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White Dwarf Atmosphere Recreated In The Lab

To better understand the way white dwarfs emit light, experimental astronomers have recreated the photosphere of these stars using the world’s most powerful X-ray machine

White dwarfs are the glowing lumps of carbon left over after stars have used up all their fuel. They are hot, dense and small, typically with the mass of the Sun packed into the volume of the Earth.  

The structure of these objects is complex. Astronomers cannot see the glowing carbon embers because white dwarfs are always surrounded by a thin, dense layer of gas, drawn in by the star’s intense gravity. 

It is this gas that glows with an intense white light at temperatures  usually between 8000K and 16,000K–by comparison the Sun’s atmosphere is about 6000K. The gas is largely hydrogen but can also contain helium, various metals and carbon. 

These elements are all easy to identify by looking at the characteristic frequencies at which the elements emit and absorb light, a technique known as spectroscopy. However, the intense heat and pressure at the surface of these stars distorts the spectra, causing the lines to spread, for example. 

That’s handy for astronomers because they can use these distortions to work out the pressure at the surface, which depends on the surface gravity. When combined with other data, such as temperature measurements, this allows them to work out the radius and mass of the star. 

So spectroscopy is a hugely powerful tool.

There’s a problem, however. The mass and radius calculated in this way do not always agree with the values calculated in other ways, such as  by measuring the star’s motion through space. 

So astronomers want to better understand the processes that influence the spectra of white dwarf atmospheres.

Enter Ross Falcon at Sandia National Laboratories in New Mexico and few buddies. Sandia happens to have the planet’s most powerful x-ray machine, a device known as the Z Pulsed Power Facility.

These guys use x-rays from this machine to heat a thin wall of gold at the end of a test tube containing hydrogen. The gold rapidly heats the hydrogen creating a high density plasma at a temperatures of 10,000K or so. 

That more or less exactly reproduces the conditions in the atmosphere of a white dwarf. Falcon and co then measure the spectrum of the gas to see how the conditions influence it..

Using the data, these guys have been able to refine astronomer’s models of white dwarf atmospheres to get a better understanding of the stars themselves, their mass and radius, for example.  

The work is far from over however. Falcon and co now want to include some of the other elements that appear in white dwarf spectra. They’ve begun preliminary experiments with helium and hope to examine the spectra of carbon and oxygen in the near future.

Beyond that, Falcon and co want to recreate the powerful magnetic fields that exist around some white dwarfs, to see what influence these have on the spectra. 

That’s easier said than done. The Z Pulsed Power Facility has no problem generating powerful magnetic fields–the difficulty is in using them in a controlled way in an experiment. So there is work to be done by Falcon’s group and others also working at Sandia.

But recreating the surface of stars on Earth–that’s cool.

Ref: arxiv.org/abs/1210.0832: Creating White Dwarf Photospheres in the Laboratory: Strategy for Astrophysics Applications

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