C27, Crescent Nebula in Cygnus
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C27 (NGC 6888) | Bright Nebula | 20h 12m 30s | +38° 25′ 00″ | Cygnus | +7.4 | 20×10 arcmin |
Overview
To me, the image of the Crescent Nebula, below, looks a bit like a brain. Of course, in our eyepieces we see none of this detail and color, but it is still a great object to discover for yourself.
It was first discovered in 1792 by William Herschel and is a fascinating example of an emission nebula. A star called WR 136 emitted a solar wind when it became a red giant a few hundred-thousand years ago. Since then, later stellar winds emitted by the same star have travelled at up to 3,000 km/s and caught up with and ionized the red giant winds to form this otherworldly feature.
The magnitude 7.5 creator of this object lies at its heart, easily visible in a telescope view. It was, before shedding its skin(s), a star 25-40 times larger than our own sun.
The nebula is positioned 5,000 light-years away from us, in the constellation of Cygnus. It’s a small object and, despite having an overall magnitude of 7.4, is actually quite hard to see because this light is spread over a large area.
Finder Charts
The first sky chart, from SkySafari 6, shows the view at 10 pm in the middle of July, looking east. Stars are shown to magnitude 5.0 but the moon and planets are not shown.
We need to locate the obvious cross-shape of Cygnus, the swan constellation. It’s shown below in the large orange circle, with the Little Dipper and Cassiopeia also highlighted for ease.
Helpfully for us, C27 (NGC 6888) lies directly between two of Cygnus’s brightest stars, Sadr (Gamma Cygni) and Eta Cygni, shining at magnitude 2.2 and 3.9, respectively.
You can see in this second chart that C27 is slightly closer to Sadr than Eta Cygni.
For more detailed positional confirmation, look for two stars shining near magnitude five – so still visible to the naked eye under decent conditions – P Cygni and b2 Cygni. These form a right-angled triangle with C27, where the nebula is at the right angle.
Stars in this view are shown to magnitude seven.
We’re now ready to investigate this little nebula in more detail.
Individual Telescope Views
The following views will help you find C27 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 star P Cygni is shown in each image. The ring represents a 1.5° field of view – note that the nebula and P Cygni are both inside of it.
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 tricky, small object to find. While the usual mantra in the VAC is to begin with low magnification and a wide field of view, this can make it hard to know if you’re actually seeing such a small feature. Especially one so faint.
As with all nebulae, success is greatly enhanced with a very dark sky. An OIII or narrowband filter can improve the view but not markedly so.
Look for a very narrow and smaller than you’re expecting needle of light. Greater magnification shows why this is called the Crescent nebula, i.e. that needle of light is not a straight line, but a slender curve. Larger telescopes reveal the slightly more complex shape that is reminiscent of the number 3. This is also why the nebula has an alternative nickname of the Euro sign nebula (€).
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 [C27-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.