ionization front

Pelican Nebula ionization front

The light from young energetic stars is slowly transforming the Pelican Nebula's cold gas to hot gas, with the advancing boundary between the two being marked by an ionization front. Most of these bright stars lie off the top of the image, but part of the bright ionization front crosses on the upper right.

An ionization front is a transition region where interstellar gas changes from a mostly neutral state to a mostly ionized state. Ionization fronts are typically found in the vicinity of hot stars and define the extent to which stellar ultraviolet has ionized the material around them.