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NGC 6715

A globular cluster that falls within the band of the Milky Way as it passes through Sagittarius. M54 lies close to the star Ascella or Zeta Sagittarii, one of the five prominent stars that make up the formation known as the Milk Dipper.

Sagittarius holds the core of the Milky Way Galaxy, including a rich seam of star clusters associated with that region, but M54 is not a part of that structure. In fact it lies far beyond the Galaxy's core and actually beyond the disc, and is not directly associated with the Milky Way Galaxy at all. Instead it forms a 'satellite' cluster of a separate minor galaxy, the Sagittarius Dwarf Galaxy, that pursues its own close orbit around the spiral of the Milky Way.

Its position in intergalactic space means that M54 is enormously distant from the Solar System: some 87,000 light years, looking through the entire Milky Way Galaxy and into the space beyond. Despite this vast distance, it was detectable by Charles Messier in the eighteenth century (and is by far the most distant globular cluster in his Catalogue, by a matter of some 20,000 light years). As this implies, M54 is a cluster on a huge scale, with a diameter of about 300 light years. Among the thousands of densely packed stars in its central regions, there appears to be at least one black hole.


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