NGC 6940 is anopen cluster in the constellationVulpecula. It was discovered byWilliam Herschel in 1784. The cluster is nearly a billion years old and it is located 2,500 light years away. It is considered the finest open cluster in the constellation.[3]
It is located a bit more than two degrees south-southeast of41 Cygni and three and a half degrees southwest from52 Cygni. The cluster is bright enough to be seen even with smallbinoculars, which can partially resolve it.[3] NGC 6940 is included in theHerschel 400 Catalogue.
NGC 6940 has hundreds of members. The cluster is quite scattered and in between its members are also visible field stars.For example, two bright stars, an 8.6 mag B8III giant star at the NE edge and a 9.1 mag A0III giant at the SW corner of the cluster are too young to be true members of NGC 6940 and are probably background stars. The brightest star (lucida) of NGC 6940 is thered giant FG Vulpeculae, a semiregularvariable star whose magnitude ranges from 9.0 to 9.5 every 80 days approximately.[3] NGC 6940 is rich in red giants, more than 20 according to WEBDA database. Based on the spectroscopic analysis of twelve of them, the cluster age was estimated to be 1.1 billion years, with a turn-off mass of 2M⊙. Itsmetallicity is close to that of the Sun.[4] In NGC 6940 have been detected 8 variable stars whose variability is consistent with thedelta Scuti variables.[5]
A study of the cluster byROSAT insoft X-rays revealed four sources that are identified as cluster members.[6] One more source was attributed to a possible cluster K0 giant star member, which was later confirmed to be a cluster member.[7] Three of these sources were identified asbinary stars, from a total of six known to exist in the cluster. The luminosity of the sources was typical ofRS Canum Venaticorum variables, close binaries that could retain very active coronal sources despite the age of the cluster.[6]
