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Loading contentThe radius to which a mass must be compressed for its escape velocity to reach the speed of light — the size of a non-rotating black hole's event horizon.
Derived by Karl Schwarzschild in 1916 from Einstein's equations, it is proportional to mass: about 3 km per solar mass. Compress any mass within its Schwarzschild radius and it becomes a black hole.
Facts on this topic will be cited from these primary and reference sources.
Mission data, planetary science, space telescopes, and public-domain imagery.
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