NGC 6888 in the constellation Cygnus is a special type of an emission nebula: a relatively rare Wolf-Rayet nebula.
|date of recording
||June 2 and 25, 2017
||7.2 h, H-Alpha: 145x100, OIII: 111x100"
||Celestron RASA F2.2
||Baader f/2 Highspeed 2" H-Alpha, Baader f/2 Highspeed 2" OIII
||250mm guide scope, MGEN
||Celestron CGE pro
A Wolf-Rayet nebula
The cause of this special type of an emission nebula is a Wolf Rayet star, the exposed core of a formerly massive star. Wolf Rayet stars were named after the French astronomers Charles Wolf and Georges Rayet and are also abbreviated WR stars in technical literature. The masses of the Wolf-Rayet stars measured so far range between 10 and 265 solar masses, although originally a theoretical upper limit of about 150 solar masses had been expected. The surface temperature is between 30,000 and 120,000 K and is thus higher than with almost all other stars. WR stars emit large amounts of matter into their environment. These stellar winds are accelerated by the radiation of the star up to 4000 km/s, which superimposes strong, very wide emission lines on the continuous spectrum. A WR star can lose several earth masses per year; episodically the mass loss rate can even increase tenfold.
Back to the Crescent Nebula. The emission nebula NGC 6888 was discovered in 1792 by the German-British astronomer Wilhelm Herschel. The nebula owes its name to its characteristic shape, which appears crescent-shaped through visual observation or on short exposures. Longer exposures reveal the true shape of the nebula, which is more reminiscent of a brain-like structure. It is formed by repelled matter of the active, central Wolf Rayet star WR 136.
The glowing of the Crescent Nebula is triggered by two independently occurring ionisation processes. The main cause is the central star WR 136 itself, which, due to its high temperatures, ionizes the nebula particularly strongly and makes it glow especially in the OIII range. Furthermore, the strong stellar wind collides with the dense interstellar medium in the region, which results in an impact front at which the material heats up and also ionizes. However, the end of this very active star has already been sealed, as it will explode as a supernova in the next millions of years.