Globular Cluster M9
M9 has an apparent magnitude of 7.9, an angular size of 9.3′, and can be viewed with a small telescope. It is one of the nearer globular clusters to the center of the galaxy as is around 5,500 light-years from the Galactic Center. Its distance from Earth is 25,800 light-years.
The total luminosity of this cluster is around 120,000 times that of the Sun, the absolute magnitude being -8.04. The brightest individual stars in M9 are of apparent magnitude 13.5, making them visible in moderately sized telescopes. There have been 24 variable stars found in M9: 21 RR Lyrae variables, plus a long-period variable, Type II Cepheid, and an eclipsing binary. No blue stragglers or SX Phoenicis variables have been discovered. Based upon the periods of the RR Lyr variables, this cluster is classified as an Oosterhoff type II globular, which precludes an extra-galactic origin.
At about 80' (1+1⁄3 degrees) to the northeast of M9 is the dimmer globular cluster NGC 6356, while about the same to the southeast is the globular NGC 6342.
Gallery
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Messier 9, from 2MASS
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Map showing the location of Messier 9