Planet around a white dwarf!!! WHAT'S A WHITE DWARF? A thread. https://twitter.com/aussiastronomer/status/1306250152341110786

A white dwarf is the small, dense remnant of a small star that's reached the end of its normal life cycle. Here you can see a comparison: a white dwarf has the mass of a star that's about the size of the Earth. https://twitter.com/aussiastronomer/status/1306251195644891136?s=20

A white dwarf is the little cousin of neutron stars and black holes. Here's a lovely chain showing how these "compact" objects form, thanks to @BlackHoleCam: https://blackholecam.org/a-massive-star-collapsing-in-upon-itself-forms-a-black-hole/

Short answer: they collapse due to gravity! But what prevents a white dwarf from collapsing to become a black hole or a neutron star? The answer is quantum mechanics!

But let's start with a simple question: why doesn't the Earth's atmosphere collapse? After all, air has mass and the Earth's gravity should pull it down, right?

The key is that the atmosphere is warm. If you remember your Ideal Gas Law from high school chemistry , you know that the pressure of a gas depends on its temperature. Let's also remember that pressure is force per unit area.

So the hotter the atmosphere is (), the more pressure there is throughout the atmosphere. Where does that come from? It's just molecules crashing into each other!

A molecule in the air feels gravity pulling it downward, and it's getting jostled from all sides by other molecules. But it's warmer below (you know this if you've climbed a mountain, for example), and so the molecule gets more bumps from below than above.

The net effect is a small upward force! So our molecule settles to a point where that force balances the force of gravity. It's called "hydrostatic equilibrium." The fluid (air) reaches a steady state.

A similar process works in the Sun, except the Sun (you'll be surprised to hear) is quite a bit hotter and more massive than our atmosphere.

So the great pressure inside the sun holds it up against its own immense gravity. On long timescales the internal heat of the sun is maintained by nuclear fusion, but on a millennium-to-millennium basis, there's plenty of residual heat! Don't believe me? https://twitter.com/SuperASASSN/status/1295101777268953088?s=20

But what happens after billions of years? The Sun can't burn hydrogen into helium forever, if for no other reason than it has only a finite supply of hydrogen. The helium may undergo fusion for a bit too, if it gets hot enough.

However far the fusion goes, you end up with layers in the interior of the star: leftover hydrogen on the outside, helium inside that, carbon and oxygen inside that, etc. Kinda looks like an onion! https://stardate.org/astro-guide/gallery/stellar-onion

ALMOST!

Because on long timescales, the star depends on nuclear reactions to keep it nice and hot. If it ends up with a core of carbon and oxygen but isn't hot enough to burn those into silicon, sulfur, or iron, those nuclear reactions will gradually cease.

Actually this is fun because now gravity gets to take over. What happens then is that the star starts to collapse under its own weight. But before it can become a black hole (assuming it's large enough), it has another hurdle to overcome: the Pauli Exclusion Principle!

Does this ring a bell from high school chemistry? I hope so! Remember that electrons are tiny particles with a property called spin. [Can a particle w/ 0 volume spin? Don't @ me.] Spin can be "up" or "down." The Pauli Exclusion Principle says that ups don't like ups for roomies.

Really it means that two electrons (fermions) can't be in the same state with the same spin. You can't put two ups or two downs in the same energy level. It's not allowed.

So your star can collapse under its own weight, but as gravity squishes the star, it starts to push the electrons in the atoms together. Eventually, the atoms get mashed together so hard that all those low-lying energy levels get filled up.

At that point, it is not possible to push the electrons closer to each other. This is called "electron degeneracy pressure," and it is what keeps the dense cores of stars and white dwarf stars from collapsing under their own weight.

In a weird twist, "degenerate" objects get smaller if you make them more massive! It's not like a ball of clay, where if you add more clay you get a bigger ball.

Now what if you've got a really big star and it's got a lot of mass? Well, degeneracy pressure is only effective up to a point (the Chandrasekhar limit). If you squeeze hard enough, you can overcome it (essentially cramming the electrons into the nuclei of their own atoms).

If you do that, via the collapse of a more massive star, the resulting particle physics processes turn those electrons and nuclear protons into neutrons. You end up with a much more dense object that is extremely rich in neutrons: a neutron star!

Neutrons, too, have an effective degeneracy pressure that allows such a star to sustain itself against gravity. But again, there's a limit too. You can estimate how massive a neutron star can be because if it's too big, you end up with a speed of sound faster than light speed!

The result is that neutron stars probably can't have more than about 3 times the mass of the sun. As far as we know, any stellar remnant with a mass greater than that limit (the Tolman-Oppenheimer-Volkoff limit) should be a black hole. We don't know of anyhting else it could be!

Anyway that's a bit of a digression today. But it's truly one of my favorite parts of astrophysics when the small (quantum mechanics) and the big (stars etc) collide in such interesting and exciting ways. Congrats to the team on this exciting discovery of a neat planet!