When black holes collide, the only thing we can detect is gravitational waves, everything else is swallowed inside. But neutron stars are relatively lighter than black holes, so when they collide and merge, a small part of their mass and radiation does escape and can be detected along with gravitational waves. [We can see that] every atom of iodine on Earth, including the iodine you put on your wound, had arrived here in the distant past from a merger of neutron stars,