Even though we often think of Perseus as a long chain of dark clouds (e.g. as it appears in CO maps), its dust emission is dominated by a giant shell, filled with H-alpha emission (above). As can be seen in the overlay below, the finger of extinction bifurcating the H-alpha emission clearly indicates that the shell is behind the molecular clouds.

Spectral-line and 8um (MSX) data (see Ridge et al. 2006) show that the shell & molecular gas are interacting on the backside of the clouds. Appreciation of the shell's existence allows understanding of the apparently bi-modal magnetic field distribution (below), with one component associated with the warm ring (blue vectors) and the other with the cool clouds (red vectors).