A black hole is a region of space where gravity is so extreme that nothing, not even light, can escape. This happens when a massive amount of matter gets compressed into an incredibly small volume, warping the fabric of space so severely that it essentially cuts itself off from the rest of the universe. Black holes are real, well-documented objects. Astronomers have detected them throughout the cosmos, and in 2019, a global network of radio telescopes captured the first direct image of one.
How Black Holes Form
Most black holes begin as massive stars. When a star many times heavier than our Sun exhausts its nuclear fuel, it can no longer support its own weight. The core collapses violently, sometimes triggering a supernova explosion that blasts the outer layers into space. What remains of the core keeps collapsing. If enough mass is left behind, no force in nature can stop the collapse, and a black hole is born.
This process isn’t always dramatic. Some black holes form during energetic, asymmetric supernova explosions. Others result from a supernova that starts but stalls, with the collapsing material falling back inward. In rarer cases, the collapse is nearly silent, producing a black hole with little or no visible explosion at all. The resulting objects range from a few times the Sun’s mass to roughly 40 times its mass, depending on the size and composition of the original star.
The Event Horizon and the Singularity
A black hole has two defining features. The first is the event horizon, a spherical boundary that acts as a point of no return. Cross it, and you cannot get back out, no matter how fast you travel. Light itself cannot escape from inside this boundary, which is why the region appears black. The interior of the event horizon is causally disconnected from the rest of the universe. Events that happen inside it can never be observed from the outside.
The size of the event horizon depends entirely on mass. A black hole with the mass of our Sun would have an event horizon just 2.9 kilometers (about 1.8 miles) in radius. A black hole with a billion times the Sun’s mass would have a proportionally enormous horizon. This boundary is sometimes called the Schwarzschild radius, named after the physicist Karl Schwarzschild, who first calculated it in the early twentieth century.
At the very center lies the singularity, a point of zero volume and, in theory, infinite density. All the matter that falls into a black hole piles up here. The singularity is where our current understanding of physics breaks down. General relativity predicts it must exist, but many physicists suspect a more complete theory will eventually replace this picture with something less extreme.
Types of Black Holes
Astronomers generally group black holes into three confirmed categories based on mass, with a suspected fourth type that hasn’t been directly detected yet.
- Stellar-mass black holes range from a few to hundreds of times the Sun’s mass. These are the remnants of collapsed stars and are the most common type.
- Supermassive black holes contain hundreds of thousands to billions of solar masses. Nearly every large galaxy harbors one at its center. The one at the heart of our Milky Way, called Sagittarius A*, has about 4 million times the mass of the Sun and sits roughly 26,000 light-years from Earth.
- Intermediate-mass black holes fall between the other two, ranging from around a hundred to hundreds of thousands of solar masses. They are the hardest to find and the least understood.
- Primordial black holes are hypothetical objects that may have formed in the extreme conditions of the early universe. If they exist, their masses could span an enormous range, from far less than a paperclip to more than 100,000 times the Sun’s mass.
What Surrounds a Black Hole
A black hole by itself is invisible. What makes many of them detectable is the material swirling around them. When gas from a nearby star or interstellar cloud gets pulled toward a black hole, it doesn’t fall straight in. Instead, it forms a hot, bright, rapidly spinning structure called an accretion disk. Matter in this disk gradually spirals inward from the outer edge to the inner edge, heating up to millions of degrees and radiating intensely before finally crossing the event horizon.
The black hole’s gravity warps the space around it so dramatically that light from the accretion disk follows curved paths. If you could view the disk from an angle, you’d see it appear to wrap over and under the black hole in a distorted shape, a phenomenon called gravitational lensing. Thin rings of light also appear at the edge of the black hole’s “shadow,” formed by photons that orbited the black hole one or more times before escaping toward the observer. Rings closer to the event horizon become progressively thinner and fainter.
Black holes that have consumed all the nearby matter and lack an accretion disk are extremely difficult to detect. The closest known black hole to Earth, called Gaia BH1, is one of these dormant types. Discovered in 2022, it sits about 1,560 light-years away in the constellation Ophiuchus and was found only because of the gravitational influence it exerts on a Sun-like companion star. It has roughly 9.6 times the Sun’s mass.
What Happens if You Fall In
The experience of falling into a black hole depends entirely on its size. The key factor is tidal force, the difference in gravitational pull between two points. Your feet, being closer to the black hole, would be pulled harder than your head, stretching you lengthwise while compressing you from the sides. Physicists call this process spaghettification.
Near a stellar-mass black hole, tidal forces become lethal long before you reach the event horizon. At a distance of just 100 kilometers from a black hole with the Sun’s mass, the tidal acceleration across a human body would reach over 51,000 times Earth’s gravity. You would be torn apart.
A supermassive black hole is a different story. Because its mass is spread across a vastly larger event horizon (hundreds of millions of kilometers in radius for the biggest ones), the tidal forces at the horizon are surprisingly gentle. At 100 kilometers from the event horizon of a 100-million-solar-mass black hole, the tidal force on a human body would be essentially zero. You could cross the event horizon without feeling anything unusual. The destruction would come later, as you continued falling inward toward the singularity.
How Scientists Photograph a Black Hole
In April 2017, the Event Horizon Telescope (EHT) observed the supermassive black hole at the center of the galaxy M87 using a network of radio telescopes spread across the globe, effectively creating a single Earth-sized instrument. The observations were made at a wavelength of 1.3 millimeters. Four independent teams processed the data using different techniques, each blind to the others’ results. All four produced the same core feature: a bright ring of light about 40 microarcseconds in diameter surrounding a dark central shadow. That ring remained stable across four consecutive nights of observation.
The image doesn’t show the black hole itself. It shows the glowing gas of the accretion disk and the distorted light orbiting near the event horizon, framing the dark region where light cannot escape. The ring’s brightness is uneven, appearing brighter on one side, which results from the gas rotating at close to the speed of light. Material moving toward the observer appears brighter due to a phenomenon similar to the Doppler effect.
Can Black Holes Die?
In the 1970s, Stephen Hawking proposed that black holes aren’t perfectly black. Quantum effects near the event horizon should cause black holes to emit a faint glow of thermal radiation, now called Hawking radiation. This radiation carries energy away from the black hole, meaning it slowly loses mass over time. Given enough time, a black hole would theoretically shrink and eventually evaporate entirely.
For any black hole that exists today, this process is unimaginably slow. A stellar-mass black hole would take far longer than the current age of the universe to evaporate. Hawking radiation has never been directly observed, but it remains one of the most important theoretical predictions linking gravity and quantum physics. It also raises a deep unsolved puzzle: if a black hole can evaporate completely, what happens to the information about everything that fell into it? That question, known as the black hole information paradox, is one of the biggest open problems in theoretical physics.