A black hole is a place in space where gravity is so strong that even light cannot get out. A stellar black hole forms when the center of a very big star collapses after the star runs out of fuel. A supernova is a star that explodes, and some black holes are born when a big star explodes. Sources disagree on how massive a star must be to leave a black hole, with estimates from about 20 to about 25 Suns; smaller stars end as neutron stars or white dwarfs. Black holes come in different sizes. The biggest black holes are called supermassive black holes. Some black holes have as much mass as millions of Suns.

A black hole is a lot of matter squeezed into a very small space. The edge of a black hole is called the event horizon. Nothing can come back out once it crosses the event horizon. Gravity can be too strong to escape from. To leave Earth you need to go about 11 kilometers per second, but inside a black hole's event horizon you would need to go faster than light. Near a black hole, gravity bends space so much that light cannot leave, and the same bending makes images of the disk behind the black hole look stretched and doubled. The event horizon is not a hard surface like the ground.

The black hole in the middle of the Milky Way, called Sagittarius A*, has about 4 million times the mass of the Sun. Stars near the center of our galaxy orbit an unseen mass; the star S2 came closest to Sagittarius A* in May 2018, which helped show the black hole is there. Reinhard Genzel and Andrea Ghez shared half of the 2020 Nobel Prize in Physics for finding a supermassive compact object at the center of our galaxy by tracking stars. The 2022 picture of Sagittarius A* was the first visual confirmation that the dark object at the center of our galaxy is a black hole. Sagittarius A* is hard to image because it changes while the data are collected; the Event Horizon Telescope needs many hours of observing, like a long camera exposure.

In 2019 scientists showed the first picture of a black hole, in a galaxy called Messier 87. The bright ring in the picture of M87's black hole is light from hot gas that is bent around the black hole, and the dark middle is its shadow. The shadow in black hole images is about twice the size of the black hole's actual surface; for M87 the shadow is about 38 billion kilometers across. The mass of the M87 black hole was first estimated from the motion of stars (about 5 billion Suns in 1978, 6.6 billion in 2011), and the shadow later gave about 6.5 billion, a close match.

The glowing disk of gas around a black hole, called the accretion disk, is the main source of light we can see. Matter falling toward a black hole is heated to millions of degrees, which makes accretion one of the most energetic processes known. Most stellar-mass black holes we have found are paired with a normal star, and the hot gas pulled from the star glows in X-rays and gives the black hole away. A black hole with no gas near it has no accretion disk, so it makes no light and is almost impossible to see directly; most black holes are found by the glow of nearby gas. Space telescopes can help find black holes, even though we cannot see the black holes themselves.

Einstein's general theory of relativity (1915) is where black holes come from, and in 1916 the same theory predicted gravitational waves. On September 14, 2015, detectors in Louisiana and Washington caught ripples in space from two black holes crashing together. The two black holes in the first LIGO detection (about 29 and 36 Suns) merged into one of about 62 Suns, and about 3 Suns of mass left as gravitational waves. The first black hole merger ever detected happened about 1.3 billion years ago, roughly a billion light-years away.

Supermassive black holes can merge when their galaxies collide, and they can grow by eating gas and stars. As of November 2026, black holes of medium size should exist between small stellar black holes and giant supermassive ones, but they are hard to confirm. Some supermassive black holes already existed in the universe's first billion years, and how they grew so big so fast is still unsolved.

Words to know

• shadow: dark middle
• exposure: long look
• estimated: roughly worked out
• accretion: falling together
• orbit: move around