Black Holes:
Where Gravity Rules

A black hole is a region of space where gravity is so strong that nothing—not even light—can escape from inside its event horizon. Black holes are not empty holes in space. They are objects containing mass concentrated in an extremely compact region.

According to Einstein's general theory of relativity, mass and energy affect the geometry of spacetime. A black hole forms a region from which every possible future path for light remains inside the event horizon once light crosses that boundary.

Black holes can have different masses. Some are formed from massive stars, while supermassive black holes contain millions or billions of times the mass of our Sun.

The event horizon is the boundary around a black hole beyond which light cannot escape to the distant universe. It is not a solid surface that a spacecraft would hit. Instead, it is a boundary defined by the structure of spacetime.

For a simple, non-rotating black hole, the Schwarzschild radius gives the radius of its event horizon. The radius increases in proportion to the black hole's mass.

The environment around a black hole can be very different from the region inside it. Hot gas outside the event horizon may emit light and X-rays that telescopes can observe.

Stellar-mass black holes

Some black holes form when a sufficiently massive star reaches the end of its life. After the star can no longer support itself against gravity, its core may collapse. Depending on the star's mass and other conditions, the result can be a black hole. Some such events are associated with supernova explosions, but not every massive star ends in the same way.

Supermassive black holes

Supermassive black holes are found at the centers of many galaxies. They may grow by accumulating gas and merging with other black holes over cosmic time. Their full formation history is still an active area of research.

A black hole does not shine like an ordinary star. Scientists therefore look for its effects on nearby matter and light. Several independent methods help astronomers investigate them.

• Hot gas: Gas falling toward a black hole can form a hot accretion disk that emits radiation.
• Star orbits: Stars moving around an invisible, compact object can reveal its mass.
• Gravitational waves: Merging black holes can create ripples in spacetime detected by instruments such as LIGO.
• Gravitational lensing: A black hole's gravity can bend light from more distant objects.

The first black hole images

In 2019, the Event Horizon Telescope collaboration released the first image of the shadow of the supermassive black hole M87*. In 2022, the collaboration released an image of Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy.

These are not ordinary photographs of the dark interior. The images were reconstructed from radio observations and show a dark central shadow surrounded by emission from hot material and light bent by strong gravity.

Myth: Black holes suck in everything in the universe.

Fact: A black hole's gravity depends on its mass and distance. If the Sun were replaced by a black hole of the same mass, Earth would not suddenly be pulled into it. Earth would lose sunlight, however, and life as we know it would not survive.

Myth: Scientists can see the inside of a black hole.

Fact: No signal from inside the event horizon can reach a distant observer. Scientists study surrounding matter, orbital motion, gravitational waves, and other observable effects.

Myth: Every black hole is the same size.

Fact: Black holes span a wide range of masses, from stellar-mass objects to supermassive black holes at galactic centers.

Black holes are supported by strong observational evidence, but important questions remain. Scientists are still investigating how the earliest supermassive black holes formed, how matter behaves under extreme conditions, and how gravity should be described where quantum physics and general relativity both become important.

A good scientific explanation distinguishes measured evidence from ideas that remain uncertain. New observations may refine or change our understanding as research continues.