M54, THE FIRST INTERGALACTIC CLUSTER DISCOVERED

M54 was first discovered in 1778 by Charles Messier, who included it in his famous deep sky catalogue. For more than two centuries, M54 has been considered a globular cluster of the Milky Way, situated 50,000 light-years from Earth.

In 1994, however, a small satellite galaxy of the Milky Way, the Sagittarius Dwarf Elliptical Galaxy, was discovered on the same line of sight as the cluster. Further observations revealed that M54 is actually located 90,000 light-years from Earth, within the dwarf galaxy. Despite having been known for hundreds of years, M54 did not become the first intergalactic globular cluster until 1994.

M54 is located at the center of the dwarf galaxy and so some authors believe that it is not a cluster, but its core. The most likely and universally accepted option, however, is that M54 was a cluster located initially at the periphery of the dwarf galaxy and then transported to its center due to decay of its orbit due to interaction with the interstellar medium.

M54 in the future is destined to become a cluster of the Milky Way. The Sagittarius dwarf galaxy is actually moving slowly toward the Milky Way and will eventually become part of it in a few million to billion years.

Credit: NASA, ESA, Hubble.