Black Holes
A black hole is a region of spacetime whose gravity is so strong that nothing, not even light, can escape past its boundary, the event horizon. First imagined as 'dark stars' in the 18th century and predicted by Einstein's general relativity in 1915, black holes were long considered unobservable. Today they are studied through gravitational waves, X-ray binaries, and the orbits of stars — and were photographed directly for the first time by the Event Horizon Telescope in 2019 (M87*) and 2022 (Sagittarius A*).