What Are Black Holes? A Guide to the Strangest Objects in Physics
A black hole is a region of spacetime so warped that nothing — not even light — travels fast enough to escape once it crosses a boundary called the event horizon. They are predictions of Einstein's general theory of relativity, and astronomers have now observed them both indirectly and directly.
How They Form
Most black holes form when a massive star (roughly twenty or more times the mass of the Sun) exhausts its nuclear fuel. With radiation pressure no longer holding the star up against gravity, the core collapses. If the remaining mass is large enough, no known force can stop the collapse, and a singularity forms: a point of effectively infinite density surrounded by the event horizon.
Stellar-mass black holes are created this way. Supermassive black holes — millions to billions of times the Sun's mass — sit at the centers of galaxies and likely grew over cosmic time through mergers and accretion.
Why They Bend Space and Time
General relativity treats gravity not as a force but as curvature of spacetime. A black hole's mass deforms spacetime so severely that within the event horizon all paths curve inward. To outside observers, clocks near a black hole appear to run slow, an effect called gravitational time dilation.
Gas falling toward a black hole forms a hot, glowing accretion disk — the source of the radiation astronomers detect even though the hole itself emits no light.
This article is an introductory explainer, not a rigorous treatment. For technical detail, consult a general relativity textbook or peer-reviewed sources.