Hey Crazies As we saw in the last video, white dwarfs
are pretty awesome.
They’re the left over core of a low-mass star like our Sun shrunk to the size of a
planet or moon.
But we can get even smaller than those white dwarfs.
White dwarves, white dwarves, white dwarves
The next group of stellar corpses is the neutron stars.
Let’s do this!
As you can see from this chart, the original star that makes a neutron star needs to have
at least 8x the mass of our Sun.
Anything less and it’ll only make a white dwarf.
Anything more than 20 and it becomes a black hole.
Neutron stars are the sweat spot in-between.
How come an 8 solar-mass star doesn’t make an 8 solar-mass neutron star?
Because the whole star doesn’t become the neutron star.
Just its core.
A star’s core is where the fusion is happening, but that core is surrounded by several other
layers that are just hot.
Those outer layers are where you’ll find the majority of the star’s mass, but those
layers get blasted away during the star’s death throes.
Only the core is left behind to collapse into a stellar corpse.
So an 8 solar-mass star only makes a 1.4 solar-mass corpse and a 20 solar-mass star only makes
a 3 solar-mass corpse.
A Neutron Star is exactly what it sounds like: a star made of neutrons.
You know those little particles you usually find in an atomic nucleus.
Except the whole star is made of neutrons.
Well, almost.
How is that even possible?!
Because gravity’s a thing!!
That left-over stellar core is heavy, so it collapses under its own gravity, which speeds
up the particles inside.
Mostly the electrons.
The faster they’re going, the more kinetic energy they have.
If the core collapses enough, the total energy of the electrons will make up for the mass
difference between the protons and neutrons.
At that point, the protons will react with the electrons and become neutrons.
This inverse beta decay, as it’s called, happens through most of the collapsing core.
After the star’s millions or billions of years of life, this last process is very violent
and very fast.
In a matter of hours-to-days, the extreme temperatures release gamma-rays that smash
larger nuclei like Iron into smaller nuclei like Helium.
The inverse beta decay turns protons into neutrons releasing a flood of neutrinos into
space.
That core will continue to collapse until something stops it.
Cue the Pauli Exclusion Principle!!!
That principle doesn’t just apply to electrons.
It applies to all fermions.
Neutrons are also spin-1/2 particles.
They’re fermions too.
But because they’re neutral, they can get a lot closer together.
They reach densities comparable to atomic nuclei before they stop collapsing.
Unlike the white dwarf, which is the size of a planet or moon,
a neutron star would be the size of a small city.
Is it true that a teaspoon would weigh as much as a mountain?!
Uh not exactly.
Ok, so a teaspoon (prop) of neutron star matter, also known as neutronium, would have the mass
of a small mountain.
But mass and weight are not the same thing.
On the surface of a neutron star, it’s going to weigh a lot more than a mountain because a
neutron star's gravity is much stronger than Earth’s.
A trillion times stronger!
What if we scooped it up and brought it to Earth?
It would explode.
The only thing keeping that neutronium stable is the gravity of the neutron star.
You won’t get that teaspoon very far before most of it decayed back into protons and electrons
releasing unimaginable amounts of radiation very quickly.
You Dead!
Plus, if you’re scooping that matter from the outside of the neutron star you’re probably
not getting just neutrons.
The electrons on the outside of the collapse weren’t moving fast enough to make more
neutrons, so that stuff still behaves more like white dwarf matter.
This also explains how it’s possible for neutron stars to make magnetic fields.
Remember, magnetic fields require charge.
Neutral things can’t make them.
Thankfully, neutron stars have some protons and electrons left over on the outer edges.
Those particles are the source of a neutron star’s strong magnetic field.
But that field doesn’t necessarily line up with the star’s rotation.
At the north and south poles, you’ll find outward jets of charged particles and, since
those charges are accelerated, they produce light that beams into space.
Light that could be anywhere on the spectrum from radio to gamma-rays.
This creates a kind of lighthouse effect, and if things line up just right, we can receive
a regular pulse here on Earth.
If that’s the case, we call it a “Pulsar” which is just short for pulsing star and these
things spin fast fast!!!!
So the pulses are more like this or even faster.
This used to be the only way we could discover them, but, now with LIGO, we can detect more
of them by their gravitational waves.
Especially, when they hit each other like this!
Ow!
The BEEP was that for?
On top of that, because of gravitational warping neutron stars would look really weird up close.
You could actually see part of its back when looking at the front.
These things are weird.
So, got any other questions about neutron stars?
Ask in the comments.
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