Degree of difficulty 3 (of 5)

Minimum aperture 30mm

Designation NGC 6809

Type Globular cluster

Class XI

Distance 19,300 ly (RR Lyr, 2001) 19,660 ly (RR Lyr, 1999)

Size 110 ly

Constellation Sagittarius

R.A. 19h 40.0min

Decl. –30° 57′

Magnitude 6.3

Surface brightness –

Apparent diameter 19′

Discoverer Lacaille, 1752

History M 55 was discovered already between 1751 and 1752 by Nicholas-Louis de Lacaille, when he carried out his southern star survey from South Africa. He registered this globular cluster as his 14th object in the class “nebula without stars” and likened it to an obscure core of a large comet.

Messier was hoping to repeat Lacaille's observation on the 29th of July 1764 for his first catalog, but he could not find this very southern object. However, almost exactly 14 years later, on the 24th of July 1778, he was finally successful, right after discovering M 54 by accident the same night. He noted on M 55: “Nebula, which is a whitish patch of about 6' in extent, its light is even and appears not to contain any star.”

The debated Australian observer James Dunlop also failed to resolve this globular cluster into individual stars. He described M 55 as a “beautiful large round bright nebula, about 2' diameter, slight condensation to the center.” Only William and John Herschel resolved cluster stars and saw a better part, 10', of the pretty large angular diameter of M 55.

Curtis gave this description, according to photographic images: “A fine globular cluster 10' in diameter. The background of faint stars is less dense near the center than in most clusters of this type.”

Astrophysics With its generous angular diameter of 19', M 55 is among the (apparently) largest globular clusters in our sky. However, with its distance of just over 19,000 light-years, it has a fairly average physical diameter of 110 light-years. Its moderate mass has been estimated as equivalent to that of 250,000 Suns, and its luminosity to that of 100,000 solar luminosities.