When a supernova forms, most of its light comes from the radioactive decay of elements created in the explosion, which decreases and fades over time. As you can see in the image above, 1987a is surrounded by a ring of detritus that flew off the progenitor star before it exploded. Inside the ring, the fish-shaped cloud of star guts is expanding ever outward. Some of this material is starting to hit the surrounding ring, which is creating shock waves that produce X-rays. These X-rays, combined with shock heating, are the new power source that is causing the supernova remnant to brighten.