Free Astronomy Magazine November-December 2019

NOVEMBER-DECEMBER 2019 SPACE CHRONICLES T his montage of spectra taken using the X-shooter instrument on ESO’s Very Large Telescope shows the changing behaviour of the kilonova in the galaxy NGC 4993 over a period of 12 days after the explosion was detected on 17 Au- gust 2017. Each spectrum covers a range of wavelengths from the near- ultraviolet to the near-infrared and reveals how the object became dra- matically redder as it faded. [ESO/E. Pian et al./S. Smartt & ePESSTO] On Earth, strontium is found natu- rally in the soil and is concentrated in certain minerals. Its salts are used to give fireworks a brilliant red colour. Astronomers have known the physi- cal processes that create the ele- ments since the 1950s. Over the following decades they have uncov- ered the cosmic sites of each of these major nuclear forges, except one. “This is the final stage of a decades-long chase to pin down the origin of the elements,” says Wat- son. “We know now that the processes that created the elements happened mostly in ordinary stars, in supernova explosions, or in the outer layers of old stars. But, until now, we did not know the location of the final, undiscovered process, known as rapid neutron capture, that created the heavier elements in the periodic table.” Rapid neutron capture is a process in which an atomic nucleus captures neutrons quickly enough to allow very heavy elements to be created. Although many elements are pro- duced in the cores of stars, creating elements heavier than iron, such as strontium, requires even hotter en- vironments with lots of free neu- trons. Rapid neutron capture only occurs naturally in extreme environ- ments where atoms are bombarded by vast numbers of neutrons. “This is the first time that we can directly