Messier 55
Template:Short description Template:Use mdy dates Template:Infobox globular cluster Messier 55 (also known as M55, NGC 6809, or Specter Cluster) is a globular cluster in the south of the constellation Sagittarius. It was discovered by Nicolas Louis de Lacaille in 1752Template:Efn while observing from what today is South Africa.<ref name=thompson2007/> Starting in 1754, Charles Messier made several attempts to find this object from Paris but its low declination meant from there it rises daily very little above the horizon, hampering observation.Template:Efn He observed and catalogued it in 1778. The cluster can be seen with 50 mm binoculars; resolving individual stars needs a medium-sized telescope.<ref name=thompson2007/>
It is about 17,600 light-years away from Earth. It contains about 269,000 solar masses (Template:Solar mass).<ref name=apj742_1_51/> As with other Milky Way globular clusters, it has few elements other than hydrogen and helium compared to the Sun. Messier 55 therefore has "low metallicity". This quantity is normally listed as the base 10 logarithm of the proportion of the Sun; for NGC 6809 the metallicity is given by: [Fe/H] = −1.94 dex, whereby −2 would be 100 times less iron than the Sun.<ref name=apj742_1_51/> This means the cluster has 1.1% of the proportion of the Sun's iron compared to hydrogen and helium.
Only about 55 variable stars have been found in the central part of M55.<ref name=aa60_3_245/>
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External linksEdit
- Messier 55, SEDS Messier pages
- Messier 55, Galactic Globular Clusters Database page
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