Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one idea. The numbers come from the escape-velocity formula; the pictures are not to scale.
Tap the buttons and slide the sliders.
Change the black hole's mass. What happens to the size of its dark shadow?
The Schwarzschild radius is proportional to mass. Slide from 1 to 10 billion Suns and watch the horizon scale. M87* sits at 6.5.
Make the black hole heavier. What happens to the dark shadow?
Sizes are computed from the mass; the glow is only a sketch.
Pick a speed and throw. How fast do you need to go so the ball never comes back?
Below 11.2 km/s the ball rises and falls back; at or above it, it escapes. The height scale is logarithmic.
Pick slow, fast or super fast, then throw. Can the ball get away?
No air in this model, just gravity.
Drag the Sun or use the slider. How small does it have to get before light is trapped?
Same mass, smaller radius: v = √(2GM/R) climbs until it passes light speed at R ≈ 3 km.
Squeeze the Sun smaller and smaller. When does it turn into a black hole?
Set the star's mass. Which stars end as a black hole?
Above ~8 Suns: supernova and a neutron star. Around 20 Suns or more: a black hole. The boundaries are approximate and still debated.
Make the star small or giant. Which one can make a black hole?
Mass ranges from NASA; real stars don't all follow the lines exactly.
Swap the Sun for a black hole with the same mass. Does Earth's path change?
Far away, only the mass matters, so the orbit is unchanged. The inset shows the tidal stretching you'd only meet up close.
Tap to swap the Sun for a black hole. Does the Earth keep going around?
Not to scale, and the lap is much faster than a real year.
Pick Earth, the Sun or a star's core. How small must each get before light can't escape?
Compare the Schwarzschild radii: about 9 mm for Earth, 3 km for the Sun, 9 km for a 3-Sun core.
A black hole is lots and lots of stuff squeezed into a tiny space. Its pull is so strong that nothing can get out, not even light. That's why it looks dark.
Throw a ball up, and the Earth pulls it back down. You would have to throw it super, super fast for it to get away into space.
If you could squeeze the whole Earth smaller, its pull would get stronger. Squeeze it as small as a marble, and nothing could get away. Not even light! That's a black hole.
Nobody can squeeze the Earth. But gravity can squeeze a giant star. When a giant star runs out of fuel, its middle falls in. The rest of the star blasts away with a big boom. The squeezed-up middle can become a black hole.
Black holes don't suck things up. If the Sun turned into a black hole just as heavy, the Earth would keep going around it. It would just be dark and cold. But don't worry: our Sun is too small to ever become a black hole.
A black hole forms when a lot of mass gets squeezed into a very small space. It gets so small that its gravity won't let anything out, not even light.110 Most of the black holes we know about began as giant stars. When a star with about 20 times the Sun's mass or more runs out of fuel, its core collapses, the rest of the star explodes as a supernova, and the core keeps falling in on itself until it becomes a black hole.58
To see why squeezing does this, you need one idea: escape speed.
A black hole is an object so dense that gravity near it is strong enough that nothing, not even light, can escape.14 Despite the name, it isn't really a hole. NASA describes black holes as "huge concentrations of matter packed into very tiny spaces."1 Because they give off no light of their own, we find them by watching what they do to things around them: stars orbiting something invisible, or gas glowing as it swirls in.16
Throw a ball up and gravity pulls it back. Throw it faster and it goes higher. Throw it at about 11.2 kilometres per second and it never comes back. That is Earth's escape velocity, the speed needed to break free of its gravity.64
Escape speed depends on two things: the object's mass, and how far you are from its centre.6 So here's the trick. Keep the mass the same, but make the object smaller, and the surface gets closer to all that mass. Escape speed goes up:
| Earth's mass squeezed to… | Escape speed |
|---|---|
| Normal size (12,742 km wide) | 11.2 km/s |
| Half as wide | 15.8 km/s |
| A ball 9 mm in radius (about a marble) | the speed of light, about 300,000 km/s |
| Even smaller | faster than light: a black hole |
Numbers from NASA's Imagine the Universe.6 The speed of light is the cosmic speed limit, so once the escape speed is higher than that, nothing can get out.6 For the Sun, the same thing happens if you squeeze it to a radius of about 3 km. Right now its radius is nearly 700,000 km.7
escape speed = √(2GM / R) → black hole when R < 2GM / c²
Here G is the gravitational constant, M the mass, R the radius and c the speed of light. The radius where the escape speed equals light speed is called the Schwarzschild radius.7 It grows with mass: a black hole with 10 times the Sun's mass has a horizon radius of about 30 km.12
The event horizon is the boundary where escape speed equals the speed of light. It's the black hole's "point of no return."6 It isn't a solid surface you could stand on. It's more like an invisible line in space, and anything that crosses it can't come back out, including light.12 That's why the middle of a black hole picture is dark.
