🕳️ What is a black hole?
Where gravity is so strong that not even light escapes.
A black hole is a place in space where gravity is so strong that nothing can escape, not even light! Many form when very big stars collapse at the end of their lives.
A black hole is a region where matter is packed so densely that its gravity stops anything, even light, from getting out. Its boundary is called the event horizon. Stellar black holes form when massive stars collapse. Supermassive black holes, millions to billions of times the Sun's mass, sit at the centers of most large galaxies, including our Milky Way. We detect them through their effects: glowing disks of gas, stars orbiting them, and ripples in spacetime called gravitational waves.
In general relativity a black hole is a spacetime region bounded by an event horizon; for a non-rotating mass its size is set by the Schwarzschild radius r = 2GM/c², while real black holes are described by the Kerr metric (mass and spin). Accretion disks radiate efficiently and can launch relativistic jets. LIGO first detected gravitational waves from a binary black-hole merger in 2015, and the Event Horizon Telescope imaged M87* (2019) and Sagittarius A* (2022). Hawking radiation implies that black holes slowly evaporate, which raises the unresolved information paradox.
⚡ Quick facts
- The first image of a black hole, M87*, was published in 2019.
- The Milky Way's central black hole weighs about 4 million Suns.
- If the Sun became a black hole, it would be only about 6 km wide.