How Are Black Holes Made?
The short answer
A black hole is made when a dead star's core is too heavy for anything to hold it up. Gravity always pulls in. Pressure pushes back, but only while it can. Electrons give way above about 1.4 Suns. Neutrons give way above a little over 2 Suns. Past that, the core collapses and never stops.
Only big stars get that far. The Sun is far too light and will end as a white dwarf. The film calls three fights: the Sun, a star of 20 Suns that leaves a neutron star, and a star of 40 Suns whose leftover core is too heavy even for neutrons.
Why a star needs fuel
Inside a star, the push of the gas at every depth holds up the weight of all the layers above. The ocean works the same way. At the bottom of the Mariana Trench, water pushes with more than 16,000 psi, about 1,100 atmospheres. In the Sun's core the pressure is about 245 billion atmospheres.
The difference is where the push comes from. Water resists being squeezed on its own. A star's gas pushes because it is hot, about 15 million °C (27 million °F) in the Sun's core, and it keeps leaking that heat into space. Fusion has to keep replacing it. Every second, the Sun turns about 4 million metric tons of itself into energy. That energy is slow to get out: NASA estimates about 170,000 years from the core to the top of the convection zone.
The Sun is about 4.5 billion years old and has about 5 billion years left. Then it swells into a red giant, its outer layers drift away, and its core is left as a white dwarf: about half the Sun's mass in a ball only slightly bigger than Earth. On Earth, a teaspoon of typical white dwarf matter would weigh 5.5 tons. With no fuel left, it is held up by electrons that refuse to be packed any closer. In 1930, Subrahmanyan Chandrasekhar, then 19, was on a ship from India to England when he worked out that this trick has a limit. Today the limit is put at about 1.4 Suns. Arthur Eddington called the idea absurd, in public, in 1935. Chandrasekhar won the Nobel Prize in 1983.
How a big star ends
More weight squeezes harder, so a heavy star burns fast. A star of 20 Suns gets about 10 million years. It burns hydrogen, then helium, carbon, neon, oxygen and silicon. The late stages are short: in a star of 20 Suns, carbon lasts close to 1,000 years and silicon about 11 days. What is left is iron, and fusing iron costs energy instead of paying it out.
The iron core, about 8,000 km across, is held up only by its electrons. Near 1.4 Suns they give way. In less than a second the core falls in, at up to a quarter of the speed of light, and ends up about 19 km across. Packed to about twice the density of an atomic nucleus, it turns stiff, and the rest of the star falling onto it bounces. That supernova can shine brighter than a billion Suns, yet about 99% of its energy leaves as neutrinos. In 1987, detectors in Japan, the United States and the Soviet Union caught about two dozen of them, three hours before anyone saw the star explode. The neutrinos simply left first. Left in the middle is a neutron star, typically about 1.4 Suns.
When nothing is left to push
Neutrons have a limit too, and nobody knows it exactly. The heaviest neutron star measured so far is 2.35 Suns, give or take 0.17, so the limit can be no lower than that star's true mass. The film calls it a little over 2 Suns. In 1939, Robert Oppenheimer and his student Hartland Snyder showed that a heavy enough star, once it starts to collapse, never stops. Their paper came out on September 1, 1939, the day Germany invaded Poland. If a dead core is past the neutron limit, that endless collapse is what happens, and the result is a black hole.
Its edge is the event horizon. The Sun as a black hole would be about 6 km (4 miles) across, and Earth would be about the size of a marble. A black hole doesn't suck. Swap the Sun for a black hole of the same mass, and Earth keeps the same orbit. For years the evidence was indirect. Stephen Hawking bet Kip Thorne that Cygnus X 1 was not a black hole, and in 1990 he conceded. On September 14, 2015, LIGO picked up two black holes of about 30 Suns each merging 1.3 billion light years away. About 3 Suns of mass became ripples in space. In April 2019 came the first picture of one: M87*, 6.5 billion Suns, 55 million light years away.
