Event Horizon.

The event horizon is the outer boundary of an interstellar object known as a black hole. A black hole is the collapsed remains of a massive dead star where gravity is so strong that not even light can escape. The event horizon marks a point of no return, past which nothing inside the black hole can ever be seen or detected from outside. Although an event horizon is spherical in shape, it is not a physical boundary; it simply marks the point at which the velocity needed to escape from the black hole’s gravity exceeds the speed of light—a speed that nothing can exceed. As would be expected, the size of a black hole’s event horizon depends on the size of the black hole itself. In relatively small black holes, anything passing the event horizon would be stretched out and ripped apart by gravity. However, in larger black holes, it would theoretically be possible to survive passing the event horizon, but such an explorer, and news of their fate, would be forever trapped inside the black hole.