Galaxy NGC 4631. Combined X-ray and optical wavelength image of the spiral galaxy NGC 4631. The galaxy is seen edge-on from Earth, with its nucleus at centre. The red areas correspond to X- ray emission from gases at high temperatures, while the green areas (at the optical wavelength of hydrogen emission) indicate recent star forming regions. The star forming regions are mainly confined to the plane of the galaxy, and the halo of hot gas is large and diffuse, and extends out great distances from the central region. In the centre, the yellow regions indicate interactions between star forming areas and hot gases.