M55 (NGC 6809)Globular cluster
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| Constellation | Sagittarius |  |
| Right Ascension | 19h 36.9m | |
| Declination | -31° 03' | |
| Distance | 20000 a.l. | |
| Visual Mag. | 7.6 | |
| Ų (') | 15 | |
| O.R.S.A.'s Newton reflector
412 mm. f/4.3 15 min. on Fuji Super G 800 Plus Osservatorio di Pizzo Słaro Authors: Giorgio Puglia e Fredi De Maria |
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M55 was discovered by Lacaille in 1752 while observing at Cape of Good Hope. He described it as "...the shadowy nucleus of a large comet".
Messier observed it on July 24th, 1778, and described it as "...a nebula which is a whitish spot...not appear to contain a star..."..."; John Herschel, instead, found it "a globular cluster, pretty bright, large, round, very rich, very gradually brighter in the middle; all clearly resolved into stars, 11-13 mag. 6' in diameter".
M55 is located in a region of Sagittarius quite poor in bright stars, far from other "deep sky" objects. The best approach is from z Sagittarii: M55 lies about 7° south and 1° east from it.
It is a large but loose-structured globular. Shapley in 1930 did not include M55 in his catalogue of globular clusters; later, he included it in his class XI: it is one of the most open of globular clusters; of course, it is easily resolvable into stars. Good binoculars show it as a hazy star of about 7th magnitude.
Up to 1968 six RR Lyraetype short-period variables have been found in M55.
Its angular diameter of about 15' corresponds to 81 light-years (Becvar). Its age has been estimated as 14 billion years.
M55 is believed to be among the nearer globular clusters, about 20,000 light years.
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