Black holes are locations where spacetime is bent so severely that nothing can escape. They form when massive stars can no longer support themselves with the energy released in nuclear reactions. The resulting runaway collapse compresses the star's core into a region of enormous gravitational pull while ejecting most surrounding material in a massive supernova. This explosion produces elements that comprise up to 73% of the human body's mass.
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Predicted by Einstein in 1916, these invisible objects can be indirectly seen through their accretion discs and X-ray bursts. Over 40 million trillion black holes may be scattered throughout the universe.
For nearly 50 years, scientists dismissed black holes as implausible oddities, despite solutions to the equations indicating their existence. But the 1970s discovery of intense X-rays from Cygnus X-1 and its massive hidden companion flipped the consensus.
Because black holes are so hard to spot directly, astronomers rely on models of galaxy and star evolution to estimate their population. A recent study used simulations and galactic data to show that about 1% of all matter in the universe is locked inside black holes.
Over time, less energy is released from the cores of massive stars as heavier elements are created through fusion. Once iron has been fused, energy is consumed rather than created, and runaway collapse creates a dense enough region where the stellar core existed to warp spacetime into a black hole.
The breakthrough came when Albert Einstein realized that freely falling is indistinguishable from weightlessness, leading him to the principle of equivalence between gravity and acceleration. His prediction that gravity bends light was confirmed in 1919 during a solar eclipse.
The 1964 discovery of Cygnus X-1 provided the first strong candidate, but it took decades—and a famous bet between Stephen Hawking and Kip Thorne—before scientists were certain it was a black hole. Hawking's thumbprinted concession was delivered after breaking into Thorne's office.
Complex synchronization and novel imaging algorithms transformed eight radio telescopes worldwide into an Earth-sized virtual observatory. Before the real thing, the team used synthetic data from fake sources—including one shaped like Frosty the Snowman—to test their imaging methods.
For years, it was known as a strong source of radio signals, but there wasn't enough data to adequately describe or identify it. Orbital movements of nearby stars would indicate it had 4.6 million times the sun's mass, proving it was a black hole.
Three types are known to exist: stellar mass black holes, supermassive black holes, and intermediate black holes. A fourth type—micro black holes—is theoretical and would have the same mass as Mount Everest in a space the size of an atom.
The horizon is often surrounded by an accretion disc of superheated material that orbits the black hole while emitting radiation. Light emitted behind a black hole can be bent around it to create the appearance of a ring of material known as a photon sphere.
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