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The event horizon is the boundary surrounding a black hole that marks the point of no return. It is not a solid surface or a shell made of matter; it is a location in spacetime defined by what can and cannot reach the outside universe. A person falling toward a very large black hole might not notice a sudden barrier at the instant of crossing it, at least locally. However, they could no longer send a signal that reaches distant observers. To someone watching from far away, light sent by the falling person would become increasingly stretched to longer wavelengths and dimmer because of intense gravity. This effect is called gravitational redshift. The horizon’s size grows with mass: a black hole with the Sun’s mass would have a Schwarzschild radius of roughly 3 kilometers, while a supermassive black hole can have a horizon larger than the orbit of a planet. Outside the horizon, matter and light may still orbit, fall in, or sometimes escape after being heated in surrounding gas.

General relativity predicts that matter falling inside an ideal black hole continues inward toward a singularity, a place where density and spacetime curvature are described as becoming infinite. This prediction signals that the theory has reached a limit rather than proving that nature truly contains an infinitely tiny point. Scientists expect that a future theory combining general relativity with quantum physics will give a more complete description of conditions at the center. What is well tested is the behavior outside black holes: mass curves spacetime, changing the routes followed by objects and light. Tidal forces become important near a black hole because gravity may be much stronger at a person’s feet than at their head. This difference can stretch an infalling object, an effect often called spaghettification. Around a supermassive black hole, the event horizon is much larger, so tidal forces at the horizon can be less severe than around a small stellar black hole. The center remains hidden because signals from within the horizon cannot return.

Many stellar black holes form when a massive star runs out of nuclear fuel. Without outward pressure from fusion to resist gravity, the star’s core collapses. A supernova explosion may blow away the outer layers, while the remaining core can become a neutron star or, if it is massive enough, a black hole. Stellar black holes commonly have masses several times greater than the Sun. Supermassive black holes, containing millions or billions of solar masses, sit at the centers of many galaxies and likely grew through mergers and long periods of swallowing gas and stars. Scientists detect black holes by studying their surroundings. Gas falling inward can form a rapidly spinning accretion disk, where collisions heat it until it shines strongly, often in X-rays. The motions of nearby stars can reveal an unseen massive object, and colliding black holes create ripples in spacetime called gravitational waves. In 2019, the Event Horizon Telescope released the first image of a black hole’s shadow against glowing gas, giving striking evidence for these extreme objects.

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Supermassive black holes contain millions to billions of times the mass of the Sun and are found near the centers of many large galaxies. The Milky Way contains one called Sagittarius A*, whose mass is about four million Suns. Although that sounds enormous, it is still tiny compared with the full galaxy, which contains hundreds of billions of stars. Scientists think these giant black holes and their galaxies may have grown together over cosmic time. A supermassive black hole can gain mass by pulling in gas, swallowing stars that pass too near, and merging with other black holes after galaxies collide. In the early universe, some grew especially quickly, producing brilliant objects called quasars. A quasar may shine more brightly than its entire host galaxy, not because light escapes from inside the event horizon, but because gas outside the horizon is heated as it falls inward. Studying stars orbiting galactic centers gives astronomers strong evidence for these otherwise invisible objects.

Black holes themselves do not glow, but matter near them can become some of the brightest material in space. Gas, dust, or pieces of a nearby star often do not fall straight inward because they carry sideways motion, or angular momentum. Instead, this material forms a flattened, rapidly rotating accretion disk around the black hole. Friction and magnetic forces within the disk make neighboring layers rub and interact, converting gravitational energy into heat. The inner disk can reach millions of degrees and emit ultraviolet light, X-rays, and other radiation. As matter loses energy, it spirals closer to the event horizon. In some systems, strong magnetic fields channel a small fraction of the infalling material into narrow jets that shoot away from the disk’s poles at speeds close to that of light. Jets can extend far beyond their host galaxy and affect surrounding gas, sometimes slowing or triggering new star formation. The disk and jets, rather than the black hole’s interior, provide visible clues about intense gravity.

Astronomers detect black holes by observing their influence on matter and light nearby, rather than by seeing the black holes themselves. One useful method is to track a visible star orbiting an unseen companion. If the star moves as though it is being pulled by a very massive, compact object that produces no light, a black hole is a strong possibility. In binary systems, a black hole may draw gas from its companion star. The gas heats up in an accretion disk and releases X-rays, which space telescopes can measure. At galaxy centers, scientists map the speeds and paths of stars and gas; very fast orbits around a dark central point reveal a concentrated mass. Black hole mergers offer another method. When two black holes collide, they send out gravitational waves: tiny ripples in spacetime that stretch and squeeze distances by an incredibly small amount. Detectors such as LIGO measure these ripples, allowing scientists to calculate the merging objects’ masses and learn about black hole populations across the universe.

A black hole’s gravity changes the behavior of time, light, and matter near its event horizon. According to general relativity, clocks closer to a strong gravitational field run more slowly when compared with clocks far away. Light climbing away from a black hole loses energy and is shifted toward longer, redder wavelengths, an effect called gravitational redshift. For an observer falling toward a large black hole, crossing the event horizon might not feel like hitting a solid boundary. However, a distant observer would receive increasingly delayed and redshifted signals from that traveler. Near smaller black holes, differences in gravitational pull between a person’s head and feet could become enormous, stretching objects in a process nicknamed spaghettification. Important questions remain. Physicists do not yet have a complete theory that combines quantum physics with gravity inside a black hole. They also debate how information about matter that falls in can be preserved if black holes slowly lose energy through Hawking radiation. These puzzles make black holes valuable laboratories for testing fundamental laws.