Strontium was spotted in the aftermath of a collision between two neutron stars. This is the first time a heavy element has ever been identified in a kilonova, the explosive aftermath of these types of collision.
The kilonova, AT2017gfo, emerging from the gravitational-wave–discovered neutron star merger, GW170817 was the first kilonova where detailed spectra were recorded.
The European Southern Observatory (ESO) pointed several of their telescopes at it to observe it in different wavelengths. In particular, they pointed the Very Large Telescope (VLT) and its X-shooter instrument at the kilonova.
Those results are presented in a study titled “Identification of strontium in the merger of two neutron stars.” The paper was published in the journal Nature on 24 October 2019.