- M 54
M 54 (NGC 6715) is usually described as a globular cluster in the constellation Sagittarius. However, discovery of the Sagittarius Dwarf Elliptical Galaxy (Sag DEG) in 1994 suggests that M 54 may have a different interpretation. Sag DEG is the second-closest companion galaxy to our Milky Way. It orbits the much larger Milky Way galaxy, and previous close encounters (up to 10) have led to its disintegration and loss of stars and gas to the Milky Way. The location of M 54 coincides with the center of Sag DEG, so some astronomers believe that it represents what is left of the core of the dwarf galaxy and is not a globular cluster at all.
M 54 lies at a distance of 85,000 LY. It contains the equivalent of 1.5 million solar masses— among globular clusters, only Omega Centauri is larger. However, M 54 is a small telescopic target due to its great distance, and also because it has a very compact structure without the extended halo of outlying stars usually found in globular clusters.
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