What Is a Black Hole? — For Kids

Elementary: A Place With a Very Strong Pull

NASA has a page for young kids. It is called What Is a Black Hole? for Grades K-4. This part follows that NASA page.

What is a black hole?

NASA says 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. NASA says this can happen when a star is dying.

Think of it like this. A lot of stuff is packed into a very small spot. That packed spot pulls very, very hard.

Why can we not see one?

Because no light can get out, people can not see black holes. NASA says they are invisible.

NASA also says 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 still learn about them. Space telescopes with special tools can help find black holes.

The tools can see how stars that are very close to black holes act in a different way than other stars. Scientists 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. Human eyes can not see this kind of light. Scientists use satellites and telescopes in space to see it.

Big and small black holes

Black holes can be big or small.

Scientists think the smallest black holes are as small as just one atom. NASA says these tiny black holes still have the mass of a large mountain.

Mass is the amount of matter, or stuff, in an object. That is NASA's own plain-words meaning.

One kind is called stellar. Its mass can be up to 20 times more than the mass of the Sun. NASA says 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 is called Sagittarius A. It has a mass equal to about 4 million suns.

How does a stellar black hole form?

Stellar black holes are made when the center of a very big star falls in upon itself. NASA's word for that is collapses.

When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

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

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

Is Earth in danger? NASA says no.

Black holes do not go around in space eating stars, moons and planets. That is how NASA puts it on the kids' page.

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 does NASA learn more?

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.

Words to know — Elementary

Gravity: The pull that holds us to the ground on Earth. In a black hole, NASA says gravity pulls so much that even light can not get out.

Matter: Stuff. NASA says black-hole gravity is so strong because matter has been squeezed into a tiny space.

Mass: The amount of matter, or stuff, in an object. That is NASA's meaning on the kids' page.

Stellar: A kind of black hole. NASA says its mass can be up to 20 times more than the mass of the Sun.

Supermassive: The largest kind of black hole. NASA says these have masses of more than 1 million suns together.

Supernova: An exploding star that blasts part of the star into space. That is NASA's meaning on the kids' page.

Galaxy: A huge group of stars. Earth's galaxy is called the Milky Way.

Orbiting: Flying around something. Scientists study stars to find out if they are orbiting a black hole.

Middle: Packed Matter, a Boundary, and Clues Around It

This part is written in a different way. It follows NASA's page for Grades 5-8, NASA Science's Black Holes page, and NASA's What Are Black Holes? page. Each fact names the NASA page it came from.

What a black hole is

NASA's Grades 5-8 page says a black hole is a region in space where the pulling force of gravity is so strong that light is not able to escape. The strong gravity occurs because matter has been pressed into a tiny space.

NASA Science puts it this way: black holes are not really holes. They are huge concentrations of matter packed into very tiny spaces.

NASA's What Are Black Holes? page adds the boundary. A black hole's surface, called its event horizon, defines the boundary where the velocity needed to escape exceeds the speed of light, which is the speed limit of the cosmos. Matter and radiation fall in, but they can't get out.

NASA Science says the event horizon isn't a surface like Earth's or even the Sun's. It's a boundary that contains all the matter that makes up the black hole. Gravity just beneath that boundary is strong enough that nothing — not even light — can escape. This page only uses that strong claim tied to the event horizon, the way NASA scopes it.

NASA Science is honest about the limits, too. There is much we don't know about black holes, like what matter looks like inside their event horizons. But there is a lot that scientists do know.

Three kinds, by mass and size

NASA's Grades 5-8 page says black holes can come in a range of sizes, but there are three main types. The black hole's mass and size determine what kind it is. Mass means the measurement for the amount of matter in an object.

Primordial — the smallest. Scientists believe this type of black hole is as small as a single atom but with the mass of a large mountain. The Grades 5-8 page says primordial black holes are thought to have formed in the early universe, soon after the big bang.

Stellar — the most common medium-sized kind. The mass of a stellar black hole can be up to 20 times greater than the mass of the Sun and can fit inside a ball with a diameter of about 10 miles. Dozens of stellar mass black holes may exist within the Milky Way galaxy.

Supermassive — the largest. These black holes have masses greater than 1 million suns combined and would fit inside a ball with a diameter about the size of the solar system. Scientific evidence suggests 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. On the Grades 5-8 page, NASA says it has a mass equal to about 4 million suns. NASA Science gives the same mass in a shorter form: Sagittarius A* (pronounced ey-star) is 4 million times the Sun's mass.

How each kind forms — only as NASA states it

Stellar: The Grades 5-8 page says stellar black holes form when the center of a very massive star collapses in upon itself. This collapse also causes a supernova, or an exploding star, that blasts part of the star into space.

NASA's What Are Black Holes? page gives more detail, kept in NASA's own numbers: a stellar-mass black hole forms when a star with more than 20 solar masses exhausts the nuclear fuel in its core and collapses under its own weight. The collapse triggers a supernova explosion that blows off the star's outer layers. But if the crushed core contains more than about three times the Sun's mass, no known force can stop its collapse to a black hole.

Supermassive: The Grades 5-8 page says scientists think supermassive black holes formed at the same time as the galaxy they are in, and that the size of the supermassive black hole is related to the size and mass of the galaxy it is in. NASA's What Are Black Holes? page is more careful: the origin of supermassive black holes is poorly understood, but we know they exist from the very earliest days of a galaxy's lifetime. Both NASA forms are printed here. This page does not merge them into one story.

