Endpoints of Stellar Evolution:
White Dwarf Stars:
White dwarfs are the endpoints of evolution of stars like the Sun (near
one solar mass).
In fact, the mass limit for a white dwarf is the
Chandresekhar Limit, 1.4 solar masses.
The radius of a white dwarf is about 10
km, that is, planet-sized (the
radius of the earth is about 6000 km).
If a star were to shrink from
about the size of the sun, 10
km, down to 10,000 (a factor of 100), its
) would shrink by 100
If little or no mass
is lost and the volume shrinks by this factor, the density,
would increase by a factor of 10
So the density of a white dwarf star
is on the order of one million grams per cubic centimeter (since the
Sun’s density is about 1 g/cm
The weight of one teaspoon of wd
star material, at the earth’s surface, would be about one ton.
What halts the collapse of a white dwarf star?
A white dwarf can be
thought of as a gas of electrons (plus nuclei that we won’t worry about
Electrons are “fermions” which obey the Pauli Exclusion
principle, which says that only one fermion can be in a given state.
When the electrons fill all the lowest energy states, none can lose any
energy (since all lower states are filled), so the electrons are at T
They are called “degenerate.”
It is also true that two
fermions cannot be in the same place, so they resist being pressed
closer and closer together, which results in “electron degeneracy
The collapse of a white dwarf star is halted by electron
The more massive a wd star is, the smaller it is.
Also, if a star contracts
factor of 100, it should spin 100
= 10,000 times faster, by
conservation of angular momentum.
The sun rotates with a period of