Free Astronomy Magazine July-August 2020

33 JULY-AUGUST 2020 SPACE CHRONICLES the period of 40 days had to be spread over several months. This was only possible thanks to ESO’s pio- neering service-observing scheme under which observations are made by ESO staff on behalf of the scien- tists needing them.” The hidden black hole in HR 6819 is one of the very first stellar-mass black holes found that do not inter- act violently with their environment and, therefore, appear truly black. But the team could spot its presence and calculate its mass by studying the orbit of the star in the inner pair. “An invisible object with a mass at least 4 times that of the Sun can only be a black hole,” concludes Rivinius, who is based in Chile. As- tronomers have spotted only a cou- T his artist’s impression shows the orbits of the objects in the HR 6819 triple system. This system is made up of an inner binary with one star (orbit in blue) and a newly discovered black hole (orbit in red), as well as a third object, another star, in a wider orbit (also in blue). [ESO/L. Calçada]