Messier 71: Globular Cluster in Sagitta

Messier 71 sits in the shaft of Sagitta, the little arrow that flies through the summer Milky Way. It is a star cluster that puzzled astronomers for decades: too loose to look like a typical globular, yet too old and rich to be an ordinary open cluster.

What it is

M71 is a globular cluster about 13,000 light-years away, one of the nearer globulars to us. Its bright part covers about 7 arcminutes, which at that distance is only about 27 light-years across, small for a globular cluster. SEDS notes that faint members may extend over a much wider area.

For much of the 20th century many astronomers, Harlow Shapley among them, treated M71 as a dense open cluster similar to M11. Its lack of a dense core and its unusually high content of heavy elements muddied the picture. The consensus today is that it is a loose globular cluster, in the same family as M68.

When to see it

M71 is a summer and early autumn object. It is highest in the south around midnight in late July and around 10 p.m. in late August. From 40°N it rises to about 69°, well clear of haze. It is visible from much of the Southern Hemisphere as well, low in the northern sky during the southern winter.

How to find it

Find Sagitta about halfway between Altair in Aquila and Albireo in Cygnus. Its four main stars form a small arrow pointing east. Delta Sagittae, in the shaft, and Gamma Sagittae, at the arrowhead, are about 3° apart. M71 lies nearly halfway between them, about 1½° east of Delta and slightly south of the line joining the two stars. The 6th-magnitude star 9 Sagittae, also between Delta and Gamma, helps you land in the right spot.

What you will see

In binoculars M71 is a small, round haze set among the rich Milky Way star fields. A 70–100 mm telescope shows a soft glow about 5 arcminutes wide with a few faint stars sparkling across it and no bright center. In a 200 mm telescope the cluster resolves well, even in the middle, into a grainy patch of faint stars that looks like a very rich open cluster. Its western side is brighter and more sharply edged, and some observers see the whole as a curving V. It holds up reasonably well in suburban skies.

History

Philippe Loys de Chéseaux first recorded M71 in 1745–46. Johann Gottfried Koehler found it again in the 1770s, and Pierre Méchain spotted it on June 28, 1780. Charles Messier observed it on October 4, 1780, describing a nebula without stars, and added it to his catalog. William Herschel was the first to resolve it into stars, in 1783.