Sh2-278 is an emission nebula in the constellation of Orion. This is a fragment of the gas clouds that make up the Orion Cloud. Like the other peripheral clouds of the complex, this one also shows signs of erosion due to the shock wave caused by the stellar wind of the bright stars of spectral class O and B belonging to the Orion OB1 association, which are also a source of ionization. Basically, the nebula is composed of two gas emanations, the head facing the association and the scattered emanation facing the opposite side. Part of the gas that makes up the cloud is not ionized, therefore remaining dark, while other parts are illuminated but not ionized, therefore bright by reflection.  From Wikipedia.