Nobody can squeeze a planet down to a marble. But gravity can squeeze a star. Here's how it happens:
| Star's starting mass | How it dies | What's left |
|---|---|---|
| Like the Sun (up to about 8 Suns) | Swells into a giant and puffs off its outer layers | White dwarf, about the size of Earth |
| More than 8 Suns | Supernova | Neutron star, about the size of a city |
| About 20 Suns or more | Supernova | Black hole |
Mass ranges from NASA. Scientists still argue about the exact boundaries.58 Our Sun will never become a black hole. It will end as a white dwarf.10
Movies often show black holes as cosmic vacuum cleaners that swallow everything nearby. They aren't. From far enough away, a black hole's gravity is just like any other object with the same mass.1
Imagine the Sun suddenly turned into a black hole with exactly the same mass. It would only be about 6 km across, but Earth and the other planets would keep going around in the same orbits. The solar system would just get very cold and dark.1710
Things only get extreme up close. Near a black hole, gravity pulls much harder on the near side of an object than the far side. It gets stretched out one way and squeezed the other, like a noodle. Scientists really call this spaghettification.13
Honestly, nobody knows. Einstein's general theory of relativity predicts that everything falling in ends up at a single point of infinite density called the singularity.2 But that prediction may just mean the theory stops working there, and that a better theory of gravity, one that includes quantum effects, is needed.26 Because nothing from inside the event horizon can reach us, we can't look.2
Almost every large galaxy, including our Milky Way, has a supermassive black hole at its centre. These giants weigh hundreds of thousands to billions of Suns.5 Ours is called Sagittarius A* ("A-star"). It weighs about 4 million Suns and is about 27,000 light-years away.114
How they got started is still an open question.5 Some were already huge less than a billion years after the Big Bang, which is too soon to have grown from ordinary star-sized black holes. They may have had a head start from the collapse of enormous stars or gas clouds in the early universe.54 What we do know is that black holes grow by swallowing gas and stars and by merging with each other.25
We can even hear merging black holes, sort of. On 14 September 2015, the LIGO detectors picked up gravitational waves, ripples in space itself, from two black holes of about 29 and 36 Suns colliding 1.3 billion years ago.15
In 2019 the Event Horizon Telescope, eight radio telescopes linked into one Earth-sized virtual telescope, released the first picture of a black hole. It's the giant in the galaxy M87, 55 million light-years away, with a mass of 6.5 billion Suns.13 The picture shows a bright ring of hot gas around a dark centre called the black hole's shadow. The event horizon itself is about 2.5 times smaller than the shadow, and just under 40 billion km across.13 Later, the same team photographed Sagittarius A*, the black hole at the centre of our own galaxy. From Earth it looks about as big as a donut on the Moon.14
The glowing ring comes from the accretion disk, gas that spins around the black hole and heats up so much that it glows, even in X-rays.2
The same supernova explosions that make stellar black holes scatter elements like carbon, nitrogen and oxygen through space. Those elements later become part of new stars, new planets and, eventually, living things.10 In a way, NASA says, we owe our existence on Earth to long-ago explosions that formed black holes.10
When a very big star runs out of fuel, nothing holds it up against its own gravity anymore. Its core collapses in less than a second and the rest of the star explodes as a supernova. If the star was about 20 times the Sun's mass or more, the core keeps collapsing until its gravity is so strong that not even light can escape. That's a black hole.
Everything has an escape speed, the speed you need to break free of its gravity. Squeeze the same mass into a smaller space and the escape speed goes up. Inside a black hole's event horizon, the escape speed is faster than light, and nothing can go faster than light.
The evidence is very strong. Astronomers have tracked stars orbiting an invisible 4-million-Sun object at the centre of our galaxy (Nobel Prize 2020), the Event Horizon Telescope has photographed the shadows of two black holes, and LIGO has detected the gravitational waves from black holes colliding.
An extremely long time. Stephen Hawking predicted that black holes slowly lose energy through Hawking radiation, but for the black holes we know about, evaporating would take far longer than the current age of the universe.
No. The Sun isn't nearly massive enough. In several billion years it will swell into a red giant, shed its outer layers and end as a white dwarf, an Earth-sized glowing cinder.
Nobody knows for sure. General relativity predicts that everything falls to a central point of infinite density called the singularity, but that prediction probably means the theory is reaching its limits. No information from inside the event horizon can reach us.
No. We have no way to destroy one. Scientists think they shrink, extremely slowly, through Hawking radiation. The good news is that the nearest known black hole, Gaia BH1, is about 1,500 light-years away, and no black hole is close enough to affect Earth.