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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.