White dwarfs don't get nearly as much attention on the stellar graveyard as neutron stars and black holes do.

A white dwarf is the dead body of what used to be a star roughly the size of the sun.

Stars come in different sizes. The smallest stars are only 1/10th of the sun’s mass and the biggest are several hundreds solar masses. Despite this broad range of mass they all share the same ability to fuse atoms and create energy. This process takes place in all stars up until a certain point when this nuclear oven in the star’s center can no longer be upheld. At this point, the star dies.

When a star dies, it can become one of 3 types of stellar remnants – or space corpses, if you will. The really big stars, those beyond six solar masses, will explode in a violent and spectacular event known as a supernova. This one mortal explosion can outshine an entire galaxy made up by billions of stars due to its extremely high energy outlet. These stars will then end their days as a neutron star or collapse into a black hole.

A spoon full of neutron star weighs as much as a billion cars and a black hole is so dense that not even light can escape. A supernova and its remnants are spectacular and mind-boggling, indeed. But what happens to the stars below six solar masses when they die?

Small Stars Get Puffy – And Turn Into White Dwarfs

Stars below six solar masses, so-called low-mass stars, don’t make as big a show when they die. In fact, in comparison to high-mass stars, a low-mass star will exhale the last breath in a rather uneventful way. The star will swell up and shred the outer layer, and then shrink to what we call a white dwarf. This is how our own sun will die 5 billion years from now, even though for that particular star the event might feel slight more spectacular to us since we have first row seats to the show. Our planet will be shredded of all life and dignity by the puffy exhale from our dying sun.

A white dwarf is what remains after the sun has exhausted nuclear fusion in the core and pushed away its outer layer. They are made up by the ashes left at the center of the sun, mainly carbon and oxygen along with small amounts of other elements. They weigh around the same as the sun, sometimes a bit less, and they never exceed 1.4 solar masses. (The reason for this upper limit is a whole story of its own and we will cover that later.)

White dwarfs don’t get nearly as much attention on the stellar graveyard as neutron stars and black holes do. Who wants to read about a small pile of ashes from an unspectacular death event, when you want get an explosive energetic supernova that ends in a dense ball best described by relativity theory? Well, I do. And you probably do too, since you are still reading this.

Quantum Mechanics Defines The White Dwarf

Black holes are best described by relativity theory, and while that surely is cool, I still find it way more exciting how white dwarfs are big balls upheld entirely by the laws of quantum mechanics.

When the star dies, atoms at the center at squeezed together by the gravitational contraction when the outer layers are shredded. While this death event might be unspectacular in comparison to a supernova, it is still very powerful. Atoms are squeezed to the maximum limit allowed by nature: quantum mechanics. In order to understand how quantum mechanics comes into play here, we need to first look at what an atom is made of. Don’t sorry, I will keep it short.

Atoms are made by a nucleus that consist of protons and neutrons. Around this nucleus is a swarm of electrons. The number of electrons depend on which element it is. Hydrogen has one, Carbon has six and Uranium has 92. That’s the number of electrons that fly around the protons and neutrons in the center. When you “squeeze” an atom, there are rules for how close the electrons can come to the nucleus, and these rules are exactly what we call quantum mechanics.

Quantum mechanics states that no two electrons can be in the same energy state in an atom. A more casual way to understand this is to imagine that a nucleus is surrounded by a ladder and there can only be one electron on one step. So, if you try to squeeze an atom, you can only push the electrons into the steps that are vacant – and you can only have one electron per step. If the inner steps are taken, you have to place the electron further out where a step if free.

This is how atoms are squeezed from the gravitational power during the dying moment of a low-mass star. The powerful event forces the atoms to be so small that only quantum mechanical effects prevents the electrons from being pushed further into the center. This is called a degenerate electron pressure or Fermi pressure. You can not squeeze atoms any further than to this point. If you try, then you destroy the atom – and hence an atom is no longer an atom.

White dwarfs are giant balls of atoms only upheld by quantum mechanics. No other object in the entire universe can say that about themselves!

That’s cool, isn’t it?

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