Messier 71 (also known NGC 6838) is a loose globular cluster in the constellation Sagitta.
M 71 was first seen by Philippe Loys de Cheseaux in 1746. It was also noted by Johann Gottfried Koehler at Dresden around 1775, and by Pierre Mechain in June, 1780. Charles Messier included it in his catalog of comet-like objects in 1780; it was first resolved into stars by William Herschel in 1783.
Amateur Observation
This globular cluster is easy to find, halfway between the stars γ and δ Sge. At magnitude 8.2, it is nicely observable, even in good binoculars. Medium-sized amateur telescopes are required to resolve this compressed 7.2'-wide ball of stars, but then they resolve it all the way to the center. The cluster is brighter and sharply terminated on the western side, forming a "curving V".
The large, but faint and sparse, open cluster Harvard 20 is situated nearby, about 1/2° to the SSW. This cluster is fairly difficult to recognize. It is of magnitude 9.6, and contains about 30 stars scattered over a field 7.8' in diameter.
Physical Properties
M 71 lies at a distance of about 13,000 light years, making it one of the nearest globular clusters to our solar system, and spans some 27 light years across. The irregular variable star Z Sagittae (mag 13.5 - 14.9) is a member of this cluster, and one of its six known M-class red giants.
For a long time, M 71 was thought to be a densely packed open cluster, similar to M 11, rather than a globular. Its color-magnitude diagram is similar to that of a galactic open cluster. Its radial velocity is poorly known, and the modern value of 23 km/sec in approach is consistent with both types. Its metallicity is one of the highest known for globulars; only M 69 has a higher abundance of elements heavier than hydrogen. M 71 contains only eight variable stars, and none at all of the RR Lyrae type commonly found in other globulars.
Nevertheless, there's now some consensus that M 71 is a loose globular. Modern photometry has detected a short "horizontal branch" in the H-R diagram of M 71, which is characteristic of a globular cluster. The shortness of the branch explains the lack of the RR Lyrae variables, and is due to the globular's relatively young age of 9-10 billion years. The relative youth of this globular also explains the abundance of "metals" in its stars. Hence today, M 71 is designated as a very loosely concentrated globular cluster, much like M 68 in Hydra.