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The Event Horizon is the boundary around a black hole beyond which nothing, not even light, can escape. The size of the event horizon is a measure of the black hole's mass. The more massive the black hole, the larger the radius of its event horizon.
Gravity is so unimaginably strong within a black hole, that a 10-solar-mass black hole has an event horizon of about 30 kilometers, and the Earth, if compressed into a black hole, would have a radius of about 9 millimeters. That being said, the Earth would be roughly the size of a jewelry bead, yet would weigh the same.
* The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to 149,597,870,700 meters. Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its modern redefinition in 2012.
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