Neutron star merger, illustration and X-ray images. Two separate stars underwent supernova explosions, leaving behind two ultra-dense cores (neutron stars) that eventually collided (top). Gravitational wave radiation (observed by the LIGO and Virgo detectors) is shown rippling outwards from the merger. The end result was a black hole. The system, known as GW170817, is shown in X-ray wavelengths (purple) a few days after (lower left) and several months after (lower right) the merger, which occurred on 17 August 2017. The observations were made by the Chandra X-ray Observatory (CXO). No brightening was observed in the August/September observation, but by December the system was emitting X-rays. This is though to be due to the shockwave of the merger impacting the surrounding gas and dust. GW170817 is 130 million light years away, in the galaxy NGC 4993, in the constellation of Hydra. Images published in 2018.