Object of the Week September 07, 2014 – NGC 6888 The Crescent Nebula

NGC6888 SH2-105

Cygnus

Emission nebula

20 12 01.0
38 23 00

Mag: 7.4

Size: 18' x 8'


Discovered by William Herschel in 1792, NGC6888 is an emission nebula approximately 2º SW of γ Cygni, or Sadr in Cygnus. Researching this object, I've learned that the host star had evolved to a red giant about 300,000 years ago. The extremely massive Wolf Rayet (WR) star began losing large amounts of mass via very strong interstellar winds. When these winds collide with the much slower moving matter ejected previously, the collision causes heating which excites the nebula causing it to glow. Many accounts online I found appear to claim that NGC6888 is a SN remnant. From what I can tell, it is not. Its host star WR136 will go supernova one day, but it has not yet. (If I am wrong, I encourage correction from fellow forum members.)

In a telescope the Crescent Nebula is an absolutely beautiful object. Observable in scopes as small as 6”, the intricate details of its nebulosity really comes into its own in larger apertures. I've tried a variety of filters and OIII filters seem to work best, with an UHC second. In my 25” with an OIII filter, the nebula is glows nicely with its western edge most prominent, though I have little difficulty seeing the object in its entirety. To me, NGC6888 almost has an “ear” or “jellyfish” appearance. (a jellyfish minus its tentacles) What makes this object so incredible in larger apertures is the amount of detail that is seen. The nebula is just full of beautiful wisps and tendrils that can keep you mesmerized at an eyepiece for quite a while, an absolute favorite that gets visited quite often!

So now that the sun is setting earlier, get out and take a look at this wonderful object as its well placed high in the sky a couple hours before midnight. If you've never seen it before, you're in for a real treat. And for those who have seen it before but some time has passed since you're last visit, I hope this weeks OOTW was a nice reminder of a fantastic object!


And as always....
“Give it go and let us know!
Good luck and great viewing!”

Attachment 1353Attachment 1354

Indeed a great object. I have observed it repeatedly over the last couple of months, and it is an enjoyable object even with small (4-6") scopes. But when seen through a larger aperture, it is an object that can leave you speechless. There are streamers and tendrils that come and go with the seeing. With smaller scopes use a UHC filter, but with larger scopes nothing beats the unfiltered view...

Attachment 1355

Observation on 5 October 2013, during a Cygnus nebulae observing run in France.

12" SCT, 117x / 42', filtered and unfiltered

26mm TeleVue Nagler Type 5: A curved glow, brightest on the SW side, to the north of a white mag. 7 star (V1170 Cygni), starting in the NNW and running through the north towards the ESE (the ESE part ends 1/3 FoV NE of V1170 Cygni). Using AV the curved shape very faintly continues to a point west of V1170 Cygni, interrupted in the WNW. On the NNE edge of the glow is the double star STT401. To the WNW of V1170 Cygni, just detached from the arc of nebulosity is a NNE-SSW elongated part. The faint nebulosity SE of V1170 Cygni is not visible.
1/2 FoV to the SE is a pale orange mag. 8 star with a mag. 10.5 star to its NE (not a double star in WDS).

26mm TeleVue Nagler Type 5, Lumicon UHC: Using this filter the nebula is more evident and the curved shave is visible to the SSW of V1170 Cygni, too. The WNW brighter part of the nebula is very clear, using AV a NE-SW elongated, slightly brighter part is visible slightly closer to the NNW. No nebulosity is visible WSW of V1170 Cygni, to the SW is an arc of nebulosity that curves towards the SSE and ends almost 1/2 FoV to the SSE at an orange mag. 10.5 star, with a very flattened triangle of mag. 11 stars to its west, pointing SSW.
To the north of the mag. 10.5 star, 1/3 the distance towards V1170 Cygni is a brighter, detached part of the nebula, a detached patch.
The curved shape of the nebula is clearly interrupted only to the west of V1170 Cygni. The northern part of the arc and the part west of V1170 Cygni are clearest, only interrupted NW of V1170 Cygni.

26mm TeleVue Nagler Type 5, Lumicon OIII: Using this filter the northern part of the nebula is brighter compared to using the UHC filter, while the SW is now fainter.

I can contribute two sketches and better quoted observation.

First an old sketch through an unfiltered 5" binocular. The field is 3° large.

http://www.deepsky-visuell.de/Zeichnungen/NGC6888.jpg

Interesting details beside the brighter N part around the central trapezoid is the triangle shaped OC Basel 1, 1° W of the WR nebula and the southern extensions of the gamma Cyg region.

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A few years ago I tried to see and sketch as much detail I could catch with the bigger 27" telescope. My goal was to see the thin structure within the nebula filaments. Unfortunately I could not any filament structure but many different knots - in the main bend and around it. I'm not very happy with the sketch, because it doesn't show the visual impression of this object (limited grey scale) but it shows all for me accessible structures I could get on this object.

27", 172x, [OIII], NELM 7m+, Seeing II
http://www.deepsky-visuell.de/Zeichn...NGC6888_27.jpg