NGC6888 – The Crescent Nebula

NGC6888 is an emission nebula about 7000 light years away in Cygnus, near the star Sadr which marks the centre of the Northern Cross asterism.

The nebula is produced by the star at its centre, WR136. This is a Wolf-Rayet star, named after astronomers Charles Wolf and Georges Rayet (pronounced Ray-eh) who discovered the first such stars at the Paris observatory in 1867.

WR stars are among the most massive and hottest in the universe. They are characterised by an absence of hydrogen in their spectra, this being the result of their outer envelopes having been stripped of hydrogen by the intense stellar winds these stars generate.

WR136 is 21 times as massive as our Sun and 600,000 times as bright, with a surface temperature of about 70,000K. Such stars live fast and die young, because the temperature and pressure in their cores burns through their nuclear fuel very quickly.

It is believed that WR136 ejected a shell of material about 250,000 years ago when it became a red supergiant, and this shell is expanding at about 80 km/s. At the time, the star was still burning hydrogen but since then it has started helium fusion and again, because of its extreme mass, this is proceeding at a prodigious rate.

The intense stellar wind now streaming away from the star is travelling at some 1,700 km/s and is catching up the older shell and shaping it into the shell structure seen in the image. The collision of the two shells of material is generating shockwaves moving both outward and inward, and these shockwaves – along with the ultraviolet radiation now being emitted from the star’s surface –  are energizing the nebula and causing it to glow.

This is a bi-colour image using Hydrogen-alpha and Oxygen-III narrowband filters. The H-α is mapped to the red channel and the OIII to the blue and green.

Outside of the nebula other clouds are emitting H-α and OIII light as the nebula lies in a very rich part of the milky way.

Equipment: Planewave CDK-14, FLI Proline P9000 CCD camera and Astrodon 3nm narrowband filters. Processed in PixInsight.