Key numbers
| What | Number | Source |
|---|---|---|
| Pressure at the bottom of the Mariana Trench | about 1,100 atmospheres (more than 16,000 psi) | Our arithmetic from NOAA, Mariana Trench noise release |
| Pressure at the Sun's core | about 245 billion atmospheres (2.477 × 10^11 bar); the film rounds it to 250 billion | Our arithmetic from NASA NSSDC, Sun Fact Sheet |
| Temperature at the Sun's core | about 15 million °C (27 million °F) | NASA Science, Sun Facts |
| Mass the Sun turns into energy every second | about 4 million metric tons | NASA ISTP outreach |
| Time for energy to get from the Sun's core to the top of the convection zone | about 170,000 years (NASA's estimate) | NASA Science, Sun Facts |
| Weight on Earth of a teaspoon of white dwarf | 5.5 tons | NASA Imagine the Universe, White Dwarf Cool Fact |
| Most mass electrons can hold up (the Chandrasekhar limit) | about 1.4 Suns | NASA Imagine the Universe, White Dwarfs |
| Collapse of a big star's iron core | less than 1 second, from about 8,000 km to about 19 km across | Chandra X ray Center |
| Brightness of a supernova | more than a billion Suns | Chandra X ray Center |
| Heaviest neutron star measured | 2.35 ± 0.17 Suns | Romani et al. 2022 |
| Width of a black hole with the Sun's mass | about 6 km (4 miles) | Our arithmetic from NASA Imagine the Universe, Black Holes |
| Mass turned into gravitational waves in the first black hole merger detected, 2015 | about 3 Suns | Abbott et al. 2016 |
What the film simplifies
- Gravity and Pressure are drawn as two boxers. In a real star, it is the difference in pressure from one depth to the next that holds up the layers above.
- The film rounds some numbers: 250 billion atmospheres for the Sun's core (about 245 billion), 10 million years for a star of 20 Suns (about 9.3 million), and an iron core the size of Earth (about 8,000 km across, a little smaller than Earth). It also says every fuel burns faster than the last, but in the model of a star of 20 Suns, neon (about 0.6 years) is over sooner than oxygen (about 1.3 years).
- The 170,000 years is NASA's estimate. Other models give anything from about 10,000 years to a few million, and it is the energy that makes the trip, not a single particle of light.
- On the ship, Chandrasekhar's first limit was 0.91 Suns. The 1.44 came later, from a better estimate of what the star is made of, and Edmund Stoner reached a similar limit on his own in 1929 and 1930.
- The main event's 40 Suns is the film's example. What makes the black hole is a leftover core heavier than neutrons can hold up.
- The ending simplifies where our elements come from. Iron also comes from exploding white dwarfs, not only from collapsing giants. Where gold is made is still disputed: neutron star crashes can make it, and magnetar flares may make up to 10% of the elements heavier than iron.
At a glance
Runtime 10 minutes · Topics: Life and death of stars, Black holes, Supernovas, Neutron stars · Captions on YouTube · Full script below
Test yourself
Water at the bottom of the ocean holds itself up without any fuel. Why does a star need fuel to keep its pressure up?
Answer
Water resists being squeezed on its own. A star's gas pushes because it is hot, and it keeps losing that heat into space, so fusion has to keep making more.
In 1987, neutrinos from a supernova reached Earth about three hours before its light. Did they travel faster than light?
Answer
No. They got out of the star first. The light was trapped inside until the blast reached the star's surface.
If the Sun were replaced by a black hole of the same mass, what would happen to Earth's orbit?
Answer
Nothing. From far away, a black hole pulls like any other object of the same mass. Earth would just get cold and dark.
The script
Read the script
The Sun: Not again! Every single day!
Host: Every star you've ever seen is a fight.
Host: In this corner, pulling every bit of the star inward, undefeated since the beginning of time... Gravity!
Gravity: Nothing leaves. Everything comes back down. Eventually.
Host: And in this corner, pushing it all back out, the challenger... Pressure!
Pressure: Hold still, champ. I'm taking that belt.
Host: When Pressure wins, a star survives. When Gravity wins...
Host: ...you get a black hole.
Host: And the stuff that gets knocked out of the ring? That's what you're made of.
Rocket: Rocket here, and I have never been this close to a star without catching fire!
Rocket: I know Gravity. Gravity's the reason I need 7 million pounds of thrust.
Rocket: But who's the new guy?
Pressure: Pressure, tin can. At the bottom of the sea, I hold up the ocean. In here, I hold up a whole star.
Host: Same job. But water pushes back on its own. A star's push comes from heat, and the heat has to keep coming.
Host: At the bottom of the deepest trench, Pressure pushes with about 1,100 atmospheres. In the Sun's core? 250 billion.
Pressure: ...I've been working the shallow end.
Host: The core is 15 million degrees Celsius, hot enough to smash hydrogen into helium. That's fusion, and it's what keeps Pressure pushing.
