Black Holes

Black holes are regions of spacetime where gravity is so strong that, once inside a boundary called the event horizon, nothing can escape to the outside, not even light.

The central idea

Einstein's general relativity describes gravity as the curvature of spacetime. A sufficiently compact amount of mass can curve spacetime into a one-way surface. The event horizon is not a solid shell; it is a boundary in the geometry of spacetime.

How they form

The core of a very massive star can collapse after the star runs out of fuel. If the core is heavy enough, no known pressure can stop the collapse, producing a stellar-mass black hole. Supermassive black holes, millions or billions of times the Sun's mass, occupy the centers of most large galaxies and likely grew through mergers and the accumulation of gas and stars.

What we can observe

Black holes are dark, but their surroundings are not necessarily dark. Hot gas spiraling through an accretion disk can emit intense light. A black hole's gravity can also bend light, move nearby stars, and create gravitational waves when black holes merge.

A useful correction

Black holes do not automatically suck in everything nearby. Far from the horizon, their gravity is the same as that of any other object with the same mass. An object falls in only when its path brings it sufficiently close.

Learning path

Read guide.md for the ideas in more detail, then use glossary.md to review the vocabulary.