NGC 281 – The Pacman Nebula
Key Scientific Data
Object type: Emission nebula – H II region
Constellation: Cassiopeia
Other designations: NGC 281, Sh2-184, LBN 616, Pacman Nebula
Distance from Earth: approximately 9,500 light-years
Diameter: approximately 80 light-years
Apparent size: approximately 35 arcminutes
Associated star cluster: IC 1590
Main ionising stars: HD 5005 multiple-star system
Principal emissions: Hydrogen (Hα), Oxygen (OIII), Sulphur (SII)
Location: Perseus Arm of the Milky Way
Right Ascension: 00h 52m 59s
Declination: +56° 37′
Discovered by: Edward Emerson Barnard, 1883
Light-travel time: approximately 9,500 years
Description
NGC 281, commonly known as the Pacman Nebula, is a large emission nebula and active star-forming region located approximately 9,500 light-years from Earth, in the constellation Cassiopeia.
Spanning roughly 80 light-years, it contains vast clouds of ionised gas, interstellar dust and young stars.
Its distinctive shape, featuring a dark indentation resembling an open mouth, inspired its nickname after the famous arcade-game character.
The nebula belongs to the Perseus Arm of the Milky Way, where massive young stars illuminate and gradually reshape the surrounding interstellar material.
IC 1590 – A Stellar Nursery
At the heart of NGC 281 lies IC 1590, a young open cluster containing hot, massive stars formed from the surrounding molecular cloud.
The multiple-star system HD 5005 is a major source of the ultraviolet radiation responsible for ionising the nebula.
Dense regions of gas and dust continue to collapse under gravity, potentially forming new stars and planetary systems.
Meanwhile, radiation and stellar winds from existing massive stars erode the surrounding clouds, creating cavities, ridges and complex structures.
NGC 281 therefore illustrates the continuous interaction between stellar formation and the dispersal of interstellar material.
The Hubble Palette – SHO
This image combines narrowband observations of three characteristic emission lines using the Hubble Palette (SHO):
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SII → Red: singly ionised sulphur, often associated with ionisation fronts and denser regions.
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Hα → Green: hydrogen emission at 656.3 nm, revealing much of the nebula's structure.
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OIII → Blue: doubly ionised oxygen, tracing regions with suitable high-energy ionisation conditions.
These assigned colours distinguish the different emission components rather than reproducing their natural visual appearance.
Additional RGB exposures preserve more natural stellar colours, contrasting with the narrowband representation of the nebula.
Dark Clouds and Bok Globules
The characteristic dark mouth of the Pacman Nebula is formed by dense interstellar dust and molecular material obscuring the glowing gas behind it.
NGC 281 also contains numerous Bok globules, compact dark clouds visible in silhouette against the bright nebula.
Some contain collapsing material and embedded young stellar objects, making them important sites for studying the earliest stages of star formation.
Infrared observations can reveal objects hidden within these otherwise opaque structures.
A Nebula Above the Galactic Plane
NGC 281 occupies an unusual position significantly above the Milky Way's main galactic plane.
Its location within the Perseus Arm makes it particularly interesting for studying the distribution and dynamics of molecular clouds and star-forming regions.
The visible nebula is part of a much larger complex containing molecular gas, dust and young stellar populations.
Curiosities
Discovered by Edward Emerson Barnard in 1883, NGC 281 was later included in the Sharpless catalogue as Sh2-184.
Its apparent diameter of approximately 35 arcminutes is slightly larger than the apparent diameter of the full Moon.
Despite this similar angular size, the nebula extends across approximately 80 light-years, meaning that light itself requires about 80 years to travel from one side to the other.
The region also contains dense molecular structures where new stars may still be forming, even as nearby massive stars gradually disperse the original cloud.
Looking Back in Time
The light captured from NGC 281 began its journey approximately 9,500 years ago, around 7500 BC, during Earth's early Holocene epoch.
The last Ice Age had ended, global temperatures were rising, and forests were expanding across previously glaciated regions.
Modern humans already existed, and early agricultural communities were developing in the Near East. Farming and permanent settlements were beginning to transform human societies, although the great civilisations of Egypt and Mesopotamia were still thousands of years away.
Throughout the entire development of human civilisation, from the earliest farming communities to the modern technological age, the photons recorded in this photograph were travelling through the Milky Way towards Earth.
The image therefore reveals NGC 281 as it appeared 9,500 years ago, long before the emergence of cities, writing or astronomy itself.
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 H-Alpha 3nm 2", OIII 3nm 2", SII 3nm 2", RGB 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 details and the full-resolution image, visit AstroBin: