Regions of spacetime with gravity so strong that light cannot escape
Black holes
Black holes are extremely dense objects formed when massive stars collapse or when matter is compressed into a region whose gravity reshapes spacetime and traps everything past the event horizon.
What a black hole is
A black hole is not a hole in the ordinary sense. It is a region of spacetime where gravity has become so intense that, past a boundary called the event horizon, nothing can travel back out, not even light. That does not make black holes invisible in practice. Their effects can be inferred from the motion of nearby stars, hot gas, and light that is bent and energized by the surrounding environment.
How they form
Many black holes form when a massive star runs out of fuel and its core collapses under its own gravity. If the leftover core is heavy enough, no known force can stop the collapse. Other black holes may grow by merging with smaller black holes or by feeding on gas, dust, and stars in dense regions such as galactic centers.
Event horizon and singularity
The event horizon marks the point of no return. It is a geometric boundary, not a physical surface, but crossing it changes what paths through spacetime are possible. Deep inside the simplest models, matter is compressed toward a singularity, a place where current physics stops giving a reliable description. That does not mean the singularity is fully understood; it means our theories are incomplete at extreme densities.
What we can observe
Astronomers detect black holes through indirect evidence. Matter falling inward can heat up and glow in X-rays. Stars orbiting an invisible companion can reveal a hidden mass. Gravitational waves can mark the merger of two black holes. In 2019, the Event Horizon Telescope produced the first image of a black hole's shadow in the galaxy M87, giving a direct look at the region around the horizon.
Time, light, and gravity
Black holes are one of the clearest consequences of Einstein's general relativity. Near them, time runs differently depending on position, light bends strongly, and spacetime itself is curved enough to dominate motion. That makes them useful laboratories for testing gravity under conditions that cannot be recreated on Earth.
Why they matter
Black holes shape galaxies, influence star formation, and power some of the brightest events in the universe when they feed on nearby matter. They also force physicists to confront the boundary between general relativity and quantum theory. Questions about what happens to information, matter, and entropy near a black hole remain some of the deepest in modern physics.