Crescent Nebula (NGC 6888 / Caldwell 27 / Sharpless 2-105)
Coordinates, Gaia DR3 distance (1.80 kpc), WR 136 Wolf-Rayet wind collision physics, and O-III filter observing guide for NGC 6888 in Cygnus.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Integrated visual magnitude (V) | 7.4 mag | ± 0.3 mag | Integrated CCD narrowband + continuum photometry | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Heliocentric distance | 5870 ly | ± 160 ly | Gaia DR3 astrometric parallax of central Wolf-Rayet star WR 136 (1.80 ± 0.05 kpc) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent angular dimensions | 18 arcmin | ± 1.5 arcmin | Optical H-alpha and [O III] elliptical shell (18′ × 12′) | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Crescent Nebula (NGC 6888 Wolf-Rayet Bubble) at RA 20h 12m 07.00s, Dec +38° 21′ 18.0″ within Cygnus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
The Crescent Nebula (NGC 6888, Caldwell 27, Sharpless 2-105) is a dramatic 18 × 12 arcminute (~30 × 20 light-year) circumstellar wind bubble located 5,870 light-years (1.80 ± 0.05 kiloparsecs) away in the rich heart of Cygnus. William Herschel discovered only its brightest northeastern arc on September 15, 1792, because the rest of the oval shell was too faint without modern narrowband filters.
Unlike star-forming H II regions or low-mass planetary nebulae, NGC 6888 is a Wolf-Rayet wind-blown bubble powered by the 7.5-magnitude star WR 136 (HD 192163, spectral type WN6(h)). Born roughly 4.5 million years ago as an O-type monster of ~30–40 solar masses, WR 136 first expanded into a cool red supergiant ~250,000 to 400,000 years ago, shedding ~5 solar masses of hydrogen- and nitrogen-rich gas in a slow (10–20 km/s) wind. When the star subsequently stripped down to its bare helium-burning core and became a hot (70,000 K) Wolf-Rayet star, its new 1,700-km/s supersonic stellar wind slammed violently into that older red-supergiant shell.
That collision triggered Rayleigh-Taylor hydrodynamic instabilities that shredded the swept-up shell into a Honeycomb of red H-alpha/[N II] filaments wrapped inside a razor-thin outer shock skin of blue-green [O III] emission, before WR 136 inevitably explodes as a core-collapse supernova within the next few hundred thousand years.
Practical Visual Observing Guide
- Easy to locate just 2.6° southwest of Sadr (γ Cygni), the central hub star of the Northern Cross, forming a triangle with Sadr and 34 Cygni (P Cygni).
- Without a filter NGC 6888 is notoriously elusive even in 200 mm telescopes, showing only the northeastern arc connecting a diamond of 7th-to-9th magnitude stars (including central star WR 136).
- Thread in an O-III narrowband filter at 50×–100× in a 130–250 mm+ telescope and the transformation is jaw-dropping: the entire 18′ × 12′ oval brain-like shell and interior crossbar ('the Euro € sign') snap into sharp relief.
Catalog Cross-Identifications & Aliases
Common Aliases: NGC 6888 · Crescent Nebula · Euro Sign Nebula · Caldwell 27 · Sharpless 2-105
| Catalog Authority | Identifier |
|---|---|
| NGC | NGC 6888 |
| SIMBAD | NGC 6888 |
| WIKIDATA | Q488504 |
Related Objects, Comparisons & Guides
Veil Nebula (Cygnus Loop NGC 6960/6992)
Coordinates, Gaia distance (735 pc), O-III filament structure, and visual filter guide for the Veil Nebula supernova remnant in Cygnus.
Thor's Helmet (NGC 2359 Wolf-Rayet Bubble)
Coordinates, Gaia DR3 distance (3.67 kpc), WR 7 Wolf-Rayet star HD 56925, winged-helmet shock geometry, and observing guide for NGC 2359.
Bubble Nebula (NGC 7635 in Cassiopeia)
Coordinates, Gaia DR3 distance (2.52 kpc), O6.5f star BD+60 2522 off-center wind cavity, and observing guide for NGC 7635 in Cassiopeia.
Surface Brightness & Extended Objects Guide
Why surface brightness (mag/arcsec²) governs nebula and galaxy visibility more than integrated magnitude, and how exit pupil controls eyepiece contrast.
Bortle Dark-Sky Scale & Sky Brightness Guide
Reference guide to the 9-class Bortle Dark-Sky Scale, zenith Sky Quality Meter (SQM) readings in mag/arcsec², naked-eye limiting magnitude, and light pollution.
Scientific Sources & Dataset Provenance
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC
- Revised New General Catalogue and Index Catalogue (RNGC/IC) (2024/2026 edition): Steinicke, W. 2010, Observing and Cataloguing Nebulae and Star Clusters (Cambridge Univ. Press); Verga, M. OpenNGC (CC-BY-SA 4.0 / data tables) — Wolfgang Steinicke / OpenNGC
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg