A shield of iron in the Sun

In the Sun, iron ions offer greater resistance than expected to the propagation of electromagnetic waves: these are the results of a study led by physicist Jim Bailey (Sandia National Laboratories) thanks to a machine that recreates the extreme conditions inside of our star.
The Sun is a huge nuclear furnace: at its core is produced, thanks to the fusion reactions, a huge amount of energy that propagates outward, and then up to be radiated into space. This path, however, is far from simple: in the first part of their ascent photons propagate with irradiative phenomena – a bit as it happens to the heat that is released from a radiator.



At some point, however, this mechanism loses effectiveness and takes over the convective that, through huge plasma cells, ferrying the energy from the interior of the star to the surface (in this case, the analogy with the life of all days is that of water that boils inside a pot). The main responsible for this change are the chemical elements heavier than hydrogen and helium, which astronomers call generically metals.
With theoretical models describing the internal structure of the Sun and verify their validity with comments is always one of the toughest challenges for astrophysicists. A challenge that was further complicated a few years ago. Some of spectroscopic measurements of the radiation coming from the Sun indicated fact that the abundances of elements such as oxygen, nitrogen and carbon in our star had to be lower than predicted by the models, and not a little lower but between thirty and fifty percent. A real earthquake for the experts of stellar structures: this meant they had to revise the behaviors of other fundamental parameters that describe the state of the material within a star, such as its temperature and density, the variation of their distance from the center.
But now, a group of researchers led by Jim Bailey of the Sandia National Laboratories in Albuquerque, New Mexico, presented in an article published in the Nature Journal the results of laboratory studies that seem closer to the theory to observations. Indeed, the team studied as iron, one of the metals present in the solar plasma, inhibits the transport of radiation at very high temperatures and densities, finding that its efficiency is between 30 and 400 percent greater than previously thought, according to of the wavelength of the radiation with which it interacts. With these numbers it is thus able to justify more or less half of the corrections required to restore the agreement between theoretical predictions and experimental observations.

Physicist Jim Bailey


Credit: Randy Montoya, Sandia National Laboratories

The researchers have come to these results using the Z machine, an apparatus capable of generating in a sample of material the size of a sand grain, temperatures of the order of 2 million kelvin and so observe the behavior of the atoms of the iron in conditions similar to what happens inside the stars, “among the most mysterious of the universe,” says Bailey. “Those areas – continues the researcher – are too opaque to be observed from the outside and too hot to send us any probe. Thus, the physics that studies the behavior of atoms in the solar plasma has never been put to the test. In our experiments, instead we can recreate temperatures and densities similar to those found in the region inside the Sun where you experience the transition from the system of irradiative transfer in the convective. We focused on the measurement of the opacity of iron, one of the few chemical elements that play the most important part in the processes of irradiative transfer. ”
Although these results are still partial, since they were not yet investigated, the behavior of other chemical elements present in the stellar material, however, will allow improving the accuracy and reliability of the theoretical models describing the internal structure of the Sun and stars.

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