Messier 55, NGC 6809

Messier 55 (also known as NGC 6809) is a globular cluster in the constellation Sagittarius. It was originally discovered by Nicholas Louis de Lacaille in 1752, observing from South Africa. After having looked for it in vain as early as 1764, Charles Messier finally found and cataloged it in 1778.

Messier 55 is a quite large globular cluster (about 19' across, roughly 2/3 of the Moon's apparent diameter), but difficult to observe due to its southern declination. It has a loose, very grainy appearance, even in 7x50 binoculars, where most globulars look like round nebulae. Published values for M 55's apparent magnitude vary from about 5 to 7.

As it is only 17,300 light years distant, M 55 is one of the nearest globular clusters to our solar system. It has a linear diameter of about 100 light years, and a total luminosity near 100,000 Suns. M 55 contains only five or six known variable stars. It is receding from us at about 105 miles per second.