The most luminous kilonova candidate to date (short gamma-ray burst 200522A) was detected using the Hubble Space Telescope, Swift Observatory and other telescopes. A kilonova is a "the afterglow caused by the radioactive decay of heavy elements unique to the merger of two neutron stars," according to the Harvard & Smithsonian Center for Astrophysics.br br Credit: Space.com | animations courtesy: NASA, ESA, and D.