Free Astronomy Magazine November-December 2019

NOVEMBER-DECEMBER 2019 T his wide-field image generated from the Digitized Sky Survey 2 shows the sky around the galaxy NGC 4993. This galaxy was the host to a merger between two neutron stars, which led to a gravitational wave detection, a short gamma-ray burst and an optical identification of a kilonova event. [ESO and Digitized Sky Survey 2] However, showing that this was demonstrably the case turned out to be very difficult. This difficulty was due to our highly incomplete knowl- edge of the spectral appearance of the heavier elements in the periodic table,” says University of Copen- hagen researcher Jonatan Selsing, who was a key author on the paper. The GW170817 merger was the fifth detection of gravitational waves, made possible thanks to the NSF’s Laser Interferometer Gravitational- Wave Observatory (LIGO) in the US and the Virgo Interferometer in Italy. Located in the galaxy NGC 4993, the merger was the first, and so far the only, gravitational wave source to have its visible counterpart detected by telescopes on Earth. With the combined ef- forts of LIGO, Virgo and the VLT, we have the clearest understanding yet of the inner work- ings of neutron stars and their explosive mergers. T his artist’s impression shows two tiny but very dense neutron stars merging and ex- ploding as a kilonova. Such ob- jects are the main source of very heavy chemical elements, such as gold and platinum, in the Universe. The detection of one element, strontium (Sr), has now been confirmed using data from the X-shooter instrument on ESO’s Very Large Telescope. [ESO/L. Calçada] ing to better understand neutron star mergers and kilonovae. Because of the limited understanding of these new phenomena and other complexities in the spectra that the VLT’s X-shooter took of the explo- sion, astronomers had not been able to identify individual elements until now. “We actually came up with the idea that we might be seeing stron- tium quite quickly after the event. associate newly created material formed via neutron capture with a neutron star merger, confirming that neutron stars are made of neu- trons and tying the long-debated rapid neutron capture process to such mergers,” says Camilla Juul Hansen from the Max Planck Insti- tute for Astronomy in Heidelberg, who played a major role in the study. Scientists are only now start- !