What prevents thermonuclear reactions from occurring at the center of a white dwarf? If no thermonuclear reactions are occurring in its core, why doesn't the star collapse?

Imagine you're trying to push your way through a packed crowd at a concert. There's hardly any room to move because everyone is standing so close together. Something similar happens inside a white dwarf, a stellar core remnant that's incredibly dense. Inside it, there's something called the electron degeneracy pressure. It's like the pressure from the crowd at the concert but way stronger.

This pressure comes from a quantum mechanical principle that says electrons don't like to be in the same state as one another. When a star has used up most of its nuclear fuel, the core gets squished until the electrons are so close together that they resist further squishing due to their quantum nature; this resistance is the electron degeneracy pressure. It's incredibly strong and is what keeps the white dwarf from collapsing under its own gravity, even though it's no longer producing energy through thermonuclear reactions.

You and your friend can't sit in the same seat at the same time, right? The Pauli exclusion principle is sort of the atomic version of this 'no double-occupancy' rule. It's named after Wolfgang Pauli, who said that two electrons (and other similar particles) can't be in the exact same quantum state at the same time.

What this means is that in a space as tiny as an atom, there's a limit to how many electrons can 'fit'. Once all the available quantum states are filled, no more electrons can be added unless they go to a higher energy level. This is what creates the electron degeneracy pressure in white dwarfs that we talked about earlier. The Pauli exclusion principle is super important because it helps to explain a lot of things in the physical world, like the structure of the periodic table and the stability of matter.

Stars are like living things; they're born, they mature, and eventually, they die. This whole life story is what we call stellar evolution. A white dwarf represents one possible ending for a star, specifically for stars that aren't too big. After burning through their nuclear fuel, these stars blow off their outer layers and leave behind their cores, which cool down over time.

Before a star becomes a white dwarf, it goes through various stages. Massive stars live fast and die young, going supernova and often leaving behind neutron stars or black holes. Less massive stars, like our Sun, will swell up into red giants and then shed their outer layers, leaving the core as a white dwarf. Each stage of a star's life cycle produces different elements through nuclear reactions—fusing hydrogen into helium and then helium into heavier elements, all the way up to iron in the most massive stars.

The concept of a quantum state might sound mysterious, but it's really just a way of describing an electron or any other tiny particle. Each electron in an atom has its own 'address,' which involves its energy, location, and spin. This 'address' is what physicists call the electron's quantum state.

Electrons follow strict rules about which quantum states they can occupy, and this is sort of like saying that in a tall building, you can't have two people living in the exact same apartment. In white dwarfs, all the lowest-energy 'apartments' are full, thanks to the Pauli exclusion principle we discussed earlier. This full occupancy at the most fundamental level is what contributes to the stability of these collapsed stars and reveals the deep connection between quantum mechanics and the behavior of stars.