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- Black hole classes
- Stellar black holes
- Supermassive black holes
- Black holes and gravitational waves
- Fate of black holes
- Links to Primary Literature
- Additional Readings
A region of spacetime exerting a gravitational field so strong that neither matter nor radiation can escape. Black holes are extreme cosmic objects predicted by German-born U.S. physicist Albert Einstein's theory of general relativity. Within a boundary known as the event horizon, the escape velocity needed to overcome the gravitational attraction of the black hole exceeds the speed of light, meaning that nothing that crosses over the event horizon can ever leave. Black holes are therefore by definition invisible, but because of their powerful gravitational fields, they can be indirectly observed through the highly conspicuous effects they have on their cosmic environment. These effects include the gravitational intake of matter through accretion disks, a process which generates tremendous heat and light and is well-observed at scales from binary star systems to the cores of galaxies. In the absence of ongoing accretion, black holes should also theoretically cause severe localized warping of spacetime, gravitationally lensing light from luminous sources and distorting their appearance (Fig. 1). See also: Astronomy; Escape velocity; Gravitation; Gravitational lens; Relativity
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