NGC 281 – The Pacman Nebula
Key Scientific Data
Object type: Emission nebula / H II region / star-forming region
Constellation: Cassiopeia
Other designations: NGC 281, Sh2-184, LBN 616, Pacman Nebula
Associated open cluster: IC 1590
Distance from Earth: approximately 9,500 light-years
Physical size: approximately 80 light-years across
Apparent size: approximately 35 arcminutes
Main ionising stars: the young massive stars of IC 1590, including the multiple-star system HD 5005
Main optical emissions: ionised hydrogen (Hα), doubly ionised oxygen (OIII) and ionised sulphur (SII)
Right Ascension: approximately 00h 52m 59s
Declination: approximately +56° 37′
Location in the Milky Way: Perseus spiral arm
Light-travel time: approximately 9,500 years
Description
NGC 281, popularly known as the Pacman Nebula, is a vast region of ionised gas, dark dust and active star formation located in the constellation Cassiopeia.
At a distance of approximately 9,500 light-years, it lies within the Perseus Arm of the Milky Way, considerably farther from us than many of the familiar nearby emission nebulae.
The nebula extends across roughly 80 light-years of space and contains an extraordinary mixture of glowing hydrogen, highly ionised oxygen, sulphur emission, dense dust clouds and young stars.
At its heart lies the open cluster IC 1590, whose hot, massive stars provide much of the ultraviolet radiation responsible for making the surrounding gas glow.
The combination of luminous gas and the prominent dark indentation cutting into one side of the nebula gives NGC 281 its famous resemblance to the video-game character Pac-Man.
But behind this playful name lies a complex astrophysical environment in which new stars are still being born.
A Stellar Nursery
NGC 281 is an active star-forming region.
The enormous cloud from which it formed contains gas and dust distributed very unevenly. In some regions the material is relatively diffuse, while elsewhere it becomes sufficiently dense and cold for gravity to begin pulling matter together.
Inside these denser structures, clouds can collapse and fragment.
The fragments may eventually become protostars, surrounded by discs of gas and dust from which planetary systems could potentially form.
This means that NGC 281 is not simply an illuminated cloud.
It is part of the continuing cycle of stellar birth, evolution and death that constantly reshapes the Milky Way.
IC 1590 – The Young Cluster Inside the Nebula
Near the centre of NGC 281 lies the young open cluster IC 1590.
Its stars formed from the same enormous molecular cloud associated with the nebula.
Several of them are extremely hot and massive, producing intense ultraviolet radiation that ionises the surrounding hydrogen.
One of the most important stellar systems in the region is HD 5005, visible close to the centre of the nebula and also identified in the plate-solved field.
HD 5005 is not simply a single ordinary star. It is a multiple system containing very hot, luminous stars whose energetic radiation has a major influence on the surrounding nebula.
The interaction between these young stars and their parent cloud is gradually transforming NGC 281.
Radiation heats and ionises the gas, while stellar winds push material outward and carve cavities into the cloud.
The stars are therefore both products of the nebula and agents of its destruction.
Why the Nebula Glows
The brilliant gas in NGC 281 shines primarily because of ultraviolet radiation from its massive young stars.
Energetic ultraviolet photons strike hydrogen atoms and remove their electrons.
The gas becomes ionised.
Eventually, electrons recombine with atomic nuclei and cascade through different energy levels, releasing photons at characteristic wavelengths.
One of the strongest of these is H-alpha, at approximately 656.3 nanometres.
This emission is normally perceived as deep red and is one of the principal signatures of active H II regions.
But NGC 281 contains other important emission lines as well.
The narrowband data used for this image isolate three of them: Hα, OIII and SII.
Together, they provide a way of mapping different physical and chemical conditions throughout the nebula.
The Hubble Palette
This image uses narrowband information associated with the famous Hubble Palette, commonly referred to as SHO.
In this mapping:
SII → Red
Hα → Green
OIII → Blue
The colours therefore should not be interpreted as a literal representation of what the human eye would see through a telescope.
Instead, they are a scientifically useful false-colour representation that separates emission from different ionised elements.
