Open clusters Messier 29 and 39: Cygnus’ star cities

Amid all the wonderful nebulae that Cygnus has to offer lie Messier 29 and 39, two fine open clusters which are both easily visible through a pair of binoculars against a marvellous Milky Way backdrop as they ride high on late summer nights.

The brilliant summer constellation of Cygnus, the Swan, is superbly placed on late-August and September nights. Its familiar ‘Northern Cross’ asterism of its brightest stars, led by awesome first-magnitude Deneb (alpha [α] Cygni), is visible overhead as soon true astronomical darkness is well established.

Owing to its position embedded in the centre the summer Milky Way, Cygnus naturally hosts many open clusters, including Messier 29 (NGC 6913) and Messier 39 (NGC 7092). 

Let’s start with the latter, M39, which to most observers’ eyes is the superior cluster; it’s certainly the bigger and brighter of the pair, with its 30 or so principal members shining with an integrated magnitude of +4.6 across a sprawling full-Moon-sized 32 arcminutes. Its appearance is somewhat misleading, as M39 is physically one of the closest (800 to 1,000 light years distant) and smallest of Messier’s clusters, with its member stars spread across just nine-light years of space.

Messier 39 is faintly visible with the naked eye as a brighter spot within the rich Milky Way background, lying some nine degrees north-east of Deneb and just under three degrees north of rho (ρ) Cygni. A small telescope at low power will show 30 or so stars in an obvious, triangular shape, but perhaps the best views are to be had through large binoculars.

Messier 29 is easy to spot through a pair of 10 × 50 binoculars as a tight knot of stars lying just under two degrees south-south—east of magnitude +2.2 Sadr (gamma [γ] Cygni). It shines with an integrated magnitude of +6.6 from its estimated membership of 200 plus stars and spans just seven arcminutes on the sky. A small telescope resolves its brightest stars into a pattern that gives it the appearance of a ‘mini Pleiades’ lying in a rich Milky Way field.

Unfortunately, interstellar dust and gas along its line of sight severely blights it, knocking over three magnitudes off its brightness.