A white dwarf packs the mass of the Sun into a millionth of its volume. Densities inside reach 109 g/cm3, and the only thing preventing further implosion is the pressure of “degenerate” electrons, which, obeying the Pauli exclusion principle, cannot get any closer to each other. (See the feature article by Hugh Van Horn, Physics Today, January 1979, page 23.)

Fifty years ago, Van Horn predicted that as the dwarfs radiate and cool, electrostatic interactions among ionized nuclei in their interiors cause the nuclei to freeze into a lattice through a first-order phase transition. One consequence is the release of latent heat, an effect Van Horn realized might be statistically observable. But white dwarfs are faint, and until two years ago fewer than 200 were known with precise distances. That changed with the first publication of data from the Gaia space observatory, a satellite that has now provided celestial positions of nearly 3 billion stars and increased the number of identifiable white dwarfs beyond 200 000, more than enough to analyze.

Pier-Emmanuel Tremblay (University of Warwick) and colleagues have now presented the first formal evidence that white dwarfs crystallize. They took a subset of the Gaia data—about 15 000 white dwarfs that reside in a sphere of radius 100 parsecs from Earth—and populated a Hertzsprung–Russell diagram with the stars (black dots). In the figure, the absolute magnitude is a proxy for luminosity, and the difference in magnitude of two filtered frequency bands (G BP and G RP ) is a proxy for color or temperature.

The diagram is a snapshot of the stars at different points—from top left to lower right—along their cooling track. As white dwarfs crystallize, the concomitant release of latent heat slows their cooling rate. That slowing causes a statistical pileup that manifests as a greater-than-expected number density of stars at the luminosity where the heat is released. The two orange dashed lines delimit the region where crystallization is theoretically thought to occur. Although the higher number density isn’t evident to the naked eye, the group confirmed it with a comparison between the number of crystallizing white dwarfs expected by their models and the number actually observed per cubic parsec per luminosity bin. (P.-E. Tremblay et al., Nature 565, 202, 2019; thumbnail illustration credit: University of Warwick/Mark Garlick.)