Buoyant crystals halt the cooling of white dwarf stars.

White dwarfs are stellar remnants devoid of a nuclear energy source, gradually cooling over billions of years 1,2 and eventually freezing into a solid state from the inside out 3,4 . Recently, it was discovered that a population of freezing white dwarfs maintains a constant luminosity for a duration comparable with the age of the universe 5 , signalling the presence of a powerful, yet unknown, energy source that inhibits the cooling. For certain core compositions, the freezing process is predicted to trigger a solid-liquid distillation mechanism, owing to the solid phase being depleted in heavy impurities 6-8 . The crystals thus formed are buoyant and float up, thereby displacing heavier liquid downward and releasing gravitational energy. Here we show that distillation interrupts the cooling for billions of years and explains all the observational properties of the unusual delayed population. With a steady luminosity surpassing that of some main-sequence stars, these white dwarfs defy their conventional portrayal as dead stars. Our results highlight the existence of peculiar merger remnants 9,10 and have profound implications for the use of white dwarfs in dating stellar populations 11,12 .