Crescent Nebula NGC 6888 + Soap Bubble Nebula Ju 1

NGC 6888
NGC 6888: Crescent Nebula in Cygnus; 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 280+120+120+120 min LRGB; Bernese Highlands; © 2017 Radek Chromik

Crescent Nebula (NGC 6888)

The emission nebula NGC 6888 is somewhat similar in appearance to the Cirrus Nebula, but it is not a supernova remnant. NGC 6888 belongs to the class of Wolf-Rayet Nebulae, a nebula caused by a star of the same name. Incidentally, this specimen is one of the brightest W-R nebulae.

W-R stars are massive and luminous and probably evolved from the also very luminous O stars. There are two groups of W-R stars: those dominated by nitrogen lines (W-N stars) in their spectra and those dominated by carbon lines (W-C stars) in their spectra. Another feature of W-R stars is their extremely strong stellar winds. The matter they blow off into space is made to glow by their high-energy light and in some cases appears to us as emission nebulae.

NGC 6888
NGC 6888: CCD mosaic of Crescent Nebula; 500/2500mm-Newton + SBIG ST-6; Observatory Bülach; © 1996 Stefan Meister

The central star of NGC 6888 is designated HD 192163, a prototype of the W-N6 stars with a very high effective temperature of 55'000 Kelvin. For this reason, its light contains a lot of high-energy UV and can therefore excite doubly ionized oxygen [OIII] very strongly. Although the oxygen in NGC 6888 is underabundant by a factor of four compared to the element abundances of the Sun and the interstellar gas, the emission lines of doubly ionized oxygen are very intense.

Long thought to be a supernova remnant because of NGC 6888's filamentous appearance and unusual spectral features, it was found to be a Wolf-Rayet star rather than a post-supernova (pulsar) at its center.

NGC 6888 appears in long exposure photographs as a closed nebula bubble amidst the nebulae of Sharpless 108, south of gamma Cygni. Since a bright filament at the northern edge of this bubble is visually striking, NGC 6888 was given the nickname Crescent Nebula, which may mean either a crescent moon or a semi-circular breakfast pastry. [72, 126]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 6888
TypeEN
Right Ascension20h 12m 06.5s
Declination+38° 21' 18"
Diameter18.00 × 13.0 arcmin
Photographic (blue) magnitude10.0 mag
Dreyer DescriptionF, vL, vmE, ** att
IdentificationLBN 203, Sh2-105, Crescent nebula
NGC 6888
NGC 6888: Crescent Nebula with Soap Bubble (Ju 1) bottom left; Celestron RASA 11" f/2.22; ZWO ASI6200 Pro; Tentlingen; © 2020 Peter Kocher

Soap Bubble Nebula (Ju 1)

This very faint planetary nebula (PN G75.5+1.7) was discovered by amateur astronomer D. M. Jurasevich on images taken on June 19, 2007 and July 6, 2008 using an Astro-Physics 160EDF apochromatic refractor at Mount Wilson Observatory. The nebula was independently noted and reported by K. B. Quattrocchi and M. Helm. They took eight separate 10-min images on July 17, 2008 with a 40.6 cm f/3.75 astrograph, located at the Sierra Remote Observatories in the Sierra Nevada Mountains.

The images revealed a near-spherical shell of gas, located within a faint H II region near NGC 6888, that was apparently not noticed (or published) previously due to the density of stars and gas in that area of Cygnus. This shell appears as a slightly elongated ellipse with its major axis at position angle 5° and having an apparent size of 260 x 235 arc seconds. [512]

Further infos at CDS: PN Ju1

How to find the Crescent?

NGC 6888 is located in the constellation Cygnus between the stars Sadr (γ Cygni) and η Cygni. It is advantageous to choose an eyepiece with a large field of view. An O-III filter is required if the sky is not completely dark. Then the telescope is aligned with the help of the Telrad finder according to the adjacent map. The nebula lies at a prominent "W" of five brighter 7th to 8th magnitude stars. The constellation is best visible in the months of April to November.

Chart NGC 6888
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

NGC 6888
NGC 6888: Drawing; 300 mm f/4 Newton, 80x, mit O-III Filter; © 8. 9. 1996 Bernd Nies

300 mm aperture: With a moderately dark sky without an O-III filter, the northern, brightest area of NGC 6888 can only be seen, since the nebula then does not appear much brighter than the sky background. With a filter, he then reveals his crescent shape. The nebula appears fainter than the well-known Cirrus Nebula in the same constellation. On a dark, clear night in the mountains, far from civilization and light pollution, NGC 6888 is easy to see even without a filter.

The image below was made under a dark sky of at least 6.5 mag visual limit magnitude. The contrast is exaggerated a bit to emphasize the structure more clearly. The stars are from a printout using TheSky. The correct number and position of the stars was not tracked.

— 1996, Philipp Reza Heck

References

72Hunting Down The Cygnus Bubble; Astronomy 7/94, p.53
126Deep-Sky-Objekte des Monats: visuell beobachtet: NGC 6888 von Ronald C. Stoyan; SuW 7/96, S.555
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)
512Central Bureau for Astronomical Telegrams, Electronic Telegram No. 1876, 2009 July 16; cbat.eps.harvard.edu/iau/cbet/001800/CBET001876.txt (2022-04-21)