What is an event horizon?
Quick answer
An event horizon is the boundary around a black hole beyond which nothing can escape, not even light. It’s not a solid surface but a point of no return: anything that crosses it is pulled inward forever. Its size depends on the black hole’s mass, a distance called the Schwarzschild radius.
Why light can’t escape
To escape a planet’s gravity, an object needs to reach its escape velocity; for Earth that’s about 11.2 km per second. A black hole packs so much mass into so small a space that at the event horizon, the escape velocity equals the speed of light, about 300,000 km per second. Since nothing travels faster than light, nothing can get out.
How big is an event horizon?
The radius grows with the black hole’s mass:
| Black hole mass | Event horizon radius (approx.) |
|---|---|
| Earth squeezed into a black hole | ~9 mm (a marble) |
| The Sun’s mass | ~3 km |
| Sagittarius A*, center of the Milky Way (~4 million Suns) | ~12 million km |
| M87*, a giant black hole (~6.5 billion Suns) | ~19 billion km, larger than our solar system |
Seeing the “shadow”
Black holes can’t be seen directly, but the hot, glowing gas swirling around them can. In 2019, the Event Horizon Telescope, a network of radio telescopes across Earth acting like one planet-sized dish, released the first image of a black hole’s shadow, in galaxy M87. In 2022, it imaged Sagittarius A*, at the center of our own galaxy.
What would happen if you fell in?
From far away, you’d appear to slow down and fade as you approached the horizon, never quite crossing it. For you, crossing could feel uneventful at first near a giant black hole. Closer to the center, extreme tidal forces would stretch you out, a process scientists jokingly call “spaghettification.”
Hawking radiation
In 1974, physicist Stephen Hawking predicted that black holes slowly leak energy near their event horizons, so they can very gradually evaporate. This “Hawking radiation” hasn’t been observed yet.
Stars like Betelgeuse may end their lives as black holes or neutron stars. For our galaxy’s big neighbor, see the Andromeda Galaxy.
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