LDN 933 – Dark Clouds in Cygnus
Key Scientific Data
Object type: Dark nebula / interstellar dust cloud
Constellation: Cygnus
Other designation: LDN 933
Catalogue: Lynds' Catalogue of Dark Nebulae
Region: Cygnus Rift / North America–Pelican Nebula complex
Nearby bright star: 57 Cygni
Nearby nebulae: LBN 359, IC 5070 (Pelican Nebula) and the wider North America–Pelican complex
Approximate Right Ascension: 20h 52m
Approximate Declination: +44° 11′
Nature: dense interstellar dust and molecular gas seen in silhouette against a brighter Galactic background
Location in the Milky Way: Galactic disc
Associated environment: extensive clouds of gas and dust within the Cygnus star-forming region
Description
LDN 933 is a striking dark nebula located in the constellation Cygnus, within one of the richest and most complex regions of the Milky Way.
Unlike an emission nebula, LDN 933 does not shine primarily by producing its own visible light.
Instead, we see it because dense concentrations of interstellar dust absorb and scatter the light coming from stars and glowing gas behind them.
The result is the extraordinary network of dark lanes, filaments and almost opaque structures visible throughout this image.
The darkness is therefore not an absence of matter.
Quite the opposite.
Some of the darkest regions in the photograph correspond to places where the concentration of interstellar material is greatest.
Darkness Made of Matter
At first sight, the darkest structures in LDN 933 can look like empty gaps between the stars.
In reality, they are enormous clouds containing gas and microscopic dust grains.
Interstellar dust consists largely of tiny particles containing silicates, carbon-rich material and other compounds.
Although individual grains are extremely small, enormous numbers of them distributed across light-years of space can absorb a substantial fraction of the visible light travelling through the cloud.
Stars located behind the densest regions therefore become dimmer or disappear completely from view.
This produces the remarkable impression of dark rivers flowing through the surrounding stellar field.
What appears to be emptiness is actually material obscuring what lies behind it.
The Cygnus Rift
LDN 933 lies within the broader system of dark clouds associated with the Cygnus Rift.
The Cygnus Rift is the enormous dark region that appears to divide the bright Milky Way when viewed with the naked eye during northern summer nights.
It is not a genuine gap in the stars of our Galaxy.
Instead, large clouds of interstellar dust lie between us and the more distant stellar populations of the Galactic disc.
These clouds absorb their light, producing the dark band visible against the Milky Way.
Through astrophotography, this apparently simple dark region resolves into an extraordinarily intricate network of individual clouds, filaments and cavities.
LDN 933 is part of this much larger Galactic landscape.
A Landscape of Dust and Light
This image is particularly interesting because LDN 933 is not seen against an empty background.
The entire field contains a complex mixture of dark absorption nebulae, faint reflection structures, diffuse emission and thousands of stars.
The boundaries between light and darkness are often extremely intricate.
Some regions appear almost completely opaque, while neighbouring areas become progressively translucent, allowing increasing numbers of background stars to shine through.
This variation reveals differences in the density and thickness of the interstellar material.
The image therefore acts almost like a map of the distribution of dust across this portion of the Milky Way.
The finest dark filaments trace structures within the cloud that have been sculpted by gravity, turbulence, magnetic fields and radiation from nearby stars.
57 Cygni and the Blue Reflection Nebulosity
One of the most visually striking features of this image is the brilliant star 57 Cygni, surrounded by delicate bluish nebulosity.
The blue glow is fundamentally different from the reddish light typical of hydrogen emission nebulae.
Here, dust grains scatter the light of nearby stars.
Shorter blue wavelengths are scattered more efficiently than longer red wavelengths, giving many reflection nebulaetheir characteristic blue appearance.
The same basic physical principle contributes to the blue colour of Earth's daytime sky.
The combination of brilliant starlight, blue reflection nebulosity and dark obscuring dust creates one of the strongest contrasts in this field.
It also demonstrates that interstellar dust can both hide light and reveal itself by scattering light, depending on its geometry relative to the illuminating stars and the observer.
The North America and Pelican Region
LDN 933 lies close to the enormous North America and Pelican Nebula complex, one of the best-known star-forming regions in Cygnus.
The Pelican Nebula, IC 5070, lies just beyond the main field represented here and belongs to the same extraordinarily rich Galactic environment.
The North America Nebula, NGC 7000, and the Pelican Nebula are parts of a much larger cloud of ionised hydrogen.
Their familiar shapes are produced partly by bright emission and partly by foreground dust clouds that obscure sections of the glowing gas.
This is an important point.
Many of the spectacular shapes we associate with emission nebulae are not defined only by the gas that emits light.
They are also sculpted visually by dark material lying in front of that light.
Dark nebulae such as LDN 933 are therefore fundamental components of the appearance of this entire region.
The Birthplace of Stars
Dense molecular clouds are closely connected with star formation.
Deep inside sufficiently cold and dense regions, gravity can cause concentrations of gas to contract.
