M71 is a very loosely concentrated globular cluster in the constellation of Sagitta.
The cluster is unusual for a globular as it is younger than most, having been formed only 9-10 billion years ago. The relatively young age means that it's 20,000+ stars have a higher proportion of elements heavier than hydrogen and helium, than in a typical globular cluster. But even so, it only has about 17% of the Suns proportion of heavy elements, although it is 13,200 times more luminous.
For a long time after M71 was discovered in 1746, the cluster was considered to be a dense open cluster. It wasn't until the 1970's when astronomers examined the relationship between the brightness and temperatures of the stars, that a characteristic deviation was discovered from the Hertzsprung–Russell main sequence. This short horizontal branch, created by stars like the Sun after they have passed through their Red Giant phase, was proof that the cluster was very old & therefore globular in nature. Moreover, the lack of RR Lyrae variable stars in the cluster, demonstrated that it was younger than most globular clusters.
M71 is situated about 13,000 light-years from Earth and spans a distance of about 27 light-years.
For more info. see the Wikipedia entry.