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.