The kilonova explosion was spotted in the galaxy NGC 4993, which is shown here in an image from the Hubble Space Telescope. The kilonova is visible here to the upper left of the very bright galactic center, and appears as a golden-yellow spot. Astronomers have two names for the Aug. 17 event, depending on what type of phenomenon is referred to. GW170817 refers to the gravitational waves observed from the merger, while GRB 170817A looks at the gamma-ray burst produced by the neutron-star merger. Gamma-ray bursts are the brightest known electromagnetic events in the universe, and can be created by a few different phenomena. NGC 4993 (also catalogued as NGC 4994) is a lenticular galaxy located about 140 million light-years away in the constellation Hydra. It was discovered on 26 March 1789 by William Herschel and is a member of the NGC 4993 Group.