NGC 6857: Emission Nebula – Cygnus: October Observer’s Challenge Report #153


Compiled by:

Roger Ivester, North Carolina


Sue French, New York

October 2021

Report #153

This is our “work-file report” for the Observer’s Challenge, used for organization, review and edits only. When all report submissions are received, personal photos are added and a final .pdf report will be issued by the 10th of the following month. (The cutoff date to receive submissions for the October report is November 8th).


The purpose of the Observer’s Challenge is to encourage the pursuit of visual observing. It’s open to everyone who’s interested, and if you’re able to contribute notes and/or drawings, we’ll be happy to include them in our monthly summary. Visual astronomy depends on what’s seen through the eyepiece. Not only does it satisfy an innate curiosity, but it allows the visual observer to discover the beauty and the wonderment of the night sky. Before photography, all observations depended on what astronomers saw in the eyepiece, and how they recorded their observations. This was done through notes and drawings, and that’s the tradition we’re stressing in the Observer’s Challenge. And for folks with an interest in astrophotography, your digital images and notes are just as welcome. The hope is that you’ll read through these reports and become inspired to take more time at the eyepiece, study each object, and look for those subtle details that you might never have noticed before.

This month’s target:

William Herschel discovered NGC 6857 on 6 September 1784. His handwritten journal for that date reads: A patch containing some nebulosity…irregularly long.

Heinrich d’Arrest writes of this object and his observation of it in his 1867 Siderum Nebulosorum Observationes Havnienses. My very loosely paraphrased English for the Latin text: Minute, faint; it is most probably a cluster. A 12th-magnitude star precedes it. – Rechecked shortly after: it was not so small; not all of the nebula is resolved, there is at least some cloudiness. I’m not surprised that this was missed by Rosse.

NGC 6857 is the brightest part of the larger, star-forming emission region Sharpless 2-100, which is a much more difficult visual target than NGC 6857. 

A 2010 paper by Manash Samal and colleagues in the Astrophysical Journal indicates that the main ionizing source at the center of NGC 6857 is the bright, massive star at its heart. This compact nebula is estimated to be approximately 28 thousand light-years away from us, and the star is thought to have a spectral type of about O4III. The most likely age of the nebula is in the vicinity of 1 to 2 million years. (Intro and object information by Sue French)

Mario Motta: Observer from Massachusetts

An Interesting object, initially thought to be a planetary nebula, but upon careful inspection, clearly is not. NGC 6857 is the bright “knot” to the left (East) in the following image. The nebula is 40 arc sec in size, but is embedded in Sh 2-100, a faint H-Alpha emitting region. It’s really just a bright, but very small section of that nebula. The image was taken with my 32-inch telescope, 3 hours, 1 hour each Ha, S2, O3 filters, processed pixinsight, touched up in photoshop.

Venu Venugopal: Observer from Massachusetts

The image was taken using a 8-inch Newtonian, H-alpha, H-Beta, OII filter with a Zwo533MC 20 minutes exposure, 30 second sub frames at the ATMOB clubhouse in Westford, MA on 10/7/21.

NGC 6857 is an H III Ionized region in the Cygnus constellation.  It is a small (~1’) compact nebula. 3 stars in a row NE-SW, SW one brighter, the other two equal. The middle star is surrounded by nebulosity. The “hidden treasure” in the centre of Cygnus was found in 1784 when William Herschel discovers a “faint glow, among Milky Way stars”. It shows a triangle shape with darker center and hard defined V shaped straight edges.

Glenn Chaple: Observer from Massachusetts

NGC 6857 – Emission Nebula in Cygnus (Magnitude 11.4, Size 40”)

Astronomical literature notes that this month’s Observer’s Challenge, NGC 6857, is a planetary nebula that wasn’t. It was correctly identified as a faint nebula by William Herschel, who discovered it on September 5, 1784. Because of its small size and the presence of a false central star, it was later misclassified as a planetary nebula. Only in recent decades has NGC 6857 returned to its rightful status as a nebula – an emission nebula, to be exact.

NGC 6857 is located in the heart of Cygnus at 20h 01m 48s right ascension and +33° 31’ 38’ declination. It’s just 2 degrees SSE of the 4th magnitude star eta (η) Cygni, which was my starting point for a star-hop (see accompanying finder charts).

I observed NGC 6857 with a 10-inch f/5 reflector on an evening when the magnitude limit was around 5.0. I was unable to see it without the aid of OIII and narrowband filters. Even at 139x, it was small – appearing as a pale ghostly ‘flame’ emanating eastward from the vicinity of a 13th magnitude star.

NGC 6857 is part of a much larger but fainter emission nebula Sharpless 2-100. Approximately 30,000 light years away, its 40 arc-second apparent size translates to a true diameter of 9 light years.

Roger Ivester: Observer from North Carolina

NGC 6857 – Emission Nebula – Cygnus 

Date:  September 27th 2021 

Telescope:  10-inch f/4.5 Newtonian Reflector 

Sketch Magnification:  291x

Eyepiece Combination:  11mm + 2.8x Barlow 

NELM:  ~ 4.9 

After two nights of failure to see the emission nebula, NGC 6857 I gave up, but thought I’d give it one more try on the following night.  So, on the third night after two hours of careful observing using mostly averted vision and a high magnification of 291x, the nebula made its intermittent appearance.  

The nebula could best be described as very small and having an arc shape on the NE side of a faint star. 

Pencil sketch as following:  

Mark Helton: Observer from Massachusetts

Had a short stretch of clear skies last evening, so I thought that I would go for this months observers challenge.  This is NGC 6857. A small planetary nebula in the middle of a much larger region of hydrogen red nebula in the constellation of Cygnus.  It is very bright and also very small for my 535mm Stellarvue 102T-R scope.  It becomes that MM with my .08 Reducer/Flattener.  I shot this with my ZWO533MC Pro camera which has a one inch by one inch sensor.  Using a OPT Tirade Filter.  I only had about an hour and a half before the clouds rolled in, so this is 40, 180 sec images at medium gain.  Stacked in Nebulosity, and processed by Dave Rust, a new member of our club and friend of mine out in Bloomington, Indiana.  He has been introducing me to some new processing techniques so, I wanted him to give this target a try.  We are both using Photoshop, and Topaz Labs DeNoise for final processing.  Thinking of trying out Pixinsight, even though it is a bit of a learning curve.  I think to go to the next level of astrophotography, we have to make the jump!  Cygnus is just chock full of wonderful nebulae, it was fun to find this one.

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