute magnitude of -10.0. It shines with a luminosity of roughly 850,000 Suns, much above the average globular, and is outshined only by Omega Centauri in our Milky Way.
M 54 contains at least 82 known variable stars, the majority being of the RR Lyrae type; there are also two semi-regular red giant variables with periods of 77 and 101 days. In July 2009, a team of astronomers reported evidence of an intermediate-mass black hole at the core of M 54.