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The Art and Science of Making White Dwarfs in the Desert

The Art and Science of Making White Dwarfs in the Desert

Astronomer Don Winget is making white dwarf "star stuff" here on Earth.

To re-create the surface of a white dwarf star, University of Texas at Austin astronomer Don Winget starts with roughly the electricity needed to power a few TV sets for the evening. He runs that through a ring of big old generators, all pointing inward toward the center of a machine more than 100 feet across and 20 feet high.

The generators compress the electricity into tight pulses, so it flows into containers holding gas. The gas is ionized with lasers, further compressing the pulses into finer spaces and shorter increments of time. Rinse and repeat until all the electricity flows down into a delicate array of tungsten wires clustered together inside a Twinkie-sized chamber about 35 centimeters from the center of the machine.

For a few nanoseconds, the power and density of the current is so great — more than six times the total amount of energy released by all the power plants in the world — that it vaporizes the wires, generating a gas-like substance known as hydrogen plasma. It simultaneously produces a magnetic field powerful enough to cause the plasma to implode.

For that brief nanomoment, as the magnetism “pinches” the plasma, generating a massive burst of X-radiation, Winget has a chunk of white dwarf star of his very own.

With this “star stuff,” Winget and his colleagues are poised to solve many of the mysteries of white dwarfs, which are extremely dense stars that are the ultimate end state of most stars in the universe, including our sun.

They’ll also use that knowledge to gain insight into the archaeological history of star formation in our galaxy, the nature of dark matter, and the conditions at the center of our sun, where the density is close to that of a white dwarf. The experiments may even help push humanity closer to the dream of nuclear fusion as a meaningful energy source.

“If we can solve this, it will rattle through whole fields of astrophysics and physics,” says Winget.

Until very recently, if you’d asked Winget whether he’d like to make some white dwarf stuff in the lab, his answer would have been simple: Lovely idea. Not going to happen.

“For decades I’d been telling students that what I do is purely an observational science,” says Winget, a professor in the Department of Astronomy and a pioneer in the study of white dwarfs. “It’s not an experimental science.”

Then Winget was approached by Jim Bailey, a physicist at Sandia National Laboratories in Albuquerque, N.M. Bailey was working with the facility’s Z Machine, which is the largest X-ray generator in the world.

The machine was primarily used to model nuclear weapons and investigate the potential of nuclear fusion as an energy source. But Bailey and Greg Rochau, also of Sandia, had begun to run experiments to see whether it could be used to simulate the conditions of stars.

“The environment we experience everyday on this planet is extraordinarily rare in the universe, if not unique,” says Rochau, manager of imaging and spectroscopy at Sandia. “Even though we think of these experiments as studying ‘extreme’ states of matter, the goal was actually to give us a small window into what the universe considers to be quite normal.”

Some of the light signatures Bailey and Rochau were generating looked familiar to Winget and his group, who quickly realized that with the machine they could make plasmas with the same conditions as those found at the surface — the photosphere — of a white dwarf star. They could make an actual bit of white dwarf star in the lab.

They all agreed to pursue this idea, and on April 14, 2010, Winget stepped across the threshold from being an observational to an experimental scientist.

“I stood next to the door, saw the flash, heard the boom, and felt the seismic wave,” he wrote in an email, sent that day, to his colleague Ed Nather. “Today you, and everyone else on the planet, were closer to a white dwarf photosphere than anyone has ever been ... the spectrograph was 5cm away — much better than 30-700 light years we are used to!”

For Winget it was a moment that transcended science.

“I get this feeling of awe in my life on two occasions,” he says. “One is when I go to the telescope at the McDonald Observatory at night. I feel like I’m flying a spaceship through the universe, searching for things no one has ever seen before. The other is at the Z Machine when that shot goes off. There is no way to miss the power that is happening.”

The science was nice too. Until that point, Winget and his colleagues in the field of “asteroseismology” had developed a very precise tool to infer what the insides of white dwarfs were like based on the pulsations of the light that traveled from the stars through space to humanity’s telescopes.

“It’s like how sometimes you can hear one note, and you can say, ‘That’s Carlos Santana,’ ” says Winget.

“That is what we do with the inside of the star. The star shakes from its own intrinsic pulsations, from turbulent convection at the surface of the star, and we take those frequencies and learn about everything below the surface.”

With this method the field had learned a great deal, but there were limits to what could be inferred from distant observation. The density of the hydrogen plasma surface of the star, in particular, was making it hard to properly interpret the light signatures.

“The surface of a white dwarf is on the order of 10,000 times more dense than the surface of our sun,” he says. “At these densities the whole is different than the sum of the parts.”

Without a good enough understanding of the physics of that surface, says Winget, it’s impossible to fully infer the conditions of the interior of the stars.

With the experiments at the Z Machine, he and his colleagues expect to change that. As the data from the machine accumulates, they should be able to refine their theoretical models, at which point they’ll go back to their telescopes and see whether their predictions align with the data from distant white dwarfs.

“Dr. Winget and his research team are uniquely suited to maximize the impact of these experiments on our understanding of the universe,” says Rochau.

For Winget, it’s the combination of practical application, basic scientific discovery and pure awe that makes this work so exciting.

“I make a point now, when I’m teaching,” he says. “I tell the students there has been a total transformation in the field. We can do experiments. We’re not just observing anymore.”

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Thursday, 15 April 2021

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