The golden and yellow-green regions are strongly influenced by hydrogen and sulphur emission, while the striking cyan and blue areas indicate regions where OIII contributes strongly.
This technique reveals structures that would otherwise overlap when represented only according to their natural visible colours.
The result is both aesthetically dramatic and scientifically informative.
Hydrogen, Oxygen and Sulphur
The three narrowband filters reveal different components of the nebula.
H-alpha (Hα) traces ionised hydrogen, by far the most abundant element in the cloud. It provides much of the large-scale structure of NGC 281.
OIII traces doubly ionised oxygen. Producing OIII emission requires energetic radiation and appropriate ionisation conditions, so its distribution can differ significantly from that of hydrogen.
SII traces singly ionised sulphur and often highlights regions associated with ionisation fronts, denser material and transitions within the nebula.
The colour variations across the image are therefore connected to real differences in the physical state of the gas.
The nebula is not chemically painted in turquoise, green and gold; rather, those colours allow otherwise overlapping emission signals to be distinguished visually.
The Dark Mouth of Pac-Man
One of the most striking structures in the image is the dark region cutting deeply into the luminous nebula.
This feature creates the characteristic “mouth” responsible for the Pacman nickname.
It is not an empty hole in space.
Instead, it contains dense interstellar dust and molecular material lying in front of or within the glowing nebula.
Dust grains absorb and scatter visible light from the brighter material behind them.
The result is a dark silhouette.
These dark structures are particularly interesting because dense molecular material is precisely where future generations of stars can form.
What looks like emptiness may therefore contain some of the most astrophysically important material in the entire image.
Bok Globules – Small Dark Clouds with Stars Inside
NGC 281 is particularly famous for its Bok globules.
These are compact, dense clouds of gas and dust seen in silhouette against the bright emission nebula.
Several small dark structures are clearly visible against the luminous central region in this image.
Bok globules can contain enough material to collapse gravitationally and produce one or more stars.
Infrared observations are especially valuable because infrared radiation can penetrate dust much more effectively than visible light.
Such observations have shown that apparently opaque clouds can conceal young stellar objects and active star formation.
The dark globules in NGC 281 therefore represent more than foreground decoration.
They are potential stellar nurseries embedded within a much larger stellar nursery.
Sculpting the Nebula
The structure of NGC 281 is the result of a continuing competition between gravity and stellar feedback.
Gravity attempts to collapse dense regions of the cloud and create new stars.
Meanwhile, massive young stars produce intense ultraviolet radiation and powerful stellar winds.
These forces erode surrounding clouds, compress some regions and disperse others.
In some cases, the expanding ionisation front can compress nearby molecular material strongly enough to encourage further gravitational collapse.
Star formation can therefore influence subsequent star formation.
The extraordinary pillars, ridges and dark clouds visible within NGC 281 are snapshots of this continuing interaction.
A Giant Cloud Far Above the Galactic Plane
NGC 281 has another interesting characteristic.
Although it belongs to the Milky Way, it lies noticeably away from the thin central plane where much of the Galaxy's gas and star formation are concentrated.
This elevated position makes NGC 281 an interesting target for astronomers studying the structure and dynamics of the outer Milky Way.
The region is associated with a large complex of molecular gas, ionised material and young stars in the Perseus Arm.
The Pacman Nebula is therefore not an isolated object.
It is the optically visible component of a much larger interstellar environment.
The Colours of the Stars
The broadband RGB exposures used in this image serve an important complementary role to the narrowband data.
While Hα, SII and OIII reveal the ionised gas, RGB data help preserve the colour diversity of the stellar field.
Blue stars generally have hotter surfaces, while yellow, orange and red stars have progressively cooler surface temperatures.
The resulting field therefore combines two different types of colour information.
The stars retain a more natural broadband appearance, while the nebula uses narrowband emission to reveal its physical structure.
This creates the striking contrast between the multicoloured stellar background and the highly structured SHO representation of NGC 281.
A Region Larger Than It Appears
The Pacman Nebula spans roughly the same apparent angular scale as the Moon in our sky.
Yet the similarity ends there.
