Glossary · Thiên văn học
Event horizon
The boundary around a black hole inside which the speed needed to escape exceeds the speed of light. Anything that crosses it, light included, cannot get back out. The event horizon is not a physical surface.
The event horizon is often called the "point of no return". At this boundary the escape velocity equals the speed of light; the radius at which that happens is called the Schwarzschild radius. Because light cannot get out, nothing about what happens inside can reach an outside observer.
Whether an infalling object is torn apart by tidal forces before or after crossing the horizon depends on the black hole's mass. According to NASA, stellar-mass black holes have much smaller event horizons and stronger tidal forces, which can rip objects apart before they reach the horizon. For a supermassive black hole like the one at the centre of the Milky Way, a NASA simulation shows an object crossing the horizon first and only then being stretched apart.
The event horizon itself cannot be seen, because it emits no light. Images from the Event Horizon Telescope show a dark central region, called the black hole's shadow, surrounded by a bright ring from hot gas orbiting the black hole.