Discovered 1752 by Abbe Nicholas Louis de la Caille.

M55 is a quite large globular cluster (about 19', roughly 2/3 of the Moon's apparent diameter) but has such a loose appearence, that the present author had a star cluster impression even in 7x50 binoculars, where most globulars look like round nebulae: This one appeared very grainy. As it is about 17,300 light years distant, this diameter corresponds to a linear of about 100 light years. M55 has only very few known variables, 5 or 6. The published values for M55's magnitude vary from mag 5 to 7. Its total luminosity may be near 100,000 times that of the Sun.

M55 was originally discovered by Abbe Lacaille on June 16, 1752, when he was observing in South Africa, and cataloged by him as Lac I.14. Charles Messier finally found it and cataloged it on July 24, 1778, after having looked in vain as early as 1764: This is a consequence of this object's southern declination.