M 71

Globular Cluster M71 (NGC 6838), class X-XI,

M11, e.g. Shapley, who classified it as type “g”, his

densest open cluster class, and Trumpler. James Cuffey of Kirkwood Observatory,

University of Indiana, has investigated both cluster types. In 1943 he found

that M71 was more resembling a loose globular like M68

(class X) or NGC 5053 (class XI), but in 1959 he took a Color-Magnitude Diagram

which he found more similar to that of an open galactic cluster. Even Burnham

is still uncertain of M71’s type ! 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 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.

According to newer sources (e.g., W.E. Harris’

globular cluster data file), it is 11,700 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 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.

The membership of these faint stars, however, is not yet finally confirmed,

according to Kenneth Glyn Jones.