Messier 39
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Messier 39 or M39, also known as NGC 7092, is an open cluster of stars in the constellation of Cygnus, sometimes referred to as the Pyramid Cluster. It is positioned two degrees south of the star Pi Cygni<ref name=Magda2015/> and around 9° east-northeast of Deneb.<ref name=Bone2005/> The cluster was discovered by Guillaume Le Gentil in 1749, then Charles Messier added it to his catalogue in 1764.<ref name=Magda2015/><ref name=Stoyan2008/> When observed in a small telescope at low power the cluster shows around two dozen members<ref name=Magda2015/> but is best observed with binoculars.<ref name=Magda2015/> It has a total integrated magnitude (brightness) of 4.6 and spans an angular diameter of Template:Val<ref name="webda2018a"/> – about the size of the full Moon.<ref name=Bone2005/> It is centered about Template:Convert away.<ref name=kharchenko2005/>
This cluster has an estimated mass of Template:Val and a linear tidal radius of Template:Val.<ref name=Piskunov2008/> Of the 15 brightest components, six form binary star systems; one more is 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 seem to be subgiants.<ref name=Zakirov1995/> Within are at least five<ref name=Gray2002/> chemically peculiar stars and ten<ref name=Schuff2006/> suspected short-period variable stars.
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