Recreating White Dwarf Atmospheres in the Lab
In 1863, a telescope maker named Alvan Clark discovered why a star known as Sirius seemed to wobble back and forth. It had a companion star, difficult to see because it is a white dwarf — the faintly glowing lump of carbon that is left over after stars burn through their fuel. The binary system is now known as Sirius A and Sirius B, and white dwarf stars have fascinated astronomers ever since.
Now physicists at Sandia National Laboratory in New Mexico are getting into the white dwarf game, collaborating with their astronomer colleagues to recreate the atmospheres of white dwarf stars in the lab. Their instrument? A powerful x-ray-generating machine at Sandia known as the “Z-pinch.”
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When we see the light from white dwarf stars, we’re not seeing the glow from the carbon embers because there is a thin layer of dense gas getting the way, pulled in by the star’s gravity. It is the gas — an atmosphere of sorts, made up primarily of hydrogen — that emits the light.
There are other elements present, too, most notably helium, certain metals, and of course, carbon. We know this because astronomers have used spectroscopy to identify those elements — a technique that measures the telltale frequencies at which different elements absorb and emit light.
The spectra are a bit smeared, due to the high heat and pressure at the surface of white dwarf stars. That blurring can be useful, helping astronomers identify surface pressure, for instance. Combine that data with temperature measurements, and it’s possible to determine the radius and mass of a given white dwarf star — or at least a reasonable estimate of those properties.
But those calculations don’t always agree with other methods used to determine the same properties. And that’s where the Sandia scientists come in.
The Z pinch is the world’s largest electrical generator, housed in an entire building. It creates lightning-like tangles of startling color for a few billionths of a second as it fires. An initial burst of electricity creates a magnetic field that compresses, or “pinches,” a gas of charged particles, producing x-rays.
Ross Falcon and other Sandia physicists are using x-rays generated by the Z-pinch to essentially recreate the atmospheric conditions of a white dwarf star in the laboratory.
They fire the x-rays at a thin gold target at the end of a tube filled with hydrogen, heating it up to 10,000 degrees Kelvin or so, thereby producing a plasma (ionized gas). This is a close approximation to the real thing, and subsequent analysis of the plasma’s spectra has been sufficient to further refine the models being used by astronomers to calculate the mass and radius of white dwarfs.
Thus far their spectral analysis has been limited to hydrogen, but they will soon begin experimenting with helium, carbon and oxygen. And since the Z-pinch generates magnetic fields on a par with those that exist around white dwarf stars, it should be possible to figure out how those fields might affect the elemental spectra — assuming fields of such magnitude can be sufficiently controlled. Creating them is the easy part.
When all is said and done, we should know a great deal more about how white dwarf stars emit light.
Images: (top) Artist’s impression of Sirius A and Sirius B. Credit: NASA, ESA and G. Bacon (STScI). (bottom) Sandia’s Z-pinch in action. Source: Sandia National Lab.