M71 (NGC 6838)

1. The history
2. To find M71
3. To observe M71
4. The stars of M71

M71 was probably by de Chéseaux in 1746. De Chéseaux' Catalogue of Truly Nebulous Stars, n° 13: "below and very close to Sagitta"; Koehler rediscovered it independently between 1772 and 1779.

Koehler described it as "...A very pale nebulous patch in Sagitta, long. 310° 50': lat. 39° N"; Méchain rediscovered it June 28, 1780, and Messier listed it Oct. 4th, 1780.

John Herschel, in his instruments, found M71 to be "Very large...a rich cluster, with stars mag. 11-16".

M71 lies about midway between d and g Sagittae, a little south of the line joining them.

It is a rich and compact cluster of faint stars, a large compressed mass of stars of quite triangular shape.

M71 is one of those unusual clusters whose precise classification is somewhat in doubt. It has been regarded at different times as either a very compact galactic cluster (similar to M11) or a very loose globular cluster (similar to M55). For a long time many astronomers thought it was a rather condensed open cluster: Both Shapley and Trumpler classified it as galactic cluster; Shapley listed it as type "g", his densest open cluster class .

The photos show an angular diameter of about 7', corresponding to a linear extension of only 25 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. .

In 1943, Dr. James Cuffey, who has made detailed studies of both kinds of cluster, found that M71 was more resembling a loose globular like M68 (class X) or NGC 5053 (class XI); in 1959 he found an H-R diagram very different from a typical galactic cluster; also other criteria, as the radial velocity, and the abundance of heavier elements (metalicity) is of little help in this case: The radial velocity is poorly known, the sources differ from 80 km/sec in approach (Jones) to 80 km/sec in recession, the most modern value being perhaps 23 km/sec in approach; this value is not very high and thus consistent with both types, and the metalicity is one of the highest for globulars, only M69 has higher abundances. It seems that the high metallicity is connected with a low number of RR Lyrae stars: None could be found in M71 (nor in M69), which has only 8 known variables at all.

Nevertheless, there's now some consensus that M71 is a loose globular.