Degree of difficulty 4 (of 5)

Minimum aperture 50mm

Designation NGC 6637

Type Globular cluster

Class V

Distance 36,920 ly (R2005)

Size 110 ly

Constellation Sagittarius

R.A. 18h 31.1min

Decl. –32° 21′

Magnitude 7.7

Surface brightness –

Apparent diameter 10′

Discoverer Lacaille, 1752

History Nicholas Louis de Lacaille carried out positional measurements of southern stars from the Cape of Good Hope in 1751 and 1752, and during that time he discovered 42 star clusters and nebulae, including M 69. Entry I.11 in his catalog published in 1755 reads: “nebula without star, resembles the small nucleus of a comet.” Charles Messier looked for this object without success in 1764, since the position given by Lacaille was wrong by 1.2°. 16 years later, on the 31st of August 1780, Messier finally succeeded in rediscovering this southern globular, which became his No. 69. He recognized that this was Lacaille's object and commented: “Nebula without stars, near it is a star of the 9th magnitude. Its light is very faint.”

John Herschel saw M 69 as a star cluster and characterized it as “bright, large, round, well resolved, stars from 14th to 16th magnitude.” Curtis' photographic description reads: “Bright globular cluster 3' in diameter.”

Astrophysics The distance to M 69, which is 37,000 light-years, gives this cluster a location 5000 light-years behind the galactic center, but also 5000 light-years below the galactic plane. Hence, M 69 belongs to the inner galactic bulge, which it never leaves. Its age is about 14 thousand million years, and the largest angular diameter in deep, modern photographs of 10' corresponds to a total physical size of 110 lightyears. Its mass is an estimated 300,000 solar masses. For a globular cluster, it has quite a generous abundance of heavy elements, related to its origin near the galactic center.

The individual stars, the brightest reach magnitude 13.2, are grouped around a relatively compact center. 61 variables have been found in M 69, among them 8 RR Lyrae stars, 48 variables of the SX Phoenicis type, and 10 eclipsing binaries. They include the two brightest cluster members and two Mira stars: V1894 Sgr (magnitude 13.3–18.8) 1.5' southeast of the center, and V3480 Sagittarii (magnitude 15.5–18.2) 5' west.