The Crescent Nebula

NGC 6888 was created by the powerful stellar wind of the hot central Wolf-Rayet star. The nebula is highly ionized by the star's radiation, so that there is very little SII emission; this image is constructed from observations in only the H-alpha and OIII filters (mapped to RGB as HOO). Unusually, the blue-green oxygen emission extends far beyond the red hydrogen light.

Although in deep images like this one the nebula is more or less spherical (especially in OIII), visual observers typically pick up only the brightest parts of the hydrogen emission, resulting in the moniker of 'Crescent Nebula'.

The 'spot' in the middle of the image is not a data flaw, but is astrophysical -- a single dense cloud of cold dust. It is unclear if it is intrinsic to the nebula, or is a foreground object.

Just for fun, the basic initial image is included for comparison (with just an preliminary non-linear stretch). I like my images "crunchy", and the decent signal here allowed me to indulge that preference.