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October 2021 Observing guide

John Hobbs | Published on 11/24/2021

OBSERVER’S CHALLENGE* – OCTOBER, 2021

by Glenn Chaple

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 sizetranslates to a true diameter of 9 light years.

*The purpose of the Observer’s Challenge is to encourage the pursuit of visual observing. It is open to anyone who is interested. If you’d like to contribute notes, drawings, or photographs, we’d be happy to include them in our monthly summary. Submit your observing notes, sketches, and/or images to Roger Ivester (rogerivester@me.com). To find out more about the Observer’s Challenge, log on to rogerivester.com/category/observers-challenge-reports-complete,