Black holes

A black hole is not an empty hole in space but a compact concentration of mass whose gravity has a defining boundary: the event horizon. In classical descriptions, once matter or light crosses that boundary, it cannot return to distant observers. The event horizon is not a solid surface; it marks a limit on what can escape and be observed from outside. The extreme gravity of a black hole can strongly bend the paths of light and matter around it.

Astronomers commonly describe black holes as stellar-mass, intermediate-mass or supermassive, while noting that the boundaries between these groups are approximate. A stellar-mass black hole can form when the core of a sufficiently massive star collapses at the end of its life. Supermassive black holes, with masses from hundreds of thousands to billions of Suns, are found at the centers of many large galaxies. The black hole known as Sagittarius A-star, at the center of the Milky Way, is one example. How the largest black holes first formed remains an open research question; observations show that some were already present early in the history of the universe.

Because a black hole does not emit or reflect light in the way a luminous star does, it is usually identified through its influence on its surroundings. Gas spiraling into a black hole can form a hot accretion disk that radiates across the electromagnetic spectrum. The motions of nearby stars can reveal an unseen massive object, and gravity can distort light from more distant sources. Pairs of black holes that orbit and merge can also be detected through the gravitational waves they produce. These methods provide different evidence about a black hole's mass, environment and motion.

In 2019, the Event Horizon Telescope collaboration released the first image revealing the shadow of a black hole: the black hole known as M87-star, at the center of the galaxy Messier 87. The EHT links radio telescopes around the world into an Earth-sized virtual instrument. Its image shows a bright ring of emission around a dark central region. The dark region is the black hole's shadow, produced by the capture and gravitational bending of light; it is not a photograph of a solid surface at the event horizon.

Black holes provide a way to investigate gravity under extreme conditions and to test predictions of general relativity. Observations of accretion, stellar orbits and mergers also help astronomers study the environments and development of galaxies. Despite major advances in detection, the physics of matter inside an event horizon and the origins of the most massive black holes remain unresolved questions.