NGC 6888 - Crescent Nebula

NGC 6888 — The Crescent Nebula

Catalog ID: SGO-005

OType: Emission nebula / Wolf-Rayet bubble

Distance: ~4,700 light-years

Size: ~25 light-years across (appears ~18 arcmin on sky—about half the Moon’s diameter)

Central star: WR 136 (massive, fast-losing outer layers via stellar wind)

Emission lines: Dominated by H-α and O III; red tones here show the H-α contributio

Acquisition Details

Dates: October 12–13, 2025

Location: Shank Gym Observatory (≈ 35.4° N, –80.6° W)

Instrument: Seestar S50 Smart Telescope

Mount: Integrated Alt-Az (on vibration-damped pads)

Filter: Internal Dual-Band (H-α + O III)

Exposure: ≈ 90 minutes total (two-night stack, 30 s subframes)

Processing: ASI DeepStack → Siril (planned refinement)

Calibration: Auto dark subtraction enabled; flats N/A

Conditions

Seeing: ≈ 2.5″ (moderate)

Transparency: Good; thin high haze each night

Temperature: ≈ 67 °F average

Light Pollution: Bortle 6 (suburban Charlotte sky)

Target Description

The Crescent Nebula (NGC 6888) is a wind-blown bubble produced by the massive star WR 136, which is shedding its outer layers at supersonic speed.

As the fast Wolf–Rayet wind collides with older, slower-moving gas from the star’s red-supergiant phase, shock fronts form, heating the surrounding hydrogen and oxygen gases to emission temperatures.

This produces the characteristic red and teal shell, representing ionized hydrogen (H-α) and oxygen (O III) lines.

The nebula’s light, now reaching us from nearly 5,000 years ago, marks one of the final stages before the star’s eventual supernova.

Observational Analysis

This two-night integration represents the first multi-session stacking experiment within the SGO archive.

The combined dataset exhibits noticeably lower noise and improved filament definition, particularly along the southern emission arc.

Future processing in Siril will focus on photometric color calibration, background gradient removal, and potential starless composition for structural study.

This observation also provides a benchmark for assessing Seestar’s internal dual-band filter performance under suburban sky conditions.

References

NASA/IPAC Extragalactic Database. (2024). NGC 6888 (The Crescent Nebula) – Object Data Summary. California Institute of Technology. https://ned.ipac.caltech.edu/byname?objname=NGC+6888

O’Dell, C. R., & Handron, K. D. (1996). The physical structure of NGC 6888 (the Crescent

Nebula). The Astronomical Journal, 111(4), 1630–1636. https://doi.org/10.1086/117889

Robberto, M., et al. (2023). WR 136 and the Crescent Nebula: A study of Wolf–Rayet wind interactions. Monthly Notices of the Royal Astronomical Society, 519(2), 2124–2140.

SkySafari Pro Database. (2025). Object summary: NGC 6888 — The Crescent Nebula. Simulation Curriculum Corp.