Host: And that energy takes its time. Made in the core today, it needs about 170,000 years just to get out.
Rocket: So the sunshine on my face set off before the pyramids?
Host: Way before. When that energy was made, Earth was in an ice age.
Rocket: Okay, but who's paying for all this?
Host: The Sun is. Every second, about 4 million tons of him turns into pure energy.
Gravity: I need nothing.
Host: That's the whole fight. Pressure has to keep paying. Gravity never runs out.
Host: Round 1!
Host: A star like the Sun can keep this up for about 10 billion years. He's 4.5 billion years in.
The Sun: Halfway there, and my back is killing me.
Rocket: So what happens when he runs out? Does he explode? Black hole?
Host: Neither. The Sun is way too small for either one.
Host: In about 5 billion years, his core runs low on hydrogen. Gravity squeezes it, and the outside puffs up into a red giant.
The Sun: I'm HUGE! I'm... I'm drifting apart!
Host: The outside drifts away. What's left in the middle, his old core, gets a new name.
White Dwarf: Hello.
Host: A white dwarf. About half the Sun's weight, squeezed into a ball the size of Earth.
Rocket: Aww. He's tiny!
White Dwarf: Tiny? Try lifting me.
Host: Here on Earth, a sugar cube of him would weigh more than a car.
Gravity: No fuel left. Then he's mine.
White Dwarf: Squeeze all you like. Electrons don't share seats.
Pressure: That's my new trick, champ. No fuel needed.
Host: Squeezed this tight, his electrons refuse to be packed any closer, so they push back all by themselves.
Host: On his own, he'll just sit there and glow, slowly cooling off.
Rocket: UPSET! Round one goes to Pressure!
Pressure: Undefeated, baby!
Gravity: ...More.
Host: In 1930, a 19 year old named Subrahmanyan Chandrasekhar sailed from India to England.
Host: On the ship, he worked out that the electrons' trick has a limit.
Host: Pile on more than about 1.4 Suns, and the electrons lose.
Rocket: He found Gravity's cheat code!
Host: The most famous astronomer of the day, Arthur Eddington, stood up and called it absurd. In public.
Host: Chandrasekhar was right. 53 years after that voyage, he won the Nobel Prize.
Gravity: 1.4. I like him.
Host: Round 2! A bigger ring: a star 20 times the mass of the Sun.
Supergiant: Hi. Please don't fight in me.
Host: More weight means a harder squeeze, so Pressure has to burn fuel way faster to keep up.
Host: The Sun gets about 10 billion years. A star this big? About 10 million.
Supergiant: 10 million? I just got here!
Rocket: That's not a star. That's a firework.
Host: When the hydrogen runs out, it burns the helium. Then carbon. Then neon, oxygen, silicon.
Host: Every fuel burns faster than the last. The carbon lasts centuries. The silicon? Just days.
Supergiant: DAYS?!
Host: And when the silicon is gone, the core is iron.
Supergiant: Iron? Who ordered IRON?
Host: Iron is where the money stops. Every fuel before it paid out energy. Fusing iron costs energy instead.
Host: So now the core is just like the White Dwarf: only its electrons are holding it up. But iron keeps piling onto it.
Gravity: 1.4.
Supergiant: I'm fine. I'm fine. I'm totally fi...
Rocket: What happened? I blinked!
Host: Let's see that again. In slow motion.
Host: The core passes the limit, and the electrons lose. A ball of iron about the size of Earth falls in on itself.
Host: It takes less than a second, at up to a quarter of the speed of light, and ends up the size of a city.
Rocket: Gravity wins! It's over!
Pressure: Not yet!
Host: Squeezed that hard, the core turns into neutrons, and they refuse to go any tighter. Pressure's last wall.
Pressure: Not! One! More! Inch!
Host: The rest of the star comes crashing down onto that wall... and bounces.
Host: A supernova.
Host: For weeks, that one dying star can shine brighter than a billion Suns.
Rocket: PRESSURE WITH THE COMEBACK!
Host: And almost all of its energy flies out as ghost particles, neutrinos.
Host: They get out hours before the light does.
Rocket: They beat the light? Nothing beats light!
Host: They just left first. The light was stuck inside, waiting for the blast to reach the surface.
Host: And standing in the middle of the wreck... the winner of round 2.
Host: A neutron star: more than the Sun's mass, in a ball about 20 kilometers across.
Rocket: Is it... beeping at us?
