This image is of the California Nebula, so named because the shape is reminiscent of its namesake western state in the United States. Here the state is lying down: imagine it rotated 90 degrees clockwise. Xi Persei, the bright star just above, is probably the nebula's source of energy. One or more of the nearby stars generate energetic ultraviolet light, exciting electrons in hydrogen atoms within this vast cloud. When the electrons return to their rest state they emit the extra energy as photons, generating the deep red colour.
Credit: Wayne Malkin.