White Dwarf Stars
A white dwarf is what stars
like our Sun become after 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
planetary
nebula. Only the hot core of the star remains. This core
becomes a very hot (T > 100,000K) young white dwarf, which cools
down over the course of the next billion years or so. (That is,
unless it is
accreting
matter from a nearby star (see Cataclysmic
Variables).)
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.
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An Artist's conception of the evolution of our Sun (left)
through the red giant stage (center) and onto a white dwarf (right). |
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
neutron
stars.
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 not able to create internal pressure
(e.g. from the release of energy from fusion), gravity
crushes it down until even the very electrons making up a
white dwarf's atoms are smashed 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 known 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 the electrons are forced close together; soon all the energy levels
in its atoms are filled up with electrons. If all the energy levels
are filled, and it is impossible to put more than two electrons in each
level, then 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 fusion, 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.
Subrahmanyan Chandrasekhar discovered this
limit to be 1.4 times the mass of our Sun. (This is appropriately
known as the "Chandrasekhar limit".)
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. If it were on earth, this atmosphere would be lower
than the tops of skyscrapers!
Underneath the atmosphere of many white dwarfs, scientists think
there is a 50 km thick crust. At the bottom of this crust is a crystalline
lattice of carbon and oxygen atoms. Since a diamond is just
crystallized carbon, one might make the comparison
between a cool carbon/oxygen white dwarf and a diamond!
Last Modified: November 2006
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