While black hole collisions produce almost no signature other than gravitational waves, the collision of neutron stars can be — and was — observed up and down the electromagnetic spectrum. “When neutron stars collide, all hell breaks loose,” said Frans Pretorius, a Princeton physics professor. “They start producing a tremendous amount of visible light, and also gamma rays, X-rays, radio waves…”

Princeton researchers have been studying neutron stars and their astronomical signatures for decades.