M39
Messier 39 or M39, also known as NGC 7092, is an open cluster of stars in the constellation of Cygnus, positioned two degrees to the south of the star Pi Cygni[6] and around 9° east-northeast of Deneb. The cluster was discovered by Guillaume Le Gentil in 1749, then Charles Messier added it to his catalogue in 1764. When observed in a small telescope at low power the cluster shows around two dozen members, but it is best observed with binoculars. It has a total integrated magnitude (brightness) of 5.5 and spans an angular diameter of 29 arcminutes – about the size of the full Moon. M39 is at a distance of about 1,010 light-years (311 parsecs) from the Sun.
This cluster has an estimated mass of 232 M☉ and a linear tidal radius of 8.6±1.8 pc. Of the 15 brightest components, six form binary star systems with one more suspected. HD 205117 is a probable eclipsing binary system with a period of 113.2 days that varies by 0.051 in visual magnitude. Both members appear to be subgiant stars. There are at least five chemically peculiar stars in the cluster and ten suspected short-period variable stars.
