While recently reviewing my portfolio, I realised that I had never captured the Crescent Nebula in color, only a monochrome image taken back in 2009 (which is still available on my website). That prompted me to return to this fascinating object and finally produce a colour version.
The Crescent Nebula (designated as NGC 6888, Caldwell 27, and Sharpless 2-105) is an emission nebula located in the constellation Cygnus. It was discovered by Sir William Herschel in 1792. This nebula is the result of powerful interactions between stellar winds and prior mass loss from a massive star, and lies at a distance of approximately 5,000 light-years from Earth.
Central Star and Formation
At the centre of NGC 6888 lies a Wolf–Rayet star, designated WR 136. This is a hot and luminous star, with surface temperatures estimated between 30,000 K and 50,000 K, depending on the source.
During an earlier evolutionary phase, either the red giant or red supergiant stage, the star expelled significant amounts of material into the surrounding space. More recently, fast and powerful stellar winds have collided with this slower-moving gas, shaping the surrounding nebular shell. This interaction creates two shock fronts: one propagating outward, and another inward, compressing and heating the trapped plasma to X-ray-emitting temperatures.
This dynamic interaction is what classifies NGC 6888 as a Wolf–Rayet bubble.
Visual Appearance and Composition
The nebula spans approximately 25 light-years across, though some sources describe structures extending close to 100 light-years. It presents a characteristic crescent shape, making it a popular target among astrophotographers.
X-ray Emission
Observations from X-ray observatories such as Chandra and XMM–Newton have confirmed the presence of hot plasma within the nebular shell. Temperatures in this gas reach up to several million Kelvin, providing crucial data on the evolution of both the star and its surrounding nebula.
Image Acquisition
During June and July 2025, I gathered exposure using Hα and O III filters, along with additional RGB data for the stars, in order to produce a final image in the HOO palette.The data were acquired with my Takahashi TOA‑150 telescope and SBIG STL‑1000M camera, using Astrodon filters for Hα, R, G, and B, and a Baader filter for O III. The total integration time amounts to 34 hours.
Image Processing
This image was not particularly complex to process. One key step was the use of MultiScaleGradientCorrection, which proved both easy to apply and highly effective—following, of course, the use of SpectrophotometricFluxCalibration in narrowband mode.
The general processing strategy involved separating the stars from the image using StarXTerminator, and working on the starless version through the following steps:
NarrowbandNormalization
HDRMultiscaleTransform
LocalHistogramEqualisation
Colour enhancement via Curves (to increase saturation)
After completing the processing of the starless nebula, the RGB stars (processed independently) were reinserted using the excellent script by Mike Cranfield and Bill Blanshan, which allows for seamless integration.
Click on the image for full resolution version, or go to the Gallery section for complete exposure details.
Image processing: Pixinsight.
Observatory automation and remote operation with Talon6.
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