Of the many striking stellar clusters in the great galactic clouds extending through Ophiuchus, and Scorpio, none has received as much attention as Messier II, the very rich open aggregation of stars in the small constellation of Scutum Sobieski. Discovered by Kirch two hundred and thirty-five years ago, it has since been observed and described by nearly all students of clusters, nebulae, and comets….
This is the beginning of a paper by Harlow Shapley published in 1917 on The Astrophysical Journal. He made a catalogue of 458 stars within four minutes of arc of the center of the “open” galactic cluster Messier II, claiming a resulting color-index that does not exceed a tenth of magnitude.
One very interesting thing, he states in the conclusions that “it seems probable that the cluster proper is composed of not more than 200 stars” (!!). Today we know that the stars are much more numerous, since modern estimate talk of about 100,000 stars (but we cannot blame Shapley because of the instrumentation available at his epoch).
Messier 22 imaged by Hubble Space Telescope (Credit: NASA/STScI/WikiSky)
Messier 2 was discovered by Maraldi on September 11, 1746. Rediscovered by Messier on September 11, 1760, it has an interesting observation history.
Our cluster’s entry for M2 is http://gclusters.altervista.org/cluster_4.php?ggc=NGC+7089
Globular Clusters Blog 