A cloud of ionized gas emitting light of various colors, often found in regions of active star formation and contributing to the reddish hues observed in galaxies.
Source: esa.int
09/10/1997

Comet Hale-Bopp's recent encounter with the inner Solar System allowed many breath-taking pictures. Above, Comet Hale-Bopp was photographed on March 8th in the constellation of Cygnus. Visible on the right in red is the North American Nebula, a bright emission nebula observable from a dark location with binoculars. The North American Nebula is about 1500 light years away, much farther than the comet, which was about 8 light minutes away. Several bright blue stars from the open cluster M39 are visible just above the comet's blue ion tail.