I wanted to tell a short story about a star I found interesting. It’s nothing like I’ve blogged about before, but I haven’t blogged in a long time and I need to write SOMETHING.

At the heart of the Crab Nebula is a little star that I will call Crabby. Its real name is PSR B0531+21, but no one’s going to remember that name so Crabby it is. Crabby is a star that exploded in 1054 AD, and the Crab Nebula is the remnants of that explosion.

What’s so special about Crabby is that we know *exactly* when it exploded, because Chinese astronomers recorded it. For almost every other supernova remnant in the galaxy we don’t have that kind of information, it’s hard to tell whether something exploded a million years ago or a *billion* years ago because we’re viewing it from so far away. So knowing the exact *year* that Crabby exploded is like knowing someone’s time of birth down to the millisecond. It gives us incredible precision that we normally don’t get in astronomy.

And that precision has let scientists come up with some wacky theories, such as the theory that Crabby is a “strange star.”

Now what’s a strange star exactly? Isn’t it a little rude to call a star “strange” in the first place? Well it requires a little bit of quantum mechanics to explain this, but read on because I promise it’s worth the read.

See, when a star normally collapses, the force of gravity becomes so great that it compactifies the matter within to a ridiculous extent. The core of a collapsing star is usually made of heavy elements like iron, and the collapse forces those atoms as close together as physically possible. There’s a simple rule in quantum mechanics: two things can’t occupy the same exact space, and this rule leads to an absolute limit of density: the density where atoms can’t get any closer together or they’d literally overlap.

Here on earth atoms aren’t usually that close together. Liquids and gases of course let atoms slosh about, but even solids on earth usually have their atoms arranged in intricate patterns with a little bit of space in between each atom. White dwarfs crush this down into the absolute limit of density, around a ton or more per cubit centimeter.

So white dwarfs like this are ultra compact objects with ultra compact structures. But gravity can do even better than a white dwarf.

See the strength of gravity depends on mass: more mass, more gravity. So what happens in the cases of a VERY massive white dwarf? Their high mass means they want to crush things down even denser, but we’ve already reached the limits of atomic density, so where else can we go?

Well remember that atoms themselves are mostly made up of empty space, there’s a lot of emptiness between the nucleus and the electrons, for example. Atoms themselves can be crushed down into just their neutrons, with each proton absorbing an electron to become a neutron. When we crush down atoms in this way, we remove all that empty space they contain, and that lets us crush the whole star down to *even greater levels of density*.

At this level we have a “neutron star,” a star so crushingly dense that it’s the size of a city with the mass of the sun. Our friend Crabby is theorized to be a neutron star. But it might possibly be even more special.

Some have theorized that Crabby is a bit too cold for its age, and how could it have lost all that heat? The answer may bring us to the fringes of theoretical physics.

See just like atoms can be crushed down into neutrons, neutrons can be crushed down further. Neutrons are made up of 3 quarks: 2 down quarks and 1 up quark. But those quarks are much smaller than the neutron itself, and between them is mostly empty space. So once again, if the gravity of the star is strong enough, a neutron star can be “crushed down,” until its neutrons are crushed into quarks.

The whole star itself can’t be crushed like this, or else it would get so small that it would become a black hole. But the core of a massive neutron star may be so dense that its neutrons are indeed crushed into quarks, releasing all that empty space within the neutrons, and leaving a quark-rich soup that resembles the instant after the big bang. We have no firm evidence of this happening, but if it happens, then such a star would be called a quark star.

And then some massive quark stars might have a final trick up their sleeves. Remember that rule from earlier, about how 2 things can’t occupy the same space? That rule is what holds each of these type of stars from crushing down too far. White dwarfs can’t let their atoms occupy the same space, it’s only when the force of gravity is *large enough to crush atoms* that a white dwarf gets crushed down into a neutron star.

Similarly, neutron stars can’t let their neutrons occupy the same space, and that’s what keeps them from crushing down further. But when gravity is strong enough to *crush neutrons*, then parts of the neutron star might devolve into a quark star.

But in a quark star, the neutrons are crushed into 2 down quarks and 1 up quark each. Now here’s the funny thing about that quantum rule I told you about: two things can’t occupy the same space right? Well that’s only true *if they’re the same type of thing*. Two neutrons can’t occupy the same space because they’re both neutrons. Same with 2 down quarks, they can’t occupy the same space either. But a down quark and an up quark? They can occupy the same space no problem.

So the neutrons are crushed into 2 down quarks and 1 up quark right? 1 down and 1 up quark can occupy roughly the same space, they can take up 1 “unit” of space together. But that second down quark? It needs to find a *different* space for itself to occupy.

But let’s bring gravity back into the picture: we’ve already seen how it can crush atoms down to neutrons, and neutrons down to quarks, can it crush these quarks even further? Well not directly, quarks are “fundamental particles,” there’s nothing they could crush down into. But there is an *indirect* form of crushing that could happen…

Remember that 1 down and 1 up quark can share the same space but that second down quark has to find a new space. If we could change that second down quark into something else, we could free up that space so gravity could crush further, but how?

Well while all the “regular” matter of our universe, stars, planets, bloggers, is all made of just up and down quarks, there’s more quarks then just them hiding in the fringes of particle physics.

The strange quark is another type of quark, different from up and down. It’s decidedly strange indeed, since it doesn’t exist anywhere in the universe except for fractions of a second at very high energies. But that’s not important, what *is* important is that *it’s another type of quark,” and just like how up and down quarks can share the same space because they’re different from each other, a strange quark *can also share that space* because it’s different from both up and down quarks.

So at the very highest limits of density, when neutrons themselves are crushed and when their quarks are crushed further, the sheer force of gravity might force those quarks to take drastic action. Half of the down quarks from the neutrons might convert into strange quarks, so that they can take up less space with the other quarks and crush the star yet further.

If this is the case, and these strange quarks do exist at the hearts of massive neutron stars, then this may the *only* place they exist for any significant length of time. Usually strange quarks decay because they’re so unstable, but at the crushing densities here, strange quarks are forced to *exist* because it’s the only way to crush down far enough under gravity. Despite being unstable, strange quarks here *can’t* decay, because gravity is forcing them to stay strange.

Now this is all very theoretical, but it begs the question: do strange stars even exist?

Maybe

And it begs another question: is Crabby a strange star?

Maybe maybe

Like I said if the neutrons are crushed into quarks and eventually strange quarks, it will only happen at the very center of the neutron star, a place we can’t observe and can only speculate about. But there is tantalizing evidence: we know exactly how old Crabby is thanks to those Chinese astronomers who recorded its explosion, and our evidence shows it to be a fair bit colder than we’d expect for a neutron star of its age, why could that be?

It’s been suggested that when neutrons are crushed into quarks and then strange quarks, that bursts of neutrinos are created which carry away much of the star’s heat into the cosmos. It *may be* that such a thing happened to Crabby, which is why it’s so cold.

This theory wouldn’t even be possible if we didn’t know how old Crabby is, if it were just another neutron star we would always have questions about if it’s actually just an older star that has had time to cool down, rather than being a young star that is unusually cold. And we owe it all to the astronomers from the past who first saw Crabby in their night sky, nearly one thousand years ago.