• Black holes are astronomical objects so compact that their gravity prevents anything, including light, from escaping, a prediction of Einstein's general relativity that involves an event horizon and a central singularity.
  • The idea dates back to the 18th century with John Michell and Pierre-Simon Laplace, but the first solution of general relativity came in 1916, and the first widely accepted black hole, Cygnus X-1, was identified in the early 1970s.
  • They typically form from collapsing massive stars in supernovae and grow by absorbing matter, with supermassive black holes like Sagittarius A* (about 4.3 million solar masses) at the center of the Milky Way existing in most galaxies.
  • Astronomers detect them via accretion disks that emit light, gravitational waves from mergers, and the orbital motions of stars, which reveal their mass and location.
  • Although quantum theory predicts they emit Hawking radiation and slowly lose mass, stellar black holes currently gain mass from the cosmic microwave background faster than they lose it.