The globular cluster that may be the core of the Sagittarius Dwarf Spheroidal Galaxy is Messier 54.
Messier 54, also designated M54 or NGC 6715, is a globular cluster in Sagittarius. It was discovered by Charles Messier in 1778 and was long assumed to be part of the Milky Way’s own globular-cluster system. After the Sagittarius Dwarf Spheroidal Galaxy was identified in 1994, astronomers recognized that M54 lies at its center and is associated with that satellite galaxy.
M54 is therefore an unusual object: it is a globular cluster apparently embedded in, or functioning as the nucleus of, another galaxy. Some studies interpret it as the dwarf galaxy’s genuine nuclear star cluster. Other research suggests that M54 formed independently and spiraled inward through dynamical friction, eventually settling near the galaxy’s center. That distinction explains the cautious wording “may be the core.”
M54 is also notable as the first globular cluster recognized as lying outside the Milky Way. Its extragalactic status can be confused with ordinary Milky Way globulars such as Messier 13, Messier 15, or Messier 92.