M71 - Globular Cluster in Sagitta

M71 Luminance

Found in the constellation Sagitta

Discovered 1745-46 by Philippe Loys de Chéseaux.

Messier 71 (M71, NGC 6838) is a loose but beautiful globular cluster in the small but nice constellation Sagitta.

<>For a long time, the classification of M71 as a globular cluster was uncertain, many astronomers thought it was a rather condensed open cluster, similar to M11.  Nevertheless, there's now some consensus that M71 is a loose globular. According to newer sources, it is 13,000 light years distant. Its denser, easily visible core is only about 5 to 6 arc minutes, while in photos it has an angular diameter of about 7', corresponding to a linear extension of only 27 light years, small for a globular cluster. However, faint members have been detected out to a total diameter of 24', making it measure 90 light years. The membership of these faint stars, however, is not yet finally confirmed.

M71 was first seen by De Chéseaux, who cataloged it as his No. 13. It was later rediscovered by Johann Gottfried Koehler, between 1772 and 1779, and by Pierre Méchain on June 28, 1780. Charles Messier cataloged it as his object No. 71, based on his observations of October 4, 1780, as a nebula without stars. It was first resolved into stars by William Herschel in 1783.

This globular cluster is easy to find and nicely observable even in good binoculars, by locating the 6th-mag star 9 Sagittae half-way between Gamma and Delta of that constellation. Medium-sized amateur telescopes are required to resolve this compressed mass of stars, but then even the center is resolved. 

Flesherton, Ontario
Jun 27 2009