M55
NGC 6809
Properties
Position & Identifiers
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M55 · 12.0′ diameter · N up, E left
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About M55
Description
M55 is a large, loose globular cluster in Sagittarius, about 17,600 light-years from Earth. It spans about 100 light-years and contains roughly 100,000 stars. Its low concentration class (XI) makes it one of the most diffuse Messier globulars, with stars easily resolved even in modest apertures.
Observing Tips
Located in a relatively isolated part of Sagittarius, about 7.5 degrees east of Zeta Sagittarii. This southerly position (declination -31 degrees) makes it challenging from northern latitudes, sitting low on the horizon where atmospheric extinction dims it. From mid-northern sites, binoculars show a large, faint, round glow. A 6-inch telescope resolves individual stars across the cluster due to its loose structure. Best observed from July through September from as far south as possible.
History
Discovered by Nicolas Louis de Lacaille in 1752 from South Africa. Charles Messier tried to observe it from Paris in 1764 but failed due to its low altitude. He finally cataloged it on July 24, 1778.
Fun Facts
M55 is one of the few Messier globulars that Messier himself struggled to observe. From his Paris location, it barely rose above the horizon. For Southern Hemisphere observers, however, M55 is a spectacular object — large, well-resolved, and easily visible in binoculars. It contains 55 known variable stars.