Host: That's exactly how we found the first one.
Host: In 1967, a student named Jocelyn Bell spotted a blip in her data: once every 1.3 seconds.
Host: So regular, her team called it LGM 1.
Rocket: LGM?
Host: Little Green Men.
Rocket: Was it aliens?
Host: No. Something weirder: a dead star the size of a city, blinking at us.
Host: The Nobel Prize for it went to her supervisor. Not to her.
Rocket: ROBBED!
Host: Round 2 to Pressure. Barely.
Gravity: Neutrons. How much can neutrons hold?
Host: A little over 2 Suns. Nobody knows exactly.
Host: And past that? In 1939, Robert Oppenheimer and his student Hartland Snyder worked it out.
Host: A heavy enough star collapses. And it never stops.
Gravity: Forever. I like forever.
Host: Their paper came out on September 1, 1939.
Host: The day World War II began. For decades, almost nobody paid attention.
Rocket: Worst release date ever.
Host: Main event. The biggest ring yet: 40 times the Sun. Its core collapses, just like before.
Host: But this time, the leftover core is too heavy. Past the neutrons' limit.
Pressure: Neutrons! Hold the wall!
Host: The neutrons give way. And there's nothing left in Pressure's corner.
Pressure: ...Anybody?
Host: Nothing can stop it now. The core keeps falling, forever.
Host: A black hole.
Rocket: KNOCKOUT! Gravity wins!
Rocket: Say something, Black Hole! Anything!
Host: It can't. Nothing gets out of a black hole. Not light. Not sound. Not even a comeback.
Gravity: See this line? Cross it, and you're mine.
Host: The event horizon. Outside it, you can still get away. Inside it, every road leads in.
Rocket: How big is that circle?
Host: Depends how much you squeeze in. Squeeze the Sun into a black hole, and it's about 6 kilometers across.
The Sun: 6 kilometers? I've got freckles bigger than that.
Host: Squeeze the whole Earth, and it's the size of a marble.
Rocket: Okay, I'm backing the booth up. That thing's a cosmic vacuum cleaner!
Gravity: I don't suck.
Host: It doesn't. Swap the Sun for a black hole with the exact same weight, and Earth keeps the exact same orbit.
Host: It would just get very cold, and very dark. You only fall in if you get too close.
Rocket: Hang on. If they're black, how do we know they're even real?
Host: For years, we only had clues.
Host: Stephen Hawking even bet his friend Kip Thorne that the best candidate, Cygnus X 1, wasn't a black hole.
Pressure: Smart man.
Rocket: He bet against black holes? He studied black holes!
Host: That was the point. If his life's work turned out wrong, at least he'd win the bet.
Host: He lost. Happily.
Host: Then, in 2015, days after new detectors were switched on, we heard two of them crash.
Host: In a split second, 3 Suns' worth of mass turned into a ripple in space.
Host: By the time it reached Earth, it stretched our detectors by less than the width of a proton.
Host: Turn that ripple into sound, and it goes like this.
Rocket: Two black holes crash... and it goes whoop?
Gravity: It's a big whoop.
Host: Dishes all over the Earth teamed up as one telescope, and took the first picture of one.
Rocket: We got a picture of the champ!
Host: Final scorecards. Pressure won 2 rounds. Gravity won the belt, by knockout.
Pressure: So I lost.
Host: You lost the belt. Now look at what flew out of the ring.
Host: Every round Pressure won threw star stuff out into space. Carbon. Oxygen. Calcium. Iron.
Host: That stuff made new stars, new planets... and the calcium in your bones. The iron in your blood. The oxygen in every breath.
Rocket: And my gold chain?
Host: Probably two of those neutron stars, crashing into each other.
Rocket: Wait. So we're made of... the leftovers?
Host: The best ones.
Pressure: You're welcome, kid.
Gravity: Enjoy it. I can wait.
Pressure: Then I'll keep paying.
The Sun: Not again!
