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How some white dwarfs stay young

How some white dwarfs stay young

15 March 2024

An intricate crystallization process may rejuvenate dying stars for billions of years.

All-sky map of the white dwarfs observed by the Gaia spacecraft.
About 230 000 white dwarf stars identified by the European Space Agency’s Gaia satellite are charted in this all-sky map. Credit: ESA/Gaia/DPAC

The languishing remnants of once-vibrant stars, white dwarfs are destined to fade to darkness. They are commonly used as age markers in galactic surveys because of the seemingly straightforward mechanism governing their existence: With no internal energy source, they are essentially glowing embers that get redder and dimmer at a predictable rate as they cool (see the article by Hugh M. Van Horn, Physics Today, January 1979, page 23).

Yet a select group of those stellar embers may have a powerful means of delaying the inevitable. Detailed in Nature by Antoine Bédard, Simon Blouin, and Sihao Cheng, a newly proposed antiaging mechanism could explain a puzzling population of ostensibly ageless stars and help solidify researchers’ understanding of how white dwarfs evolve.

The new study addresses one of several white dwarf–related surprises that were embedded in a 2018 data release from the European Space Agency’s Gaia mission. The star-mapping satellite uncovered tens of thousands of the small, faint cinders that are left behind when all but the most massive stars run out of fusion fuel and collapse. When researchers charted the properties of the new finds, they noticed a glut of relatively bright white dwarfs and concluded that some were cooling about a billion years more slowly than expected.

The white dwarf stars identified by the Gaia mission are plotted by color and luminosity.
Some of the white dwarf stars detected by the Gaia satellite defy the conventional wisdom that they get steadily redder and dimmer as they age. The white dwarfs in the Q branch are particularly quirky, appearing as blue and as bright as objects billions of years younger. Credit: Adapted from ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

Those researchers accounted for the anomaly with a half-century-old theory. White dwarfs pack a solar mass’s worth of ionized nuclei into an Earth-size sphere, forcing the ultradense matter into a liquid-like state. In 1968 Hugh Van Horn predicted that those nuclei would become increasingly sensitive to electrostatic forces as they cooled. Starting in the star’s center, where matter is packed the tightest, and spreading outward, those forces would spur the nuclei to freeze into a lattice (see Physics Today, March 2019, page 14). Latent heat released during the liquid-to-solid phase change would slow the cooling of the star.

Yet even that energy boost could not explain the properties of a small percentage of white dwarfs that appear to have hit pause on cooling for some 8 billion years. To explain that outlier population, Bédard, Blouin, and Cheng considered a class of relatively massive white dwarfs that are composed primarily of carbon and oxygen with impurities in the form of heavier neon nuclei.

A comparison of a white dwarf crystallizing from the inside out and one with crystals that float toward the surface.
(a) White dwarfs can receive infusions of latent heat as their component nuclei crystallize from the inside out. But some of those stars may undergo a more complex process (b) in which crystallized nuclei rise and melt, hastening a migration of mass that generates gravitational energy. Credit: Adapted from A. Bédard, S. Blouin, S. Cheng, Nature 627, 286 (2024)

The researchers surmised that as such white dwarfs cooled, the crystals that emerged in their centers would expel much of the neon, much like sea ice does salt as it forms in the ocean. The carbon–oxygen crystals would be less dense than the surrounding neon-containing liquid, and so they would rise toward the surface. During their ascent they would melt, fortifying the white dwarf exterior with carbon and oxygen. The once-homogenous white dwarf would gradually develop a neon-rich core surrounded by a lighter liquid layer. Along with the latent heat from crystallization, the white dwarf would receive infusions of gravitational energy as mass concentrated inward, and the already-compact star would contract even further.

Bédard, Blouin, and Cheng calculated that the combined thermal and gravitational energies could curb a white dwarf’s cooling and delay the start of inside-out crystallization for 7 billion–13 billion years. They also simulated populations of white dwarfs that employed such a process and were able to reproduce the color–luminosity distribution of the ageless stars observed by Gaia. Next the researchers plan to analyze binary white dwarf systems and look for differences in the crystallization-induced aging delays between the partner stars.

Bédard, Blouin, and Cheng hope to pin down the intricacies of white dwarf evolution so that they can update the models that astronomers use to estimate the ages of stellar populations throughout the Milky Way. The ubiquity of crystallization-induced delays, whether the 1-billion- or 13-billion-year variety, suggests that some of the reference white dwarfs used in galactic surveys are older—perhaps far older—than they appear. (A. Bédard, S. Blouin, S. Cheng, Nature 627, 286, 2024.)

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