Electron and Neutron Degenerate Pressure

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Forces that Hold Collapsing Stars Together

Solar Mass: <1.44

Degenerate Electron Pressure Counteracts the Force of Gravity

Results in a White Dwarf

Between 1.44 and 2 or 3 Solar Masses

Degenerate Electron Pressure fails, Degenerate Neutron Pressure Counteracts the Force of Gravity

Results in a Neutron Star

Solar Mass >  ~2-3

Degenerate Neutron Pressure is Not Enough to Overcome the Force of Gravity

Results in a Black Hole

   Information from sources [15] and [16]


In stars the force of gravity is constantly attempting to compress the star while the force of pressure is constantly attempting to balance out this compressing force.  For a stable star, these two forces are equal.

By the Fermi Exclusion Principle it is accepted that no two fermions of an atom can occupy the same energy state.  This means that no two electrons can be in the same state, unless they have different spins.  Also, neutrons in the nucleus cannot be in the same energy level as other neutrons.  This all is illustrated below:



Electrons have different spins, which allows

two electrons  to occupy each energy level,

provided they have opposite spins.

Image From: http://www.physicscentral.com/explore/action/gas-1.cfm[18]

 For Fermions, even as pressure builds, no two

can be located in the same energy state.  This causes

them to "stack up" in effect.  Only under great force

can this be overcome.

Image From:http://www.ipodphysics.com/thermal-states-of-matter.php [17]


This means that if a star is under pressure it may be possible that this degenerate electron or neutron pressure will support the star.  So, as stars begin to collapse, atoms are compressed and electrons move to lower energy states.  However, as electrons fill up energy states, they reach a point that they cannot be compressed any further without two electrons being in the same state.  This means that eventually degenerate electron pressure equal the force of gravity and the star becomes stable again.

            If the star is massive enough, and the force of gravity great enough, electron degenerate pressure can break down.  Then, as the atoms are further compressed, degenerate neutron pressure begins to build in an attempt to counteract the force of gravity.  If the neutron pressure is great enough to balance out the force of gravity than a neutron star is created.  If however, the degenerate neutron pressure breaks down under the force of gravity, then a black hole is created.

            If a star is less than 1.44 solar masses then electron degeneracy will be strong enough to counter act the forces of gravity and the star will become a white dwarf [15].  If the star is more massive than 1.44 solar masses than electron degeneracy will eventually break down and neutron degeneracy will be supporting the stellar body [15].  This is called a neutron star.  If the star is massive enough, even neutron pressure can break down.  This is what occurs when black holes are formed.  For neutron pressure to break down masses must be larger than about 2 or 3 times that of the sun [15].







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