[music playing] As if black holes and neutron stars aren't weird enough, physicists have very good reason to believe that there are even stranger stellar remnants out there, stars made entirely of quarks.

[music playing] The mathematics of modern physics that emerged through the 20th century explained so much about our universe.

But the same mathematics also hid some surprises.

There are monsters lurking in that math, predictions of phenomena so extreme and bizarre that, as with any monster, it was hard to know whether to credit them with reality.

The most wonderfully monstrous of these are the remnant corpses of the most massive stars, stellar zombies like neutron stars and black holes.

Einstein's general theory of relativity tells us that the core of a dead star must collapse under its own incredible weight.

What happens to the resulting ultra-dense material depends on quantum theory.

We've already talked about how quantum processes save a neutron star from collapse, but ultimately also doom the most massive to collapse into a black hole.

But just shy of that final transition, and on the fringe of our understanding of the quantum universe, a star may become very strange indeed.

Literally, I'm talking about strange stars.

Before we can understand strange stars, we need to start with a stellar remnant that we know for sure exists, the neutron star.

These are weird enough all on their own.

They are created in the final collapse of a very massive stellar core after it has exhausted all possible fusion fuel supplies.

In that collapse, most of the electrons and protons are crunched together to form neutrons.

At a radius of around 10 kilometers, the collapsing core is suddenly halted when those neutrons hit an absolute limit of density, which I'll come back to.

The rest of the in-falling star collides with the new neutron star and ricochets outwards in the most powerful explosion in the universe, a supernova.

The remaining neutron star is millions of Kelvin in temperature, and may be spinning thousands of times per second.

Its immense magnetic field drives jets of material at extreme speeds.

These jets may sweep across the Earth due to the spinning-top-like procession of the neutron star.

We see these as pulsars.

Our understanding of neutron stars seems to fit the behavior of pulsars very well, at least for most of them.

But for some, we see hints of weird things happening deep beneath the star's surface, which we'll get to.

For ordinary neutron stars, that surface is a thin crust of iron, which quickly gives way to a fluid of almost pure neutrons, neutronium, the densest known substance in the universe.

A cubic centimeter weighs a billion tons.

Like I said, this is the absolute limit of density.

Well, nearly, as we'll see soon.

Neutronium is degenerate matter, and I don't mean that in the same way that your parents probably used the word.

Degenerate matter is so compressed that particles can't get any closer together without occupying the same quantum states.

The Pauli exclusion principle states that this is forbidden for fermions, the family of particles that neutrons belong to.

We don't know nearly as much as we'd like about the nature of neutronium.

We can't make this stuff in labs.

And we certainly can't test what happens to it when subjected to the insane pressures at a neutron star's core.

In those conditions, individual neutrons are packed so tight that they begin to overlap.

This may cause neutrons to dissolve into their component quarks.

This so-called quark matter is its very own type of bizarre.

We think that a type of gas-like quark matter, a so-called quark-gluon plasma, filled the entire universe until around a millionth of a second after the Big Bang, the Quark Epoch.

And we have good reason to think that, because we can actually make this stuff in our largest particle accelerators.

Minuscule flecks of quark-gluon plasma exist for tiny fractions of a second after very high-speed particle collisions.

We can study its nature based on the particles that decay from it.

However, the quark matter in a neutron star is forged by insane pressures, not by the greater-than-a-trillion-Kelvin temperatures of the Quark Epoch or the Large Hadron Collider.

In that state, it forms a superfluid rather than a plasma, a superfluid even denser than neutronium.

We sometimes call a neutron star with such a quark matter core a quark star.

Now, neutrons are comprised of one up and two down quarks.

Quark matter made of these quark types would need to be confined by incredible pressures to maintain stability outside the atomic nucleus.

So that probably rules out having an entire star made of this stuff, unless the quark matter is also strange.

It may be that when neutrons disintegrate under high pressure, half of the down quarks are converted to strange quarks.

The result is strange matter.

It's a special type of quark matter.

It has three quark types instead of two, and that means more particles can occupy the lowest quantum energy states.

It's as though the quarks trick their way around the Pauli exclusion principle by having some of them put on silly disguises.

This lower energy state means that strange matter may be the most stable form of matter in the universe, more stable even than iron, which is the most stable atomic nucleus.

A star made entirely of this stuff should be completely stable.

And if they exist, they could exist forever.

We call these strange stars.

Not content even with this level of weirdness, physicists have proposed even more mad ideas for neutron star cores.

If the density is high enough, the conditions at the core may be so extreme that they resemble the time even before the Quack Epoch.

At less than a billionth of a second after the Big Bang, the fundamental forces of nature were not as we know them today.

The electromagnetic force and the weak nuclear force were unified as the electroweak force.

It could be that neutron stars have an electroweak core, an apple-sized ball with the mass of two Earths in which quarks themselves burn.

The outflowing energy is almost all in neutrinos.

And those may provide the final pressure that halts the collapse of some stars into a black hole, at least for another million years or so.

OK.

This is fun stuff.

And it keeps theoretical physicists off the streets.

But can we test any of this madness?

Well, yes.

Have we?

Maybe.

Take the case of 3C58.

In the year 1181, Chinese and Japanese astronomers recorded a new star in the constellation of Cassiopeia.

It faded over six months.

But nearly 1,000 years later, after some small technological advancements, we pointed our radio telescopes and then the orbiting Chandra X-ray Observatory to that spot and found a young pulsar, a rapidly rotating neutron star 10,000 light years distant.

Now, this wasn't so unexpected.

After all, the pulsar in the Crab Nebula was also observed as a supernova by Chinese astronomers in 1054, except something was weird in the case of 3C58.

The x-ray data revealed a surface temperature of a more million Kelvin, much cooler than expected for a neutron star of its age.

A possible explanation is that a quark matter core formed at the heart of this neutron star and is slowly transforming into strange matter.

As down quarks flip into the more massive strange quarks, they have to absorb energy from somewhere to provide for that extra mass.

That energy would be the heat energy of the neutron star.

There are other candidates that could be quark and/or strange stars.

Some appear a little bit too small for their mass, suggesting quark matter densities.

Then there are supernovae that appear way too bright and last too long.

And it's been hypothesized that these may be due to a second explosion as the neutron star collapses further into a quark star.

Even the famous supernova that exploded in the Large Magellanic Cloud in 1987 has been hypothesized to have left behind a quark star.

The dying star shouldn't have been massive enough to leave a black hole, yet astronomers still haven't found the expected neutron star at the location of the supernova.

Nothing is confirmed yet, but there are tantalizing hints that these exotic stars, these monsters in the math, may be very real.

Who knows what other strange denizens lurk in yet to be discovered laws of physics?

And who knows which are actually out there, waiting to be discovered in the expanse of spacetime?