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  • Neutron stars are one of the most extreme and violent things in the universe.

  • Giant atomic nuclei, only a few kilometers in diameter,

  • but as massive as stars.

  • And they owe their existence to the death of something majestic.

  • [Intro music]

  • Stars exist because of a fragile balance.

  • The mass of millions of billions of trillions of tons of hot plasma

  • are being pulled inwards by gravity,

  • and squeeze material together with so much force that nuclei fuse

  • Hydrogen fuses into helium.

  • This releases energy which pushes against gravity and tries to escape.

  • As long as this balance exists, stars are pretty stable.

  • Eventually, the hydrogen will be exhausted.

  • Medium stars, like our Sun, go through a giant phase,

  • where they burn helium into carbon and oxygen

  • before they eventually turn into white dwarfs.

  • But in stars many times the mass of our Sun,

  • things get interesting when the helium is exhausted.

  • For a moment, the balance of pressure and radiation tips,

  • and gravity wins, squeezing the star tighter than before.

  • The core burns hotter and faster,

  • while the outer layers of the star swell by hundreds of times,

  • fusing heavier and heavier elements.

  • Carbon burns to neon in centuries,

  • neon to oxygen in a year,

  • oxygen to silicon in months,

  • and silicon to iron in a day.

  • And then

  • death.

  • Iron is nuclear ash.

  • It has no energy to give and cannot be fused.

  • The fusion suddenly stops, and the balance ends.

  • Without the outward pressure from fusion,

  • the core is crushed by the enormous weight of the star above it.

  • What happens now is awesome and scary.

  • Particles, like electrons and protons, really don't want to be near each other.

  • But the pressure of the collapsing star is so great

  • that electrons and protons fuse into neutrons,

  • which then get squeezed together as tightly as in atomic nuclei.

  • An iron ball, the size of the Earth,

  • is squeezed into a ball of pure nuclear matter, the size of a city.

  • But not just the core; The whole star implodes,

  • gravity pulling the outer layers in at 25% the speed of light.

  • This implosion bounces off the iron core,

  • producing a shock wave that explodes outwards

  • and catapults the rest of the star into space.

  • This is what we call a supernova explosion, and it will outshine entire galaxies.

  • What remains of the star is now a neutron star.

  • Its mass is around a million times the mass of the Earth

  • but compressed to an object about 25 kilometers wide.

  • It's so dense that the mass of all living humans

  • would fit into one cubic centimeter of neutron star matter.

  • That's roughly a billion tons

  • in a space the size of a sugar cube.

  • Put another way, that's Mount Everest in a cup of coffee.

  • From the outside, a neutron star is unbelievably extreme.

  • Its gravity is the strongest, outside black holes,

  • and, if it were any denser, it would become one.

  • Light is bent around it,

  • meaning you can see the front and parts of the back.

  • Their surfaces reach 1,000,000 degrees Celsius, compared to a measly 6,000 degrees for our Sun.

  • Okay, let's look inside a neutron star.

  • Although these giant atomic nuclei are stars,

  • in many ways, they're also like planets,

  • with solid crusts over a liquid core.

  • The crust is extremely hard.

  • The outermost layers are made of iron left over from the supernova,

  • squeezed together in a crystal lattice,

  • with a sea of electrons flowing through them.

  • Going deeper, gravity squeezes nuclei closer together.

  • We find fewer and fewer protons, as most merge to neutrons.

  • Until we reach the base of the crust.

  • Here, nuclei are squeezed together so hard

  • that they start to touch.

  • Protons and neutrons rearrange,

  • making long cylinders or sheets,

  • enormous nuclei with millions of protons and neutrons

  • shaped like spaghetti and lasagna,

  • which physicists call nuclear pasta.

  • Nuclear pasta is so dense that it may be the strongest material in the universe,

  • basically unbreakable.

  • Lumps of pasta inside a neutron star

  • can even make mountains

  • at most a few centimeters high,

  • but many times as massive as the Himalayas.

  • Eventually, beneath the pasta, we reach the core.

  • We're not really sure what the properties of matter are when they're squeezed this hard.

  • Protons and neutrons might dissolve into an ocean of quarks,

  • a so-called quark-gluon plasma.

  • Some of those quarks might turn into strange quarks,

  • making a sort of strange matter, with properties so extreme,

  • that we made a whole video about it.

  • Or, maybe they just stay protons and neutrons.

  • No one knows for sure, and that's why we do science.

  • That's all pretty heavy stuff, literally, so let's go back out into space.

  • When neutron stars first collapse,

  • they begin to spin very, very fast, like a ballerina pulling her arms in.

  • Neutron stars are celestial ballerinas, spinning many times per second.

  • This creates pulses

  • because their magnetic field creates a beam of radio waves,

  • which passes every time they spin.

  • These radio pulsars are the best-known type of neutron star.

  • About 2,000 are known of in the Milky Way.

  • These magnetic fields are the strongest in the universe,

  • a quadrillion times stronger than Earth's after they're born.

  • They're called magnetars until they calm down a little.

  • But the absolute best kind of neutron stars are friends with other neutron stars.

  • By radiating away energy as gravitational waves, ripples in spacetime, their orbits can decay,

  • and they can crash into and kill each other in a kilonova explosion that spews out a lot of their guts.

  • When they do, the conditions become so extreme

  • that, for a moment, heavy nuclei are made again.

  • It's not fusion putting nuclei together this time,

  • but heavy neutron-rich matter falling apart and reassembling.

  • Only very recently,

  • we've learned that this is probably the origin of most of the heavy elements in the universe,

  • like gold, uranium, and platinum, and dozens more.

  • So there now two neutron stars collapse and become a black hole, dying yet again.

  • Not only do stars have to die to create elements, they have to die twice.

  • Over millions of years, these atoms will mix back into the galaxy,

  • but some of them end up in a cloud, which gravity pulls together to form stars and planets, repeating the cycle.

  • Our solar system is one example,

  • and the remains of those neutron stars that came before us are all around us.

  • Our entire technological modern world was built out of the elements neutron stars made in eons past,

  • sending these atoms on a thirteen-billion-year journey to come together and make us and our world.

  • And that's pretty cool.

  • Until then, we can look at them on paper.

  • The 12,020 Human Space Era Calendar has arrived.

  • You can order it now until we sell out,

  • and then never again.

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  • Get it for Christmas for your friends, families, and kids,

  • or to distract yourself from the fact that there are

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  • A happy interstellar year 12,020.

  • [Outro music]

Neutron stars are one of the most extreme and violent things in the universe.

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