The Life and Death of Stars#

Stars hang around because of a delicate balancing act.

• A massive amount of hot plasma (millions of billions of trillions of tons) is constantly being pulled inward by its own gravity.
• This squeezing is so strong it forces atomic nuclei together – that’s fusion.
• Usually, hydrogen fuses into helium, which releases energy.
• This energy pushes outwards, resisting gravity.
• As long as this push and pull are balanced, stars are pretty stable.

Eventually, a star runs out of hydrogen.

• Medium stars (like our Sun) puff up into a giant phase, burning helium into carbon and oxygen, before ending up as a white dwarf.
• But in much more massive stars, things get wild once the helium is gone.

The Collapse: Gravity Takes Over#

For a moment, the balance between the outward pressure from fusion and the inward pull of gravity breaks. Gravity wins.

• The star’s core gets squeezed even tighter.
• The core burns hotter and faster.
• Meanwhile, the star’s outer layers swell hundreds of times bigger.
• Heavier and heavier elements start fusing:
  • Carbon burns to neon (takes centuries).
  • Neon burns to oxygen (takes about a year).
  • Oxygen burns to silicon (takes months).
  • Silicon burns to iron (takes only a day).

The End: Iron and Supernova#

And then, it’s death.

• Iron is the end of the road for fusion. It’s like nuclear ash; you can’t squeeze energy out of it by fusing it.
• Fusion suddenly stops. The balance is completely gone.
• Without the outward pressure from fusion, the core is absolutely crushed by the immense weight of the star’s outer layers.

What happens next is both awesome and terrifying:

• Normally, particles like electrons and protons don’t want to be crammed together.
• But the pressure from the collapsing star is so intense that electrons and protons are forced to fuse into neutrons.
• These neutrons get squeezed together as tightly as particles are packed in an atomic nucleus.
• Imagine an iron core the size of the Earth getting squashed into a ball of pure nuclear matter the size of a city.

And it’s not just the core. The entire star implodes!

• The outer layers are pulled in by gravity at a frightening 25% the speed of light.
• This massive implosion slams into the incredibly dense iron core.
• The impact creates a powerful shock wave that blasts back outwards.
• This shock wave catapults the rest of the star’s material into space.
• This huge explosion is what we call a supernova.
• A supernova explosion can actually outshine entire galaxies for a while.

Extreme Density#

Neutron stars are unbelievably dense.

• They are so dense that the mass of all living humans would fit into one cubic centimeter of neutron star material.
• That one cubic centimeter weighs about a billion tons.
• Think of it like putting Mount Everest in a cup of coffee.

Extreme Properties#

From the outside, a neutron star is mind-bogglingly extreme:

• Its gravity is the strongest known, second only to black holes.
• If it were any denser, it would collapse and become a black hole.
• Gravity is so strong that it bends light around the star, meaning you can see the front and even parts of the back simultaneously.
• Their surface temperature can reach a staggering 1,000,000 degrees Celsius.
• Compare that to our Sun’s surface, which is a relatively cool 6,000 degrees Celsius.

The Crust#

Let’s go layer by layer through the crust:

• Outermost layers: Made of iron left over from the supernova. This iron is squeezed into a crystal lattice structure. A ‘sea’ of electrons flows through this lattice.
• Deeper in the crust: Gravity squeezes atomic nuclei even closer. You find fewer and fewer protons because most have merged into neutrons due to the immense pressure.
• Base of the crust: Nuclei are squeezed so hard they start touching. Protons and neutrons rearrange into weird shapes: long cylinders or sheets. These are enormous nuclei containing millions of protons and neutrons. Physicists affectionately call these shapes nuclear pasta (like spaghetti and lasagna).

Nuclear Pasta and Neutron Star Mountains#

• Nuclear pasta is thought to be the strongest material in the universe, basically unbreakable.
• These lumps of pasta inside a neutron star can actually form ‘mountains’.
• These mountains are tiny, maybe only a few centimeters high at most.
• But because of the density, these tiny mountains are many times more massive than the Himalayas!

The Core: A Mystery#

Eventually, below the nuclear pasta, you reach the core.

• We aren’t really sure what matter is like when it’s squeezed this hard.
• Possible states of matter in the core:
  • Protons and neutrons might dissolve into an ocean of quarks, forming a quark-gluon plasma.
  • Some of these quarks might change into “strange quarks,” creating a type of exotic strange matter (which is weird enough for its own video!).
  • Or, maybe the protons and neutrons just stay protons and neutrons, just packed incredibly tightly.
• No one knows for sure, and that’s precisely why scientists are still researching them.

That’s a lot of heavy stuff (literally!), so let’s head back out into space.

Pulsars and Magnetars#

This rapid spin, combined with intense magnetic fields, creates interesting effects:

• Their magnetic field forms a beam of radio waves.
• As the star spins, this radio beam sweeps past us, like a lighthouse beam.
• We detect these pulses of radio waves every time the beam points our way.
• These are called radio pulsars. They are the best-known type of neutron star.
• About 2,000 pulsars are known in our Milky Way galaxy.

Neutron stars also have the strongest magnetic fields in the universe.

• Right after they are born, these fields can be a quadrillion times stronger than Earth’s magnetic field.
• Neutron stars with these super-strong fields are specifically called magnetars until their fields calm down a bit.

The Origin of Heavy Elements#

When two neutron stars merge in a kilonova, the conditions get unbelievably extreme for a moment.

• These conditions are perfect for making heavy nuclei again.
• But this time, it’s not fusion putting things together. It’s heavy, neutron-rich matter coming apart and reassembling into different heavy elements.
• Very recently, we’ve learned that these kilonova explosions are probably where most of the heavy elements in the universe come from.
• This includes famous ones like gold, uranium, and platinum, plus dozens of others.

So, after the kilonova, the two neutron stars collapse and become a black hole, dying yet again.

• It’s kind of wild – stars have to die to create elements, but to get the really heavy ones, they seem to have to die twice.