As material collapses, density and temperature increase until a protostar begins to form.
Eventually, if enough mass accumulates, conditions in the stellar core become sufficiently extreme for hydrogen fusion to begin.
A new star is born.
The same dark material that hides stars from our view can therefore contain the raw ingredients from which future stars and planetary systems will emerge.
Dark nebulae are not simply clouds blocking our view of the Universe.
They are part of the stellar life cycle.
Why We See So Many Fewer Stars in the Dark Areas
The effect of interstellar extinction is particularly clear in this photograph.
In the more transparent regions, enormous numbers of faint stars are visible.
Inside the darkest structures, the stellar population appears to drop dramatically.
This does not necessarily mean that there are fewer stars physically located in those directions.
Many of them are simply hidden behind the cloud.
The dust both dims and reddens background stars because shorter wavelengths are generally scattered and absorbed more efficiently than longer wavelengths.
Astronomers call these effects interstellar extinction and reddening.
By measuring how strongly the brightness and colours of background stars are altered, researchers can estimate how much dust lies between us and those stars.
Barnard and Lynds – Cataloguing the Darkness
Dark nebulae became an important field of astronomical study through the work of astronomers who realised that the apparent holes in the Milky Way were actual obscuring clouds.
One of the pioneers was Edward Emerson Barnard, whose photographic surveys in the early twentieth century produced a famous catalogue of dark nebulae.
Several decades later, American astronomer Beverly T. Lynds created a much more extensive catalogue using photographic survey plates.
Published in 1962, the Catalogue of Dark Nebulae identified thousands of obscuring clouds across the sky.
The letters LDN therefore mean Lynds Dark Nebula.
LDN 933 is object number 933 in that catalogue.
These catalogues transformed dark patches from apparent absences into astronomical objects worthy of study in their own right.
Seeing Something Because It Is Dark
LDN 933 illustrates an unusual concept in astronomy.
Most astronomical objects are detected because they emit or reflect light.
A dark nebula is often discovered precisely because it removes light from the scene.
We infer its presence from what we cannot see behind it.
The principle is similar to seeing the silhouette of an object standing in front of a bright window.
Without the luminous Milky Way behind these clouds, many of their structures would be almost impossible to detect in visible light.
The extraordinarily rich star fields of Cygnus therefore provide the luminous background needed to reveal the shape of the foreground dust.
Curiosities
Dark nebulae can be among the coldest environments found in the interstellar medium.
Deep inside dense molecular clouds, temperatures may fall to only a few tens of degrees above absolute zero.
At visible wavelengths they appear dark, but this does not mean that they are invisible at all wavelengths.
Infrared radiation penetrates interstellar dust much more effectively than visible light.
Infrared telescopes can therefore reveal stars and protostars hidden inside or behind clouds that appear almost completely opaque in optical photographs.
Radio and millimetre observations can also detect molecules within these clouds, allowing astronomers to study their composition, temperature, density and motion.
A region that appears almost black in visible light can therefore become extraordinarily complex when observed at other wavelengths.
From Dust to Stars, Planets and Life
The material contained within clouds such as LDN 933 is part of the continuous recycling of matter within the Milky Way.
Gas and dust expelled by previous generations of stars become incorporated into molecular clouds.
Some regions of those clouds eventually collapse.
New stars form.
Around some of those stars, discs of dust and gas produce planets.
The Solar System itself formed approximately 4.6 billion years ago from such a collapsing interstellar cloud.
The dark dust visible in this photograph is therefore not merely material obscuring the stars.
It represents the same fundamental type of cosmic raw material from which stars, planets and ultimately the chemical ingredients of life can emerge.
Looking Back in Time
Unlike a single star cluster or galaxy, LDN 933 does not provide one simple light-travel time.
The photograph contains several layers at different distances.
The dark cloud lies in the foreground, while many of the stars whose light it blocks are located considerably farther away within the Milky Way.
The light from each visible star therefore began its journey at a different moment.
Some photons may have travelled for hundreds of years.
Others may have travelled for thousands.
Then, during the final part of their journey toward Earth, some encountered the dust of LDN 933 and were absorbed or scattered away.
The dark structures in this photograph are therefore created partly by photons that never reached the telescope.
This makes the image particularly fascinating.
We are seeing the stars whose light successfully crossed the interstellar medium, while simultaneously mapping enormous clouds through the light they prevented us from seeing.
In LDN 933, darkness itself carries astronomical information.
We are not simply photographing light from the Milky Way. We are photographing the places where interstellar matter has intercepted it.
Image Data
Total integration time: 11h 28m
Integration by filter:
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L-Pro: 5h 52m
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Red: 1h 45m (35 × 180")
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Green: 1h 51m (37 × 180")
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Blue: 2h (40 × 180")
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 L-Pro 2", Optolong Red 2"
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Accessories: Pegasus Astro Prodigy Microfocuser, ToupTek AFW-L 7x2", WandererAstro WandererBox Pro V3
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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: