Challenge #2 – C9, The Cave Nebula in Cepheus
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C9 (Sh 2-155) | Bright Nebula | 22h 56m 48s | +62° 37′ 00″ | Cepheus | +7.7 | 50×30 arcmin |
Overview
It was Sir Patrick Moore, creator of the Caldwell catalog, who gave this nebula its name. It’s not hard to see what his inspiration was for that name when you look at the image below.
The Cave Nebula was first discovered by Stewart Sharpless (note the Sharpless catalog number for this object) in 1953 and is a hydrogen-alpha and dust emission nebula. It’s about 2,400 light-years away from us and has a diameter of about seventy light-years. The brightest region is lit by the energy from a group of new stars called the Cepheus OB3 association.
This is a hard object to track down and requires persistence, dark skies and a good telescope. It is only included in the VAC for completeness because, even if you do find it, it is a very ghostly ‘faint fuzzy’ indeed.
Finder Charts
The first sky chart, from SkySafari 6, shows the view at 10 pm in the middle of September, looking east. Stars are shown to magnitude 5.0 but the moon and planets are not shown.
Cepheus is the constellation that looks like a child’s drawing of a house. You can see it in the first chart below with the apex of the house’s ‘roof’ pointing towards the ground and running parallel to the more obvious ‘W’ of Cassiopeia, circled orange.
In Cassiopeia, identify the stars Shedar and Caph. These line up and point to the magnitude-five star 30 Cephei in Cepheus. We’ll find C9 on that line, just outside of the main body of the Cepheus constellation.
Another star on the same line, identified above and below, is 4 Cassiopeiae. Also a magnitude five star, we can find C9 just over halfway from 4 Cass. to 30 Cephei.
Stars in this view are shown to magnitude seven and the area of C9 is about 1°, however it is incredibly faint and not at all easy to observe.
We’re now ready to attempt observation of this nebula.
Individual Telescope Views
The following views will help you find C9 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 4 Cass and 30 Cephei 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
This is a very difficult object to see – probably the hardest in the entire VAC. Attempt it using a UHC filter if you have one and only under the darkest skies and the best seeing conditions.
You’ll need at least a 6″ telescope and a lot of patience. Sweep the area gently with averted vision and a relatively low magnification to try and eke out the faint difference in light between the areas of nebulosity and the general background sky.
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 [C9-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.