  The Orion-Eridanus superbubble, formed by the nearby Orion high mass
star-forming region, contains multiple bright H$\alpha$ filaments on the
Eridanus side of the superbubble. We examine the implications of the H$\alpha$
brightnesses and sizes of these filaments, the Eridanus filaments. We find that
either the filaments must be highly elongated along the line of sight or they
cannot be equilibrium structures illuminated solely by the Orion star-forming
region. The Eridanus filaments may, instead, have formed when the
Orion-Eridanus superbubble encountered and compressed a pre-existing, ionized
gas cloud, such that the filaments are now out of equilibrium and slowly
recombining.

Astrophysics of Galaxies