ESO/L. Calçada/M. Kornmesser

Some of the universe’s heavier elements are created by neutron star collisions

Some of the heavier elements in the periodic table are created when pairs of neutron stars collide cataclysmically and explode, researchers have shown for the first time.

Light elements like hydrogen and helium formed during the big bang, and those up to iron are made by fusion in the cores of stars. Some heavier elements like gallium and bromine need something more, such as a supernova. Others—such as gold and uranium, which are the most neutron-rich—require a process called rapid neutron capture. Here, an atomic nucleus is bombarded with neutrons so it swells to an unstable size, but the whole thing happens so fast the element doesn’t have time to split apart.

Scientists have long suspected that neutron stars, the superdense remnants of burned out suns, are needed for this sort of rapid neutron capture. But until 2 years ago, they had never witnessed such an event. That’s when the GW170817 merger happened. Taking place 140 million light-years away (and imagined above, with strontium in yellow), astronomers first detected it from the gravitational waves generated by the stars crashing together.

In the new study, published today in Nature, researchers took a closer look at the event. Computer modeling revealed that strontium in the expanding ball of gas would absorb light at wavelengths of 350 and 850 nanometers. When they looked again at the X-shooter spectra, they found dips in the spectra at those wavelengths. The end result: five Earth masses worth of strontium.

The work confirms that at least some of the heavier elements are produced by merging neutron stars, and that neutrons stars really are made of neutrons. So next time you watch a firework display, remember that the red flashes—provided by strontium—may have started life when two dense stellar remnants crashed together before the Solar System existed.