Sources
- The Sun's central temperature is 1.571 × 10^7 K and its central pressure is 2.477 × 10^11 bar.NASA NSSDC, Sun Fact Sheet
- The Sun's core is about 15 million °C (27 million °F); energy takes about 170,000 years to get from the core to the top of the convection zone; the Sun is 4.5 billion years old and will last another 5 billion years or so before it becomes a white dwarf.NASA Science, Sun Facts
- The pressure at every point inside a star holds up the weight of the layers above it.OpenStax Astronomy 2e, section 16.3
- The pressure is more than 16,000 psi at the bottom of the Mariana Trench.NOAA, Mariana Trench noise release
- Each second, the Sun turns about 4 million tons of its mass into energy.NASA ISTP outreach
- The Sun will end as a white dwarf holding about half its present mass.Stanford Solar Center
- Big stars burn fast, layer by layer, and fusing iron takes more energy than it releases.Las Cumbres Observatory
- The iron core approaches 1.4 solar masses and collapses in less than a second from about 8,000 km to about 19 km across; within a day or two the exploding star becomes brighter than a billion Suns.Chandra X ray Center
- The collapsing core falls nearly freely at about a quarter of the speed of light, and the collapse of the inner core halts at nearly twice the density of an atomic nucleus.Woosley and Janka, 2005
- A supernova can outshine its entire galaxy for a few days or even months.NASA Space Place
- About 99% of a core collapse supernova's energy leaves as neutrinos.arXiv 1605.02204, 2016
- In 1987, Kamiokande II saw 12 neutrinos, IMB 8 and Baksan 5, about three hours before the supernova became visible.The Astrophysics Spectator
- A typical white dwarf is half as massive as the Sun and only slightly bigger than Earth; fusion has ceased; the Chandrasekhar limit is 1.4 times the mass of the Sun.NASA Imagine the Universe, White Dwarfs
- A teaspoon of typical white dwarf matter would weigh 5.5 tons on Earth.NASA Imagine the Universe, White Dwarf Cool Fact
- Chandrasekhar sailed from Bombay on 31 July 1930.Resonance, 2011
- Chandrasekhar's first limit was 0.91 Suns and 1.44 came later; Stoner reached a similar limit; Eddington called the result absurd.MacTutor, Chandrasekhar and Eddington
- Chandrasekhar left for England at 19 and won the 1983 Nobel Prize in Physics.Physics Today, Chandra: a biographical portrait
- Neutron degeneracy pressure helps hold a neutron star up.NuSTAR (Caltech), Neutron Stars
- A typical neutron star is about 1.4 solar masses.University of Maryland, neutron star page
- PSR J0952 0607 weighs 2.35 ± 0.17 solar masses, the largest well measured neutron star mass to date.Romani et al. 2022
- Jocelyn Bell found the first pulsar in 1967; it was nicknamed LGM 1; the Nobel Prize went to Antony Hewish and not to her.Physics Today, Detection of the first pulsar
- The first pulsar was nicknamed Little Green Men.Space.com, Little Green Men
- One type of black hole is born when a massive star runs out of fuel; black holes are not really holes and don't suck in matter.NASA Science, Black Holes
- Oppenheimer and Snyder, On Continued Gravitational Contraction, Physical Review 56, 455 (1939).Oppenheimer and Snyder, 1939
- The Oppenheimer and Snyder paper was published on September 1, 1939, the day World War II began.3 Quarks Daily
- Cygnus X 1 was discovered in 1964; Hawking and Thorne bet on it in 1974 or 1975, and Hawking conceded in 1990.Astronomy magazine, Cygnus X 1
- The Hawking and Thorne bet on Cygnus X 1.National Academies Press, Stephen Hawking: A Life in Science
- A black hole with the Sun's mass has a radius of 3 km, and Earth's orbit would be unchanged if the Sun were replaced by one.NASA Imagine the Universe, Black Holes
- Earth squeezed into a black hole would be about one inch across.PBS NOVA, The Schwarzschild Radius
- Black holes don't suck; the Sun will never become a black hole.NASA Science, 10 Questions About Black Holes
- On September 14, 2015, LIGO detected two merging black holes, each about 30 times the Sun's mass, 1.3 billion light years away.LIGO, GW150914
- About 3 Suns of mass were radiated as gravitational waves in GW150914.Abbott et al. 2016
- The first image of a black hole, M87*, was released on 10 April 2019: 6.5 billion Suns, 55 million light years away.ESO, 2019
- Basically all the iron in the Universe is made in explosions; where gold is made is in dispute.SDSS blog (Johnson), 2017
- Neutron star mergers can make gold; magnetar flares could make up to 10% of the elements heavier than iron.NASA Science, Where Does Gold Come From
- A star of 20 Suns burns hydrogen for 8.1 million years and helium for 1.2 million; carbon lasts about 980 years, neon 0.60 years, oxygen 1.3 years and silicon 0.031 years (about 11 days).Heger et al. 2003