Challenge #1 – C56, the Skull Nebula in Cetus
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C56 (NGC 246) | Planetary Nebula | 00h 47m 04s | -11° 52′ 22″ | Cetus | +10.4 | 4.0×3.5 arcmins |
Overview
Herschel made the first observation of this planetary nebula in November 1785. He observed “…four or five…stars forming a trapezium. The enclosed space is filled up with faintly terminated nebulosity.” What we’ll easily see are four bright stars, with one of them centered in the nebulosity.
This is a relatively large nebula but, although well located near bright stars in Cetus, it remains challenging to see unless you have a very dark sky. You should expect to need at least a 6″ telescope to see this object.
Planetary nebulae are the remains of a star’s outer shell cast off in a dramatic explosion, called a nova. The Skull Nebula is no exception, being birthed thousands of years ago from a sun-like star about 1,600 light-years from here. Over that time, the gas shell has expanded to a diameter of about 1.5 light-years. The remaining central star has dimmed from magnitude 9 to 12 over the past century or so.
Finder Charts
The first sky chart, from SkySafari 6, shows the view looking south at 10 pm in the middle of November. The moon and planets are not shown and stars are only shown to magnitude 5.0.
Orion and the Great Square of Pegasus are the two most obvious constellations visible at this time and they’re circled orange in the first chart. Where we need to begin our hunt for the Skull nebula is in Cetus. The constellation of the whale swims to the south of the Great Square. Once you’ve found it, move on to the second, more detailed chart.
C56 (shown as NGC 246 in the charts) is due north of the very bright star Deneb Kaitos (Beta Ceti). That star forms an equilateral triangle with two stars of similar brightness: Eta and Iota Ceti. We’ll find the Skull Nebula slightly to the south of the center of the line that joins these two stars.
This third and final finder chart shows stars to magnitude seven. Sticking with the theme of equilateral triangles, identify the stars Phi1 and Phi2 Ceti. These form an inverted equilateral triangle with C56 at the third point.
Binoculars or a magnifying finderscope make these two stars much easier to identify. Unfortunately, your lowest magnification eyepiece probably doesn’t quite give a large enough field of view to get either star and the nebula in the same view, but the tiny cluster of stars that sits over the nebula will likely be what you notice before the nebula itself.
Now you’ve found this faint nebula, what should you expect to see?
Individual Telescope Views
The following views will help you find C56 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 circle is a 1.5° field of view.
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
Armed with good seeing conditions and a dark sky, you’ll see the skull nebula’s faint, almost circular glow is close to being drowned out by the brightness of the stars sitting in front of it.
The three brightest stars shine around magnitude 12, and it is the left-most one of the skinny base that lies almost dead-center in the nebula.
An OIII or narrowband filter both reveal more detail, and careful study shows that the nebula is brighter along the arc of its west-facing rim, which seems to join to the brighter star at the peak of the triangle.
Most nebulae don’t respond well high magnification as their already faint light is spread too thinly, and that’s the case here too. However, the Skull can be observed with moderate magnification if you have a larger aperture telescope. That said, what look like obvious ‘eyes’ in photos – the two dark, circular voids – are practically impossible to detect through a backyard telescope.
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 [C56-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.