For the first time, pressure more than 100 times found in the Earth’s core has been generated in a lab, setting a new record.
Using the highest energy laser system in the world, physicists have subjected short solid hydrocarbon samples to pressures up to 450 megabars, which means 450 million times the Earth’s atmospheric pressure at sea level.
This is equivalent to the pressure found in the carbon-dominated envelopes of a rare type of white dwarf star – some of the closest objects in the known Universe. It could help us to better understand the effect that this pressure has on changes in the brightness of stars.
Most of the stars in the Universe will end their lives as a white dwarf, including our Sun. As they reach the end of their main sequence, hydrogen-fusing days, they will puff in red giants, eventually expelling most of their material into space as the nucleus falls into a white dwarf – a ‘dead’ star no longer able to support merger.
White dwarfs are close. They can be a maximum of 1.5 times the mass of the sun, packed in an atmosphere the size of Earth. Only what holds so-called pressure for electron degeneracy does not keep the star from falling under its gravity.
At about 100 megabars of pressure, electrons are repelled from their atomic nuclei – and, because identical electrons cannot occupy the same space, these electrons provide the external pressure that the star does not hold in the collage.
This pressure does not affect how compressible the material is, it also reduces the opacity of the plasma that is ionized by the loss of electrons. And the links between these properties are described by the equation of state material, which can also be used to calculate such properties as the temperature profile and the rate.
However, there are some similarities compared to state-of-the-art (EOS) models for extreme pressure; for white dwarf stars, the EOS models may differ by 10 percent along what is known as the shock Hugoniot – the curve that places the increase in pressure and density under compression.
This can be a problem as you try to understand the fundamental properties of the Universe, because white dwarf stars need to be fairly predictable. Although they shine, light is only of residual heat, not fusion, and its cooling rate can therefore be used as a kind of clock to confirm, for example, the age of the Universe, and the ages of the stars around them.
That’s what the research team is trying to solve, using the laser system at the Lawrence Livermore National Laboratory’s National Ignition Facility (NIF).
“White dwarf stars provide important tests of stellar physics models, but EOS models in these extreme conditions are largely untested,” said physicist Annie Kritcher of the Lawrence Livermore National Laboratory.
“NIF can duplicate conditions, ranging from the nuclei of planets and brown dwarfs to those in the center of the sun. We can also deduce in NIF experiments the opacity along the Hugoniot shock. This is a necessary component in stellar studies. structure and evolution. “
The experimental setup consisted of a small, solid, millimeter hydrocarbon (plastic) bead in a hollow gold cylinder about the size of a pencil gum called a cavity. This was then irradiated with 1.1 million joules of ultraviolet light supplied by the lasers, creating a uniform X-ray bath that heated the plastic atmosphere to nearly 3.5 million Kelvin.
The outer layer of the bead was destroyed by ablation, creating a spherical abrasion wave that traveled up to 220 kilometers per second that converted spherically, resulting in increasing pressure as it propagated through the bead.
In fact, all of this happened extremely fast – the shock wave took only 9 nanoseconds to go through the entire sample – but, using X-ray radiography, the research team was able to record the shock Hugoniot, and measure pressure of 100 megabars on the outside of the bead up to 450 megabars by the time it reaches the center.
The pressure inside the Earth’s core is 3.6 megabars. And earlier, the highest pressure reached in this kind of controlled experiment was 60 megabars.
The pressure generated in their experiment, the team said, is consistent with the carbon envelope – the convection region around the core – seen in what are known as “hot DQ” white dwarfs. These are relatively rare; Unlike ordinary white dwarfs, whose atmospheres are primarily composed of hydrogen and helium, hot DQs are primarily carbon atmospheres, and they are unusually warm and clear.
Some of them also pulsate as they spin, resulting in brightness variations. To understand and model these pulsations, we need an accurate understanding of how matter behaves in the star under pressure.
In addition to X-ray radiography, the physicists used X-ray scattering Thomson scattering to measure the electron temperature and measure of ionization in the sample. It also took hot DQ on.
“We measured a reduction in opacity at high pressure, which is associated with a significant ionization of the carbon inner shell,” Kritcher said.
“This pressure range along the Hugoniot corresponds to the conditions in the carbon envelope of white dwarf stars. Our data are consistent with comparison-of-state models that include the detailed electronic shell structure.”
What this means is that ionization will eventually make the material more compressible than models that do not have electronic shells. This places new constraints on the compressibility and opacity of the carbon envelope in hot DQs, which in turn may contribute to a better understanding of their properties and evolution. All this, from a lab experiment on our own planet.
The study was published in Nature.
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