M54 - NGC 6715

Messier 54 (NGC 6715) is a globular cluster located in the constellation Sagittarius, notable for being one of the first extragalactic globular clusters ever discovered. Although it was cataloged by Charles Messier in 1778, it was not until 1994 that astronomers realized M54 does not actually belong to the Milky Way but is instead associated with the Sagittarius Dwarf Elliptical Galaxy (SagDEG), a small satellite galaxy that is currently being tidally disrupted and absorbed by our own galaxy. M54 lies approximately 87,000 light-years from Earth and serves as the core cluster of the Sagittarius dwarf galaxy, making it significantly more distant than most Messier globular clusters. The cluster is intrinsically very luminous, with an absolute magnitude of about -10, making it one of the most luminous globular clusters known. It has a dense, concentrated core and contains several hundred thousand stars packed into a region about 150 light-years across. Due to its great distance, M54 is a challenging object for amateur astronomers to resolve into individual stars. In small telescopes it appears as a bright, compact fuzzy ball that stubbornly resists resolution even at high magnification. Telescopes of 10 inches or more may begin to show a granular texture at the edges under excellent seeing conditions. M54 is located just 1.5 degrees south of the star Ascella (Zeta Sagittarii), making it easy to find but placing it low in the sky for northern hemisphere observers, where atmospheric turbulence can further hamper resolution.

M54 spans about 150 light-years in diameter, contains several hundred thousand stars, and lies approximately 87,000 light-years from Earth.