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NGC 6888 (Caldwell 27) · Emission Nebula (Wolf-Rayet bubble)
Crescent Nebula
A wind-blown bubble of shocked gas in Cygnus, sculpted by the powerful stellar wind of the Wolf-Rayet star WR 136. The bright shell is the leading edge where the wind ploughs into older material the star shed before it became a Wolf-Rayet.
NGC 6888 is one of the best examples in the sky of what happens shortly before a massive star explodes as a supernova. The central engine, WR 136, is a Wolf-Rayet star — a stripped, intensely hot remnant of a once-much-more-massive O-star. WR 136 is shedding mass at roughly a million times the rate of the solar wind, at velocities approaching 1,700 km/s.
That fast wind is currently sweeping up the slower, denser material the star ejected during its earlier red-supergiant phase. The collision shocks the gas to millions of degrees, producing the bright crescent-shaped shell visible in Hα and OIII. The filaments are clumps where the swept-up shell has become Rayleigh-Taylor unstable. In another few hundred thousand years WR 136 is expected to end its life as a Type Ib supernova — meaning the Crescent is, essentially, a slow-motion preview of a supernova progenitor.
The object is small (about 18 × 12 arcminutes) and sits in a rich part of Cygnus near Sadr (γ Cygni). It responds strongly to narrowband and dual-band filters, but even broadband OSC imaging from a dark sky can pull out the bright leading shell.
// imaging sessions
session // 01
02.10.2025 OSC broadband — 200P + 183CA from Plau am See
- Gear
- Sky-Watcher Explorer 200P Newtonian · Sky-Watcher EQ5 Pro SynScan GoTo · ToupTek 183CA · TS-Optics 2" Newtonian coma corrector · Pegasus Astro Gemini EAF · 50/90 mm guide / finder scope · ToupTek 327C Mini guide camera
- Filters / frames
- None — bare OSC broadband
- Total integration
- ~2 h 50 m (10,200 s)
- Frames
- drizzle stack; ~120 s subs
- Sensor
- gain 100
- Location
- Klebe, Plau am See, Mecklenburg · Bortle 3
- Software
- N.I.N.A. (Nighttime Imaging 'N' Astronomy) · Siril · GIMP · PHD2
Early-October session at Plau am See — bare OSC 183CA at f/5, no filter. Even broadband the bright leading shell carved by WR 136 came through, and StarNet star reduction on the final pass let the bicolor-style orange-brown filaments speak.
