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When you think of a frozen object in space, you might think of Pluto, but stars themselves actually freeze.

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If I ask you to imagine something frozen in space, you might think of distant objects like comets or Pluto. Objects that are far, far away from a star. But an object doesn’t have to be at the fringes of its system, far away from any star, to freeze.

Under the right circumstances, stars themselves actually freeze. In fact, in several billion years, even our own Sun will become a frozen crystal sphere. It all starts when a star like our Sun runs out of fuel.

Right now, the Sun and other mid-sized stars are fusing hydrogen into heavier elements in their cores. And that produces all their heat and light. And it also creates the pressure that keeps them from collapsing under the pull of their gravity.

But once a star uses up its fuel, there’s no outward pressure to fight against gravity, and it collapses into a small, denser star called a white dwarf. White dwarfs are typically about as big as Earth, but they are about as massive as the Sun, which makes them ridiculously dense. There’s basically no space between the atoms that make up a white dwarf.

Those are usually carbon and oxygen atoms, the remnants of that initial hydrogen fuel. In fact, these atoms are so tightly squeezed together that the electrons surrounding each atomic nucleus merge into one big sea of electrons. And they leave behind a bunch of naked carbon and oxygen nuclei, which are positively charged.

Despite being so dense, those nuclei are also moving around. That’s because every substance has what’s called thermal energy. That’s energy that’s contained within its atoms and molecules, causing them to vibrate and knock into each other.

The hotter a substance is, the more its particles move around. At first, these particles are zipping past each other so quickly that they don’t exert very strong forces on each other, so they form a liquid. But that does not last forever.

Since the star is no longer fusing atoms together in its core, it’s also not producing heat, so it begins cooling, and the motions of the carbon and oxygen nuclei in its core slow down. At slower speeds, the positively charged nuclei that were whizzing past each other start interacting like magnets that repel each other. But because of gravity, they can’t completely get away from each other, so the best way for them to minimize their repulsive forces is to arrange themselves into a crystal lattice.

That’s a repeating pattern where atoms lock into position, basically holding each other at arm’s length. Since that effectively maximizes the space between them, the nuclei fall into place. Starting at the core, which is the densest, and moving outward.

In the end, the entire star becomes a frozen crystal. In fact, white dwarfs that are largely crystallized carbon are not that different from diamonds on Earth. Yeah.

Diamond stars are a thing. As exotic as it sounds, the process is pretty similar to how water freezes on Earth. The main difference is that the freezing point for a white dwarf is somewhere in the millions of degrees Celsius.

Now, white dwarfs are the only stars that freeze all the way through, but other stars can freeze to some extent. Specifically neutron stars. Like white dwarfs, neutron stars are the remnants of stars that have run out of fuel.

Except these are the collapsed cores of gigantic stars that exploded as supernovas. And they are even denser than white dwarfs. In neutron stars, gravity is so strong that it doesn’t just collapse the space between atoms, it collapses the atoms themselves.

When that happens, electrons merge with protons in the nucleus to form neutrons. And what’s left is a star made almost entirely of neutrons. In these stars, even the outer layer is denser than the core of a white dwarf.

And there, atomic nuclei freeze into a crystal lattice a lot like what is in a frozen white dwarf. Unlike white dwarfs, the crust of a neutron star is made mostly of iron nuclei, rather than carbon and oxygen. So it’s no diamond, but it is still ridiculously solid.

Like, simulations have found that it’s about ten billion times harder to break than steel. But oddly enough, while the rest of the star is even denser, the deeper layers don’t freeze. That’s because, deeper down, the nuclei squeeze so closely together that they start to touch and interact with each other, forming all sorts of different shapes.

And those interactions keep them from arranging themselves into a nice, neat lattice. They’re just way too dense! Still, the bottom line is that we have frozen stars just floating around the universe.

For the most part, our understanding of freezing stars has come from models and computer simulations. It’s hard to actually see white dwarfs and neutron stars, since they’re really dim. So probing for evidence that they freeze is tricky.

But it has been done! For instance, one team of researchers looked for evidence of what’s called latent heat in the freezing of white dwarfs. Latent heat is the amount of energy it takes for a substance to change phase.

That’s what we call switching between states of matter, like liquid to solid. And this process can release heat as atoms slow down and settle into place. So, astronomers had long hypothesized that, as white dwarfs cool, they reach their freezing temperature and then release latent heat as they begin to freeze.

That release of latent heat would temporarily pause their cooling. So the astronomers expected to see white dwarfs linger right around the freezing temperature before cooling further. To test their hypothesis, the authors of a 2019 study examined data on 15,000 white dwarfs, and they actually identified a population of these stars that appeared to hit a pause in their cooling, just as expected.

On neutron stars, scientists have found even more direct evidence of the solid crust… in the form of starquakes. Starquakes are violent waves that can crack open the crusts of neutron stars. And basically, they wouldn’t happen the way they do if the crusts weren’t solid.

The science it takes to explore these extraordinary objects is almost as extraordinary as the stars themselves. But what it shows is that you don’t need to go into the fringes of a star system to find a frozen world. Right at the center, there could be a frozen crystal ball.

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