A pair of neutron stars after the collision thrown out the debris not only synthesized gold, silver and other stable elements, but also synthesized a large number of radioactive elements, they decay, fission, release a lot of energy, the debris itself heats up to make it glow,although not as bright as a supernova, it can reach about 1000 times the brightness of a star  So it's called a thousand new stars In August 2017, astronomers first observed the binary neutron star and combined it with gravitational waves, which were discovered about 10 hours later
The phenomenon of thousands of new stars formed by neutron star fragments completely confirms the hypothesis that neutron stars can be combined to synthesize large numbers of heavy elements
Supernova is brighter than a new star