M71 is a very loose but attractive globular cluster located in the small constellation of Sagitta, "the Arrow". At magnitude +7.1, it's not visible to the naked eye but can be seen with binoculars. The cluster is best placed in the sky during the months of June, July and August.
M71 was discovered by wealthy Swiss landowner Philippe Loys de Cheseaux in 1746 and then re-discovered by Dresden based astronomer Johann Koehler in 1775 and Pierre Méchain in June 1780. Méchain informed his friend Messier who searched for the cluster, found it and finally catalogued it on October 4, 1780. He described it as "very faint…containing no stars…the least light extinguishes it".
Of course, the view through modern backyard telescopes is considerably better, but lets first find M71. The finder chart below depicts its position in Sagitta. The globular is very easy to locate as it lies midway between gamma Sge (γ Sge - mag. +3.5) and delta Sge (δ Sge - mag. +3.7). These stars form part of the distinctive, although not particularly bright, "Arrow" asterism. Two degrees southwest of γ Sge is M71.
Through binoculars, M71 appears as a reasonably large fuzzy patch of light. It's a faint object for small telescopes and is best seen with averted vision. For example, through a 80mm (3.1-inch) scope little or no detail is discernible but increase the aperture just a small amount and what a difference! Through a 150mm (6-inch) or 200mm (8-inch) scope the globular appears large and loose with many 11th and 12th magnitude stars resolvable across the cluster surface. Comparisons are often made with M11, the larger and brighter Wild Duck open cluster and it's easy to see why M71 was earlier questioned (and sometimes still is) as an open cluster.
Larger telescopes still reveal many more stars. A 300mm (12-inch) telescope resolves over 100 stars across a seven arc minute diameter disk. It's also apparent that M71 lacks the typically strong central condensation of normal globular clusters.
M71 is of the smallest and youngest globular clusters know and also one of the nearest at only 13,000 light-years distant. It has a spatial diameter of only 27 light-years and is estimated to be 9.5 billion years old. In total, it contains at least 20,000 stars and is a nice globular cluster with a remarkable history, well worth a look.
M71 Data Table
|Object Type||Globular Cluster|
|RA (J2000)||19h 53m 46s|
|DEC (J2000)||18d 46m 42s|
|Apparent Size (arc mins)||7.2 x 7.2|
|Number of Stars||>20,000|
|Other Name||Collinder 409|
|Notable Feature||The irregular variable star Z Sge (Z Sagittae) is a member of this cluster|