NGC 6888 (Cygnus)
Fig. 1 - An unusual class of nebulae that are byproducts of Wolf-Rayet stars: The emission nebula NGC 6888 in Cygnus, photographed with an 8-inch f/5 Newtonian reflector on a ZWO AM5 mount.
| Object name: | Constellation: | Coordinates: | Apparent size: | Visual brightness: |
|---|---|---|---|---|
| NGC 6888 | Cygnus | 20h12m / +38°21' | 18' x 12' | 7.4 mag |
The emission nebula NGC6888 (Crescent Nebula, Caldwell 27, Sharpless 105) in the constellation Cygnus. The nebula is shaped by the stellar wind of the central Wolf-Rayet star WR 136 (HD 192163). This massive, evolved star is in a late stage of stellar evolution, characterized by intense mass loss through high-velocity winds.
Roughly 250,000 to 400,000 years ago, WR 136 passed through a red supergiant phase, during which it expelled a slower-moving shell of gas. As the star evolved into its current Wolf-Rayet phase, it began emitting a much faster wind which collided with the earlier ejected material.
A Wolf-Rayet star (WR star) is a rare, extremely hot and massive type of star known for its intense stellar winds and unusual spectral features. WR stars typically start out with more than 20 times the mass of the Sun and are in a late stage of stellar evolution. They display broad emission lines of ionized helium, nitrogen, carbon, or oxygen - indicating powerful outflows of material. Their surfaces often are hydrogen-poor since many WR stars have lost their hydrogen layers, exposing deeper layers rich in heavier elements.
WR 136 has already shed its hydrogen layers and is now blowing off heavier elements. It is expected to end its life in a supernova explosion within the next few hundred thousand years. It's unclear whether WR 136 will end in a neutron star or a black hole.
NGC 6888 lies at a distance of approximately 4,700 to 5,000 light-years from Earth. The nebula spans about 25 to 26 light-years in diameter. It was discovered by William Herschel in 1792 (source: Wikipedia).
Exposure time: 5h 45min (115x3min) at gain 100 and -10°C, taken on June 29, July 5 and on August 30/31, 2025. Processing with Astro Pixel Processor (APP) and Photoshop. Darks, flats, and bias were used.
Equipment: Cooled ASI 2600MC Pro camera, TeleVue Paracorr Type II coma corrector, 8" f/5 "ONTC" Newtonian telescope riding on a ZWO AM5 Strain Wave Mount, ZWO OAG-L off axis system, ASI 174MM Mini guide camera, ASIAIR Plus.
NGC 6888 was also captured with a 16-inch dobsonian on an equatorial platform. The image can be seen here.
Fig. 2 - Labeled image.
Fig. 3 - Search chart for NGC 6888. Copyright 2025 'The Mag-7 Star Atlas Project', www.siaris.net.