White Dwarf Stars
A white dwarf is what stars
like our Sun become when they have exhausted their nuclear fuel. Near
the end of its nuclear burning stage, such a star expels most of its outer
material (creating a
nebula), until only the hot core remains, which then settles down to
become a very hot (T > 100,000K) young white dwarf.
Since a white dwarf has no way to keep itself hot unless it is accreting
matter from a nearby star (see Cataclysmic
Variables), it cools down over the course of the next billion years or so.
Many nearby, young white dwarfs have been detected as sources of soft
(i.e. lower-energy X-rays); recently, soft X-ray and extreme ultraviolet
observations have become a powerful tool in the study the composition and structure of the
thin atmosphere of these stars.
An Artist's conception of the evolution of our sun|
through the red giant stage and onto a white dwarf.
A typical white dwarf is half as massive as the Sun, yet only slightly
bigger than the Earth. This makes white dwarfs one of the densest forms of
matter, surpassed only by
What's Inside a White Dwarf?
To say that white dwarfs are strange is an understatement. An earth-sized
white dwarf has a density of 1 x 109 kg/m3. In
comparison, the earth itself has an average density of only 5.4 x
103 kg/m3. That means a white dwarf is 200,000
times as dense!
Because a white dwarf is no longer able to create internal pressure, gravity
unopposedly crushes it down until even the very electrons that make up a
white dwarf's atoms are mashed together. In normal circumstances,
identical electrons (those with the same "spin") are not allowed to occupy
the same energy level. Since there are only two ways an electron can spin,
only two electrons can occupy a single energy level.
This is what's know in physics as the Pauli Exclusion Principle. And in a
normal gas, this isn't a problem; there aren't enough electrons floating
around to completely fill up all the energy levels. But in a white dwarf,
all of its electrons are forced close together; soon all the energy levels
in its atoms are filled up with electrons. Well, if all the energy levels
are filled, and it is impossible to put more than two electrons in each
level, than our white dwarf has become degenerate. For gravity to compress
the white dwarf anymore, it must force electrons where they cannot go.
Once a star is degenerate, gravity cannot compress it any more because
quantum mechanics tells us there is no more available space to be taken up.
So our white dwarf survives, not by internal combustion, but by quantum
mechanical principles that prevent its complete collapse.
Degenerate matter has other unusual properties; for example, the more
massive a white dwarf is, the smaller it is! This is because the more mass
a white dwarf has, the more its electrons must squeeze together to
maintain enough outward pressure to support the extra mass. There is a
limit on the amount of mass a white dwarf can have, however. It was found
by Subrahmanyan Chandrasekhar to be 1.4 times the mass of our Sun, and is
is call the "Chandrasekhar limit" after its discoverer.
With a surface gravity of 100,000 times that of the earth, the atmosphere
of a white dwarf is very strange. The heavier atoms in its atmosphere
sink and the lighter ones remain at the surface. Some white dwarfs have
almost pure hydrogen or helium atmospheres, the lightest of elements.
Also, the very strong gravity pulls the atmosphere close around it in a
very thin layer, that, if were it on earth, would be lower than the tops
of our skyscrapers!
Underneath the atmosphere of many white dwarfs, scientists think
there is a 50 km thick crust, the bottom of which is a crystalline
lattice of carbon and oxygen atoms. One might make the comparison
between a cool carbon/oxygen white dwarf and a diamond! (After all,
a diamond is just crystallized carbon!)
Last Modified: November 2004