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Spec Tech: White Dwarfs

August 5, 2010

White dwarfs are the fossilized remains of once proud stars. Just because they are past their prime doesn’t mean they are boring by any means. Many humans, and presumably other creatures out there too, find them among the most interesting objects in the universe.

Stellar Life Cycles

Evidence is growing that most stars form the same way. What gives a star its character is how much and what kind of gas it formed from. If you cram enough gas into a star it gets huge, and consequently the pressures in it’s core are gigantic. Stars are held together by gravity. The combined mass of 1030 kilograms of gas is constantly trying to collapse in on itself! As the pressure builds up in the core it heats up and eventually starts a nuclear furnace. The energy expelled by the fusion of elements manages to counteract this immense gravitational pull, at least until it runs out of fuel. Really huge stars tend to burn through their fuel in mere hundreds of millions of years (instead of many billion). They are destined for exciting conclusions; the core of the star collapsing in on itself into a black hole while simultaneously exploding the outer layers into space in what is called a Type II supernova.

But the lighter stars just don’t have the chutzpah to go supernova. Once out of fuel they too can no longer prop themselves up against the constant strain of gravity, and start to collapse. As this happens they heat up and blow off the outer layers into what is called a ‘planetary nebula’. This has nothing to do with planets, the term in a vestige from the days of squinting through telescope eyepieces and guessing what one was looking at. But what happens to the leftover core of the star is very interesting.

White Dwarfs, Stellar Laboratories

White Dwarfs are so interesting because they are a real world example of some very strange quantum mechanic behavior. Most people are familiar with the idea of conservation laws, like conservation of energy. It’s one of the “unbreakable” laws of physics that define how systems behave. But there also exists a lesser known rule called the Pauli exclusion principle.

The Pauli exclusion principle boiled down states that certain kinds of particles cannot be at the same place at the same time, which if read in reverse means that other kinds of particles can be at the same place at the same time. It turns out that this principle neatly explains a lot of modern chemistry, electrical resistance in metals, and—among other things—the existence of white dwarfs.

It’s All About Electrons

Electrons are one of those particles that have to obey the Pauli exclusion principle. And there a lot of electrons in stars. In fact there are a lot of electrons in everything. So as a normal star starts to collapse, eventually it will get so dense that the electrons will simply refuse to get any closer together! This is a phenomenon called ‘electron degeneracy’. Once in this state the star’s core will not collapse any further despite the now unimaginable gravity and density. If left alone this new white dwarf will stick around forever, slowly cooling off as the universe expands around it.

Diamonds and Density

White dwarfs, being squeezed to the point of degenerate electrons, lend themselves to all kinds of large numbers. Most are about the size of the Earth, but have an average density of around 109 kg/m3, meaning that one cubic meter (about the size of a household washing machine) of it would weigh one billion kilograms, or a little more than 2 billion pounds! The gravity on its surface would be about 100,000 times stronger than on Earth. White dwarfs probably have an atmosphere of leftover hydrogen, but with gravity like that the gas above it would be squished down, only extending a hundred feet or so above the surface.

Since most stars turn a lot of their remaining hydrogen and helium into carbon, nitrogen, and oxygen just before turning into white dwarfs the interior may be mostly composed of carbon. Once it cools down that carbon is in a crystallized state, in other words, a giant diamond! Objects like V886 Cen have been theorized to be mostly made of a diamond like form of carbon inside. Imagine a diamond the size of the Earth!

Boom! Supernova

White dwarfs are very unlikely candidates for life. There is a habitable zone around them, but it would constantly move as the star cooled off. Furthermore any planet that existed before the collapse into a white dwarf would have most likely been destroyed by the collapse itself. Nonetheless it is possible for some enterprising aliens to drop into orbit and set up a home. Perhaps to mine diamond? Like most intergalactic mining schemes there are usually much more practical (and boring) ways to get materials, but why not!

They might not want to stay there for long though, if a white dwarf is near another star or source of gas, it can slowly build up mass until even degenerate electrons can’t stop it from collapsing anymore. If enough mass is piled on the electrons actually merge with protons and form neutrons, collapsing the star into a neutron star. The resulting release of energy is enormous. It can be seen across the universe, temporarily outshining a billion suns! It’s called a type Ia supernova and is a fitting end to a remarkable object.

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