When two neutron stars collide, they produce a flare that shines like a million Suns. In 2017, astronomers saw such a flare for the first time — event AT2017gfo. Now scientists have built computer models to understand exactly how this light arises.

In the new study, they simulated different collision scenarios: varying the stars' masses, their total mass, and the properties of the material. They focused on light escaping along the rotation axis — the light we see when looking at the flare from above. It turned out that the spectrum shows bright lines from the elements strontium, lanthanum, gadolinium, and cerium. These lines resemble those observed in AT2017gfo, though in the models they are shifted toward the blue.

Interestingly, the flare's brightness depends on the amount of ejected material, but the spectrum's shape barely changes with the details of the collision. This means that even just the dynamical ejecta, material thrown out at the moment of impact, can explain many features of AT2017gfo at early stages. But to fully understand it, future models with other ejecta components are needed.

Such studies help us understand where heavy elements like gold, platinum, and uranium come from in the universe. After all, they are born in these very collisions.