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NGC 6637

A globular cluster in the constellation Sagittarius, near the star Kaus Australis at the southern end of the Archer's bow in the sky. This places it in the general direction of the centre of the Galaxy, placing it in the busiest and most densely clustered part of the Milky Way.

Near M69 in the sky is another globular cluster, only marginally fainter, catalogued as M70. In fact both these clusters were observed by Charles Messier on the same evening: that of 31 August 1780. M69 and M70 are not merely close to one another in the skies of Earth: they actually fall less than 2,000 light years apart, making them near neighbours on a galactic scale. These two clusters lie in the central parts of the Galaxy, probably within the central bulge. A third globular cluster, NGC 6652, was too faint to be observed by Messier, but also appears to lie in the general vicinity of M69 and M70.

M69 falls about 29,700 light years from the Solar System, and is at least 40 light years across (estimates vary depending on the criteria used, but it is rather larger than its neighbour M70). M69 has an age measured at a little more than thirteen billion years, meaning that it formed in the earliest history of the Milky Way Galaxy. This makes its composition unusual for such an old cluster, for it shows greater concentrations of metals than most globular clusters, a feature typically associated with younger stars. The cluster is also notable for the rarity of variable stars within it - only the merest handful of such stars have been found among the estimated 125,000 that make up M69.


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