Challenge #2 – C37, Open Cluster in Vulpecula
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C37 (NGC 6885) | Open Cluster | 20h 12m 01s | +26° 28′ 00″ | Vulpecula | +8.1 | 20 arcmin |
Overview
There are two open clusters so nearby to each other that they are often superimposed on star charts: NGC 6885 (C37) and NGC 6882. For our purposes – in a nutshell – we need to look for the loose cluster centered on the star 20 Vulpeculae and we’ll have found the object we seek.
It was discovered by William Herschel in 1784 and comprises about twenty loosely connected stars.
This object is almost 2,000 light-years away from us.
Finder Charts
The first sky chart, from SkySafari 6, shows the view at 10 pm in the middle of September, looking south. Stars are shown to magnitude 5.0 but the moon and planets are not shown.
The brightest, most obvious constellations visible are the Great Square of Pegasus and Cygnus, both of which are circled orange in the chart below. Note the position of C37 in the bottom left quarter of Cygnus and shown as a green circle labelled NGC 6885.
Albireo is the bright, magnitude-three star at the head of Cygnus and Gienah, which is brighter still, is shown on one of its wings. Our cluster’s approximate location can be found using these two stars. In the chart below, note the two fainter stars in the purple box – one directly above and the other to the right of the cluster.
With stars now shown to magnitude seven, you can see that the two brighter stars are 23 and 15 Vulpeculae. Both of these are visible to the naked eye and form two angles on a right-angled triangle that has C37 on the right-angle.
For added detail, note the two fainter stars 18 and 19 Vulpeculae. Both of these are naked-eye visible under a dark sky and sit inside the same 1.5° field of view (blue circle) as the cluster.
Although not shown in this chart 20 Vulpeculae is at the center of the green circle and marks the precise location of the cluster. It shines at magnitude 5.9.
We’re now ready to attempt observation of this cluster.
Individual Telescope Views
The following views will help you find C37 in different telescope types by presenting the images as your telescope will show them. The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Each image can be clicked on for a full-screen version. Stars are shown to magnitude 10.0 and the stars 18 & 19 Vulpeculae (as well as 20 Vul) are shown in each image.
Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope
Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal
Mirrored View – Refractors and Cassegrain models with a star diagonal show this view
Observation
At lower magnifications under less than ideal conditions, this cluster is hard to see. As you’ll have seen from the image above, it is sparse and dominated by the stars 18, 19 and 20 Vul.
A ‘gray fuzzie’ bar of light near 20 Vul is the most obvious sign that there’s more at play here than just the three brighter stars. There’s also a trapezoid of stars southwest of 20 Vul that is somewhat reminiscent of that asterism in the Orion Nebula.
Greater magnification will pull out the tiny pinpricks of light contributing to that fuzzy bar. How many of the 60 stars brighter than magnitude 13 does your telescope reveal?
PDFs for Printing
Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [C37-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.