Precursor in a Shock Within the Cepheus A Bubble

The greyscale is a difference between H-alpha and [S II] images, white being strong [S II]. The contours (molecular H) in this image tend to lie along regions where [S II] is strongest, and lie ahead of the H-alpha that denotes where the shock occurs. This unexpected result shows that both molecular H and [S II] exist in a precursor ahead of the main shock.