M55

M55, also known as Messier 55 and NGC 6809, is a globular cluster in the southern constellation Sagittarius. It lies in the Milky Way’s halo and is one of the larger, more diffuse globular clusters visible from the southern hemisphere. Its apparent magnitude is about 7.0, allowing visibility with binoculars under good conditions, and its angular diameter is roughly 32 to 40 arcminutes.

Discovery and designation: M55 was discovered by Nicolas Louis de Lacaille in 1752 during his southern sky survey and was later included in Charles Messier’s catalog as M55.

Distance, size, and composition: The cluster is located about 18,000 light-years from the Sun (approximately 5.5 kiloparsecs) and spans roughly 60 parsecs (about 200 light-years) in diameter. It is metal-poor, with a metallicity around [Fe/H] ≈ -1.9, and is among the older components of the Milky Way, with an estimated age near 12–13 billion years.

Observational and scientific notes: M55 exhibits a modest central concentration with a diffuse halo of stars. It has been studied to understand globular cluster evolution, stellar populations, and dynamical processes such as mass segregation. The cluster is best observed from southern latitudes during the months when Sagittarius is prominent in the night sky.

See also: Messier objects, globular clusters, Sagittarius (constellation).