Modern observational techniques using surveys at multiple wavelengths have allowed astronomers to piece together stories of exceptional objects such as NGC 6888, also known as the Crescent Nebula. The sole energy source of the Crescent Nebula is the powerful Wolf-Rayet star HD 192163 (WD 136) visible as a blue star in the center of the shell-like nebula in the image. HD 192163 began its stellar life in the Cygnus OB1 stellar association some 4.5 million years ago as an extremely luminous and hot O- type supergiant. A few hundred thousand years ago it left the main sequence as it began to exhaust its dwindling reserve of hydrogen fuel. Swelling to tremendous proportions it became a red giant, releasing the last vestiges of its hydrogen fuel in a fierce stellar wind at speeds up to 20,000 miles per hour. With its exterior stripped and its inner helium layers laid bare the massive star became unstable. The resulting instability led to the Wolf-Rayet phase of prodigious mass loss from the stars surface. The mass loss occurs at a furious pace in the form of a powerful, high energy stellar wind traveling at speeds up to 3 million miles per hour. During the WR phase the star loses the equivalent of our suns mass every 50,000 years. The Wolf-Rayet phase will last several hundred thousand years (similar to the red giant phase). According to astronomical models the Wolf-Rayet nebula should be visible for about 10,000 years, somewhat less than the life of a planetary nebula. HD 192163 is expected to end its tumultuous existence in a great supernova blast within 100,000 years. NGC 6888 has given astronomers a unique opportunity to study a pre-supernova star.
Wolf-Rayet stars were first described in 1867 by Charles Wolf and Georges Rayet who detected their broad emission lines. Only later did it become clear that the broad emission lines were indicative of a thick expanding atmosphere. The huge expansion velocity of the outflowing stellar material from the stars surface exceeds the escape velocity of the star, producing a fierce stellar wind and copious mass loss. The Wolf-Rayet stage represents the late evolutionary stage of stars having an initial mass greater than 30 solar masses. Over 50% of WR stars are associated with nebular emission, many of which are wind blown bubbles or shells. The first three W-R nebulae reported were NGC 2359, NGC 6888 and S306 (Johnson and Hogg 1965).
The rapidly expanding shell of hot gas blown out by the stellar wind ultimately collides with the slower moving ambient gas cloud, much of which was ejected thousands of years earlier during the stars red giant phase. The force of the collision creates a complex shock front releasing energy in many different wavelengths including the brilliant colors of the visual spectrum. The result of this dynamic process is the shell-shaped glowing gas cloud we know as the crescent nebula.
The physical structure of NGC 6888 is an oblate shaped shell measuring roughly 25 x 16 light years in dimension. The material lost by HD 192163 during its red giant phase was swept up by the subsequent fast wind during the WR phase. Subsequently the shell thinned and fragmented and became ionized by the ultraviolet flux of the central star leaving the nebula as we see it today. Presently the nebula consists of a network of clumps and filaments which shine from the ionizing field of the central star. The shell appears to be extremely leaky to ionizing emission as only 2% of the photons from HD 192163 are processed by the nebula. The escaping photons ionize the thin neutral clouds which surround the nebula.