OBSERVERS’ CHALLENGE – SEPTEMBER, 2021
by Glenn Chaple
NGC 6823/20 Open Cluster/Emission Nebula in Vulpecula
(Mag: 7.1, Cluster Size: 7’, Nebula Size: 40’ by 30’) ,
A popular celestial designation for clear, moonless September evenings is M27, the “Dumbbell Nebula.” Next time you visit the Dumbbell, take a side trip three degrees west and slightly north to the open cluster NGC 6823 and its surrounding nebula NGC 6823.
The 2000.0 coordinates for NGC 6823 are R.A. 19h43.2m, dec. +23°18’. I star-hopped there using a 10-inch f/5 Dobsonian-mounted reflector and a magnifying power of 80X. My starting point was alpha (α) Vulpeculae - a wide optical double situated about 3 degrees south of Albireo (beta [β] Cygni).
Before arriving at NGC 6823, I came across an eye-catching half-degree-long chain comprised of 6 magnitude 9 and 10 stars. It began about a half degree west of the cluster and ended just north of it. NGC 6823 itself proved to be a neat little group dominated by a bright double star. In all, I saw some dozen stars down to about 14th magnitude.
A scan of the online edition of the Washington Double Star Catalog uncovered a stellar pair of magnitudes 9.4 and 10.4 and 37 arc-second separation at NGC 6823’s coordinates. This is most likely the double I saw. The brighter component has a spectral class of O6.5V, which makes sense as NGC 6823 is dominated by hot, young stars.
What about NGC 6820? There was no sign of the surrounding nebula, even when I used a narrowband filter. Resources note that it is extremely difficult to see visually. This is understandable when you consider that William Herschel discovered the cluster in 1785 and the nebula remained unknown until seen by Albert Marth nearly 80 years later. A noticeable feature of NGC 6820 is a dark trunk-like pillar similar to the “Pillars of Creation” seen in the Hubble image of the Eagle Nebula (M16) in Serpens.
NGC 6823 and NGC 6820 are approximagtely 6000 light years from the earth. The cluster is about 50 light years in diameter.
Finder Charts for NGC 6823/6820
Finder chart from AAVSO’s Variable Star Plotter (VSP). Numbers are stellar magnitudes, decimals omitted. Bright star at upper right is alpha Vulpeculae. Field size is 4° by 3° with north up. Stars plotted to 10th magnitude.
Close-up view of NGC 6823/6820. Image by Mario Motta. MD (ATMoB) Taken with 32 inch f6.5 scope, with ZWO ASI6200 camera, using 2 hours Halpha, 1 hour each O3 and S2 NB filters. Processed Pixinsight.