The Moon is only about 3,474 kilometres across and lies roughly 384,000 kilometres away.
NGC 281 lies approximately 9,500 light-years away and spans around 80 light-years.
Its apparent size therefore conceals an almost incomprehensible physical scale.
A beam of light could travel around Earth more than seven times in one second.
The same light would require approximately 80 years to cross NGC 281 from one side to the other.
From Nebula to Stars – and Back Again
NGC 281 illustrates one of the fundamental cycles of the Universe.
Cold clouds of gas collapse to create stars.
Stars illuminate and reshape their birth clouds.
The most massive stars live comparatively short lives, eventually enriching their surroundings through powerful winds and supernova explosions.
The expelled material mixes again with the interstellar medium.
Eventually, some of it may become part of another molecular cloud and participate in the formation of a new generation of stars.
The atoms forming planets — and ultimately living organisms — are part of this same cycle.
A star-forming region such as NGC 281 therefore represents not simply the birth of stars, but one stage in the continuous recycling of matter within the Galaxy.
Curiosities
NGC 281 was discovered by Edward Emerson Barnard in 1883.
Its modern nickname, the Pacman Nebula, comes from the distinctive dark indentation that gives the nebula an appearance reminiscent of the famous arcade-game character.
The nebula is also catalogued as Sh2-184 in the Sharpless catalogue of H II regions.
The young cluster IC 1590 lies within the nebula and contains the hot stars responsible for much of its ionisation.
The dark structures visible against the bright nebula are not holes but dense clouds containing gas and dust.
Some of these structures are associated with ongoing star formation.
The field therefore contains stars at dramatically different evolutionary stages: ordinary foreground and background stars, extremely young stars recently formed within NGC 281, and protostellar objects still hidden inside dense clouds.
The image contains 11 hours of narrowband exposure alone: 5h 10m in Hα, 1h 10m in SII and 4h 40m in OIII. This deep narrowband integration is what allows the complex internal structure and faint outer regions of the nebula to emerge so clearly.
Looking Back in Time
NGC 281 lies approximately 9,500 light-years from Earth.
The light recorded in this photograph therefore began its journey toward us around 7,500 BC.
At that time, humanity was living in the early Neolithic period in parts of the world.
Agriculture was beginning to transform some human societies, but the great civilisations of Egypt and Mesopotamia were still thousands of years in the future.
There were no pyramids.
No classical Greece.
No Roman Empire.
No telescopes.
For approximately 9,500 years, the photons captured in this image travelled through the Milky Way before finally reaching Earth and being recorded by the camera.
But this also means that the photograph does not show NGC 281 as it exists today.
It shows the nebula as it was approximately 9,500 years ago.
During the time its light has been travelling toward us, massive stars inside the region have continued to evolve.
Ionisation fronts have moved.
Dust clouds have been eroded.
New stars may have formed.
The nebula has already changed.
We simply cannot see those changes yet.
Their light is still travelling toward us.
NGC 281 is therefore not only an image of a stellar nursery — it is a 9,500-year-old view of star formation taking place deep within the Milky Way.
Image Data
Total integration time: 14h 49m
Integration by filter:
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Red: 1h 29m (89 × 60")
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Green: 30m (30 × 60")
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Blue: 1h 50m (110 × 60")
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H-alpha: 5h 10m (31 × 600")
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SII: 1h 10m (7 × 600")
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OIII: 4h 40m (28 × 600")
Equipment:
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Telescope: Artesky ARTEC 250 Pro
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Camera: ToupTek ATR2600M
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Mount: 10Micron GM2000 HPS II
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Filters: Optolong Blue 2", Optolong Green 2", Optolong H-Alpha 3nm 2", Optolong OIII 3nm 2", Optolong Red 2", Optolong SII 3nm 2"
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Accessories: Pegasus Astro Prodigy Microfocuser, WandererAstro WandererBox Pro V3, ZWO EFW 7 × 2″
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Software: Adobe Photoshop, Pleiades Astrophoto PixInsight, Stefan Berg Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA)
For further information and the full-resolution image, visit AstroBin: