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Q&A: Black Holes

Q:
Black holes are usually made from collapsing stars because these stars exceed the Chandrasekhar limit. However, I've found that in other types of stars when they begin to collapse, the exclusion principle stops the atoms from getting too close to each other. I know that the exclusion principle has to do with the electrons not having the same charge, spin, mass, and position but what exactly does this principle really state? Also, when a star collapses into a black hole, why does the exclusion principle not apply? I saw the show "Live from the Edge of Space and Time" that was on the NASA channel and found it very helpful, but I still had some questions.

A:
The Pauli exclusion principle states that no 2 electrons with the same properties can exist in the same orbit (same orbit means the same place inside the atom). Every electron has the same mass and charge, the only property that can be different is the spin. An electron can have spin up or spin down. Spin can be thought of as a kind of internal angular momentum. So when electrons are filling up the orbits or "shells" of atoms, each orbit can have only 2 electrons in it, one of spin up, and one of spin down. The next electron that enters the atom will have to go in another orbit, or, if all orbits are full, it has to go away.

But what that really means is that when the electron approaches the filled up orbit, it will feel a lot of resistance. It won't be impossible to stick another electron into the orbit, but it will take a lot of energy. So the Pauli exclusion principle just states how electrons normally behave, not how they behave in very extreme environments.

An example of an extreme environment is inside a massive star that is collapsing. If the star is greater than 3 times our Sun's mass, once it starts to collapse it will end up as a black hole. Inside this collapsing star the gravitational energy is so big that all the particles get smashed together, despite the exclusion principle of electrons.

For more information on black holes, how they form and their different masses, see the Chandra black hole field guide:
http://chandra.harvard.edu/xray_sources/blackholes.html

The following pages should be helpful to you, they explain white dwarfs, which are the kinds of stars that don't have enough mass to collapse past the exclusion principle energy barrier:
http://chandra.harvard.edu/xray_sources/white_dwarfs.html

There is a particularly good explanation of the exclusion principle in the previous link.