Free Astronomy Magazine July-August 2020

34 JULY-AUGUST 2020 SPACE CHRONICLES T his wide-field view shows the region of the sky, in the constellation of Tele- scopium, where HR 6819 can be found, a triple system consisting of two stars and the closest black hole to Earth ever found. This view was created from im- ages forming part of the Digitized Sky Survey 2. While the black hole is invisible, the two stars in HR 6819 can be viewed from the southern hemisphere on a dark, clear night without binoculars or a telescope. [ESO/Digitized Sky Survey 2] ple of dozen black holes in our galaxy to date, nearly all of which strongly interact with their envi- ronment and make their presence known by releasing powerful X-rays in this interaction. But scientists es- timate that, over the Milky Way’s lifetime, many more stars collapsed into black holes as they ended their lives. The discovery of a silent, invis- ible black hole in HR 6819 provides clues about where the many hidden black holes in the Milky Way might be. “There must be hundreds of mil- lions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” says Rivinius. Baade adds that finding a black hole in a triple system so close by indicates that we are seeing just “the tip of an excit- ing iceberg.” Already, astronomers believe their discovery could shine some light on a second system. “We realised that another system, called LB-1, may also be such a triple, though we'd need more observations to say for sure,” says Marianne Heida, a post- doctoral fellow at ESO and co-au- thor of the paper. “LB-1 is a bit further away from Earth but still pretty close in astronomical terms, so that means that probably many more of these systems exist. By find- ing and studying them we can learn a lot about the formation and evo- lution of those rare stars that begin their lives with more than about 8 times the mass of the Sun and end them in a supernova explosion that leaves behind a black hole.” The discoveries of these triple sys- tems with an inner pair and a distant star could also provide clues about the violent cosmic mergers that re- lease gravitational waves powerful enough to be detected on Earth. Some astronomers believe that the mergers can happen in systems with a similar configuration to HR 6819 or LB-1, but where the inner pair is made up of two black holes or of a black hole and a neutron star. The distant outer ob- ject can gravitationally im- pact the inner pair in such a way that it triggers a merger and the release of gravitational waves. Although HR 6819 and LB-1 have only one black hole and no neutron stars, these systems could help scientists understand how stellar col- lisions can happen in triple star systems. T his animation shows the orbits and movements of the objects in the HR 6819 triple system. This system in- cludes an inner binary with one star (orbit indicated in blue) and a newly discovered black hole (orbit indicated in red). As we move away from this inner pair, we see the outer object in the system, an- other star in a much wider orbit (in blue). [ESO/L. Calçada] !