Massive stars typically die in an explosion called a supernova, seeding the cosmos with heavy metals and producing a distinct signature. Sometimes, the blasts leave behind an ultra-dense core called a neutron star, which contains more mass than the Sun in a sphere roughly the size of a city. Neutron stars can be found in binary pairs, and when their incredible gravity draws them together, their collision causes an equally catastrophic explosion called a kilonova. The signal suggested a merger between two massive objects, like the collision between neutron stars that causes kilonovas. In this scenario, the original star exploded in a supernova and essentially split in twain to birth two, smaller neutron stars, not just one.