Growing: Once born, black holes can grow by accreting matter that falls into them, including gas stripped from neighboring stars and even other black holes. That is from NASA's What Are Black Holes? page.

The Sun: The Grades 5-8 page says the Sun does not have enough mass to collapse into a black hole. In billions of years, at the end of its life, it will become a red giant star, then throw off its outer layers, and finally be left as a cooling white dwarf star.

How scientists find something invisible

The Grades 5-8 page says a black hole cannot be seen because of the strong gravity pulling all of the light into the black hole's center. However, scientists can see the effects of its strong gravity on the stars and gases around it. If a star is orbiting a certain point in space, scientists can study the star's motion to find out if it is orbiting a black hole.

NASA Science agrees: black holes don't emit or reflect light, making them effectively invisible to telescopes. Scientists primarily detect and study them based on how they affect their surroundings. NASA Science lists four ways:

1. Black holes can be surrounded by rings of gas and dust, called accretion disks, that emit light across many wavelengths, including X-rays.

2. A supermassive black hole's intense gravity can cause stars to orbit around it in a particular way. Astronomers tracked the orbits of several stars near the center of the Milky Way to prove it houses a supermassive black hole, a discovery that won the 2020 Nobel Prize.

3. When very massive objects accelerate through space, they create ripples in the fabric of space-time called gravitational waves. Scientists can detect some of these by the ripples' effect on detectors.

4. Massive objects like black holes can bend and distort light from more distant objects. This effect, called gravitational lensing, can be used to find isolated black holes that are otherwise invisible.

The Grades 5-8 page explains the glowing disk in more steps. A black hole's gravity can sometimes pull off the outer gases of a star and grow a disk around itself called the accretion disk. As gas from the accretion disk spirals into the black hole, the gas heats to very high temperatures and releases X-ray light in all directions. NASA telescopes measure the X-ray light. Astronomers use this information to learn more about the properties of a black hole.

NASA's What Are Black Holes? page adds two big moments. In 2019, astronomers using the Event Horizon Telescope — eight ground-based radio telescopes networked into a single Earth-size dish — captured an image of a black hole for the first time. It appears as a dark circle silhouetted by an orbiting disk of hot, glowing matter. That supermassive black hole is at the heart of a galaxy called M87, about 55 million light-years away, and weighs more than 6 billion solar masses. In 2015, scientists first detected gravitational waves from two orbiting black holes that spiraled into each other and merged 1.3 billion years ago.

NASA's tools for this work, named on the What Are Black Holes? page, are the Hubble, Chandra, Swift, NuSTAR, and NICER space telescopes, as well as other missions. The Grades 5-8 page names the Chandra X-ray Observatory, the Swift satellite, and the Fermi Gamma-ray Space Telescope.

What black holes are not

NASA Science has a short list headed Black Holes Are Not. Two items on it matter for kids:

They are not wormholes. They don't provide shortcuts between different points in space, or portals to other dimensions or universes.

They are not cosmic vacuum cleaners. Black holes don't suck in other matter. From far enough away, their gravitational effects are just like those of other objects of the same mass.

The Grades 5-8 page says the same safety idea in school words. Black holes do not wander around the universe, randomly swallowing worlds. They follow the laws of gravity just like other objects in space. If a black hole with the same mass as the Sun were to replace the Sun, Earth would not fall in. The planets would still orbit the black hole as they orbit the Sun now.

Record holders — NASA Science's own list

Closest: The nearest known black hole, called Gaia BH1, is about 1,500 light-years away.

Biggest: The most massive black hole observed, TON 618, tips the scales at 66 billion times the Sun's mass.

Smallest observed: The lightest-known black hole is only 3.8 times the Sun's mass. It's paired up with a star. Note: this is the lightest one scientists have observed. It is a different claim from the Grades 5-8 page's primordial idea — a type scientists believe could be as small as a single atom. This page keeps the observed record and the believed type apart.

Spin: All black holes spin. The fastest-known — named GRS 1915+105 — clocks in at over 1,000 rotations per second.

One type of black hole is born when massive stars run out of fuel and explode in supernovae. NASA Science lists that under the heading Star booms.

Words to know — Middle

Event horizon: The boundary of a black hole. NASA's What Are Black Holes? page defines it as the boundary where the velocity needed to escape exceeds the speed of light. NASA Science says it isn't a surface like Earth's — it's a boundary that contains all the matter that makes up the black hole.

Mass: The measurement for the amount of matter in an object. That is the Grades 5-8 page's definition.

Accretion disk: A disk, or ring, of gas and dust around a black hole. NASA Science says accretion disks emit light across many wavelengths, including X-rays. The Grades 5-8 page says gas spiraling in heats to very high temperatures and releases X-ray light in all directions.

Supernova: An exploding star. The Grades 5-8 page says a stellar black hole's collapse causes a supernova that blasts part of the star into space.

Gravitational waves: Ripples in the fabric of space-time, created when very massive objects accelerate through space. That is NASA Science's wording.

Gravitational lensing: The bending and distorting of light from more distant objects by a massive object like a black hole. NASA Science says this effect can be used to find isolated black holes.

High-energy light: Light produced when a black hole and a star are orbiting close together. The Grades 5-8 page says scientific instruments can see this high-energy light, even though human eyes cannot.

Red giant star: A star that is larger than the Sun and red because it has a lower temperature. That is the Grades 5-8 page's definition.

White dwarf star: A small star, about the size of Earth — one of the last stages of a star's life. That is the Grades 5-8 page's definition.