White Dwarf Stars
A white dwarf is what stars like the Sun become after they have
exhausted their nuclear fuel. Near the end of its nuclear burning
stage, this type of 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 white dwarf, with a temperature exceeding 100,000 Kelvin.
Unless it is accreting
matter from a nearby star (see Cataclysmic
Variables), the white dwarf cools
down over the next billion years or so. Many nearby, young white dwarfs
have been detected as sources of soft, or 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).
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A typical white dwarf is half as massive as the Sun, yet only slightly
bigger than Earth. An Earth-sized white dwarf has a density of 1
x 109 kg/m3. 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. This makes white dwarfs one of
the densest collections of matter, surpassed only by neutron
stars.
What's inside a white dwarf?
Because a white dwarf is not able to create internal pressure
(e.g. from the release of energy from fusion, because fusion has
ceased), gravity compacts the matter inward until even the electrons
that compose 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.
In a normal gas, this isn't a problem because there aren't enough
electrons floating around to fill up all the energy levels completely.
But in a white dwarf, the density is much higher, and all of the
electrons are much closer together. This is referred to as a
"degenerate" gas, meaning that all the energy levels in its atoms are
filled up with electrons. For gravity to compress the white dwarf
further, it must force electrons where they cannot go. Once a star is
degenerate, gravity cannot compress it any more, because quantum
mechanics dictates that 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.
However, there is a limit on the amount of mass a white dwarf can have.
Subrahmanyan
Chandrasekhar discovered this
limit to be 1.4 times the mass of the Sun. This is appropriately
known as the "Chandrasekhar limit."
With a surface gravity of 100,000 times that of 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, gravity pulls the atmosphere close around it in a very thin
layer. If this occurred on Earth, the top of the atmosphere would be
below the tops of skyscrapers.
Scientists hypothesize that
there is a crust 50 km thick below the atmosphere of many white dwarfs.
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: December 2010
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