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A starry sky with the Milky Way galaxy(thanks to Bernd Nies)

A starry sky with the Milky Way galaxy (thanks to Bernd Nies) – there are probably black holes in there somewhere!

What is a black hole?

A black hole is a place in space where there is a huge amount of mass in a very small area.

More about space
What is mass?
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Why are black holes black?

Gravity is the same as mass multiplied by 1/volume. So when you have a lot of mass in a small volume it makes a lot of gravity. Apparently, it makes so much gravity that even light can’t get away from the black hole! That’s why it looks black to us: no light comes towards our eyes from the black hole. So black holes happen when the speed you would have to be going to overcome the gravity and get away from the black hole is bigger than the speed of light.

What is gravity?
And what is light made of?

Supernova from space (thanks to NASA and the Hubble Telescope)

Supernova from space (thanks to NASA and the Hubble Telescope) – later, it might collapse and make a black hole.

Black holes and spacetime

Or think of space and time the way physicists do, as being both part of one thing, spacetime. In that case, a black hole might be a place where spacetime bends in a special way. So when things near the black hole go forward in time, as everything does, they also have to go forward in space. That pulls them deeper into the black hole.

What is time? (Physics of time)

How big is a black hole?

Astronomers think most black holes are about the size of large stars. They probably get started after a supernova when big stars collapse in on themselves. There may also be a lot of teeny-tiny black holes, called micro black holes. But nobody has seen one or proved that they must exist. These micro black holes might have formed during the Big Bang.

More about the Big Bang
What is a supernova?
More about stars

LIGO gravitational wave detector

LIGO gravitational wave detector

Are there even bigger black holes?

Many astronomers think that there are black holes at the center of galaxies of stars, including our galaxy, the Milky Way. These would be extra-big black holes, millions or billions of times the mass of our Sun.

What is a galaxy?
What’s the Sun made of?

Ripples in a pond

Gravitational waves from a black hole spread out like these ripples on a pond

How do we know black holes exist?

Nobody’s ever seen a black hole, even with a telescope. So the only way we know they exist is that we need them to make calculations about space come out according to the laws of physics. New detectors here on Earth, though, recently picked up tiny vibrations. These vibrations are probably the gravity waves created when two black holes run into each other billions of miles away – like the last ripples from throwing a stone into a pond.

See how she spins faster when she pulls in her arms?

Do black holes spin around?

Because stars spin around, black holes made from old stars probably also spin around. Probably these black holes spin very fast, because they are so much smaller than the stars they came from, just as an ice skater or ballet dancer spins faster when she pulls her arms tight in to her sides, because of conservation of momentum.

Learn by doing: throw a pebble into a pond and watch the ripples, then spin around with your arms out and suddenly pull your arms in tight.
More about supernovas
More about gravity

Bibliography and further reading about black holes:


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