Astronomers studying what they thought were a double star system 1,000 light years from Earth in the southern constellation Telescopium stumbled on what must be a stellar mass black hole, an unseen companion with four times the mass of the Sun that gravitationally shapes the orbit of its nearest companion. It is the nearest known black hole to Earth. The star system, known as HR 6819, was observed as part of a study of binary star systems. Using the FEROS spectrograph with the European Southern Observatory’s MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile, researchers were surprised to find one of the visible stars, one with at least five times the mass of the sun, was orbiting an unseen body every 40 days. The second visible star is at a larger distance from the inner pair. Analysing the inner star’s motion, the team concluded it is orbiting a black hole. Only a few dozen stellar-mass black holes have been found in the Milky Way to date. Most of them interact violently with their environment, releasing bursts of detectable X-rays. But quiescent black holes may be a commonplace end state for massive stars across the galaxy’s history.