NGC 281: The Eye of Sauron...

NGC 281 - The "Pac-Man" Nebula...

Object: NGC 281 (also commonly known as the Pacman Nebula, IC 11 or Sharpless 2-184 (Sh2-184), with open cluster IC 1590)
Type: Emission Nebula 
Constellation: Cassiopeia
Distance: 9200 light years

Date: 18th and 29th November 2019
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 25 x 300s H-alpha, 12 x 600s OIII, no flats/darks (hot pixel removal in Astroart).

Discovered in August 1883 by E. E. Barnard, NGC 281 is an emission nebula forming part of a larger HII region in the Perseus Spiral Arm of the Milky Way. Lying against the backdrop of constellation of Cassiopeia, it can be found virtually overhead on late autumn evenings but is extremely difficult to see visually in telescopes from suburban locations. 

Location of NGC 281 in night sky, looking north

At its centre, this faint, full-moon-sized nebulosity contains the open star cluster IC 1590, whose stars provide the radiation that causes the fluorescence of the surrounding gas cloud.  The nebula also contains several Bok globules, small dark dust clouds that can be seen in the image above. 

Colloquially, NGC 281 has suffered the misfortune to have been saddled with the ghastly, hackneyed moniker of “the Pacman Nebula” for its supposed resemblance to the eponymous character from the primitive 1980’s video game. 

The name actually put me off trying to hunt down this object, daft though that may seem.  It just seems so incredibly tacky (rather like the computer game) and certainly not something that should be associated with such a splendid and complex wonder of the night sky.

This particular evening was the first clear, dark night for over a month but as is always the way, the atmosphere was very unsteady.  I use an electronic focussing motor that I manually nudge to obtain a sharp star focus, but the image of the star could be seen to be twisting and flexing, making focussing very difficult.  The subsequent guiding of the telescope was thus also pretty poor, leading to bloated star images and I despaired of getting a decent result. 

Indeed, I packed it in after a couple hours. Nevertheless, some aggressive star reduction and sharpening in PSP yielded a reasonable hydrogen-alpha image (shown above).

NGC 281: hydrogen alpha data.

The RGB data was completed with the acquisition of some OIII data a week or so later (R = Ha, G = 70:30 OIII/Ha, B = OIII)  and the result recombined a colourised Ha luminance layer (technique described here) to give the image at the top.

By way of a counterpoint to the naffness of its accepted nickname, this image of NGC 281 is presented in a rotated format, with north to the right. I think that this orientation serves to draw the eye to the dramatic and highly-structured dark dust lane that runs across the face of the nebula and which resembles the pupil of a vast, sinister celestial eye – hence my title of this post: the Eye of Sauron…

This nickname has already been bagged by a dim and distant galaxy (11th magnitude NGC 4151) in the constellation of Canes Venatici.  NGC 281 is a much more accessible object, however, and does actually resemble (and deserves the link to) its cinematic avatar far more than the obscure galaxy does. I've since been told that the Helix nebula in Aquarius (NGC 7283) has also been called the "Eye of Sauron" (as well as the "Eye of God": conflating the two may be offensive to some folk!). The Helix is a long-standing and not-at-all-naff nickname which describes that large planetary nebula well, so giving it other names seems rather silly and pointless.

But I would dearly like to bury the "Pacman".  However, below is the nebula shown in its natural orientation, for those folk who are fans of naff eighties computer games.  Or just like seeing things the way they really are, at least in spatial terms (the colours are subjective, of course)...

NGC 281; Oriented with North up (as seen in sky)