What Is a Black Hole?

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

So Why is it called a Hole?

Albert Einstein’s 1915 General Theory of Relativity deals largely with the effects of gravity, and in essence predicts the existence of black holes and singularities. Einstein hypothesized that gravity is a direct result of mass distorting space. He argued that space behaves like an invisible fabric with an elastic quality. Celestial bodies interact with this “fabric” of space-time, appearing to create depressions termed “gravity wells” and drawing nearby objects into orbit around them. Based on this principle, the more massive a body is in space, the deeper the gravity well it will create. Therefore, an object with enormous mass but the infinitely small size would create a bottomless pit—a black hole.

How big are Black Holes?

Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object.

Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar-mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

What does a Black Hole look like?

Because of their nature, black holes cannot be seen. Black holes do not have a physical surface. Instead, they begin at a central point of singularity and continue out to a spherical boundary. The event horizon is the “dividing line,” beyond which anything that crosses cannot escape. Outside the event horizon, material falling into the black hole collects into a 3 band of hot gas and dust called an accretion disk. Narrow jets of gas shoot out from the accretion disk, emitting detectable radiation. The physical size of black holes is measured with a special unit called the Schwarzschild radius. This radius is defined to be the distance from the point of singularity to the event horizon. The larger the Schwarzschild radius, the more massive the black hole.

How Do Black Holes Form?

Scientists think the smallest black holes formed when the universe began.

Stellar black holes are made when the center of a very big star falls in upon itself or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

Scientists think supermassive black holes were made at the same time as the galaxy they are in.

If Black Holes Are "Black," How Do Scientists Know They Are There?

A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.

When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see high-energy light.

If we can't see them, How do we know they're out there?

Black holes—by definition—cannot be seen directly. The only way to find a black hole is to look for its effects on other objects in space around it. Observation of gas jets, radiation, rapidly orbiting objects, and other methods are used to indirectly detect the locations of black holes. Astronomers have observed evidence this way for dozens of black holes in our own galaxy. Scientists who study black holes focus on how other bodies are affected in the space around them. The first approach to locating black holes involved observing binary star systems. In these systems, two stars orbit each other, moving in generally predictable ways because of the gravitational attraction between the stars. Scientists knew that if they saw a single star moving as if there were a massive object nearby, but with no other star in evidence, then its invisible companion could be a black hole. Scientists also realized that if the invisible object in a binary system was a black hole, there would be huge gravitational force associated with it. The gas from the visible star—or any nearby gas and dust—would spiral at very high speeds around the black hole before disappearing into it. This action would create enormous heat and X-ray radiation, which could be detected through observations. In the 1970s, scientists took great interest in gamma-ray bursts as a way to detect black holes. One hypothesis suggested that a binary system consisting of a normal star and a black hole creates gamma-ray bursts when the black hole finally consumes all of its companion star’s material. Another widely-accepted theory suggests that gamma rays are released when black holes or neutron stars collide. Gamma-ray bursts are probably also released when a giant star collapses and a black hole is formed.

Could a Black Hole destroy the earth?

Black holes do not go around in space eating stars, moons, and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that. Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now. The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.

How Is NASA Studying Black Holes?

NASA is using satellites and telescopes that are traveling in space to learn more about black holes. These spacecraft help scientists answer questions about the universe.