Messier 55 (also known asM55,NGC 6809, orSpecter Cluster) is aglobular cluster in the south of theconstellationSagittarius. It was discovered byNicolas Louis de Lacaille in 1752[a] while observing from what today isSouth Africa.[8] Starting in 1754,Charles Messier made several attempts to find this object fromParis but its low declination meant from there it rises daily very little above the horizon, hampering observation.[b] He observed and catalogued it in 1778. The cluster can be seen with 50 mm binoculars; resolving individual stars needs a medium-sized telescope.[8]
It is about 17,600light-years away fromEarth. It contains about 269,000solar masses (M☉).[3] As with other Milky Way globular clusters, it has few elements other than hydrogen and helium compared to the Sun. Messier 55 therefore has "lowmetallicity". This quantity is normally listed as the base 10logarithm of the proportion of the Sun; for NGC 6809 the metallicity is given by: [Fe/H] = −1.94dex, whereby −2 would be 100 times less iron than the Sun.[3] This means the cluster has 1.1% of the proportion of the Sun's iron compared to hydrogen and helium.
Only about 55variable stars have been found in the central part of M55.[9]
^Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters",Harvard College Observatory Bulletin,849 (849):11–14,Bibcode:1927BHarO.849...11S.
^Kaluzny, J.; et al. (September 2010), "The Cluster AgeS Experiment (CASE). Variable Stars in the Globular Cluster M55",Acta Astronomica,60 (3):245–260,arXiv:1011.0831,Bibcode:2010AcA....60..245K
^Specifically in the south of this constellation which makes it visible from everywhere below about the50th parallel north. However the Sun passes through Sagittarius (or technically the Earth orbits so as to make the Sun seem to do so) throughout December. This also makes the cluster mostly risen during day, not night, in the nearest months.
