OBSERVER’S CHALLENGE* – October, 2022
by Glenn Chaple
Messier 39 Open Cluster in Cygnus (Magnitude 4.6; Size 31’)
Cygnus is a relatively large constellation centered on the star-rich fields of he Milky Way. It’s surprising, therefore, that it’s home to just two Messier objects – the open clusters M29 and M39. The latter, the larger and brighter of the pair, is this month’s Observer’s Challenge.
Credit for its discovery goes to Charles Messier himself, who observed the cluster on October 24, 1764. Some sources suggest that it may have been seen by Messier’s fellow countryman Guillaume Le Gentil 14 years earlier, while others note a possible naked eye observation by Aristotle in 325 BC.
M39 is located at the 2000.0 coordinates RA 21h31m48.0s, Dec. +48o26’00”. I found it by star-hopping 3 degrees roughly north of 4th magnitude rho (ρ) Cygni. This star can be found by tracing an imaginary line from delta (δ) Cygni through Deneb and extending it an equal distance beyond (refer to the two finder charts).
My first encounter with M39 came on the evening of November 11, 1977, when I observed both it and M29 with a 3-inch f/10 reflector and a magnifying power of 30X. I was able to prove for myself that M39 is indeed larger and brighter – bright enough to be visible in the scope’s primitive 3X25mm finderscope. During a small-scope survey of all Messier objects conducted between the years 1996 and 2013, I revisited M39 with a 3-inch f/6 reflector and 39X eyepiece. In my logbook, I wrote “Large, sparse cluster, triangular in shape. Over 2 dozen stars down to 11th magnitude.” For a fresh impression of M39, I viewed it on September 15, 2022, with a 60mm (2.4-inch) refractor, again with low power (this time, 25X) to capture its entire full-moon-sized span. I counted about 20 stars, which is two-thirds of the recognized cluster membership.
On all three occasions, I sketched M39. Reviewing them, I noticed a common denominator besides the triangular shape. Near the middle was a faint (for my small-sized instruments) double star. A search of the Washigton Double Star Catalog (WDS) identified it as ARN 78, whose magnitude 7.6 and 8.8 components are separated by 50.0 arc-seconds. The WDS listed several other pairs within the bounds of the cluster – all too faint or close for ordinary backyard scopes.
At a distance of 800 light-years, M39 is one of the nearest Messier objects. Its true diameter is around 7 light-years.
*The purpose of the Observer’s Challenge is to encourage the pursuit of visual observing. It is open to anyone who is interested. If you’d like to contribute notes, drawings, or photographs, we’d be happy to include them in our monthly summary. Submit your observing notes, sketches, and/or images to Roger Ivester (firstname.lastname@example.org). To find out more about the Observer’s Challenge, log on to rogerivester.com/category/observers-challenge-reports-complete.
Positioned only about 800 light years away from our solar system, this 300 million year old group
Messier 39 Finder Chart
Messier 39 Finder Chart B
Created using the AAVSO’s Variable Star Plotter (VSP). Numbers are stellar magnitudes, decimals omitted. The magnitude 4.0 star is rho (ρ) Cygni. Stars plotted to 9th magnitude. North is up in this 2 by 4 degree field.
Mario Motta, MD. “M39 is too large for my 32 telescope inch to image it (would only see half of it.) So, I took it with my 600mm F8 Sigma lens for my Nikon 7100 camera piggybacked. (effective FL is 900mm on this camera) Stack of 5 images of 1 minute duration each, then cropped a bit, mildly processed in Pixinsight.”