A black hole is a mysterious place where the laws of physics people are familiar with stop working. Black holes appear when massive stars collapse under their own weight. The gravitational field of the newly formed object is so powerful that even light, including X-rays, can't escape it. Every black hole has an invisible line-in-the-sand. Cross it - and you won't be able to escape, even if you're a beam of light. Beyond the point of no return, the gravity is just too strong. It's called the event horizon.

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