M54

M54 is a bright, dense globular cluster in Sagittarius, about 87,400 light-years from Earth. It is remarkable because it is not actually a Milky Way globular cluster — it belongs to the Sagittarius Dwarf Elliptical Galaxy, a small satellite galaxy that is being tidally disrupted and absorbed by the Milky Way. M54 is believed to be the core of this dwarf galaxy.

Located about 1.5 degrees south-southwest of Zeta Sagittarii (Ascella), the bottom of the Teapot's handle. In binoculars it appears as a small, bright, round patch similar to many other globulars. A 4-inch telescope shows a bright, strongly concentrated core. Due to its extreme distance, resolving individual stars is virtually impossible in amateur telescopes. Best observed from July through September.

Discovered by Charles Messier on July 24, 1778. For over 200 years it was assumed to be a normal Milky Way globular cluster. In 1994, it was identified as belonging to the Sagittarius Dwarf Elliptical Galaxy — the nearest known galaxy to the Milky Way (closer than the Magellanic Clouds).