The supernova is right in the center of a star-like galaxy. Therefore, the galaxy appears brighter than in the POSS II image (right) shown for comparison. Compare stars in the left image that are as bright as the supernova and then find the same stars in the right image and compare their brightness to the galaxy. You will find that the galaxy is in the left image brighter and more star-like than in the right image. The gain in brightness is due to the supernova.

Left image by M. Mrotzek