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A crystal of calcium ions in a trap, illuminated by a laser, is a tiny model of the matter inside a white dwarf. There, in the depths of a dying star, atomic nuclei arrange into a rigid lattice as the temperature drops. In the lab, physicists recreated this moment: they cooled a cloud of ions to a few millikelvins and watched it freeze into an ordered pattern.

Then they mixed in xenon ions with a higher charge, like inserting a few heavy boulders into a neat brick wall. As long as impurities are scarce, the lattice doesn't notice the intruders and freezes at the same temperature. But when the xenon fraction exceeds a few percent, the behavior changes: crystallization requires stronger cooling, and the shift grows almost linearly with the amount of impurity.

This effect matters for astrophysics: white dwarfs and neutron stars cool, and the moment their interiors crystallize affects how they shine and how long they live. Standard models often treat stellar matter as pure — made of a single type of nucleus. The experiment shows that even a small fraction of heavy elements can shift the freezing point, and thus the entire thermal history of the star.

Now theorists have an experimentally verified correction: exactly how impurities hinder crystallization. This will allow more accurate calculations of white dwarf ages, and by them, like clocks, we determine the ages of star clusters and even the entire Galaxy.

Original abstract

as written by the authors arXiv:2603.14980

We report a laboratory measurement of how impurities shift Coulomb crystallization in a strongly interacting ionic system. This is achieved by using laser cooled Ca$$^+$$ crystals doped with a controlled number of Xe$$^{12+}$$ highly charged ions. We find that the crystallization threshold is unchanged at low impurity concentration, but shows a clear crossover once the impurity content becomes sufficiently large, after which the shift grows approximately linearly. Complementary measurements reveal that this global effect originates from a local pinning of the crystal around the impurities. We further show how the measured shift could impact standard models of crystallization in white dwarfs and neutron stars. Our results provide an experimental route to incorporating impurity effects into models of multicomponent Coulomb matter, relevant to stellar crystallization and strongly coupled plasmas.