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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 planetary 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 X-rays (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.

Evolution of a main sequence star
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 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 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

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