This picture is a 2.5 hour exposure (HaRGB) taken with an STL-1301E CCD camera thru Kopernik's 20 inch telescope working at F/5.
"Faint, very large, very moderately extended, double star attached."
NGC 6888 (aka The Crescent Nebula) is an oddly shaped emission nebula in the Constellation of Cygnus. It is associated with the bright Wolf-Rayet star located in the center of the image. Wolf-Rayet stars are believed to have evolved from massive (>25 solar mass) O-type stars via a period of major mass loss probably involving a very close companion star. One or more shells of expanding gas surround them. The expanding gas from the star sweeps up additional gas from the inner stellar medium. Some astronomers believe that the gas in a nebula like NGC 6888 is excited to glow by the ultraviolet light from the Wolf-Rayet star. Others suggest that the gas is excited by the expanding shock wave. Recently, some astronomers suggest that the odd shapes of some NGC 6888 type nebulae results from the Wolf-Rayet star's shell running into expanding shells from other nearby stars. In the image above the other three bright stars are all B-type blue giants of a similarly young age and they are only slightly less massive. B-type stars have a less vigorous but still massive stellar wind. Much additional research is needed before we can have a full understanding of Wolf-Rayet stars and the bubble shaped nebulae that surround them.
George Normandin, KAS
February 7th, 2012