Challenge #4 – C11, Bubble Nebula in Cassiopeia
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C11 (NGC 7635) | Bright Nebula | 23h 20m 42s | +61° 12′ 00″ | Cassiopeia | +11.0 | 15.0×8.0 arcmin |
Overview
Inside the same one-degree field of view as the open star cluster M52, the 12th magnitude Bubble Nebula forms a stunning subject for the Hubble Space Telescope.
Remarkably, the legendary William Herschel discovered this diffuse emission nebula in his simple telescope back in November 1787, calling it “A star of 9th magnitude with very faint nebulosity of small extent around it.”
Normally when we see an image like this, we expect it to be of a planetary nebula, where the central star has blown off layers of its material into space. The bubble, however, is the inverse of that: it is a vacuum bubble in a swamp of nebulosity. The void inside it has been caused by the hot blue star at its heart ejecting radiation pressure and pushing back the boundaries of the surrounding nebula.
The nebula lies about 10,000 light-years away but astronomers have not accurately tied down a distance for it yet.
This charismatic view is only available to backyard telescopes 8″ and above and with dark skies to play in, although the magnitude 8.7 star at its center is quite a bit easier to discern.
Finder Charts
In the first star chart (from SkySafari 6) you can see that NGC 7635 (C11) is on the border between Cepheus and Cassiopeia, which is circled in orange.
This first chart shows how the sky looks facing west at 10 pm in mid-September. Stars are shown to magnitude five and the moon and planets are not shown.
Join the two bright stars Shedar and Caph with an imaginary line and extend it onwards for the same distance again to be in the vicinity of C11, as shown in the next chart.
The easiest way to land in the correct area is to identify the star cluster M52. This bright cluster shines at magnitude 6.9 and is readily visible with any kind of magnification.
Stars in this final chart are shown to magnitude seven.
In the last section, you’ll discover how to attempt to see this notoriously hard object.
Individual Telescope Views
The following views will help you find C11 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. Stars are shown to magnitude 10.0. The large circle is a 1° field of view and M52 is shown in each image.
Each image can be clicked on for a full-screen version.
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 difficult object to see and requires a larger telescope and dark sky to attempt it. You’ll need at least an 8″ model to see it for yourself.
What you will see is the nebula NGC 7635, shown on the charts above, which is a large area of nebulosity. The Bubble is a small part of that nebula, and it is this that is practically impossible to distinguish without a large telescope and good conditions.
NGC 7635 spans 15 x 8 arcminutes, whereas the Bubble Nebula is three arcminutes and centred on the star SAO, a magnitude 8.7 star on the southern rim of the nebula. Look for that star and try to distinguish a faintly brighter circle of nebulosity tied to it but ever so slightly off-center from it.
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 [C11-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.