Wednesday, August 18, 2010

White Dwarf Workshop Day 2: Extreme Physics


Yesterday was the second day of this year's white dwarf workshop.   I gave my presentation yesterday, and it went well.  (In a few weeks, video of the talks will be online here. Just not yet.)

Much of the first half of the day focused on physics.  While a lot of astronomy often may appear to involve describing objects, one of our main goals in astronomy is to understand the physics behind all of the beautiful objects in the sky. 

White dwarfs are a great physics laboratory.  Because white dwarfs can contain as much matter as the sun squeezed into a ball only the size of the Earth, the material is very dense.  It is so dense, in fact, that white dwarfs create forms of matter that do not exist on the Earth.  White dwarfs are one of the few ways to study the physics of these extreme environments.

As I mentioned briefly yesterday, white dwarfs slowly cool over time.  The matter in their cores, which starts off as a dense hot plasma, also cools off.  When it cools enough, the matter changes from a plasma to a solid, in fact a crystal.  Since white dwarfs are made out of carbon, and since on Earth crystalline carbon is also known as "diamond", we often claim that crystallized white dwarfs are true diamonds in the sky.  This is not strictly true, since a polished piece of crystalline white dwarf the size of a normal Earth diamond would weigh several hundred pounds. 

Yesterday, one of the talks focused on crystallization.  Since white dwarfs crystallize from the inside out, you might think it isn't possible to catch white dwarfs in the process of crystallizing.  But as white dwarf material crystallizes, it releases heat.  Most crystals, like ice, do the same thing.  The amount of heat released is small, but when you add up a sun's worth of material, it adds up to a lot of heat.  This release of heat temporarily slows the white dwarf's cooling.

When you look at a group of white dwarfs, you will tend to see a whole bunch of different temperatures, since the individual white dwarfs formed at different times and have therefore been cooling for different times.  This slow down in cooling from crystallizing will cause there to be more white dwarfs at one temperature (the crystallizing temperature) than at other temperatures.  It is sort of like a car hitting the breaks on a busy freeway -- cars will quickly pile up where the first car slowed down, and someone watching from an airplane or helicopter can easily spot the slow down.

When we look at groups of older white dwarfs, we indeed see a pile up at certain temperatures.  The exact temperature of the white dwarf pile up depends on what is crystallizing (carbon and oxygen) and the physics of the crystallization.  And we find that the pile up occurs at a different temperature than the physics theorists predicted!

This finding has made a lot of physicists angry.  Some have claimed that the astronomers have made the measurement incorrectly, because everyone who has calculated the crystallizing temperature agrees on the "right" answer.  But this is science - we test theories with observations.  And if the observations don't fit the theory, and if the observations are done properly, then something must be wrong with the theory.

Some physicists have done the scientifically right thing, which is to go back and look at the theory again.  And they've found that some of the assumptions made in the old calculations were wrong.  For example, the old calculations assumed that the crystallizing material was pure carbon.  But white dwarfs are a mix of carbon and oxygen.  And just like adding salt to water changes the freezing point of water, adding oxygen to carbon changes its freezing point. 

This is good science at work.  The observers test scientific hypotheses, and the theorists re-visit ideas that are proven wrong.  Now it is up to us observers to go back to the white dwarfs, measure the crystallization of even more stars, and more tightly constrain the revised theoretical calculations.

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