NGC 281 - The "Pac-Man" Nebula

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About this Image

Subject NGC 281 - The "Pac-Man" Nebula
Objects NGC 281 with assorted embedded Bok globules, NGC 281A molecular cloud, IC 1490 central open cluster, HD 5005 bright star
Info Wikipedia entry
Data SAO/NASA Astrophysics Data System (ADS) database entry

NGC 281 is sometimes known as the Pac-Man nebula. It can be hard to visualize why it was given the moniker of this video game character - but just imagine that the brighter part of the nebula above is a circle that makes up the head and the large extended dark nebula is its gaping mouth. It's chomping down towards the star at the bottom of the image.

The large dark nebula (NGC 281A) that makes up this "gaping mouth" is a molecular cloud made up predominantly of molecular hydrogen, but also dust and other gases. New generations of solar systems are being forged within its cold interior. Once these young stars' fusion engines switch on they will irradiate their surroundings - heating up, ionizing and eroding away the remaining dark material from which they formed.

NGC 281A
Molecular cloud NGC 281A, or alternatively and just for fun, H. P. Lovecraft's Shub-Niggurath,
one of the Old Ones, an "evil cloud like entity".

Originally the whole Pac-Man nebula would have been one large dark molecular cloud - the stars that formed early on at its center having progressively hollowed out the center of the nebula. The gas in and around this central region is ionized by the copious UV radiation emitted by the central open star cluster (IC 1490), causing it to glow and providing the light by which this narrowband image was taken.

NGC 281A
The intense radiation environment at the center of NGC 821.
The open star cluster IC 1490 situated at the top left, is dominated by the massive star HD 5005.
Dark Bok globules float above, and deeper in vast fingers of molecular hydrogen point upwards.
...or keeping with the Lovecraftian theme, perhaps this is the many tentacled mouth of Cthulhu!

The massive multiple-star system at the center of the star cluster, know as HD 5005, is by far the brightest source of ionizing radiation within the cluster. You can see how remnants of the original molecular cloud point like accusing fingers at the source of their natal cloud's demise. Due to the higher density of material at their tips these "fingers" or "pillars" of material manage to cling on while the surrounding dark nebula is stripped away. This higher density material is gravitationally collapsing and forming yet more stars.

Backlit by the glow of ionized gas within the nebula, several small isolated black patches of material are evident. These are known as "Bok globules" after Bart Bok, a dutch astronomer who first observed and studied them. Most Bok globules contain one or more protostars that will eventually become visible once the globule dissipates.

The Mountains Of Madness!
A slice through NGC 281 - a vista as viewed from the "Mountains of Madness" at its base

What are "narrowband" images?

This is a narrowband image, which put simply, means that the different colours in the image represent different elements. Red is sulphur gas, blue is oxygen and green is hydrogen. Various hues of colour represent different mixes of these gases. For example cyan areas are a mixture of hydrogen and oxygen, yellow areas a mix of sulphur and hydrogen, orange areas a mixture of more sulphur and a less hydrogen, and so on.

How are narrowband images created?

The data used to create these images is gathered using specialist "narrowband" filters. These filters pass only an increadibly narrow frequency band (colour) of light through to the camera. When gas is ionized, for example by ultraviolet light from a star, the different atomic elements within that gas each emit light with a particular set of narrow frequencies - in much the same way as a neon light glows with a particular colour. Narrowband filters provide a way to isolate particular frequencies of light that correspond to particular elements. One filter can capture images from light emitted by hydrogen, another the light from oxygen and so on.

Images captured using narrowband filters are inherently monochrome and are combined together in order to create composite "false" colour images. To create a false colour image you have to choose a particular mapping of each monochrome filtered image to a specific primary colour. In this set I have used a common mapping referred to as the "hubble palette" - where red is used for sulphur filtered images (light from singly ionized sulphur AKA SII), green for hydrogen (hydrogen alpha AKA Ha) and blue for oxygen (light from doubly ionized oxygen AKA OIII). The colours are then balanced to approximately normalize the brightness contribution of each element within the image. Without this adjustment the image would be overwhelminly green due to the predominant signal from ionized hydrogen gas.

Technical Details

Date 20111222-20121210
Location Pumpkin Patch Observatory, Bourn, Cambridge, UK
Optics Astro Optik 400mm Cassegrain @ cassegrain focus (f=3600mm)
Filters Astrodon 6nm narrowband, YR Cyclops filter wheel
Mount Paramount ME
Guiding SBIG ST402ME
Camera FLI Microline 16803
Exposure Total exposure 18.7hrs - R/SII 8.6hrs, G/Ha 3.6hrs, B/OIII 6.5hrs, 10 & 15 minute subs
Processing Maxim/DL 5, CCDStack, Photoshop CS
Notes Unfortunately many of the SII frames aquired were of poor quality and had to be excluded from final stack. Many more good quality SII frames were ideally needed as the overall SII signal is relatively low, hence the higher levels of noise evident in the red channel. The image was processed with an emphasis on contrast with some negative trade-offs in resolution and apparent noise.
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